Sag Your Glass

Today’s chatter about glass has been a long time coming, relating to glass sagging experiments that were done about 4 years ago. Despite a whole year of covid-19 boredom, only now do I put finger to keyboard to bring you the results of the experiments.

The aim of the experiments was to better understand how and why different colours and types of glass begin to melt at different temperatures. My hope is that the results and my conclusions will in some ways assist you with your own glass work.

The Experimental Setup

The underlying method for the experiments is very simple and illustrated in cross-section by the diagram you see here. Click it to see it full size in a new window, if you wish.

Put simply, two mullite dams were laid parallel to each other with a 4.5 inch gap between them. Over the tops of the dams were laid strips of Bullseye thinfire paper which not only stopped the overlaid glass from sticking to the dams but also provided a suitable base on which the sagging glass could slide.

Five inch lengths of glass were laid at right-angles to the dams, on top of the thinfire paper, with a quarter-inch overlap at each side.

Glass Used and Placement

The same type of glass was placed in the exact same position within the kiln across all the experiments to ensure that subtle temperature differences within the kiln did not cause spurious effects in the results.

The positional sequence of glass was as follows and you will notice that the list contains a selection of thirteen transparent and opal glasses from Bullseye and includes an orange transparent glass from the Wissmach Corella range (which to my eyes looks more like a red glass with a slight orange tint). The numbers in brackets are Bullseye’s product codes, should you wish to repeat my experiments.

Clear Tekta (1101)
Petal pink opal (0421)
Black opal (0100)
White opal (0113)
Pink opal (0301)
Turquoise opal (0116)
Dusty Lilac opal (0303)
White opaque opal (0013)
Turquoise transparent (1116)
Aventurine blue (1140)
Red opal (0124)
Canary yellow opal (0120)
Wissmach Corella Orange

Some types of Bullseye glass were chosen deliberately whilst others were arbitrarily chosen to provide a wider mix of colours. The logic behind my choices was as follows:

The clear Tekta was chosen because we often use clear glass and it provides a baseline for when no “colourants” have been added to make glass colourful. The black was chosen as we’re often told that black melts the fastest. There are two kinds of white, one of which is intense and the other slightly translucent and I had already noticed that they behaved differently. Aventurine blue and pink were chosen because I’d observed that they often seemed to be more highly processed than I had expected in some of my glass work. The opal and transparent turquoise glasses were chosen partly because I had plenty of that kind of glass but mostly to ensure that at least one colour could be compared in opal and transparent forms. The Wissmach Corella Orange glass was deliberately chosen as an example of a different manufacturer’s glass to compare against the Bullseye range. Remaining colours of glass were chosen to fill in gaps in the colour spectrum.

You will notice that most of the types of glass are opal. There is no particular reason why opal rather than transparent glasses were chosen other than a vague suspicion that it is opal glasses that tend to melt at a lower temperature. The aventurine blue glass is a strange case because it is technically a transparent glass but has been “overloaded” with “colourants” to the point that it looks more like an opal colour.

Had I been doing the experiments again I think the kinds of glass I would use would be subtly different, as would be their placement. With age and practise comes greater experience!

The type of glass used in a particular experiment could be pieces of 1mm stringer, 2mm stringer or pieces of 2mm or 3mm sheet glass of varying widths. Most of the experiments used 0.5 inch wide strips of 3mm glass.

The Kiln Schedule

Other than the ‘top temperature’ the same kiln schedule was used for all the experiments and is given in the table you see here.

   Step  Rate       Target   Hold
   1     300°C/hr   520°C    0:05
   2     150°C/hr   Varies   0:20
   3     END

The essence of the kiln schedule was to provide a reasonably rapid climb in temperature until 520°C, followed by 5 minutes to allow for temperatures to equalise for all experiments, then a further increase, dependent on the particular firing, at a relatively slow rate until the target temperature was reached (varying from  520°C to 590°C)  and held for 20 minutes before allowing the kiln to cool down at its natural rate.

Whether or not you approve of the kiln schedule does not matter as much as it is consistently applied across all the experiments.

A Hint of Physics

It would be remiss of me to not introduce a little maths or science to turn you all into nerdy glass workers, so today you will learn (or be reminded) of a small amount of physics that relates to the arrangement of the glass on top of the dams. We’re talking about “Forces” and “Moments” and unusually I will not give you any Wikipedia references because they are ridiculously horrible to read. Instead I point you to a BBC web page here  where you will find ‘revision’ information presented in a gentler form.

In the diagram you see here is what we might call a physicist’s perspective of the way the pieces of glass will be affected by gravity. Remember that it’s the force of gravity that will be causing the melting glass to sag. Click it to see it full size in a new window, if you wish.

What you need to recognise by comparing this diagram with the earlier diagram is that the inside edges of the dams are going to act as places where the glass will pivot as it begins to sag. For the sake of simplicity we will ignore the fact that as the glass sagging increases these dam edges are also what the glass will have to slide over.

The triangles in the diagram represent points at which the glass will pivot, so another word to describe these points is to call them fulcrums. Notice also that most of the glass is between the two pivots and that there are short “dangly bits” hanging over the ends at each side.

Next I want you to notice the downward pointing arrows which represent the pulling forces caused by gravity. The arrows at the ends are small because they represent a small force resulting from gravity acting on a small amount of glass. The arrow in the middle is much longer because it is representing a larger force resulting from gravity acting on the greater bulk of the glass.

Although our knowledge of the lengths, combined with the weight of the glass, would allow us to precisely calculate the three forces we will not bother. All you need to understand is the downward force can be calculated by multiplying weights by the distances away from a fulcrum and that most of the effect acts as if at the “centre of gravity” of the piece of glass.

So, what does the physics tell us? How does it help explain what should happen?

Well, we have very small downward forces at each end and a much bigger downward force in the middle. This means that when the glass begins to sag we can expect it to mostly sag at the middle and, perhaps, just a tiny little amount at the left and right extremes.

We also expect the inside edges of the dams to act like pivots, at least for smaller amounts of sagging. But we also have to realise that when the glass has sagged a considerable amount the ends will tend to slide over the pivots and will ultimately tend to “drop off the edge” at one side or the other, landing on the kiln shelf.

Recording the Results

The mullite dams used for the experiment are 38mm tall so it would not be possible to measure any sagging that is, or would have been, more than 38mm. At this point the glass will be touching the kiln shelf. But we have to also remember that the overhang at each end of the glass pieces is only a quarter of an inch.

In the course of the kiln firings it was found that a sagging of more than 30mm tended to result in the glass piece slipping and sometimes falling off the dams at one side or the other resulting in uncertainty about exactly how much the sagging actually was. So, degrees of sagging of more than 30mm were all recorded as 30mm.

Degrees of sagging were measured by putting the sagged piece of glass on its side on top of millimetre graph paper, butted against a flat edge that was carefully laid along one of the graph lines. It was then a simple task to measure the largest amount of sagging to the nearest millimetre.

Before I bore you to tears, if not already, let’s get down to business and look at some of the results from the glass sagging experiments.

Some Illustrative Results

The picture you see here is a full thirteen glass sample set of 0.5 inch strips for the 565°C firing. The glass strips are sequenced in order of how much they sagged.

This picture is a very visual demonstration that different kinds of glass, even from the same manufacturer’s range, do not behave exactly the same once they start to soften in a kiln.

Experiment 1 – Strips of Clear Glass

In this experiment 1.0 inch and 0.5 inch strips of 3mm clear Bullseye Tekta glass are sagged alongside a 0.5 inch strip of thinner 2mm Bullseye Tekta glass. Look at the graph for the results. Click it to see it full size in a new window, if you wish.

It is very interesting to see that the 1.0 inch strip of 3mm glass sagged in a remarkably similar way to the 0.5 inch strip of 3mm glass. One might have expected the greater weight of glass in the 1.0 inch strip to cause it to sag more, or perhaps we might expect the reverse situation if it is the strength of the strips to be of greater significance.

Notice also that the thinner 2mm thick strip of glass sagged at a lower temperature than the equivalent 3mm thick strip of glass. We might say the difference is about 10°C because it is the same shape of curve, but shifted to the left. This begins to hint that it is not the weight but rather it is the strength of a strip of glass that affects its sagging behaviour.

Something to bear in mind is that glass is unusual in that it does not suddenly melt. It does not suddenly change from a solid to a liquid like most other materials. In this sense, when our glass is at a temperature where is is just about able to sag slowly, we’re seeing the glass in a “mostly solid” state.

Experiment 2 – Turquoise Stringers and Strips

In this experiment a 2mm stringer, a 1mm stringer and a 0.5 inch strip of 3mm transparent turquoise Bullseye glass were sagged alongside a 0.5 inch strip of 3mm Bullseye clear Tekta glass.

This experiment is allowing us to make two comparisons. The first comparison is to look at the sagging of stringers against a 0.5 inch strip of glass of the same colour. The second comparison is to look at the sagging of a transparent turquoise coloured 0.5 inch strip of glass against an uncoloured clear 0.5 inch strip of Tekta glass. It is also possible for us to compare the results of this experiment to those of experiment 1.

Look at the graph for the results of this sagging experiment. Click it to see it full size in a new window, if you wish.

In experiment 1 we noticed that a 2mm thick strip of glass sagged more than a 3mm thick strip of glass for a given temperature. In this experiment we notice that a 1mm stringer sags more than a 2mm stringer which in turn sags more than a 0.5 inch strip of 3mm glass for a given temperature.

We are therefore beginning to get a clearer picture that it is the strength of the glass rather than the weight of glass that seems to be most important. Remember that we’re dealing with glass in a “nearly solid” state. We can see that stringers will begin to melt at lower temperatures than their equivalent sheet glass and that we might say that the 1mm stringers melt at a temperature around 5°C lower than the equivalent 2mm stringer which in turn melts at a temperature around 5°C lower than the equivalent strip of 3mm sheet glass.

Something else to observe with this experiment is that the turquoise glass always begins to soften and sag at a lower temperature than the 3mm clear Tekta glass. At lower temperatures there is little difference but as the temperature increases the differences are more marked.

Comparing these two types of sheet glass begins to hint that perhaps the colour, if not specific colours, of glass will also cause the glass to sag and melt at a lower temperature than clear glass. This needs more investigation!

Experiment 3 – Transparent and Opal Strips

In this experiment 0.5 inch strips of 3mm transparent and opal turquoise Bullseye glass were sagged to better understand at how transparent and opal types of glass differ in their behaviour. The results are shown in the graph. Click it to see it full size in a new window, if you wish.

The results from this experiment give us a hint that transparent glass will sag and soften at a lower temperature than opal glass for a given colour. However, only turquoise glass has been considered and it might not be representative. More experiments would be needed to confirm that what we see here is representative for all colours of glass.

Despite only testing turquoise glass, we can at least say with certainty that transparent and opal glasses for a particular colour will not behave identically.

Experiment 4 – Coloured Strips

The fourth experiment uses a selection of 0.5 inch wide strips of 3mm Bullseye glass for two purposes. The first purpose is to see if the degree of sagging mirrors the sequence of colours in the rainbow (which would correspond to a sequence of decreasing light wavelength) and to get a better picture of how glasses of different colours behave in general.

The coloured strips of glass are all opal colours with the exception of the aventurine blue glass which, technically speaking is a transparent glass. The aventurine blue glass was added to the samples because it was already known that it has a habit of “over-processing” compared to other colours so would likely give us our “left hand side curve”.

From the graph you will see that we do not see the red, yellow, turquoise, lilac sequence that we might expect if the softening and sagging processes were influenced by light wavelength being absorbed or reflected by the different types of glass. So, we can not use the sequence of colours in a rainbow to predict anything about which colours will soften and sag at a lower or higher temperature. Click it to see it full size in a new window, if you wish.

It is interesting to see confirmation that the aventurine blue glass does indeed soften and sag at a considerably lower temperature than all the other colours tested here. What is perhaps more important to notice is that the aventurine blue glass is softening and sagging at a temperature that is anywhere from around 10°C to about 25°C lower than the other samples of glass that were used. Also worth noting is that the red opal glass is at the other extreme of the samples used for this experiment.

Although we have learned that there is no simple way to estimate softening and sagging effects by colour alone, we can at least infer that the consequence of mixing different colours and types of glass in our creations can have an undesirable consequence. For example, an artefact using both aventurine blue and red opal glass will result in one of them being over-processed or the other under-processed – and never ever will they both be processed to the same degree within a single kiln firing.

That aventurine blue glass, like aventurine green glass, is super-saturated with the “colourant” material leads to an interesting question… Is it the amount of “colourant” materials (typically metals and metal oxides) added to clear glass that are the cause for glass softening and sagging at different temperatures?

Perhaps there is some merit in the “amount of colourant materials” hypothesis but we have to recognise a contrary situation with Experiment 3 where it was the transparent turquoise glass that sagged more than the opal turquoise glass. The problem is that although both types of turquoise glass must have been produced by adding “colourant” materials, the notable difference is the addition of yet more materials to make turquoise glass opalise.

Experiment 5 – Brand of Glass

This experiment compares a small selection of Bullseye glasses that have shown extreme sagging results against a single sample of Wissmach Corella orange glass. The reason for this experiment is to gain an insight into whether or not different brands of glass will differ. Here is a graph of the results. Click it to see it full size in a new window, if you wish.

Although identical colours were not used in this experiment, it is clear that the Wissmach Corella orange glass softens and sags in a similar way to the Bullseye glass samples but at a lower temperature. From previous experiments we know that the aventurine blue glass softened and sagged at the lowest temperature amongst the Bullseye glass samples being used and yet the Wissmach sample is softening and melting at an even lower temperature.

We also know from earlier experiments that the red opal and clear Tekta glasses from Bullseye required higher temperatures to soften and sag amongst the glass samples being used. Notice that they are softening and sagging at temperatures about 40°C higher than the Wissmach glass – a rather considerable difference.

But of course we would never want to mix Wissmach glass with our Bullseye glass, nor would we mix Spectrum and Bullseye glass together. This leads to another observation…

As noted in a document at Glass Campus, and many other places, it is well known that Spectrum (now Oceanside) glass is softer and requires kiln schedule temperatures to be about 25°F (or 14°C) lower than for Bullseye glass. That the Spectrum glass is softer and melts at a lower temperature than Bullseye glass, and that the Wissmach glass melts at an even lower temperature is an important clue about glass behaviour in a kiln.

I think it is safe to assume that the brand of glass matters when it comes to temperatures at which glass will soften and sag. These brands of glass are no doubt made from clear base glasses that are formulated differently. It must be that differing raw material proportions used to make the raw uncoloured glass contribute to differing melting points in broad terms.

And we know from this and earlier experiments that adding “colourants” to glass seems to make the resulting coloured glasses soften, sag and melt at lower temperatures than the “basic” clear glass. We seem to be reaching a hypothesis that the temperature at which a glass will melt is governed the formulation of the underlying clear glass and that the “colourants” used to make them into transparent and opal colours seem to lower that temperature.

Experiment 6 – All At Once

This experiment is not so much an experiment as it is the “full dataset”. By this I mean it is a graph of all the temperatures and all the degrees of sagging for all thirteen kinds of glass I used for the “over-arching experiment”. The previous experiments I have written about are, in truth, nothing more than “extracts” based on the graph given to you here. Click it to see it full size in a new window, if you wish.

There is a lot of information stuffed into this one graph so I leave it to you to decide if you want to study it in detail. You will notice it mentions some extra colours of Bullseye opal glass not mentioned in the other experiments, such as petal pink and black, or that there are two kinds of white glass.

One interesting thing you might notice is that the curve for black opal glass is within the middle of curves from other kinds of glass. This is interesting because it dispels the myth that black glass melts at the lowest temperature.

Making Some Conclusions

From the foregoing experiments and their graphs I hope you are beginning to appreciate that the temperature at which a particular piece of glass will begin to soften and sag depends on the formulation of the underlying basic glass as well as the “colourants” added to the glass. We know this by comparing glasses from Bullseye, Wissmach and Spectrum (Oceanside).

Comparing aventurine blue against other colours of Bullseye glass, and their clear Tekta glass, seems to hint that it is perhaps the quantity of “colourants” added to a clear base glass that causes the melting point of glass to become lower. The “colourants” used in glass are many and varied but in general terms they tend to be metals and metal oxides so we might expect that the particular materials added to the clear base glass might have subtly different effects. The most spectacular comparison we can make is aventurine blue with clear Tekta.

It is however interesting to notice that transparent turquoise Bullseye glass softens and sags at a lower temperature than the opal black glass. This seems to contradict the conclusion given in the previous paragraph because we would expect a black glass to require more “colourant” than the transparent turquoise glass.

Incidentally, if you are interested in learning a little more about what materials are used to colour glass then pop have a look at the Wikipedia page here.

So our conclusions are not as clear as we might hope for. But at least some progress has been made in understanding some of the principles about why different kinds of glass might soften and sag at different temperatures.

But most of all, it is the practical consequences for our kiln work that needs to be given some thought. This is, I suppose, the most useful thing to take from my endless chatterings.

Some Practical Consequences

The experiments have led us to definitively understand that black glass does not melt at the lowest temperature. A few careful experiments have revealed this supposed fact to be incorrect. One more spooky magic “fact” consigned to the wastebasket of blind assumptions.

The experiments have also led us to understand that coloured glasses seem to soften and sag at lower temperatures than clear glass. We suspect it is the “colourant” materials that are causing the difference. This leads to a practical consequence – we should not assume that a test piece made using Bullseye Tekta will allow you to predict what will happen when you make the “real thing” from more expensive coloured glass.

The experiments also lead us to understand that there are combinations of coloured glass that are going to be problematic, especially when one is going to soften, sag and melt at a much lower temperature than the other. To help visualise this problem more easily I leave you with just one more graph, which shows you the curves for the four extreme cases studied in my experiments for Bullseye glass. Click it to see it full size in a new window, if you wish.

From this graph we learn that putting a piece of aventurine blue with a piece of opaque white Bullseye glass into your kiln will result in one being over-processed and one under-processed no matter what kiln schedule you use. With the extremes examples around 30°C to 40°C apart it is not possible to choose any temperature which gives you nice evenly processed results.

But what about a full fuse you might ask? Will that not result in equal processing? Unfortunately not…

Even if you are aiming for a full fuse you will still notice differences when using glass types that appear at opposite sides of the graph. Consider making a coaster from strips of opaque white and aventurine blue, in which there is an even number of alternating strips. In this situation you will have an aventurine blue strip at one edge and opaque white at the opposite edge. After a full fuse I predict you will find the curvature of the aventurine blue corners to be more pronounced than the opaque white corners. If you don’t believe my prediction then try it for yourself and let me know!

Of course, it should be borne in mind that the two highest and two lowest samples in that graph are not necessarily the two extremes you might find if you had the time and patience to compare all the offerings from Bullseye. The graph for Experiment 6 provides you with all the information I have available. If nothing else you’ll begin to realise just how much waste glass I have created by the experiments, not to mention the electricity.

Something else we have learned is that the strength of the piece of glass has a bearing on how it will soften and sag in a kiln. Clearly we have a sequence of increasing strength from 1mm stringers to 2mm stringer then 2mm sheet and then 3mm sheet glass. Another way of thinking about “strength” is to consider their cross-sectional areas.

For those of you that are not in the “Bullseye camp” I must warn you that specific details I have given in this blog might not be the same for your kiln work, but you can at least take note of the more general points I have made. Do try some experiments to get a better understanding of your kind of fusing glass.

Oh, and one more thing. Did you notice that I was putting Wissmach Corella Orange glass into my kiln? Yes, it seems that some non-fusible glasses can be re-melted in a kiln without causing major devitrification problems. This happens to be one of the types of glass that I know behaves reasonably well. But of course I don’t mix it with other kinds of glass.

Well, that’s all for now. I hope I haven’t bored you too much with my chatterings.

Coming Soon

In addition to the experiments described in this blog I also have results for other related experiments that were also done about 4 years ago. In those other experiments I looked into how and why different colours of glass polish and fuse at different temperatures and I hope to chatter about it in the not too distant future. Let’s hope it doesn’t take me another 4 years!

Posted in Experiment, Melting Glass, Sagging | Tagged , , | 4 Comments

Making Bubbles with Metal Oxides

Today’s chatter is about making bubbles inside kiln-fired glass using metal oxides rather than borax, bicarbonate of soda or other commercial bubble mixtures. I hope you find it a productive distraction, especially if you’re getting tired of being in a “covid-19 lock-down”.

And before I start, what I’m talking about in this blog works for microwave kilns as well as for “proper” glass kilns.


Looking Backwards

Way back in 2013 I posted some related chatter in Bubbles – Borax or Bicarb? about some early experiments with borax and bicarbonate of soda, trapping them between layers of glass to create bubbles, rather than using commercial bubble mixtures.

The problem you are likely to encounter with borax and bicarbonate of soda is that the size and quantity of bubbles that get produced is difficult to control and that simply producing bubbles inside glass is not especially exciting.

Today’s chatter takes this bubble production story forwards by exploring the use of metal oxides which not only produce bubbles but also to tint the glass that’s trapping the bubbles.


Some Technical Background

Necessary conditions to produce bubbles between two pieces of glass are that

  1. we must be able to fuse two pieces of glass together before the chosen chemical substance breaks down, and that
  2. when the substance breaks down it will produce a gaseous substance that can form bubbles between two layers of glass.

Both these conditions explain why borax was not very effective. As noted in my Bubbles – Borax or Bicarb? chatter, the problem was that the water of hydration for borax gets boiled-off at around 75°C, long before the surrounding pieces of glass are soft enough to fuse together at somewhere around 500°C. The sparse bubbles produced with borax are produced at a high temperature when the borax undergoes another change. If you want to better understand what water of hydration means, and also have some fun, then I recommend you look at another of my chatterings from 2013 called Science Meets Art – The Borax Bead Test.

By contrast, I found that bicarbonate of soda was better able to produce copious bubbles, because it decomposes into sodium carbonate  at about 50°C, loses water of hydration at around 100°C, then it breaks down some more at a high temperature, possibly around its melting point of 851°C, and certainly well above the temperature at which the two layers of glass have softened and fused together.

So, we now know what to look for when considering metal oxides that might produce bubbles between two pieces of glass in a kiln. We want it to break down and produce a gaseous product at a temperature that’s higher than the softening temperature of the glass we’re using but lower than the maximum temperature our glass kilns can achieve.


Some Lateral Thinking

In the main, though not exclusively, it’s metal oxides that are used to produce coloured glass and it’s often those same metal oxides that are used by potters for their coloured glazes. So, a metal oxide used to produce coloured glass or as a pottery glaze is worth considering.

We can refine our candidate metal oxides by considering “colour chemistry”. It is rather interesting to realise that metal oxides used to colour glass, produce colour pottery glazes, the pigments in paints and the colour effects in fireworks will all produce the same colour (or sometimes a range of colours) given the same metal oxide. So, for example, using copper oxide tends to produce a turquoise blue coloured glass just as it produces a turquoise blue firework.

We can continue the colour chemistry thread by considering scientific diagnostic methods such as the flame test you probably did at high school and the now rarely seen borax bead test. Both methods produce characteristic colours for different metals. Incidentally, you’ll find my “take” on the borax bead test to make little antennae and flower stamens here.

In the context of glass manufacture and pottery glazes, these metal oxides are subjected to very high temperatures compared to what we would normally use in a glass kiln, and in those contexts the aim is to “develop the colour” rather than “generate some bubbles”. So, we might expect some of them to not work at the lower temperatures we use with ordinary glass kilns.

So, our task is to look for metal oxides that are used by glass manufacturers and potters that might break down at a reasonable fusing temperature into metal and oxygen gas. The liberated oxygen produces the bubbles. The metal diffuses into the glass to produce a characteristic colour tint that depends on the metal that’s involved.


Three Candidate Metal Oxides

I have already mentioned the creation of bubbles with borax and bicarbonate of soda, and they aren’t metal oxides, so I will not mention them again. Instead we will focus on just three metal oxides that I have experimented with, for no better reason than they’re the ones I’ve tried and tested.

If you have heard of the paint pigment and glass colour called “cobalt blue” then you’re already ahead of the game because you already implicitly know of at least one metal oxide that can be used to produce a colour. The cobalt blue colour is produced by the breakdown of cobalt oxide in coloured glass manufacture and pottery glazes. This is what is supposed to happen to cobalt (II,III) oxide at 950°C according to Wikipedia:

2 Co3O4 → 6 CoO + O2

Similarly, turquoise blue is produced by the breakdown of copper (II) oxide in coloured glass manufacture and in pottery glazes. This is what we hope happens to copper (II) oxide when it is heated:

2 CuO → 2 Cu + O2

And as a third example, you may have heard of the “chromium green” or the old name “viridian”. Here it is chromium (III) oxide that is used in glass manufacture and pottery glazes as well as a paint pigment. This is what happens to chromium (III) oxide when it is heated:

Cr2O3 +3 CrO3 → 5 CrO2 + O2

In each case we are hoping, with fingers crossed, that the oxygen-liberating decomposition will happen at a sensible fusing temperature. Of course, I already know which of them will work and which one of them doesn’t. With this little teaser you should now be in a mild state of excited anticipation, are you not?

If you enjoy researching on the Internet, or know a wise and friendly potter, you’ll maybe want to explore the possibilities of other metal oxides that are used in coloured glass manufacture and as pottery glazes. Some of your discoveries might be suitable for use in a glass kiln but others might require temperatures far higher than we would tend to use in a glass kiln. Perhaps the easiest way to do your research is to make friends with an experienced potter and start by asking them about the coloured glazes they use at lower pottery-firing temperatures.


Using the Metal Oxides

Just as it was way back in my Bubbles – Borax or Bicarb? chatter, the basic concept is that we sparingly apply some of the chosen metal oxide between two layers of glass and heat the whole assembly to a full-fuse temperature to cause the bubble effect and colour diffusion. But simple concepts usually hide little tricks and nuances that need further exploration. Life is never so simple…

For your initial experiments I suggest you make use of a clear glass base and a clear glass capping layer, otherwise you will not be able to clearly see what you achieve in your experiments. Later you can experiment with coloured base glass and maybe some tinted capping glass.

Start with your piece of base glass. It does not matter whether it is 2mm or 3mm but as usual we are aiming for a stack thickness of about 6mm.

On the top of your piece of base glass you must apply a very thin layer of your chosen metal oxide. Keep the metal oxide away from the very edges of the base glass and ensure none has found its way onto the sides or you may get undesirable black marks in the finished piece. This picture shows what you should be trying to achieve for a copious amount of bubbling (and you can click the image to look closer).

In the Applying the metal oxide layer section below I will give you some practical advice about how to apply such a nice even thin layer of the metal oxide. The best method I’ve found is not very obvious!

When you have your thin layer of metal oxide on the top of your base glass, and cleaned away any over-spill, place your clear glass cap on top. This picture shows what you should be trying to achieve:

Your test piece is now ready for kiln firing using a typical full-fuse schedule. To some extent the time spent at the top temperature will determine the size of bubbles produced and the amount of colour diffusion.

A longer hold time at the top temperature means more time for individual bubbles to coalesce into bigger bubbles. This is no different than the way as bubbles in a sudsy foam also coalesce over time. And just like the situation with a sudsy foam, bubbles in glass can coalesce to the point when they burst, sometimes leaving a crater.

A longer hold time at the top temperature allows a little more time for the metal atoms to diffuse further into the glass, though the difference is barely noticeable.

Here is a picture of what you should expect to produce:

The turquoise blue example used copper oxide. The deeper blue example used a mixture of copper oxide and cobalt oxide. In the Mixing the metal oxides section below I will briefly chatter about mixing metal oxides and in the Some metal oxides fail section below I will chatter about why you don’t see a picture of a chromium oxide example. Yes, it was the chromium oxide that didn’t work in my glass kiln.

When you come to your own experiments, explore different amounts of metal oxide and vary your kiln’s firing schedule in terms of top temperature and hold time to get a feel for how to reliably get the results you’re looking for. Experimentation is always a key to learning and success!

To demonstrate there are some easy and simple creative possibilities with little bubbles, here are some coasters I made.

The pale blue comes from the copper oxide and the cobalt blue colour comes from a mixture of copper oxide with some added cobalt oxide.


Mixing the metal oxides

I have not had any trouble producing nice turquoise blue bubbles with copper oxide. The only question is how much or how little to use for the size of bubbles and degree of colouring you are looking for. Just a few experiments will tell you how much to use and at what firing temperature. If you use too much then you are likely to produce massive bubbles that either burst whilst still in the kiln or are so fragile after cooling that they easily pop when you press on them, or you will encounter craters where massive bubbles have already popped during the kiln firing process.

Early experiments with cobalt oxide were not pleasing, producing a mess of crazy fizzing and not the deep cobalt blue bubbles I was hoping for. It was clear than using cobalt oxide on its own was not an effective strategy. Further experiments revealed a simple answer to the problem…

To obtain nice cobalt blue bubbles I now use a mixture of copper oxide and cobalt oxide. It takes some experimenting to get the mixture just right but the best results seem to come from mainly copper oxide with just a small touch of cobalt oxide in the mixture. Again it is experiments that lead to success.

The two bubbly glass blobs and the coasters you saw in the picture above are simple examples of what you are aiming for – sensible sized bubbles and a reasonably amount of colour.


Applying the metal oxide layer

It might seem strange that I have chosen to devote a whole section to describe how to apply some black powder to the surface of a piece of glass, but I’m sure you will find my comments and experiences useful.

Copper oxide and cobalt oxide are fine black powders. If you try to apply them to a glass surface you will soon realise that attempting to apply a dry, thin, even layer is very difficult. It is also very messy because these fine powders also have a habit of “getting everywhere” except where you actually want them. So, trust my experiences and don’t try to use these metal oxide powders in their dry form.

An obvious solution might be to mix some of the metal oxide in water and, with a paint brush, attempt to paint the mixture onto your glass. In theory this should work but in practise it is not so good because there are two problems.

The first problem is that the metal oxide powders are suspended rather than dissolved in the water, so have a nasty habit of clumping and settling out. The more serious problem is that water clumps into droplets on the glass surface, just as it does outside you window on a rainy day. This droplet problem is a consequence of surface tension and the solution is to reduce the surface tension by using a surfactant or by some other trickery, but this is a big topic that I will not explore in any great detail. Instead, I’ll just tell you my answer to these problems…

The best solution I found, which allows a thin even layer of metal oxide to be painted onto a glass surface, is to make a mixture of the metal oxide in a weak CMC solution.

But what do I mean by “weak CMC solution” I hear you cry. And why should it be important or useful? Let me explain with a short explanation and references to some of my previous chatterings…

I have mentioned CMC in several of my previous chatterings and have done so because it’s really useful in several areas stained glass crafts. CMC is an acronym for carboxymethyl cellulose and an example trade name for it is “Tylo”. Sugar crafters use “Tylo” with icing sugar to form a sugar paste because CMC can be used to thicken a liquid or to form a stiff gel. In effect it is able to “suspend” water and icing sugar in its tangled matrix. We can do similar things within our craft as well…

As glass crafters we can use CMC to good effect. If you need some convincing then notice that I have chattered about how it might be used to make a safety flux gel over here, how it can be used to thicken glastac into a gel over here, how it could be used to bind together glass grinder waste (or powdered frit) for freeze-fuse work over here and how to squirt powdered frits over here. The last of these is perhaps the most important for you to read because it explains everything you need to know about what CMC is, how it behaves and how we can exploit it for our craft. Trust me, it’s a useful addition to your glass crafting toolkit. And I’ve not even approached the possibility of using CMC with frits to produce powder wafers on a sheet of acetate.

CMC is not expensive and you don’t need much, so get onto Ebay and look for “CMC powder” or “Tylo”. In these “covid-19 lock down” days it’s important to keep the economy ticking over so buy some now!

Back to the topic at hand…

What you need to create is a smooth runny mixture from the powdered CMC and water, then add a little of the metal oxide you have chosen. The aim is to take advantage of the stickiness of the CMC to address the surface tension problem and take advantage of the way the CMC suspends the metal oxide evenly in its sticky matrix.

It takes some experimenting to get the optimum mixture, but when you’re able to apply the mixture onto your glass with a paintbrush you know you’re on the right path. Do remember that in addition to using a “colour wash”, you can do swirls, squiggles or whatever else you might choose to do with the paintbrush. For example the outlines of the hearts in the coaster were done with a fine paint brush.

I store my ready-made CMC, water and metal oxide mixtures in tiny little lidded acrylic pots of the kind that often get used for lip balm. Over time the mixtures will dry out but simply re-adding some water and stirring them back into their original state is all that’s needed.  The pot on the left is ready to use but the pot on the right has dried out.

Some metal oxides fail

You have maybe noticed that I mentioned chromium oxide earlier on but have said nothing more about it. This is because it was a dead end experiment. The chromium oxide did not break down between piece of clear glass even with a hotter-than-usual full-fuse schedule.

So, I was disappointed to find that although chromium oxide can be used to produce green glass (using the pot melt method) and can be used as a pottery glaze, it does not work at the lower temperatures we tend to use with glass fusing kilns.

My philosophy is that we learn more from failures than successes. Maybe you agree.


Buy Cautiously

From my chatter you will now realise that metal oxides used in coloured glass manufacture and for pottery glazes are good candidates for experimentation, but do not guarantee success because we are working at lower temperatures. Implicit in this is a warning not to buy larger quantities of a metal oxide unless you know they will work.

You should now also realise that copper oxide is perhaps the easiest to use and that cobalt oxide can also be used but is best used very sparingly in a mixture with copper oxide. I have a 100g tub of each of them, purchased mail order from my stained glass supplier. But you don’t need this much – I expect to die of old age before I finish using them.  Implicit in this is a suggestion to buy the smallest quantity you can find.

More importantly you should now understand that chromium oxide did not work in a glass kiln, even though it is used in pottery glazes and in coloured glass manufacture. This was a costly mistake because I have very nearly 500g of chromium oxide and no further use for it. Implicit is the warning not to repeat my mistake with chromium oxide.

The key message is therefore to buy only small quantities. If you are friendly with other glass fusers, try to share the cost of buying a 100g tub of copper oxide and perhaps also a 100g tub of cobalt oxide.

Better still, perhaps, is to make friends with your local high school chemistry teacher then ask if you can “borrow” a gram or two of your chosen metal oxide for experimental purposes. Draw them in to your ruse with chatter about wanting to explore colour chemistry, talk about the use of metal oxides in coloured glass manufacture and as pottery glazes. Scientists are naturally curious so exploit their weakness mercilessly!

Oh, and remember to also buy some CMC. Better still, make friends with a sugar crafter, but not until your “social distancing” rules are relaxed. No crafting endeavour is worth dying for!


Unexpected Delights

The small circular blobs of bubbly glass and the coasters I have shown you earlier in those pictures are more exciting than you might suppose. Although they might only look like coloured pieces of bubbly glass, they have hidden delights to bestow.

Hold them up to the sun (or some other kind of bright source of light) and all those little bubbles of oxygen act like lenses, focusing the light, producing spectacular sparkling starbursts of light rays.

You can exploit this sparkly trait by constructing a “watery panel” (ideally slumped over an arch) with fused-on representations of  aquatic plants and fishes over a bubbly blue underwater background and put it in a sunny bathroom window. Or as an alternative, experiment with squiggly brush work and squiggles of glass in a bubbly blue arch panel like this example:

Another hidden delight that the blue bubbles open up a magical scientific story. Tell your friends, and especially children, about how the metal oxide breaks down at high temperatures into metal atoms and oxygen gas. Tell them that the metal atoms diffuse slowly into the glass to create their characteristic colours, the exact same colour that a firework would produce if made using the same metal oxide. Then tell them that the bubbles in the glass contain nothing but pure oxygen, trapped for eternity inside the glass.



It’s time for me to conclude my chattering and I suppose the key things to say are that adding copper oxide, cobalt oxide and some CMC to you your glass crafting toolkit opens up many new possibilities.

Not only will those two metal oxides provide you with a means to add blue bubbles to your creations, the CMC will help you explore the freeze-fuse technique, thicken your glastac glue, and more besides.

And finally, here’s a picture of a silly little piece I made some time ago. It’s a representation of a photograph of an particle collision in a bubble chamber where an atom gets smashed into sub-atomic particles. Glass art can even be made interesting to a atomic physicist!

Stay safe everyone and have a productive “lock-down”.


Posted in Borax, Borax bead test, Bubbles, CMC, Experiment, Tylo | Tagged , , , , , , | 24 Comments

Leaded Light Cement Formulation

The process of waterproofing a leaded glass panel is a time-proven process using simple raw materials formulated as leaded light cement. In this blog I will consider several formulations for leaded light cement and discuss the various constituents that should and should not be part of the formulations.

The object of cementing a stained glass panel is to make the leaded panel weatherproof, to support the glass within the lead framework and to strengthen the whole structure in a way that makes it rigid and sturdy. When done properly the result is a leaded glass panel that could remain strong and weatherproof for more than a hundred years.

Let’s get cracking. There’s lots of chatter to get through and although some of it’s not going to be exciting I can at least promise to try and cover the subject in reasonable depth.

As an aside I apologise for not doing much chattering through this blog recently and observe with some wry amusement that much of the material for this blog was accumulated back in 2011 – meaning that many of the web references I found back then are long since gone. Oh, how ephemeral this Internet stuff can be!


Cement or Putty?

My first task is to explain why this blog is about Leaded Light Cement and why I do not call it Putty. Using the right word is important because words have meanings.

The words “putty” and “cement” are not interchangeable because there’s a real difference in what they mean, what they are, how they behave and how they are used. Put simply, we use a putty to hold glass in a wooden window frame but we use a cement to waterproof and bond glass into a leaded light window panel.

As the Wikipedia entry for Putty explains, putty is a generic term that describes highly plastic materials, similar in texture to clay or dough, typically used in domestic construction and repairs as a sealant or filler. Notice the words “sealant and filler” do not imply any sense of binding or adherence.

By contrast Wikipedia’s entry for Cement describes a substance that sets and hardens and adheres to other materials so that it can bind them together.

Leaded light cement is therefore a product that we expect to bind to the glass and lead work and we can also expect it to harden as part of a setting process. By contrast putty does neither. This subtle difference explains why “putty” and “cement” are not interchangeable terms even though you’ll soon realise they both have a formulations substantially based on whiting and linseed oil in our sphere of interest.

As I’ve just mentioned, putty and leaded light cement both contain whiting and linseed oil but that’s as far as the similarity runs. So, it’s the additional materials you will find leaded light cement that changes it from being a putty, causing it become a cement. You will learn more about these differences as you wade through my seemingly endless chattering.

Later you will begin to understand why materials such as Plaster of Paris and Portland Cement should never be part of any formulation for putty or leaded light cement.


What Is Glazing Putty?

Although this chatter should be about leaded light cement, it does no harm to briefly mention glazing putty if only to understand more about how it simultaneously has similarities and yet differs from leaded light cement. I’ve already given you some outline hints. But as usual I have more chatter to complete the picture.

Traditional glazing putty, also known as Common Putty, is whiting and linseed oil mixed in a proportion that forms a firm dough that is neither too hard nor too runny to glaze windows. This is just one kind of putty described at Wikipedia. Later in this blog I will look more closely at Whiting and at Linseed Oil as ingredients.

Although I have occasionally encountered comments suggesting leaded lights can be waterproofed with glazing putty, a common complaint is that it is not blackened and that it is the wrong consistency to force under the leaves of lead cames.

A further problem can be seen when comparing old windows in-situ. Leaded light cement deteriorates far more slowly than glazing putty. Leaded light cement is expected to last a hundred years whereas glazing putty will typically last tens of years.

In summary it’s clear that you shouldn’t be using ordinary glazing putty to waterproof a leaded light panel.

With a clear idea of what common glazing putty is, let’s have a look at some leaded light cement recipes. To start with I will generalise. Then we will look at published recipes. We will discover there are many variations on the same theme, some good, some bad and some to be avoided.


Leaded Light Cement – Generalisations

There are numerous recipes for leaded light cement, both proprietary and published, but the core common ingredients are always the same – linseed oil and whiting – so a similarity with traditional glazing putty is not in dispute. However, beyond these two core ingredients are other materials that make it more suitable for waterproofing and strengthening leaded lights. Principle amongst the additional materials are a volatile drying agent and a blackening colourant – more about these additions below.

The consistency of leaded light cement is a matter of personal preference, but should be firm enough that it can be forced under the leaves of lead cames without running through to the back. So, although leaded light cement could be as stiff as Plasticine or Play-Doh, which tends more towards the consistency of glazing putty, it has a more useful consistency when it is less firm than traditional glazing putty, variously described as peanut butter, porridge, a very thick pancake batter which slowly oozes off your hand, treacle or like molasses on a cold morning.

Some published formulations use only boiled linseed oil and others only raw linseed oil. There are others that use half boiled linseed oil and half raw linseed oil. Decisions about which to use or in what proportion relates to their drying properties, more about which I will chatter about later. For the moment, just remember that raw linseed oil takes a longer to dry and stiffen up in leaded light cement than boiled (or refined) linseed oil. These two oils are known as “drying oils” and the word “drying” isn’t in the sense that we’d normally consider to be “drying”. It is very much an “industrial” meaning for the word which actually relates to a hardening process that is the result of chemical oxidation.

Turpentine was traditionally used as a “volatile drying agent” but it is now an expensive product for which turpentine substitute is a more cost-effective alternative. A more modern synthetic alternative that is equally effective but not as pungent as turpentine is spirit white, also known as mineral spirit. Again the word “drying” isn’t quite what we mean in ordinary conversation. The word “volatile” hints that we’re dealing with a substance that evaporates quickly.

The reason for also using a “volatile” drying agent (such as turpentine) in leaded light cement to evaporate quickly from the cement causing it to thicken more quickly than would normally occur with just linseed oil. It therefore accelerates the drying process. This is why leaded light cement hardens within a day or so but common putty (which has no turpentine or equivalent) takes many days to harden.

The primary reason for adding extra liquid drying agents to the cement is to thin the cement to less-firm consistency, so that the cement can be brushed more easily under leaves of lead cames. A less desirable consequence of too much of the liquid components is that it can mean the cement runs out of the back of the panel being waterproofed and remains soft for several days unless measures are taken to assist the drying and stiffening process. The volatility of turpentine, or one of its alternatives, will always results in a faster drying time when compared with an equally runny formulation that uses only linseed oils (whether raw, boiled or in combination).

Although the use of volatile drying agents, such as white spirit, will aid rapid drying, the proportion used should not overwhelm or replace the linseed oil component because it is the long-term polymerisation of the linseed oil that is important to the end result. It is a matter of conjecture that the use of too much volatile drying agent may cause shrinkage or increase porosity of the cement in a leaded light panel.

Leaded light cement typically contains a black colourant which traditionally would be lampblack (black carbon powder, soot) but modern alternatives that are equally effective are “Universal colorants” (powder or liquid), concrete colouring powders, oil-based paints, powdered poster paint or stove blackening compounds. The resulting cement is a medium grey colour that approximates the colour of weathered lead. Be aware that the use of water-based pigments (such as ready-mixed poster paint, acrylic colours or latex) will cause problems because linseed oil and water don’t mix easily.

Some leaded light cement formulations contain small amounts of lead and lead compounds such as white lead, red lead, or litharge. Although no clear references have be found to explain why these lead compounds are sometimes used in leaded light cement formulations, it is reasonable to admit three plausible reasons, any or all of which may have explain their historic use:

  • Lead compounds in the cement may impede deterioration by biological agents (such as fungal or microbial attack) due to lead toxicity. Comparing the longevity of leaded light cement in comparison with typical traditional window glazing putty supports this possibility.
  • The inclusion of lead compounds may lend to the cement a colour and general appearance more reminiscent of lead came than the use of a black colourant alone. It is plausible to suggest that the long-term oxidation of lead compound inclusions in the cement will mirror long-term oxidation of the lead came of a leaded light.
  • The lead salts might be used as a “metal drier” or an anti-oxidant. The use of red lead has been noted as a “hardener” in a recipe from the start of the 20th century and white lead has been used in the past in paints.

In addition to first noting that all lead compounds are toxic, outline notes about common lead compounds is perhaps useful, if only for future reference:

  • White lead has the chemical formula (PbCO3)2Pb(OH)2 and is the pigment used in old paints and old cosmetics. It is now largely banned due to the lead poisoning it causes.
  • Red lead has the chemical name lead tetroxide and has the chemical formula Pb3O4 (or more strictly 2PbO·PbO2) and may become yellow lead following exposure to damp air over time (as the monohydrate).
  • Litharge is the red form of lead(II) oxide which has the chemical formula PbO. The other form of lead (II) oxide is yellow and is called massicot.
  • Lead acetates also exist, and may be produced as a white bloom on new lead cames after a leaded light has been installed using modern synthetic grouts that emit acetic acid vapours when they cure.

With a foundation understanding of the main constituents found in leaded light cement under your belt, it’s now time to consider a few of the many published formulations for leaded light cement.

Some of the recipes below are eminently suitable whilst others contain questionable materials such as Plaster of Paris or Portland Cement. After each recipe I will add some comments to give you a deeper understanding of what I think is good and bad about each one of them. You, of course, are entitled to do your own research, come to different conclusions and disagree entirely with my opinions.


Recipe 1

This is a simplified traditional style formulation. It is the essence of what I read at Ehow.


  • Measure boiled and raw linseed oil in equal parts (for example, 1/2 cup of each) into a bucket.
  • Mix in whiting a cup at a time with a spoon. If you used 1/2 cup each of boiled and raw linseed oil, use up to about four cups of whiting, depending on the desired consistency.
  • Mix in about 1 tbsp of stove or lamp black, depending on how dark you want the cement.
  • Mix everything together until there are no streaks. Use your gloved hands when it gets difficult to mix with the spoon.


The use of boiled linseed oil suggests an intention to increase the drying time of this formulation. However the lack of a volatile drying agent (such as turpentine or white spirits) means this will not have a particularly fast drying time which may cause disappointment. This recipe is substantially the same formulation as Recipe 2 and it is likely that one is a re-statement of the other.


Recipe 2

This is another simplified traditional style formulation. It is the essence of what I read here.


  • 1/2 cup boiled linseed oil
  • 1/2 cup raw linseed oil
  • 4 cups whiting
  • Stove black or black colouring for cement
  • A container for mixing the [not] putty and rubber gloves or nitrile gloves.


Keep adding whiting to the linseed oil mixture, a cup or so at a time to begin with then smaller increments towards the end. Mix using a spoon with gloved hands, much like making bread dough, until lumps are gone and the mixture is smooth.

If you want your black cement, add the black colouring at the end. If you do not use black, the cement will be a light beige colour like traditional glazing putty.


The use of boiled linseed oil suggests an intention to increase the drying time of this formulation. However the lack of a volatile drying agent (such as turpentine or white spirits) means this will not have a particularly fast drying time which may cause disappointment. This recipe is substantially the same formulation as Recipe 1 and it is likely that one is a re-statement of the other.


Recipe 3

This is a faster drying traditional formulation. It comes from a blog.


  • 7 parts by volume whiting
  • 1 part by volume boiled linseed oil
  • 1 part by volume white spirits (turpentine or substitute or mineral spirits)
  • 1-2 Tablespoons lamp black or other colorant


Add the whiting (reserving about one quarter) to the linseed oil and white spirit. Mix this well, by hand or with a domestic mixer capable of mixing bread dough. When these are mixed thoroughly, check the consistency is barely fluid. At this point, add the colorant then add more whiting as required to get the consistency you want.

The author of this recipe says you should make only what you will be using on the current project, as the whiting separates from the linseed oil and spirit relatively quickly, and that since making your own is cheap and quick to make, there is no saving in making a lot.

The author of the recipe also says that commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product. More about this later!


Recipe 3 reveals a different (higher) proportion of whiting to drying agents than the previous recipes. The higher whiting proportion probably addresses the consequence of using a less viscous mixture of boiled linseed oil with volatile white spirit. The consequence of both boiled linseed oil and white spirit will be a faster setting than the first two recipes. This is a good starting point for your own experiments.


Recipe 4

This fast-setting recipe came from here but disappeared off the Web. It has been found again as the first of two recipes here. The author originally said “This putty sets up faster than commercial putty” which I take to mean it hardens more quickly than commercial leaded light cement.

Dry Ingredients:

  • 4 parts whiting(chalk, calcium carbonate)
  • 2 parts plaster of Paris
  • 1 1/2 parts Portland cement
  • 1/4  part lamp black

Wet ingredients:

  • 1 part – boiled linseed oil
  • 1 1/2 – 2 parts pure gum turpentine


The method implied is that dry and wet ingredients can be stored and then mixed to a desired consistency when required. This is a good idea, but the formulation isn’t.


Recipe 4 is curious for several reasons. It is also bad for several reasons. Use it at your peril.

Specifying pure gum turpentine is needlessly prescriptive because it is expensive and will perform no better than ordinary turpentine or white spirits.

The use of Plaster of Paris and Portland Cement is not recommended, despite this formulation despite the claim it has a much faster setting time. The problem is that plaster of Paris and the Portland cement deteriorate in a damp atmosphere in the long term. A further problem with these materials is that they contribute to an excessively stiff cement which in turn raises the possibility of glass breakage. Another reason not to use Portland cement is that it strongly attaches itself to glass making future restoration activities more difficult. And in relation to your health and safety a problem is that Portland Cement becomes strongly caustic and can cause skin burns as well as severe eye or respiratory irritation. Health and safety problems also arise with the used of plaster of Paris.


Recipe 5

This is another fast setting recipe and is the second of two recipes found here.

Dry Ingredients:

  • 1 part calcium carbonate (whiting/chalk)
  • 1 part plaster of Paris
  • 1/4 part lamp Black

Wet Ingredients:

  • 1 part boiled linseed oil
  • 1 part – white spirit (paint thinner)


Recipe 5 is an improvement over recipe 4 but is still not recommended. One improvement is that less-expensive white spirits are now used. Another improvement is that Portland cement is no longer being used. The problem is with the Plaster of Paris.

The wet ingredients hint at a fast drying time and although the use of Plaster of Paris may be less hazardous than Portland cement, it is still not appropriate for reasons outlines in the previous recipe. Use this recipe at your peril.


Recipe 6

This recipe came from a book called Decorative Glass by Nance Fyson (page 164) and is interesting because it starts with traditional glazing putty and turns it into leaded light cement.


75% linseed oil putty – traditional glazing putty
10% white spirit and 5% linseed oil mixed
10% lampblack
Whiting as required


Place in a bucket. The white spirit and linseed oil mixture is added to the  lampblack then putty is added, a little at a time. To adjust consistency, more white spirit to thin or whiting to thicken. To darken add more lamp black.


Recipe 6 is a practical and convenient suggestion in that the formulation starts with inexpensive traditional glazing putty and adds other materials to formulate a sound and sensible leaded light cement.

The use of white spirit is cost-effective and the extra linseed oil could be boiled linseed oil would improve the drying time. The addition of the white spirit and linseed oil will soften the mixture to make it more suitable as a leaded light cement – and the exact amounts can be varied to achieve the desired consistency.

Though not stated in the original recipe, using black powder paint (or some other suitable oil-based blackening agent) in place of lampblack would make the recipe more easily accessible to any modern stained glass craft worker who doesn’t have a ready supply of lampblack or soot.

This recipe is recommended if you want a quick, convenient and economical method of making your own leaded light cement, particularly if you have some left over glazing putty and have easy access to the other ingredients.


Recipe 7

This recipe was found at Project Gutenberg in a book called  Stained Glass Work – a text-book for students and workers in glass, by C. W. Whall. When you’ve downloaded it, search for “How to Cement a Leaded Light“.


Whitening, 2/3 to plaster of Paris 1/3; add a mixture of equal quantities of boiled linseed-oil and spirit of turpentine to make a paste about as thick as treacle. Add a little red lead to help to harden it, some patent dryer to cause it to dry, and lamp-black to colour.


This recipe comes from the start of the 20th century so it’s old. It is interesting in that it does lend some credence to the traditional (though not necessarily common nor sensible) use of Plaster of Paris in leaded light cement. It resembles recipe 5 in that they both use Plaster of Paris.

The use of red lead is confirmed as a “metal drying agent” but is not a practical proposition for most stained glass workers given the toxic nature of lead compounds and widespread legislative restrictions on their use. However, the mention of lead compounds does link nicely to some modern leaded light cement formulations that mention lead as one of their ingredients.

The reference to a ‘patent drier’ is an unhelpfully vague reference perhaps to a catalytic drying agent (that accelerates the oxidative polymerisation of the linseed oil) rather than to a volatile drying agent such as turpentine (that simply evaporates quickly).

This is an old recipe that isn’t recommended, mainly because it uses Plaster of Paris and red lead.


Commercial Formulations

Seeking out manufacturer’s safety data sheets (MSDS) and looking closely of packaging labels sometimes provide us with useful hints about  the formulation of their products. From such sources we get some hints about modern commercial leaded light cement products.

Here we learn that their “putty” is a dough-like compound of whiting and linseed oil which may contain small amounts of white lead, red lead, lampblack and/or litharge (which is lead(ii) oxide). Those lead compounds are probably intended as metal drying agents.

And here we discover a formulation that contains naptha, orange lead, mineral filler, linseed oil, a plasticiser and white spirit. The term “mineral filler” we can assume to mean “whiting”. Notice we find linseed oil and white spirit in this recipe. Notice also that there is orange lead as a metal drying agent but no mention of lampblack or carbon black.

As a simple confirmation, this web page suggests that commercial glazing putty is exactly what we’d expect – nothing more than a ” Homogeneous mixture of mineral filler and linseed oil” for which we again assume that “mineral filler” actually means “whiting”.

Arbo leaded light cement used to be described here but the web page has now disappeared. It used to say that their leaded light cement contained a vegetable oil, mineral fillers, white spirit and metal driers. For the term “vegetable oil” we can assume “linseed oil”, for “mineral fillers” we can assume “whiting” and for “metal driers” we can assume a special metal drying agent. This metal drying agent will be chosen to catalyse (ie accelerate) the drying process by a process of oxidative polymerisation of the linseed oil (which makes the linseed oil thicken and harden) and may or may not be a lead compound of the kind seen in other formulations.

Comments here suggest there may be commercial formulations of “putty made for stained glass” that contain Portland cement and/or Plaster of Paris. The comments are, I assume, meant to offer us advice relating to commercial formulations of “leaded light cement” rather than “leaded light putty”:

If you are using a commercial putty made for stained glass, look for one that does not contain cement or plaster of paris. A good indicator of what not to buy is if the product contains the word cement in the name or description.

Although the author of that web page is correct to suggest that [Portland] cement and Plaster of Paris are a bad idea, the remaining comments are of dubious value because of the confused use of “putty” and “cement”. Think about it – the recommendation is not to purchase a properly formulated leaded light cement because there’s the word cement in the name – this is patent nonsense.

Although the web page here is now long gone, the forum thread had a similar comment about commercial “putty” (as usual, actually meaning leaded light cement):

Everyone has their personal favourite formula or brand. If you elect to buy premade putty [meaning leaded light cement], take care to read the list of ingredients. Reject ANYTHING that contains plaster or cement. Both are NOT waterproof but will absorb moisture and generate mold and mildew.

This time the author correctly limits product rejection to just those with ingredients that contain the words plaster or cement rather than in the name of the product. The comment about the water absorbing nature of Plaster of Paris and of Portland Cement is both useful and correct and will be described in more detail later. Again the key message is not to use leaded light cement with Plaster of Paris or Portland Cement as ingredients.

The web page here from which Recipe 3 was taken, says that “commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product”. I have found no evidence from any MSDS documents to support this claim. I also think it is unlikely to be correct because two immiscible liquids are required to form an emulsion and although we have oil-based liquids we don’t have water-based liquids in leaded light cement.

Another commercial stained glass cement formulation is a two-part American product called Miracle Mudd. You will find related MSDS documents here for the dry component and here for the liquid component.

Information gathered from the MSDS documents is typically sparse. This is not unusual because they don’t want you to know what you’re paying for, especially not when it’s “nothing special”.

The MSDS document for the dry part tells us it’s a grey powder that’s not reactive and not appear to be particularly hazardous to our health. If there were Plaster of Paris or Portland Cement involved I would expect the health hazard data and control measures to be greater than described. I would therefore speculate that it’s a mixture of whiting and some kind of colourant powder such as poster paint, brick colourant or perhaps even lampblack. So, nothing special here so far as I can tell.

The MSDS document for the liquid part tells us that it is an “amber to dark brown oily liquid with a pine aroma” with a boiling point of 307°F and flash point of 105°F. Also, an advertising claim is that it isn’t so smelly as competing products. Pulling these facts together is suggestive of the liquid part having a formulation that involves turpentine and linseed oil or derivatives of them. So, nothing special here so far as I can tell.

In conclusion, my guess is that Miracle Mudd is a traditional leaded light cement that you happen to purchase as separate dry and liquid parts and mix yourself. In this respect it is reminiscent of recipes 4 and 5 but probably without the nasty Plaster of Paris or Portland Cement. So, the miracle of Miracle Mudd would appear to be that it comes in two parts which doesn’t really seem to be such a great miracle.

Before I completely bore you to tears I think it’s time to stop looking at commercial formulations and pull together some general comments about the formulation of leaded light cement.


General Formulation Comments

A potentially good idea hinted-at by the authors of recipes 3 and 4, as for Miracle Mudd, is that dry and wet ingredients can be prepared and stored separately. This allows you to only make-up the amount of leaded light cement that you intend to use which in turn should reduce the amount of waste.

If you are making your own leaded light cement try to make only what will be needed for the current project. If you purchase ready-made leaded light cement try to buy quantities that are not excessively large. The problem with leaded light cement is that the whiting slowly separates from the oily liquid portion, resulting in a solid mass hiding under an oily surface. This separation is not permanent and can be reversed– loosening the solid portion, breaking it down and re-mixing it back into the oily portion with something like a palette knife or wallpaper scraper works but it takes a lot of time and patience.

Since making your own leaded light cement is quick and cheap, there is no saving in making large amounts in a single batch. If necessary, as hinted-at by some recipes, make-up and store dry ingredients and wet ingredients in distinct containers. Or consider the recipe from the book which adds ingredients to ordinary glazing putty.

One commentator suggests that a batch of leaded light cement has a shelf life of just one day but does not say why this should be so or the conditions that cause this to be the case. It is however plausible that formulations containing Plaster of Paris or Portland Cement may chemically ‘go off’ in consequence of the effects of atmospheric water moisture. If you’re sensible and are not using Plaster of Paris or Portland Cement then this should never be a problem.

Whether you are using a commercial glazing putty or a commercial leaded light cement, look for one that does not contain Portland Cement or Plaster of Paris. Although none of the MSDS documents I examined suggested that commercial formulations contain them, it does not mean that none of them do.

If your cement mixture is too stiff add just  a little more white spirit to get it to your desired consistency. Conversely, if the mixture is too soft or runny add more whiting to stiffen the mixture.

If you are making a big batch of leaded light cement the mixing task can get physically difficult. In such circumstances using a bucket and an electric drill-powered paint stirrer becomes preferable to using a kitchen blender, kitchen mixer or a spoon.

And that’s all the advice I can give other than to suggest you experiment, do your own research and come to your own conclusions. I am not the fount of all knowledge!

For the remainder of this blog I’ll change the focus to an examination of the various component materials. Expect some repetition but I hope there’s not too much for you to bear.


What Is Whiting?

Whiting forms the bulk of the leaded light cement used to waterproof leaded lights in much the same way as whiting forms the bulk of glazing putty used for a traditionally glazed window.

Whiting is also used as a desiccant or drying agent to remove excess oils from leaded light cement during the “cleaning-up” process, thus promoting a faster setting time. Whiting serves as a mild abrasive to help remove excess cement and to polish the glass and leading as part of the final cleaning process.

Whiting is an archaic name for finely powdered white chalk. It has the chemical name calcium carbonate and the chemical formula CaCO3. However, not all “chalk” you’ll find being sold is calcium carbonate.  So, be careful when purchasing “chalk” because it can also refer to other compounds including magnesium silicate and calcium sulphate. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulphate).

Chalk is used to marking lines on playing fields and although some of these products are still made with calcium carbonate, it is titanium dioxide that is often used nowadays. Sports such as gymnastics, rock-climbing, weight-lifting and tug-of-war also make use of chalk though nowadays it is usually magnesium carbonate.

Whiting should not contain any calcium oxide (quick lime) or calcium hydroxide (slaked lime or hydrated lime). Calcium carbonate is a neutral substance that can be handled easily and safely but calcium oxide and calcium hydroxide are caustic alkaline substances that can be hazardous.

The building trade also use line marking chalk which may (or may not) be calcium carbonate. Painting and DIY outlets may have calcium carbonate (in stock or to order) where it will probably be sold as whiting or chalk dust. Pottery supplies stores may sell calcium carbonate where it will probably be called calcium carbonate or chalk dust.

Agricultural or feedstuffs merchant and garden centres may have whiting but be sure it is calcium carbonate (chalk) and not calcium oxide (quick lime) or calcium hydroxide (slaked lime) or some non-specific formulation such as “field lime” that may contain calcium oxide or calcium hydroxide. Also be wary of “field lime” because it can be very gritty and scratch your glass.

Lime plaster is a mixture of calcium hydroxide, sand and other fillers. The calcium hydroxide reacts exothermically with water and ultimately forms calcium carbonate. Whitewash is the same chemistry. As both involve calcium hydroxide they should be avoided in stained glass crafts. Despite this, some workers use lime, Plaster of Paris and related materials in their stained glass cement to achieve a faster setting time even though such formulations are more hazardous and less suitable than those that use whiting.

So, perhaps the best place to buy whiting for stained glass crafts is from a reputable stained glass store where the purpose for its use is well understood.


What about Plaster of Paris?

Plaster of Paris is also known as Gypsum Plaster because is produced by heating a type of rock called Gypsum. Plaster of Paris is more properly known as calcium sulphate semi-hydrate, with the chemical formula CaSO4·0.5H2O which tells you it contains just a little water. Adding water to Plaster of Paris turns it back to Gypsum, which has the chemical formula CaSO4·2H2O which means it now contains more water. So, in essence, Plaster of Paris relies on water to turn it back to “rock hard” gypsum.

Therefore the first and obvious question to ask is how it could be possible that Plaster of Paris might be a suitable component for leaded light cement when all the other ingredients (linseed oils and the drying agents such as white spirits) do not contain water. The chemistry of Plaster of Paris tells us that water that is needed to make it harden but the only source of water we have available must be rain and damp air. Something is not right here – we can not expect Plaster of Paris to contribute to waterproofing a leaded light panel if it needs water to make it harden. The only way the Plaster of Paris can “work” in leaded light cement is if it “compromises” the ability of the cement to repel water and moisture.

Another less obvious problem with Plaster of Paris is that it hardens to a hard and very inflexible state which is not desirable in a well-formulated leaded light cement. In well-formulated leaded light cement the resulting matrix of whiting in polymerised linseed oil is waterproof and strong but not so rigid as solidified Plaster of Paris. So, leaded light cement containing Plaster of Paris is more likely to crack the glass in leaded lights because it is too rigid. This is reflected in comments here where it says that “recipes that use plaster of Paris or [Portland] cement are not recommended because the product is too hard and stiff when dry, providing no cushioning for the glass inside the lead frame. A slight rubberiness is desirable to provide support without the stiff restraint of the cement, which can cause the glass to break.”

In summary, Plaster of Paris should never be a component in your leaded light cement.


What about Portland Cement?

Portland cement is a “hydraulic cement” meaning it hardens when exposed to water, becoming water-resistant (rather than waterproof). It is what you use to make concrete.

Quicklime, or calcium oxide, CaO, is a major component of Portland Cement and as such its nasty stuff. This makes Portland Cement highly alkaline which means it is strongly caustic and will cause skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membrane can cause severe eye or respiratory irritation.

Everything that you just read about the use of Plaster of Paris also applies to Portland Cement.

In summary, Portland cement should not be a component in your leaded light cement.


What is Linseed Oil?

Linseed oil is a major component of both glazing putty and leaded light cement and is also known as flax seed oil. You can read lots about it at Wikipedia.

Linseed oil is what is known as a “drying oil”, which means it polymerises over a period of time (days or weeks) to form a solid mass. This polymerisation causes glazing putty and leaded light cement to harden to a rigid (rather than brittle) state over the course of several days. As a triglyceride, the water-repellent (hydrophobic) nature of linseed oil is also an advantage for its water-repellent weatherproofing properties. Read more about “drying oils” here if you have the enthusiasm because it’s the key to how leaded light cement achieves what we’re after – water-repellent strength without excessive rigidity.

Linseed oil is available either as raw linseed oil or boiled linseed oil. Boiling linseed oil causes it to partially polymerise and oxidise, making it thicker which in turn means it has a shorter drying time. Most products labelled as “boiled linseed oil” are now a combination of raw linseed oil, a volatile petroleum-based solvent (which evaporates quickly) and metallic dryers (which are chemical catalysts that accelerate drying).

As noted at the long-gone Stained Glass Town Square forum, raw and boiled Linseed oils are available (in-stock or to order) from a painting and decorating or DIY outlet. Art suppliers are also a source but are usually more expensive. Some agricultural or animal feed merchants might sell raw linseed oil as a feed supplement for horses.


What About Volatile Drying Agents?

Leaded light cement formulations often include turpentine, turpentine substitute or white spirits. There is little (if any) practical difference between these different products when used as a volatile drying agent in leaded light cement, the significant differences being how they smell and how much they cost.

Turpentine is a natural product derived from pine trees. As a natural product it tends to be expensive, particularly when highly purified.

Turpentine substitute (mineral turpentine) is an inexpensive petroleum-based replacement for turpentine. It consists of a complex mixture of highly refined hydrocarbon distillates mainly in the C9-C16 range, typically containing at least 60% white spirit and 20-40% naptha (petroleum). Low odour turpentine is close to 100% white spirit.

White spirits are the inexpensive petroleum-based replacement for turpentine, consisting of a complex mixture of highly refined hydrocarbon distillates. So, in addition to being cheaper, white spirits are less flammable and less toxic than turpentine and turpentine substitute.

Therefore, it seems, any of turpentine, turpentine substitute or white spirit will suffice but white spirit causes less smell and is the cheapest.

Paint thinners are a solvent commonly held to mean white spirits, turpentine or turpentine substitute. It can also be taken to include other solvents such as acetone, xylene and toluene. So, be aware that when you want to clean your glazing brushes paint thinners may not always mean white spirits.


What about Colourants?

Traditional glazing putty is a pale brown colour but leaded light cement tends to be shades of grey tending towards black. The colour was traditionally introduced by the use of lampblack which is nothing more than soot, which in turn is just powdered carbon.

Comments from the long-gone Stained Glass Town Square forum suggest that leaded light cement can be colored with stove blackening products (such as tubes of Zebo or Zebrite), concrete colouring (powder), powdered poster paint, oil paint and the traditional lamp black (powder).

This forum thread also warned against the use of any water-based pigment because they cause problems with linseed oil putty (because water doesn’t mix well with oils!). This also means that latex and acrylic colours are not acceptable.


What about Other Additives?
Lead and lead oxides were used in traditional formulations for leaded light cement but due to the toxicity of lead compounds their use is less common in modern formulations.

Why lead and lead oxides were used is not clear but one rational possibility for using them might be that they are toxic to all kinds of life including algae, moss and lichens that might attempt to establish themselves on your leaded light. Of these lichens would perhaps be the most difficult to avoid as they are well-known for their ability to tolerate concentrations of heavy metals, such as lead, that kill other forms of plant life.

Another possible reason for using lead compounds in leaded light cement relates to their possible use as a drying agent. In this context they may be catalysing the oxidative polymerisation of the linseed oil – making the linseed oil harden more quickly.

Unless you have a particularly strong justification for using lead or lead oxides in your own formulations for leaded light cement I recommend that you stay clear of lead compounds in the interests of your health.

If you have the enthusiasm for a brain-stretching read, have a closer look at Oil Drying Agents at Wikipedia to understand more about how some additives help to accelerate the drying of oils.


Cheaper Oils and Solvents

Boiled linseed oil is cheaper and ‘cures’ faster than raw linseed oil. This is why some formulations use a mixture of both. Ultimately they both become the same polymerised material in your leaded light panel .

White spirits are cheaper and just as effective as turpentine substitute which in turn is cheaper and just as effective as pure natural turpentine. Ultimately they all serve the same purpose and will all evaporate out of your leaded light panel.


That’s All Folks!

Thank you for your patience reading this blog. I hope my chatterings have been useful. Do try some experiments of your own and let us all know how successful they were.


Posted in leaded light, Leaded light cement, Putty, Whiting | Tagged , , | 10 Comments

Make Your Own Glass Cleaner

Today I want to chatter about glass cleaning products that are used on kiln-fired glass, discovering who’s using what and talking about what works and why, and then talk about how you can make or choose the best glass cleaner.

If you’re hoping to read about ordinary household window cleaning products then this blog is unlikely to be of interest, although I will make some passing comments that you may find helpful.

The Motivations

A key motivation is to ask why we pay for a brand name when a generic product or home-made product does the same job, sometimes better, and is cheaper? You might suppose that there’s something special in the branded product but often you’ll discover it’s nothing more than an uninspired formulation of standard raw materials.

Another motivation is to consider why we use glass cleaning product. There are people who use commercial products just as there are others that do not clean their glass before putting it into a kiln. It’s all rather confusing when all you want to do is reduce the incidence of devitrification on you glass masterpiece.

A third motivation is to consider some of the questionable advice found on the Internet when one cleaning product is often recommended in preference for another but without clear reasoning or justification.

Why Bother Cleaning Glass?

The boiled-down essence of “perceived wisdom” is that we need to do whatever is necessary to reduce the number of nucleation sites on the surface of our fusing glass which could lead to devitrification. Basically, it’s any kind of “muck” on the surfaces of your glass that form these nucleation sites that can provoke the devitrification process. The process of devitrification is the result of normally amorphous crystalline glass changing progressively into regular crystalline glass. If you are a keen and enthusiastic learner have a look here to understand more about crystal formation, paying particular attention to the section about “heterogenous nucleation”.

The reason that some people get obsessive about cleaning glass before use is that they believe even the merest hint of a fingerprint or dust on glass will result in devitrification. Others suspect it’s perhaps just a big conspiracy theory, believing that any contaminant on glass will simply burn away, just as white PVA glue or Glastac is supposed to do. Think about both sides of the argument because both sides can’t be simultaneously true but both have some truth to them.

The bottom line is that if we really want to clean our glass then something we must hold in our minds is that a glass cleaner should evaporate and not leave any residues whatsoever if it is to leave your glass in a pure unsullied state.

Who’s Using What?

It’s maybe a good idea to start by finding some examples of who is using what to prepare their glass for firing. We can then look at what’s in those products to understand why they work and why they might not be ideal formulations.

Over here in 2010 we find Spartan Glass Cleaner being recommended by someone from Bullseye in response to a mention of a professional glass cleaner from Bohle being used by someone who has a problem. This forum thread is particularly interesting because the advice is well-meant but smacks of “product placement” because Bullseye sell Spartan Glass Cleaner.

Elsewhere I see another “product placement” in the same forum in 2010 where a lack of understanding that “rubbing alcohol” and “denatured alcohol” are not the same thing leads to another “product placement” for Spartan Glass Cleaner, again by someone from Bullseye. Overall, this forum thread gets my vote for whacky advice and mis-information.

But we do now have two named commercial products and two “alcohols” to start the ball rolling for our investigation…

  • Bohle glass cleaner
  • Spartan Glass Cleaner
  • rubbing alcohol
  • denatured alcohol

Let’s look at them in more detail.

If you look for Bohle glass cleaner you find you’ll find that they actually produce more than one product. Find their safety data sheets here and study them, looking for the section in which you find their ingredients. You will find their glass cleaners contain isopropyl alcohol and a surfactant called 2-butoxyethanol. We will individually consider these chemical components later.

We now move on to the aforementioned “Spartan Glass Cleaner” and you will also find that Spartan Chemicals make several glass cleaners. The particular Spartan product that Bullseye sell here seems to be this product from Spartan Chemical and lower down on the same page you’ll find the safety data sheets in three languages, the American language version being here. From the manufacturer’s safety data sheet we learn that Spartan Glass Cleaner contains mostly water, some isopropanol and a little acetic acid. Curiously the safety data sheet for the product suggests a “floral fragrance” which I can only assume means the addition of a small amount of unlisted perfume to mask the acrid smell of acetic acid.

Rubbing alcohol has been mentioned in the forums. It has a strange name because it describes the intended use rather than a specific chemical. So, as you’ll find at Wikipedia’s page for rubbing alcohol it’s likely to be some water with either isopropyl alcohol or ethanol with the possible addition of various additives, some some of which may make it taste bitter and undrinkable and others that have mild medicinal qualities. This lends some credence to a forum contributor here that says rubbing alcohols “sometimes contains impurities, like oils”. Therefore, in summary, rubbing alcohol is an ill-defined product that is mainly made using alcohols that will quickly evaporate but may also contain additions that will not evaporate after cleaning our glass, leaving a small amount of residue. These ill-defined additions are a possible trigger for devitrification if they do not cleanly burn off in a kiln.

Denatured alcohol is also known as methylated spirits and is a mixture of chemicals though typically it is ethanol that dominates, with the addition of some water. It is difficult to pin down precisely what the formulation is because many possible additives are added to the ethanol to make it taste bitter and undrinkable. Whatever the mixture, the alcohols will evaporate completely and quickly because they are volatile, just as they will do for rubbing alcohol. Other additive, such as dyes and bitter-tasting denatonium (as with rubbing alcohol) are a possible trigger for devitrification if they do not cleanly burn off in a kiln.

Domestic Window Cleaning Products

Here I just want to make a few passing comments about what we might call a general purpose window cleaning products. By this I mean something you’d choose to routinely use in a domestic setting.

My first observation is that I don’t really see how Spartan Glass Cleaner can be considered to be special. It certainly does not appear to be specifically designed for our precious fusing glass. More to the point, the formulation appears to be what one might expect from a general purpose window cleaning product (by virtue of the vinegar and having a fragrance) and indeed it seems to be sold by Spartan Chemicals as a general purpose glass and window cleaning concentrate.

A similar observation can be made regarding the Bohle glass cleaners as they too do not appear to be specifically designed for our precious fusing glass.

There is however one very useful comment mentioned here that warns us to avoid extremes of pH (meaning acidity or alkalinity) because they can attack metallic structures such as lead or zinc cames. In other words, glass cleaners that contain ammonia or vinegar at significant concentrations should be avoided. However, because we spray our windows with products that contain only low concentrations of ammonia (or vinegar) then wipe it all away within minutes I fail to understand how any damage can be done to the glass itself and there is little time available to corrode metal work.

Looking More Closely

We have now looked at four products that have been mentioned in forum threads and I’m sure that many others could be added. But let’s press on to the interesting stuff. It’s time to consider the key chemicals from which they are made so that we can better understand what’s good, what’s not and why.

Water is a common ingredient. As we all know, pure water boils at 100 °C and should be neither acidic nor alkaline. Water is probably the most familiar solvent we use and it will not easily cause our glass to devitrify. It is a cheap chemical because it’s so abundant, even if we choose to pay extra for deionised water or even more for distilled water because we don’t have a nice clean soft water supply coming from out taps (American: faucets). The only concern here is the use of hard water because of the dissolved minerals which might come out of solution when cleaning glass because tiny mineral particles can act as nucleation points for devitrification. With regard to distilled vs. deionised water I note that although distilled water is very pure, it should be noted that deionised water has the ionic mineral components removed so is much cheaper to produce and just as usable as distilled water for cleaning glass. Sometimes spending that extra money doesn’t really give you any significant advantage.

Isopropyl alcohol has many chemical synonyms such as isopropanol, propanol and propan-1-ol. We’ve already seen isopropyl alcohol mentioned in the safety data sheets for both of the commercial products as well as for the rubbing alcohol (and possibly for denatured alcohol). So is important we know more about this particular alcohol. In essence, isopropyl alcohol is a pure chemical, not a commercial formulation, which means we can know exactly how it behaves. It is volatile with a boiling point of 82.6 °C which means it evaporates quickly and completely. It is neither an acid nor an alkali so it will not attack glass. It is an effective solvent in many situations and is also cheap to manufacture so you will find it being used in many diverse cleaning products. These are all good characteristics for a glass cleaner, explaining why each of our four example products involve isopropyl alcohol.

2-butoxyethanol has many synonyms of which 2-butoxyethan-1-ol is its favoured chemical name. In essence it’s another pure chemical, not a commercial formulation, which means we can know exactly how it behaves. It is a non-volatile solvent of low toxicity that has mild surfactant properties and has a mild sweet smell. In other words, it’s a gentle cleaning agent that doesn’t evaporate quickly so is useful for cleaning mucky glass. It has a boiling point of 171 °C which means that any of it that is not cleaned off our glass ought to evaporate completely in a kiln. Surfactants are something we’re more familiar with in washing-up liquid and other “harsher” household washing and cleaning products and the reason they’re often used is because they’re good at “lifting dirt”.

Acetic acid is also known as ethanoic acid and when diluted we know it as vinegar. When in Britain, be sure to add some to your fish and chips (American: fries) if it’s of the malted vinegar variety. Acetic acid has a boiling point of 116 °C which means it should evaporate completely but not very quickly but we must remember that acetic acid is mixed with plenty of water to form what we know as vinegar. Vinegar has been used to clean things for time immemorial and is used in some glass cleaning formulations such as Spartan Glass Cleaner as promoted by Bullseye as well as domestic products such as Windowlene in the UK. It is a cheap and abundant chemical. It is a weak acid being used in small quantities, and because we don’t spend hours cleaning a piece of glass, it is unlikely to cause any problems for our fusing glass surfaces.

Though not mentioned in the four products we’ve been looking into, it would be remiss of me to ignore ammonia which is a synonym for household ammonia and ammonium hydroxide. It does happen to be used in a Spartan glass cleaner promoted as being “fortified with ammonia” and you’ll find the MSDS here. But I digress. Ammonia is volatile and has a boiling point of around 27 °C and is a commonly used in small quantities, diluted in water, as a solvent in some household window cleaning products. Only because ammonia tends to be used in small quantities within window cleaning products, and because we don’t spend hours cleaning a piece of glass, can we assume the ammonia it is unlikely to cause any problems with our fusing glass. It just smells absolutely horrible.

We can not identify the un-named but implied fragrance perfume in Spartan Glass Cleaner and can only assume that if it’s not causing Bullseye any problems. We can however guess that it’s probably some kind of organic molecule (or mixture) that successfully burns off in a kiln. But, if we are worried about devitrification and want to avoid contaminating the glass then shouldn’t we also want to avoid things like fragrance chemicals that aren’t needed to clean our glass? This is an interesting concept to consider! All I can suggest is that the only logical reason to add a perfume to a cleaning product is to mask the smell of something unpleasant – like vinegar or ammonia.

And finally, I have one more chemical that deserves a mention because it’s rarely mentioned. It’s called acetone and is also known as propanone. Add some colour and perhaps some fragrance and we call it nail polish remover. Acetone is very volatile and has a boiling point of 100 °C which means it will quickly evaporate and leave no residues. It is neither acidic or alkaline so will not attack glass. The colorants and perfumes added to nail polish remover make it impure and therefore less desirable than pure acetone. The only link on the Internet I can recommend reading that knowledgeably relates to the use of acetone to clean glass is here.

Looking Wider

Having discovered and learned a little about chemicals that are commonly used, it’s time to trawl the Internet for more examples of what people are using to clean their glass. The best source I encountered comes from a long forum thread here at Fused Glass in 2008 that reveals many examples of what people do to clean their glass.

What follows in this section is a brief summary of what caught my eye in that forum thread and I don’t suppose attitudes and behaviours have changed much in the succeeding decade.

One person uses distilled water and vinegar, commenting that they use distilled water due to having a hard water supply. Avoiding hard water is a good idea because of the dissolved mineral that might come out of solution, leaving scummy particles that might just provoke the nucleation which cause devitrification. Again we see readily available vinegar being used. What is not clear is whether the water and vinegar are separate cleaning steps or whether they are mixed together. I personally would hope this person used the vinegar as a first step then rinsed using the distilled water.

Another person uses hot water with a small amount of soap. This sounds reasonable because the soap and water will be effective with most kinds of contamination. The kind of soap is not mentioned though some (such as washing-up liquid) often contain surfactants (like the 2-butoxyethanol used in the Bohle glass cleaner) whereas others (particularly hand-made soaps) may contain an excess of oils and fats and might therefore leave a thin greasy residue which ought to burn off in the kiln. But for a good cleaning regime it is important to ensure that soap residues are completely removed with a thorough rinse.

Another related comment in the forum thread is the use of warm water with a little dish soap followed by rubbing alcohol. I like this regime better than the previous example because there is a rational two-step regime. Not only does the alcohol rinse away any remaining dish soap but it also serves as a different kind of solvent. The only problem I have with this regime is that it is two steps which implies lots of time and effort.

One contributor reveals what is, in effect, the reverse process, saying ” I use alcohol, then I wash it is very hot water & Dawn dishwashing soap”. Dawn dishwashing soap is what I’d call washing-up liquid, probably a perfumed, mixture of chemicals that includes a surfactant that should be well suited to removing a variety of contaminants. However, this regime would be more reliable and more effective if the alcohol was the second stage, if only to ensure that the washing-up liquid residues are completely removed by the alcohol wash.

And then there’s someone who uses an undefined alcohol and another who uses isopropyl alcohol. As we’ve seen with the Bohle and Spartan glass cleaners, alcohols are widely used because good for cleaning glass.

A curious comment is from someone who says they attended a workshop at Bullseye and they use a mixture of 50% white vinegar and 50% denatured alcohol. This seems to be at odds with what the people from Bullseye have been saying in the forum threads I mentioned at the top of this blog though one might rationally argue that this forum thread is from 2008 whereas the people at Bullseye were recommending Spartan’s products in 2010. But regardless of this observation, the quantity of white vinegar being used seems to be rather excessive, even when we recall that vinegar is acetic acid diluted with lots of water. A small quantity of the white vinegar added to the alcohol would be just as effective and would not smell so horrible. In fact, I’d hazard a guess that omitting the vinegar would be equally effective.

Of course, there are also some people who are using ordinary domestic window cleaning products. But, to be blunt, is that not what the Bullseye recommended Spartan glass cleaner and the Bohle glass cleaner seem to be?

And, for completeness, I should mention that there were some people in the forum thread who’ve been told not to bother cleaning glass because the contaminants will all burn off. This is questionable advice because only the chemicals and contaminants that cleanly burn or evaporate will disappear and not cause a problem. Try burning off particles of grit for example!

 Make Me Laugh

The quote below from the same forum thread at Fused Glass and I find interesting because it reminds us to question what we are told, do our own research and to not assume that a supposed expert is always right, whether the advice is from a face-to-face encounter with a real person or via the Internet.

I took classes from a ” prominent teacher ” who also gave incorrect information….lots of it actually.  Forums such as these are an excellent source of information and have taught me MORE than the prominent teacher.”

If you’re now of the opinion that forums are good and experts are unreliable then consider the advice given here at the Glass Fusing Made Easy web site. It tells us…

Detergents, dish soaps, multi-purpose cleaners, some window cleaners, ammonia and even denatured alcohol should not be used to clean glass.
The Bullseye site suggests that you purchase a cleaner called Spartan Window Cleaner.

Though well-meaning, it’s rather lacking in rigour. Where is the justification and reasoning to support any of this advice? More to the point, there’s a bland assumption that Spartan Window Cleaner must be a good choice because Bullseye recommend what they sell.

Remember folks, I regularly and deliberately tell you to question everything you read on the Internet. I even tell you to not assume I know what I’m talking about. Don’t take my advice without question and likewise don’t take advice from anyone else without question. Think things through and make your own decisions.

My Recommendations

No matter what you decide to use for cleaning glass, if indeed you choose to clean your glass, just make sure the glass is cleaned thoroughly then buffed until it is dry. If you stop buffing when the glass is still wet then the contaminants dissolved in your glass cleaning liquid will be left behind (nicely smeared) on the glass surface when the remaining glass cleaning liquid evaporates.

Use a lint-free cloth if you can for your drying and buffing, but if you can’t then try using good quality paper kitchen towels instead. Cheap paper kitchen towels can leave a residue of paper dust so try not to use such products.

If you find that a piece of glass does not seem to be completely clean after trying to clean it with one kind of cleaning product then repeat using the same cleaning product if it previously made some good progress.

Do remember that you can choose a different cleaning product for your second cleaning attempt, especially if the first cleaning product is having difficulties with the contaminant. Different cleaning products have different capabilities because they contain different solvents.

If you are going to use a cleaning product that contains a soap or surfactant then consider a second rinsing to be sure the soap residue is removed. Be especially aware that some kinds of soap (particularly “super-fatted” hand-made soaps) may leave a thin greasy residue on the glass you just cleaned.

I am an advocate of using pure solvents that are devoid of non-essential additives. I therefore tend to use neat isopropyl alcohol or neat acetone as my cleaning solvents. But remember that I’m not afraid to pre-clean mucky glass with soap and water. Of all the recommendations I give you in this section, this is the paragraph I want you to remember!

Isopropyl alcohol and acetone are volatile and flammable. Take care when you use cleaning products that contain these chemicals. Take care with acetone near plastics as it’s good at melting some of kinds of plastic!

I believe that touching glass with a finger is overstated as a risk for devitrification if your hands are reasonably clean. I have found that white PVA glues are far worse than fingerprints for causing devitrification. So the simple advice here is to try washing your hands before you work with your glass!

And finally, don’t get obsessive about glass cleaning. Just use a quick and effective cleaning regime that works.

Choose Your Cleaning Product

You should now be aware that several different chemicals can be found in glass cleaning products and that you now have an idea of what other people are using. You now also know that there’s nothing particularly special about expensive proprietary commercial glass cleaning products. You also know that water (soft, deionised or distilled), acetone and isopropyl alcohol are the key solvents you need to consider.

Ideally, you will need to find a local supplier of “industrial chemicals” or maybe “industrial solvents” to buy your distilled or deionised water, isopropyl alcohol or acetone, but they should cost you a lot less that buying them from your local stained glass supplier.

An example supplier that I’ve mentioned in the past is APC Pure. I don’t mention them because I work for them or get any commission from them. I simply mention them because they’re local to where I live and they’re an example of the kind of company you should be looking for. They also sell on eBay as do other chemical suppliers. Seek and ye shall find though it’s harder in some countries than others.

If you choose to buy some isopropyl alcohol you should now realise that you can, if you wish, do things lilke dilute it with water and perhaps add some white vinegar to make your own version of the Spartan glass cleaner type of product. But I find it is best when the isopropyl alcohol is used at full strength and pure because it evaporates quickly and cleanly.

You may also choose to buy some acetone. I find it will deal with contaminants that isopropyl alcohol struggles with (and vice versa). I always use it neat as it evaporates very quickly and cleanly.

Don’t waste your money on branded products when you know about cheaper and better alternatives!

A Curious Detour

Although a detour, it is interesting to look here because it says

“I’ve been told that [Fusemaster] Superspray has a shelf life of about 1 1/2 years. If that is true, and yours is fairly old, that might be the cause [of devitrification]”.

Well, a search for the MSDS for this devitrification spray tells me it’s pretty-much powder frit added to ethyl alcohol with a tiny amount of isopropyl alcohol with some methyl isobutyl ketone as a surfactant. What struck me as interesting is that the three chemicals used to “carry” the glass powder seem to be remarkably similar to what we’d expect for a glass cleaning product. I can see the logic for using the two alcohols but I’m baffled why the surfactant is used, other then perhaps they are simply mixing a bought-in glass cleaning product and mixing it with clear glass powder frit.

But back to the context of the original forum posting… Call me stupid if you like but I don’t think powdered frit has a shelf life. And I don’t ever recall seeing a “Best before” date on any window cleaning product I’ve ever seen in the past. In other words the devit spray can’t be the cause of the devitrification.

Another example of well-meant poor advice I think!

And Finally

That’s all my chatter for today so all that remains is to remind all you “do it yourself” fans that I’ve already done postings that tell you how to make your own copper patina solution, safety flux and glass polishing compound.

Best wishes and a happy new year to you all.

Posted in Glass Cleaning, Money-saving ideas | Tagged , , | Leave a comment

Undercut Your Dichro

Today’s blog is mercifully short. There’s no maths or science and it isn’t even a fantastic new discovery that will change the world. It’s more of a helpful tip to those who’ve maybe not realised there’s an answer to the following problem:

When firing a piece of  clear-backed dichroic glass metal side down over a base of some other kind of glass we can produce a three-dimensional “well” effect. The problem is that it results in a messy-looking metal edge when we take it out of the kiln.

On the right is a picture of what I mean.

The left side illustrates what you can expect to happen without today’s tip. The right side shows that the problem can be managed, albeit not perfectly in this particular example.

Although I earn no gold star for excellence with this particular experiment you can at least see there’s a distinct improvement. The questions to be answered are how the problem arises and how the problem can be eliminated or at least mitigated.

The problem arises because the clear-backed dichroic glass sinks into the glass base during the fusing process and leaves behind some of the metal layer at the edges.

The answer to this problem is simple. Just grind away the dichroic metal layer from the edges of the piece of dichroic glass. This can be done very quickly with diamond pads for untextured dichroic glass but will be more fiddly with textured surfaces.

To illustrate what you are aiming for, look at the diagram below. The lower area represents the base glass, the upper area represents the clear dichroic glass and the thick line represents the metal layer of the dichroic glass.

Notice the angled undercutting at the lower edges of the dichroic glass and how some of the metal layer gets removed as a consequence. From this you will realise it’s not the angle of the undercutting that is important but the amount of metal layer that gets removed. Having said that, you’ll get better control over how much metal you remove with a reasonable angle that you’ll achieve with a really shallow angle.

Exactly how much undercutting is needed to eliminate the problem primarily depends on the thickness of the dichroic glass layer. I also expect (but have not checked) that the amount of undercut will also depend on whether you are using a microwave kiln or a “proper” kiln. Start with a couple of millimetres of undercutting for your first experiment. Let practise and experience become your teacher.

That’s all for now. Happy fusing.


Posted in Dichroic Glass, Microwave kiln | Tagged , , | Leave a comment

The Glass Bookshelf

For your enjoyment I have gathered together details of well over a hundred old books that relate to glass in some form or another, all of which are easily accessible over the Internet.

Whether it’s the glass sands of Kentucky, glass manufacture, glass painting, old trade catalogues, collecting glassware, making laboratory glassware, old Church windows or one of the many other strange topics relating to glass, there’s surely something for you somewhere in the list. I hope you find something that tickles your fancy.

Downloadable Book Lists

Although you’ll find the book list below within this blog I’m sure that some of you will want to “do your own thing”. So, I’ve created three alternative files that you can download and fiddle with. You could then sort your copy of the book list by title, date or author, improve the topic lists, add more books, or whatever else you might fancy doing. Make the downloaded files your own personalised bookshelf!

Beware: Although the file names below have familiar file types like “.doc” and “.pdf”, they aren’t what they seem to be. I’ve had to do some simple “trickery” to enable you to download file types that WordPress, in its infinite wisdom, has chosen not to allow. So, be sure to read the text that follows the filenames before downloading them!

  • Bookshelf-rtf.doc   Actually this is not a “.doc” file, it’s a “.rtf” file. I had to rename it from “Bookshelf.rtf” to “Bookshelf-rtf.doc” before WordPress would to allow me to share it with you. So, please rename it back to “Bookshelf.rtf” once you’ve downloaded it. Let me explain… Unlike “.doc” files, “.rtf” rich text files are Microsoft’s only official “portable” file format for word processing documents. This means it can be loaded into many kinds of word processor applications and not just Microsoft Word. Use this file if you want to maintain and use the book list within your word processor. If this doesn’t work for you then try the Bookshelf.xls file format in your spreadsheet application and copy the table out of there into your word processor application.
  • Bookshelf.xls   This spreadsheet file really is an “.xls” file and it is saved in an old version of Microsoft Excel format so you don’t need Microsoft’s latest and greatest bloatware to use this file. It should work with some other spreadsheet applications too.  Use this file if you want to maintain and use the book list as a spreadsheet. If this doesn’t work for you then try the Bookshelf.tsv file instead (see below).
  • Bookshelf-tsv.pdf   Actually this is not a “.pdf” file, it’s a “.tsv” file. I had to rename it from “Bookshelf.tsv” to “Bookshelf-tsv.pdf” before WordPress would to allow me to share it with you. So, please rename it back to “Bookshelf.tsv” once you’ve downloaded it. This is a “tab separated variable” file that can be loaded into most kinds of spreadsheet application, word processors and even some database applications. Treat this as your “last chance” file format because you’ll probably need to do some messing about to get what’s loaded into a nice useful state. Despite this it’ll be much quicker than trying to type everything manually.

If you struggle and fail with all three file formats, or have a particular need for some other file format for use on something other than a Microslop Windoze machine then do let me know and I’ll try to help.

Some Notes

The Title, Author and Date columns should uniquely and correctly identify each book but in some cases notes are added in square braces within the Title column to indicate something noteworthy, such as telling you that a different edition elsewhere in the table. Where a the title of a book title contains an ellipsis it is an indication that the full title was so long and verbose that it has been truncated. There were occasions when I reckoned the book details didn’t seem to be correct (for whatever reason) so occasionally you’ll find that information about titles, authors or dates in my table are not quite what you’ll find at

The Topics column gives an indication of what kind of information is found in the books and can be rather vague. Sometimes I’ve given you what the archivist recorded as the topics, if anything. Sometimes I’ve added to their topic lists and sometimes I had to create the topic list myself. A consequence is that the list of topics can be rather erratic but better than a poke in the eye with a blunt stick.

The Location column contains one or more URL links that take you to a web page where you can download the book. It’s perhaps sad to note that many of the old books have been digitised multiple times whereas many other old books remain only as “hard copy”. After awhile I got bored with finding alternative sources for the books so stopped bothering.

In all cases, visiting the URL will reveal a download available in PDF format. Often there are other file formats available as well. Alternative formats might be more suitable for use on a Kindle or mobile phone, for example. If you intend to download a copy of all the books in PDF format then you will need about 4GB of free disk space.

Bear in mind that copyright laws in different countries are not the same. Distributing this list of books and their locations will not breach any copyright but distributing the scanned book images and expecting payment might.

Here are the books I’ve found so far. There are even more out there!

Oh, and sorry about the bad layout of the table. Despite messing around with the undelying HTML I can’t force WordPress to present the information tidily.

Title Author Date Topics Location
The Glass Worker, vol 4 no 46 (Jun 1907) and vol 4 no 48 (Aug 1907) Amalgamated Glass Workers’ International Association of America 1907 Glass Industry, Industrial
A Notable Collection of Ancient Egyptian, Greek and Roman Glass, Persian Potteries, Greek Painted Vases … Anderson Galleries Inc. 1924 Mr. Azeez Khayat, Catalogue
Stained glass of the middle ages in England & France Arnold, H. 1913 Glass painting and staining, Art, Mediaeval, England, France. and similar at
Arts and crafts essays

[in which many subjects are covered, including Stained Glass by Somers Clarke, pp96-105)

Arts and Crafts Exhibition Society 1893 Arts and crafts movement, Decorative arts
History of the Worshipful Company of Glaziers of the City of London, otherwise the Company of Glaziers and Painters of Glass Ashdown, C. H. 1919 Company of Glaziers, London, Guilds.
Twelfth Census of the United States: Census Bulletin no 228: Manufactures: Glass Manufacture Austin, S. P. 1902 Glass industry, Census
The painter, gilder, and varnisher’s companion containing rules and regulations in everything relating to the arts of painting, gilding, varnishing, and glass staining … [See also 1867 edn] Baird, H. C. (Pub.) 1850 Painting, Industrial, Gilding, Varnish and varnishing, Receipts.
The painter, gilder, and varnisher’s companion containing rules and regulations in everything relating to the arts of painting, gilding, varnishing, and glass staining …, 10th Edn.

[See also 1850 edn]

Baird, H. C. (Pub.) 1867 Painting, Industrial, Gilding, Varnish and varnishing, Receipts.
Schwäbische Glasmalerei Balet, L. 1912 Glass painting and staining
A Treatise on Painted Glass, Shewing its Applicablity to Every Style of Architecture Ballantine, J. 1845 Glass painting and staining
American glassware, old and new: a sketch of the glass industry in the U.S. and manual for collectors of historical bottles Barber, E. A. 1900 Glass manufacture, Glassware
Spanish Glass in the Collection of the Hispanic Society of America Barber, E. A. 1917 Glassware, Spain
English table glass

[See also later edition]

Bate, P. H. 1905 Glassware, Tableware, England
English table glass

[See also earlier edition]

Bate, P. H. 1913 Glassware, Tableware, England
President’s Address on “Stained Glass and Painted Glass” , in Transactions of the Bristol and Gloucestershire Archaeological Society, Vol 22. Editor: Rev. C. S. Taylor, Bazley, G. S. 1899 Periodical. Antiquities, Bristol and Gloucestershire, Glass painting and staining.
Catalogue Belcher Mosaic Glass Co. 1886 Mosaics, Glass painting and staining, Glass, Coloured–Catalogues
A lecture on stained glass Bell, R. 1922 Glass painting and staining
Sir Henry Bessemer, F. R. S. An autobiography

[Chapter VIII relates to glass processing]

Bessemer, H., Sir. 1905 Glass manufacture, Industrial processes, Autobiography
Elements of Glass and Glass Making Biser, B. F. 1899 Glass manufacture
The Art of Glass: shewing how to make all sorts of glass, crystal and enamel, likewise the making of pearls, precious stones, china and looking-glasses … [Translation of: De l’art de la verrerie] Blancourt, H. 1699 Glass manufacture, Enamels & enamelling, precious stones, minerals, optical glass, glass eyes
A handbook of laboratory glass‐blowing Bolas, B. D. 1921 Glass blowing and working, Glass manufacture, Technique
A memoir on British resources of sands suitable for glass‐making Boswell, P. G. H. 1916 Glass manufacture
Japanese Enamels: With Illustrations from the Examples in the Bowes Collection Bowes, J. L. 1886 Enamels and enamelling, Cloissonné, Shippo, Japan
Notes on Shippo: A Sequel to Japanese Enamels Bowes, J. L. 1895 Enamels and enamelling, Glass manufacture, Cloissonné, Shippo, Japan
Glass cup plates: A Guide to Collectors Burns, C. 1921 Glassware
Storied windows: A traveller’s introduction to the study of old church glass, from the twelfth century to the Renaissance, especially in France Bushnell, A. J. 1914 Glass painting and staining, Cathedrals, Renaissance, Mediaeval, France
Catalogue of the collection of stained and painted glass in the Pennsylvania museum Bye, A.E. 1925 Glass painting and staining, Philadelphia Museum of Art
General Electric Lighting Canadian General Electric Company 1949 Lighting, Glassware, Commercial & industrial equipment.
Stories of Industry, 1891, Vol 1

[Vol 2 relates to clothing and foodstuffs]

Chase, A. & Clow, E. 1891 Industrial Arts, Minerals, Industrial Processes, Machinery & Equipment, Glass Manufacture, Pottery manufacture
Art glass metals Chicago Metallic Sash Company 1925 Art glass, Came, Stained Glass, Glazing accessories, Trade, Catalogues
Five Black Arts: a popular account of the history, processes of manufacture, and uses of printing, pottery, glass, … Coggeshall, W. T. 1861 Printing History, Porcelain History, Glass Manufacture History, Iron Industry and Trade History, Gas Manufacture and Works History
The Crown Glass Cutter and Glazier’s Manual: Glass cutter, glazier & stained glass maker in ordinary to the king for Scotland Cooper, W. 1835 Glass manufacture
Windows: A Book about Stained & Painted Glass

[See also later edition]

Day, L. F. 1897 Glass painting and staining and also
Windows: A Book about Stained & Painted Glass

[See also older edition]

Day, L. F. 1909 Glass painting and staining
Catalogue of an Exhibition of Stained Glass from the XIth to the XVIIIth cent Demotte Inc. (Pub.) 1920 Glass painting and staining
Glass Dillon, E. 1907 Glass, Glassware
A history of English glass painting, with some remarks upon the Swiss glass miniatures of the sixteenth and seventeenth centuries Drake, M. 1912 Glass painting and staining
Valuable secrets in arts, trades, &c.: selected from the best authors and adapted to the situation of the United States [See also other editions] Duyckinck, E. (Pub.) 1802 Receipts, Workshop recipes, Industrial art, Art technique
Valuable secrets in arts, trades, &c.: selected from the best authors and adapted to the situation of the United States. [See also other editions] Duyckinck, E. (Pub.) 1809 Receipts, Workshop recipes, Industrial art, Art technique
Valuable secrets in arts, trades, &c.: selected from the best authors and adapted to the situation of the United States [See also other editions] Duyckinck, E. (Pub.) 1816 Receipts, Workshop recipes, Industrial art, Art technique
Ancient stained and painted glass Eden, F. S. 1913 Glass painting and staining, History
The collection of heraldic stained glass at Ronaele Manor, Elkins Park, Pennsylvania, the residence of Mr. & Mrs. Fitz Eugene Dixon Eden, F. S. 1927 Dixon, Fitz Eugene, Glass painting and staining, Heraldry, Devices
Graeco‐Egyptian Glass Edgar, C. C. 1905 Ancient Glassware, Greco-Egyptian Glassware
The Origin of Glass Blowing in American Journal of Archaeology, Vol XX, pp 134-143 Eisen, G. 1916 Glass blowing
Antique Glass, in The Art Bulletin, Vol2: pp 87-119. Eisen, G. A. 1919 Glass painting and staining
Stained glass windows: An essay with a report to the vestry on stained glass windows for Grace Church, Lockport, New York Faber, W. F. 1900 Grace Episcopal Church (Lockport, N.Y.), Church records and registers, Glass painting and staining, Genealogy
Notes on the Painted Glass of Canterbury Cathedral Farrar, F. W. 1897 Canterbury Cathedral, Glass painting and staining
Glass Manufacture and the Glass Sand Industry of Pennsylvania Fettke, C. R. 1919 Glass, Glass manufacture, Glass Sands, Pennsylvania and at
The drama of glass [Publication date uncertain but there are references to 1893 within text] Field, K. 189‐ Glass manufacture
Hiersemanns Handbücher, Vol VIII: Handbuch der glasmalerei für forscher, sammler und kunstfreunde, wie für künstler, architekten und glasmaler Fischer, J. L. 1914 Glass painting and staining.
Laboratory manual of glass‐blowing Frary, F. C. 1914 Glass Manufacture, Glass-Blowing, Scientific Glassware, Laboratory, Technique
An Essay on the Art of Painting on Glass from the German of Emanuel Otto Fromberg Fromberg, E. O. 1851 Glass painting and staining
Rudimentary Essay on the Art of Painting on Glass from the German of Emanuel Otto Fromberg Fromberg, E. O. 1857 Enamel and enamelling, Glass painting and staining, Glass manufacture chemistry
Histoire de la Verrerie et de l’Émaillerie Garnier, E. 1886 Glass painting and staining, Enamel and enamelling
Street Lighting Glassware General Electric Company 1925 Street lighting, Glassware
Metal Casements and Frames, Wrought Iron George Wragge Ltd. 1948 Steel windows, casement windows, Trade catalogue
Rudimentary treatise on the art of painting on glass or glass-staining: comprising directions for preparing the pigments and fluxes … Gessert, M. 1851 Enamel and enamelling, Glass painting and staining and similar at
A booke of sundry draughtes, principaly serving for glasiers … Gidde, W. 1615 Decoration and ornament, Glass painting and staining, Enamel and enamelling
Stained Glass and Metal Work of Every Description for Churches, Chapels, etc.: Catalogues, Photographs and Special Designs with… Gorham Manufacturing Co. 1895 Trade catalogue, Stained Glass, Art Glass, Ecclesiastical, Windows, Memorials,
Le Vetrate di S. Francesco in Assisi – Studio Storico Iconografico Guisto, E. M. 1911 San Francesco Church (Assisi, Italy), Glass painting and staining
Old English glasses. … glass drinking vessels in England, from early times to the end of the eighteenth century Hartshorne, A. 1897 Glassware, Great Britain, Old English Glass, Jacobite Glass, Irish Glass, Wine in England
An history of the origin and establishment of Gothic architecture : comprehending also … Hawkins, J. S. 1813 Cesare Cesariano, Glass painting and staining in Middle Ages, Gothic Architecture
Rich cut glass and fine china Higgins & Seiter 1903? Trade Catalogue, Porcelain, Glass, Cut Glass
General catalogue of the manufactures of Adam Hilger, Ltd Hilger, A. 1913 Wavelength Spectrometers, Optical instruments, Scientific instruments
Stained glass as an art Holiday, H. 1896 Glass painting and staining
A practical manual of the collodion process : giving in detail a method for producing positive and negative pictures on glass and paper, ambrotypes, printing process, also patents… Humphrey, S. D. 1857 Collodion process, Collodion process, Photography, Photographic chemistry
Invisible Glass Windows: A Dramatic New Merchandising Force

[An example of this can be seen in Byram Street, Huddersfield, West Yorkshire]

Invisible Glass Company of America 1937 Trade catalogue, Store front, Display window.
Encyclopaedia of chemistry, theoretical, practical, and analytical, as applied to the arts and manufacturers J. B. Lippincott & Co. 1877 Chemistry, Technical, Art, Manufacturing
Glass: Interesting Facts connected with its Discovery and Manufacture J. E. Caldwell & Co. 18?? Glassware, Trade Catalogues, Glass History, Cut Glass
Priced and Illustrated Catalogue of Optical Instruments, Made, Imported and Sold, Wholesale and Retail James W. Queen & Co. 1870 Trade Catalogue, Optical Instruments, Lenses, Spectacles, Eye-Glasses, Telescopes, Camera Obscura, Microscopes, Dissection Tools, Biological Specimens, Geological Specimens
Reminiscences of glass‐making, 2nd Edn. Jarves, D. 1865 Glass Manufacture, Glass History, Chemistry, Workshop, Recipes
L’art de la Peinture sur Verre et de la Vitrerie Le Vieil, P. 1774 Glass painting and staining
Valuable secrets concerning arts and trades

[Note: T. Hubbard is the publisher, not the author]

Leather, V. 1795 Decorative Arts and similar at
Department Store Mechandise Manuals: The Glassware Department Lehmann, M. A. 1918 Glassware, Glassware manufacture
Old glass and how to collect it

[See also later editions]

Lewis, J. S. 1900 Glass collecting, Glassware
Old glass and how to collect it

[See also earlier edition]

Lewis, J. S. 1916 Glass collecting, Glassware
Old glass and how to collect it

[See also earlier editions]

Lewis, J. S. 1950 Glass collecting, Glassware
Flat Glass Libbey‐Owens‐Ford Glass Company 1924 Glass, Glazing, Trade Catalogue, Glass manufacture
Experimental Glass Blowing for Boys Lynde, C. J. 1920 Glass blowing and working, Laboratory
Fifty years of glass making, 1869‐1919 Macbeth‐Evans Glass Company 1920 Glass Manufacture
Glass and glass manufacture Marson, P. 1919 Glass, Glass manufacture
Optical measuring instruments, their construction, theory, and use Martin, L. C. 1924 Optical Instruments, Optical devices, Optics, Engineering
American glass McClinton, K. 1950 Glassware
Les Vitraux Merson, O. 1895 Glass painting and staining and also at
Fifteenth-Century Glass in The Chancel Window of St. Peter Mancroft, Norwich Meyrick, F. 1911 St. Peter Mancroft Church (Norwich, England), Glass painting and staining
Stained glass Miller, E. 1900 Glass painting and staining
Window Glass in the Making: An Art, A Craft, A Business Monroe, W. L. 1926 Glass, Glazing, Industrial Arts
The secret of pictorial art, or Self instructor in painting on glass, china, satin, and paper … Morse, D. D. 1879 Decorative arts, Decoration and ornament, Painting, Drawing
International Art Glass Catalogue: Art and bevelled glass in all its branches: church, memorial, society and domestic windows, Art Nouveau, prism, mitre beveled plate, leaded bevel, etc. [See also 1924 edition] Nat. Ornamental Glass Mfrs Assn of the US and Canada. 1914 Glass painting and staining, Catalogues, Trade catalogue, Art glass
Revised International Art Glass Catalogue: Art and bevelled glass in all its branches: church, memorial, society and domestic windows, Art Nouveau, prism, mitre beveled plate, leaded bevel, etc. [See also 1914 edition] Nat. Ornamental Glass Mfrs Assn of the US and Canada. 1924 Glass painting and staining, Catalogues, Trade catalogue, Art glass
Ancient painted glass in England 1170-1500 Nelson, P. 1913 Glass painting and staining, England, Mediaeval.
A descriptive catalogue of the glass vessels in the South Kensington museum Nesbitt, A. 1878 Glassware
Glass Nesbitt, A. 1878 Glass, Glassware, History, Glass Composition
Geschichte der Schweizer Glasmalerei Oidtmann, H., Dr. 1905 Glass painting and staining
Die Rheinischen Glasmalereien vom 12. bis zum 16. Jahrhundert, Erster Band Oidtmann, H., Dr. 1912 Glass painting and staining
Modern optical instruments and their construction Orford, H. 1896 Optics, Optical Instruments, Optical devices.
Le Vitrail; Son Histoire, Ses Manifestations à Travers les âges et les Peuples Ottin, L. 1896 Glass painting and staining
The Arcana of Arts and Sciences: or Farmers’ & Mechanics’ Manual: containing a great variety of valuable receipts and useful discoveries… Parker, M., Dr. 1824 Formulae, Recipes, Receipts, Agriculture, Industrial Arts, Pigments, Dyes, Dyeing, Woods, Metallurgy, Stains, Varnish, Glass manufacture, Cements, Enamelling
Curiosities of glass making: With details of the processes and productions of ancient and modern ornamental glass manufacture Pellatt, A. 1849 Glass manufacture, Anceint glassware, Technique
The glass collector; A Guide to Old English Glass Percival, M. 1919 Glassware
The amateur’s handbook of practical information for the workshop and the laboratory, 2nd Edn. Phin, J. 1879 Workshop, Laboratory, Recipes. and at
Glass, paints, oils and painters’ sundries Pittsburgh Plate Glass Company 1901 Trade Catalogue, Art materials, Glass, Ornamental
Glass, Paints, Varnishes and Brushes: their history, manufacture, and use Pittsburgh Plate Glass Company 1923 Glass, Glazing, Mirrors, Plate glass, Leaded glass, Store front, Commercial furniture, Pittsburgh Plate Glass Company
Old Beauty in New Glass Pittsburgh Plate Glass Company 1930 Trade catalogue, Glass, Ornamental glass, Windows, Art glass, Stained Glass
A treatise on the origin, progressive improvement, and present state of the manufacture of porcelain and glass Porter, G. R. 1832 Porcelain, Glass manufacture and similar at
The Rudiments of Mineralogy, 3rd Edn. Ramsay, A. 1885 Mineralogy, Rocks & Minerals
The Glass Sands of Kentucky: A Detailed Report Covering the Examination, Analysis and Industrial Evaluation of the Principal Glass Sand Deposits of the State Richardson, C. H. 1920 Rocks & Minerals, Sand, Glass manufacture
Glorious Glass at St. John’s Church, Gouda Rijksen, A. A. J. 1900 Sint Janskerk (Gouda, Netherlands), Glass painting and staining
Glass Manufacture Rosenhain, W. 1919 Glass manufacture
Vasily Kandinsky painting on glass (hinterglasmalerei) Anniversary Exhibition Rothel, H. K. 1966 Wassily Kandinsky, Glass painting and staining, Stadtische Galerie in Munich
Structure and Kinetics of Glass Corrosion [D.Phil Dissertation] Sanders, D. M. 1973 Glass, Glass corrosion
La Verrerie depuis les Temps les Plus Reculés Jusqu’à nos Jours

[See also 1869 edn]

Sauzay, A. 1868 Glass, Glassware, Glass manufacture, Glass craft, Decoration and ornament, Glass painting and staining, Mirrors, Optical glass, Optical instruments, Eyes, Artificial, Glass beads
La Verrerie depuis les Temps les Plus Reculés Jusqu’à nos Jours, 2nd Edn.

[See also 1868 edn]

Sauzay, A. 1869 Glass, Glassware, Glass manufacture, Glass craft, Decoration and ornament, Glass painting and staining, Mirrors, Optical glass, Optical instruments, Glass eyes, Glass beads, Pigments, Water colours, Cosmetics, Fabrics and dying,
Wonders of Glass‐Making in All Ages Sauzay, A. 1870 Glass manufacture, Glassware
Marvels of Glass‐Making in All Ages Sauzay, A. 1870 Glass, Glass manufacture
Handbook of the Glass Industry Scholes, S. R. 1941 Glass manufacture, Chemistry, Physics, Orton Cones, Recipes, Conversion tables, Commercial advertising
Recipes for Flint Glass Making: being leaves from the mixing book of several experts in the flint glass trade…, 2nd Edn. Scott, Greenwood & Son (Pub.) 1907 Glass, Workshop recipes
General Catalog 1911-1912

[Mostly wood-related]

Segelke & Kohlhaus Mfg. Co. 1912 Trade Catalogue, Wooden fittings, Wood windows, Commercial fixtures, Art glass, Stained Glass
The chemistry of the several natural and artificial heterogeneous compounds used in manufacturing porcelain, glass, and pottery

[Originally published 1837]

Shaw, S. 1900 Pottery, Chemistry, Technical
The Methods of Glass Blowing ‐ For the use of physical and chemical students Shenstone, W. A. 1897 Glass blowing and working
The Methods of Glass Blowing, and of Working Silica in the Oxy‐Gas Flame: For the use of chemical and physical students [see also 1916 edn’ Shenstone, W. A. 1902 Glass blowing and working
The Methods of Glass Blowing, and of Working Silica in the Oxy‐Gas Flame: For the use of chemical and physical students [see also 1902 edn’ Shenstone, W. A. 1916 Glass blowing and working
Stained glass tours in France Sherrill, C. H. 1908 Glass painting and staining, Church buildings, Cathedrals, France, Travel
Stained glass tours in England Sherrill, C. H. 1909 Glass painting and staining, Church buildings,Cathedrals, England, Travel
A stained glass tour in Italy Sherrill, C. H. 1913 Glass painting and staining
Photography in a nut shell; or, The experience of an artist in photography, on paper, glass and silver, with illustrations. Simons, M. P. 1858 Photography, Photographic chemistry, Daguerreotype, Ambrotype
The Laboratory, or, School of Arts: Containing a large collection of valuable of secrets, experiments, and manual operations in arts and manufactures… Vol II, 6th Edn.

[See also Vol I]

Smith, G. 1799 Industrial Arts, Technology, Receipts, Recipes, Coins, Medals, Drawing, Painting, Gnomonics, Optics, Etching, Engraving, Paint pigments, Varnish
The Laboratory, or, School of Arts: Containing a large collection of valuable of secrets, experiments, and manual operations in arts and manufactures… Vol I, 6th Edn.

[See also Vol II]

Smith, G. 1799 Industrial Arts, Technology, Receipts, Recipes, Fireworks, Metallurgy, Glass making, Glass staining and painting, Metal casting,
Practical instructions in enamel painting on glass, china, tiles, etc … Snell, H. J. 1874 Glass painting and staining, China painting, Enamel and enamelling, Artists’ materials
The golden cabinet : being the laboratory, or handmaid to the arts : containing such branches of useful knowledge, as nearly concerns all kinds of people, from the squire to the peasant, and will afford both profit and delight Spotswood, W. 1793 Chemistry, Technical, Industrial arts, Technical manuals, Formulas, Recipes, Gilding, Glass, Drawing, Dyes and dyeing, Japanning and similar at
Decoration of Metal, Wood, Glass, etc.: A book for manufacturers, mechanics, painters, decorators,… Standage, H. C. 1908 Workshop recipes, receipts, Metal, Glass
Old Irish glass Stannus, G. 1921 Glassware, Irish
Procedures In Experimental Physics Strong, J. 1938 Experimental Physics, Laboratory, Glass blowing, Optics, Heat, Radiation, Vacuum, Moulding and casting
A Treatise on the Art of Glass Painting Suffling, E. R. 1902 Glass painting and staining
Memorial windows Tiffany Glass & Decorating Co. 1896 Glass painting and staining, Favrile glass, Catalogues, Trade catalogues
Tiffany Favrile Glass, Tiffany Windows, Tiffany Mosaics, Tiffany Monuments, Tiffany Granite [see also later edition] Tiffany Studios 1913 Catalogues, Mosaics, Glass painting and staining, Memorials, Sepulchral monuments.
Tributes to honor, suggested types of memorials by the ecclesiastical department of the Tiffany Studios. Tiffany Studios 1918 Catalogues, Glass paining and staining, Sepulchral monuments.
Tiffany Favrile Glass, Tiffany Windows, Tiffany Mosaics, Tiffany Monuments, Tiffany Granite [see also earlier edition] Tiffany Studios 1922 Catalogues, Mosaics, Glass painting and staining, Memorials, Sepulchral monuments.
Art in Glass: A guide to the glass collections Toledo Museum of Art 1969 Glass art
Philological Studies in Ancient Glass

[1922 PhD Thesis]

Trowbridge, M. L. 1930 Greek language, Latin language, Classical literature, Glass Manufacture, Glass uses, Glass
On some Optical Peculiarities of Ancient Painted Glass, in Proceedings of the Clifton Antiquarian Club for 1884/88-1909/12, Vol 1, pp 207-216.

[Appears to be the same as the book of the same name]

Tuckett, F. F. 1888 Trademarks, Merchants
On some Optical Peculiarities of Ancient Painted Glass. Offprint from VClifton Antiquarian Club. Tuckett, F. F. 1888 Glass painting and staining, Medieval
Journal of the Society of Glass Technology, Vol IV Turner, W. E. S. (Ed) 1920 Journal, Technical, Glass Technology
Journal of the Society of Glass Technology, Vol V Turner, W. E. S. (Ed) 1921 Journal, Technical, Glass Technology
Journal of the Society of Glass Technology, Vol VI Turner, W. E. S. (Ed) 1922 Journal, Technical, Glass Technology
Journal of the Society of Glass Technology, Vol VII Turner, W. E. S. (Ed) 1923 Journal, Technical, Glass Technology
Chemical analysis for glassmakers: Containing methods of analysis for clays and other silicates which will be found useful for the pottery industry Uhlig, E. C. 1903 Glass manufacture, Chemical analysis,
The glass industry. Report on the cost of production of glass in the United States United States Bureau of Foreign and Domestic Commerce 1917 Glass manufacture, Industry, Glass industry
Information Concerning Optical Glass and Chemical Glassware United States Tariff Commission 1919 United States, Glassware, Glass manufacture, Optical glass, Trade Tariffs
Notice historique sur les Peintres-Verriers d’Anvers du XVe au XVIIIe Siècle van Cauwenberghs, C. 1891 Glass painting and staining
University of Illinois Engineering Experiment Station Bulletin No. 118, December 1920, Dissolved Gases in Glass Washburn, E. W., Footitt, F. F. and Bunting, E. N. 1920 Glass manufacture, Glass chemistry
Report on the Manufacture of Glass Weeks, J. D. 1883 Glass Manufacture, Glass-working, Glass composition, Report, Statistics, 1880 Census, United States,
Stained glass; a handbook on the art of stained and painted glass, its origin and development from the time of Charlemagne to its decadence (850-1650 A. D.) Werck, A. 1922 Glass painting and staining, Mediaeval, Art
Irish Glass: An account of glass‐making in Ireland from the XVIth century to the present day Westropp, M. S. D. 1920 Glass, Glass manufacture, Ireland
Stained glass work: a text-book for students and workers in glass.

[part of The Artistic Crafts Series of Technical Handbooks, ed. W. R. Lethaby]

[See also 1905 edition’

Whall, C. W. 1920 Glass painting and staining, Glass manufacture, Technique and similar at and at
Stained glass work: a text-book for students and workers in glass

[part of The Artistic Crafts Series of Technical Handbooks, ed. W. R. Lethaby]

[See also 1920 edition]

Whall, C.W. 1905 Glass painting and staining, Glass manufacture, Technique
A concise account of the principal works in stained glass that have been executed Willement, T. 1840 Glass painting and staining — Great Britain
William Morris & Compy (Ruskin House) Ltd William Morris & Co. 1910 Windows, Glazing, Glass, Stained Glass, Casement windows, ornamental metals, Trade Catalogue
Notes on the development of the ruby color in glass in University of Illinois Bulletin Vol XI No. 23, 30 July 1914 Williams, A. E. 1913 Glass, Glass chemistry, Ruby
An inquiry into the difference of style observable in ancient glass paintings: especially in England: With Hints on Glass Painting, Vol. 1

[See also Vol 2]

Winston, C. 1847 Glass painting and staining and similar at and at
An inquiry into the difference of style observable in ancient glass paintings, especially in England with hints on glass painting, Vol. 2

[See also Vol 1]

Winston, C. 1847 Glass painting and staining and similar at
Memoirs illustrative of the art of glass painting Winston, C. 1865 Glass painting and staining and similar at
One Thousand Valuable Secrets in the Elegant and Useful Arts, Collected from the Practice of the Best Artists, and … Woodward, W. W. 1795 Receipts, Recipies, Art materials, Cooking, Industrial Arts, Formulae, Engraving, Metallurgy, Varnish, Glass Manufacture, Glass painting,
The Manufacture of Optical Glass and of Optical Systems: A war-time problem Wright, F. E. 1921 Optical instruments, Optical glass, Glass manufacture
Manual of Laboratory Glass Blowing Wright, R. H. 1943 Glass Blowing, Technique
Collecting old glass: English and Irish Yoxall, J. H. 1916 Glass collecting, Glassware

Well that’s all folks. Happy reading!

Posted in Books | Tagged | Leave a comment

Thickening Glastac

Today I want to have a short chat about my experiences with Bullseye’s Glastac and some experiments with using CMC to thicken it. There’s no rocket science. I simply though you might enjoy another way to make use of your CMC.


If you’re a glass fuser you’ve probably used Glastac. If you haven’t I recommend you try it.

Glastac is a deliberately weak glue and in my experience it cleanly burns away in a kiln. It is useful because you can glue together the component parts of your glass masterpiece with the certain knowledge that you can re-place the parts for quite a long time. Better still, it helps to ensure your masterpiece will not fall apart when you are moving it to your kiln.

As an aside, I caution against using white PVA glues, such as the oft-recommended Elmer’s Glue. Although I do use PVA glues occasionally, I have noticed that it does not always cleanly burn away and that it can cause damage to a glass surface. I particularly notice this when an excess of PVA is used. But I digress.

The only problem I have found with Glastac, particularly when used in excess, is that surface tension sometimes drags small pieces of glass away from where I put them. Using less Glastac helps avoid this problem but the converse situation is that sparing use of Glastac means that the small pieces of glass are not sufficiently glued to stay put.

And then I noticed that Bullseye had introduced Glastac in a gel form. Hmm. Thinks. Can I find the Manufacturer’s Safety Data Sheet (MSDS) to find out how it differs from the “old fashioned” Glastac? No, so it’s time to just experiment.

Avid readers amongst you will remember that I’ve chattered about carboxymethylcellulose (CMC) on a number of occasions. Most of my chatterings were in Squirt Your Frits but I’ve also mentioned it in passing in other places such as converting ordinary safety flux into a gel form over at Make Your Own Safety Flux.

This made me wonder if a little CMC gloop would thicken “old fashioned” Glastac and make it even better.

Thickening Your Glastac

dscf3558-glastacInstructions on where to find CMC, what it is, how it works and how to make your own CMC gloop is covered in plenty of detail over in my Squirt Your Frits blog. I will not repeat all this information so read that blog and come back again if you need to. I will however show you pictures of a bottle of Glastac and (lower down) a little 100 gram pot of CMC sold under the “Tylo” brand name typically bought by sugar crafters.

Making your own substitute for Bullseye’s Glastac Gel is a simple matter of thickening up some “old fashioned” Glastac with CMC. However the devil is, as they say, in the detail.

I found it most convenient to start by making up some “sloppy” sol-phase CMC gel. The exact consistency does not matter. All you are aiming for is a CMC gel that is thick but flows reasonably slowly when you tip the container.

dscf3555-cmc-potNext, you need to find another little container and add some Glastac to some CMC gel and give it a thorough mixing. If the outcome is too thick, either add some more Glastac or add some water. If the outcome is too runny add some more CMC gel.

As to the exact proportions of Glastac and CMC gel I can only report that making numerous little batches with differing proportions didn’t make seem to make any noticeable difference. As long as there’s a reasonable amount of Glastac in the mixture it’s going to be a glue. As long as there’s some CMC gel in the mixture it’s going to be thicker.

What could be simpler than that?

Speedy Gel?

In an attempt to make the homemade substitute dry quicker I tried using propanol in place of some of the water.

There’s nothing special about propanol (also known as isopropyl alcohol). It’s just an industrial alcohol  that is widely used as a solvent and as such it’s also useful for cleaning glass. Having said that, I mostly use acetone for cleaning glass.

However, unlike the ethanol (also known as ethyl alcohol) in your booze, it’s not safe to drink. Here in the UK I buy it from APC Pure rather than from glass suppliers simply because it’s much cheaper and they’re relatively local. I’m not connected with APC Pure other than being a customer. And once again, I digress…

I didn’t notice any adverse effects of using this modified mixture other than the smell of evaporating propanol. I can’t really say that I noticed the glue drying noticeable quicker but to be fair I wasn’t conducting a proper controlled experiment against a stopwatch.

So, I leave you with the thought that using an alcohol, such as propanol, should quicken the rate at which the glue dries but no convincing practical evidence. This is a prompt for you to continue the experiment and tell me!

Homemade Glastac Gel In Use

Although I have not compared this homemade Glastac gel mixture with the genuine  Bullseye’s Glastac Gel I can at least tell you that I’ve had good results with the homemade substitute (with or without adding alcohol).

The homemade mixture still works as a weak glue. The improvement is that it no longer seems to cause little glass pieces to slide and shift as the glue dries. Other advantages are that it’s quick, easy and cheap to produce in small quantities and you don’t need to find more space for yet another bottle of glue.

If you find your homemade Glastac gel mixture works well you might consider buying some of the real Glastac Gel. At the worst it’s going to be no better. At best it’ll be even better.

Shelf Life

I found that leaving a small open pot of my homemade Glastac Gel substitute lying around for several weeks resulted in an “infection”. The feint tint of green near the surface leads me to suspect the beginnings of algal growth. Maybe I’ve discovered a new balanced diet for bugs. Hmm.

But all is not lost. Simply make up your homemade Glastac gel mixture in small quantities when you need some.

This does not mean you have to make up the CMC gel in small quantities. There is a shortcut. You may recall that I’ve already mentioned in Freeze Your Frits that your CMC gloop will freeze successfully. Store your excess in the freezer and make use of it a little at a time.


That’s all folks.

Posted in CMC, Experiment, Glastac, Tylo | Tagged , , , | 3 Comments

Shake Your Globs

If my early chattering about making globs from scraps of ordinary stained glass in Recycling Scraps of Stained Glass and with fusing glass at Waste Not Want Not have inspired you to make your own then maybe today’s blog will be of some interest. But only if you also do copper-foiled “Tiffany” style work.

Reducing waste and saving the planet is a good ethic, but what’s the point if you don’t make use of them?

I previously mentioned that kids enjoy collecting the recycled glass globs and that sometimes they can be used for pieces of jewellery too. Also, if you live in the land of deliberate bad spellers you might consider making jewelry with them instead.

Today’s theme is all about making use of recycled globs in your copper-foiled “Tiffany” style work.

Grind Before Foiling

Wrapping copper foil around a glob is a rather fiddly process. Not only is it a curved surface but it’s also rather smooth and slippery. The simple answer to this problem is to grind the edges of the globs so that you have a roughened surface against which the copper foil sticks more easily. Grinding can also be used to subtly re-shape a glob that has a defective or irregular shape.

Once you have wrapped the globs with copper foil you’ll want to burnish the copper foil and make the globs ready for soldering. Use your fingers to gently push-in the upper and lower edges of the copper foil but do not waste too much time on this. All that you need to achieve is a crinkly edge that needs the burnishing “finished off” by a method that is less fiddly and more efficient than using a fid or Allnova tool.

Burnishing the Foil

A simple and effective way to complete the burnishing process is to put your foiled globs into a jar with some un-foiled globs, put the lid on the jar, then shake until the burnishing process is complete. This trick is something I read on a forum years ago. So long ago that I don’t recall where or when. There’s no credit due to me for this burnishing method but I can at least recommend it.

dscf3492-globs-and-jarOn the right you’ll see a picture of a “half pound” jar, some un-foiled globs and some foiled globs. Notice the relative quantities being used. Notice that the foiled-globs still have crinkly edges to their copper foiling.

The number of un-foiled globs to use is not critical. It partly depends on the size of the jar just as it depends on how many foiled-globs are to be burnished. Too many globs in the jar means there’s not enough space for the globs to do their burnishing task. Too few globs in the jar means fewer impacts resulting in a slower burnishing process.

dscf3498-foiled-globsYou need to stop shaking as soon as the copper foil is burnished, otherwise the burnishing gets over-done and the copper foil starts to peel away from the globs. The picture on the right shows you the same foiled globs after burnishing. Good, eh?

This noisy task is enjoyed by children but do bear in mind that child labour (labor) is likely to result in over-shaken globs because they are more interested in the noise than the degree of burnishing. To mitigate this problem, reduce the number of un-foiled globs (or maybe eliminate them) so that the burnishing process is slower, thus extending their acceptable period of fun and enjoyment.

Using the Foiled Globs

You might think that there’s nothing more for me to say about how to use some copper-foiled glass. But you’d be wrong. If writing some more means there’s less time for housework then I’m not going to be deterred.

The privileges of childhood include the latitude to do stupid things like stuff small things up noses or in mouths. So, here’s an example of how I tend to use under-sized globs. Incidentally, it’s also a way by which I make use of the irregular shaped rectangles that we produce when squaring-off a wonky ends of newly purchased sheets of glass that were not accurately cut by the supplier.

dscf1719-glob-use-1In this picture you see a small area of a finished abstract piece of copper-foiled work, the design of which is motivated more by the use of scrap glass than intentional artistry. Of greater note in consequence of the theme of this posting is the use of foiled globs. Of incidental interest is the use aforementioned irregular-shaped strips of glass that tend to arise from “squaring-off”.

You can see that this particular example is not random. It may be a “sort-of rainbow” but in other pieces I arrange the globs such that they “sort-of flow” down through “sort-of cracks” between the irregular glass strips in the manner of “sort-of fractured rock”. Notice also that I’ve left a big hole (negative space) in the design. I also try to introduce excitement and interest by using different textures, differing opacity, slipping in a slice of agate or messing around with incomplete patination. Sometimes the best results come from not thinking too much!

dscf2414-glob-use-2My second example is what can be done when you have a huge excess of globs and can’t bear to throw them out. From the picture you’ll see one end of a “box of bubbles”. Just like the previous example I’m using both flat glass, foiled globs and leaving some holes. Incidentally, have you noticed that I chose a Wissmach hammered green border glass to reinforce the “blobby” nature of the interior of the panel?

There are two problems with this kind of panel. The first is that it can use up an awful lot of lead solder and the panels can get very heavy as a consequence. The second is that it’s very time consuming to make. So don’t make these panels too large and don’t expect to recover the real cost of making them if you intend to sell them!

My final comments relate to the technical matter of soldering thick globs into a panel made with thinner glass. You will no doubt realise that the sheet glass will be about 3mm thick but the globs might be 6mm. So, the globs will tend to “stick out” at the front.

If you want your globs to “stick out” only on the front of the panel then simply lay out the pieces front-side uppermost and get soldering without further ado. As an alternative you might try using thick card scraps underneath the plain glass pieces which then allows the globs to “sink” backwards into a more balanced and less proud form.

Concluding Remarks

So, there you have it. A couple more ways to save the planet by making use of recycled glass globs and a simple but effective way to burnish the copper foil onto globs.

A mercifully short blog and not one hint of maths or science. Aren’t you the lucky ones?!

Happy shaking and a happy (though belated) New Year.

Posted in Copper-foiled, Recycling | Tagged , | Leave a comment

Make Your Own Glassline Paper

Today’s chatter is for glass fusers and is about Glassline Paper. I’ll discuss how and why you might make your own, either because Glassline don’t produce exactly what you want or because you have the time and inclination to make a relatively inexpensive equivalent.

I don’t think I’ve inflicted mathematics onto you in previous blogs so now’s my chance. I hope to show you that mathematics (and the scientific method) can sometimes be useful, even when you’re not a mathematician or scientist.

Some Background

If you’re not familiar with Glassline products then a visit here will put you in the picture. Today we’re interested in the Glassline Paper and Glassline Pen products. I don’t often use them myself but there are times when they’re useful.

dscf3548-glassline-paper-examplesGlassline papers are available in a variety of colours, patterns, textures and sizes. Here’s a picture of a small selection of examples. You’ll find more examples at Glassline’s online shop here and summarised here in a single page. Both links contain information about how to use the Glassline papers.

As noted in those two links, Glassline Pens can be used to draw directly on Glassline Paper and I’ll come back to this little fact later when I talk about making your own papers. You can see a colour chart here and you will see the variety of standard colours is not large – but you can mix the colours if you wish.

Something worth mentioning for the benefit of novices is that Glassline Pens aren’t pens. They’re flattish bottles of coloured materials that can be fired and you can buy “stick-on” tips so that the bottles can be used as “squirty pens”. As is my habit, I refer you to the Manufacturer Safety Data Sheet (MSDS) page to find out what materials produce which colour and the all-important advice about how to safely use them. Potters amongst you will notice similarities with materials used in glazes.

dscf3554-glassline-pen-examplesThe picture opposite shows you a couple of example Glassline Pen bottles. Although you can also use a paint brush, an airbrush or whatever else takes your fancy, today we’re only interested in these Glassline Pens in one context – as the colourant for Glassline Papers.

Incidentally, Glassline only quote temperatures in Fahrenheit. For future reference, 1500°F is about 816°C and 1000°F is 538°C and you shouldn’t worry if you want to round off the numbers.

Now that you’re familiar with Glassline Pens and Glassline Papers, we’re ready to continue

An Accidental Discovery

It is quite some time since I discovered you can put Bullseye thinfire paper between layers of glass. I don’t think I ever remembered to mention this in past blogs so I now apologise and hint that this turns out to be part of what lies behind today’s discussion.

More recently I decided to buy a few Glassline Pens and a few small packs of Glassline Papers so that I could experiment with them. Until recently most of the packs of Glassline Paper remained unopened as I rarely find a use for them and have only spent a little time experimenting with them.

dscf3560-glassline-paper-logoOne of the packs of Glassline Paper I opened last week came with a surprise discovery. I noticed that the obverse (back) of the papers had the familiar logo and markings of Bullseye Thinfire paper. Previous papers did not have the Bullseye Thinfire markings. The picture opposite shows what I mean. Three sheets out of twelve identifying their origin doesn’t sound random to me so I was left wondering if Glassline didn’t intend to  reveal what their papers were made from. This is something we will test statistically in the next section.

A final realisation was that the Glassline Paper is coloured with what looks like and what feels like Glassline Pen colourants. This is hardly surprising when you remember that Glassline tell us that we can use Glassline Pens on Glassline Paper.

Mathematics is Fun

If the papers being used at the Glassline factory were unpacked and stacked randomly then we might expect half to be plain-side up and half plain-side down. If Glassline are trying to hide the fact that they use Bullseye Thinfire paper, in an attempt to obscure how their products are made, we’d expect Glassline to deliberately stack their Thinfire paper the same way so that the plain side ends up on the backs of their Glassline Paper, covering the Bullseye logo and markings with the colourants.

I have 12 sheets of Glassline Paper so would expect to see the Bullseye logo at the back of about 6 sheets if Glassline doesn’t care which way up they use the Thinfire paper. As already mentioned, I observed just 3 out of 12 having the logo on the back. If this is deliberate bias then it would be interesting to find out more and the Chi-squared test is how we find out.

For our statistical test we need to confirm (or reject) our “null hypothesis” that 3 out of 12 is statistically no different to 6 out of 12.

If that sounds daft to you, ask yourself these next two questions. Ask yourself “how many children of the same gender can a family have before we decide something weird has been happening” and ask yourself “how many times you can flip a coin and get the same result without getting worried that there’s something wrong with your coin”.

But I digress…

We start our Chi-squared test with some calculations based on observed (O) and expected (E) results. We have 12 samples, 3 of which have a logo on the back…

Paper Side         O         E          O-E     (O-E)2  (O-E)2/E

Back is plain      9          6          3          9          1.5

Back has logo   3          6          -3         9          1.5

We add up the two numbers at the right to get 3.0 and this is our Chi-squared value.

We now need to think about “degrees of freedom” because laying a sheet of paper (or flipping a coin) involves the freedom to do something that causes something to happen.

Should we decide to lay a sheet of Thinfire with the plain side upwards then we automatically know that the only other possibility is to have had the plain side downwards. Notice that we had two possible options and that choosing one of them automatically determines what the eliminated alternative was. So, two options gives us just one “degree of freedom” to choose. Therefore 1 is the degree of freedom we need for our Chi-squared test.

Looking at a Chi-squared table in the row for 1 degree of freedom we find 3.0 is a Chi-squared value that lies somewhere between the columns headed with 0.10 and 0.05. These two column headings are upper and lower bounds on the probability we’re looking for. These numbers can be represented in more familiar ways. Instead of 0.10 you could say 10% or 1 in 10. For 0.05 you could say 5% or 1 in 20.

Therefore, in human-speak, the 0.10 and 0.05 mean that 3 out of 12 sheets of Thinfire paper with their plain side upwards will happen somewhere between 10% and 5% of the time when we do our “sampling”. These are quite large probabilities which means our null hypothesis (that there is no difference) is probable. The corollary is that our original hypothesis is not probable. If our original hypothesis is not probable then Glassline aren’t trying to hide the fact they’re using Bullseye Thinfire paper and are therefore not actively using the Thinfire paper one way or the other.

Therefore, based on my small sample of 12 pieces of Glassline Paper, 3 out of 12 is no different than 6 out of 12. The same would apply to 9 out of 12.

If the whole concept of 3 out of 12, or 9 out of 12, being no different than 6 out of 12 has your intrigued or stunned, I challenge you to analyse some of the statistics behind advertising claims you’ll see on TV or the statistics being quoted by governments or other official bodies. Often you’ll discover the claims are being made on the basis of customer preferences, responses or equivalent measures that are not statistically valid. Knowledge is power.

If all that babble left you realising you’ve forgotten all your mathematics then here is where you can revise your Chi-squared test. It’s also where you can find a Chi-squared value table.

Making Your Own

And at long last we come to the most important part of my chatter – telling you what Glassline Paper is made of and how you might make your own equivalent papers.

All you need to make your own is Bullseye Thinfire paper, some Glassline Pens and a method of production.

There are many methods by which you can apply the colour to the papers. Some of them are mentioned by Glassline. You might use a paintbrush, sponge pad, an airbrush or whatever else comes to mind. Sometimes you may wish to add water to make a thinner mixture, especially when you want to use an airbrush or want to produce a paler tint.

Remember also that Glassline tell us that we can use Glassline Pens to decorate their Glassline Papers.

dscf3561-glassline-overspillAlthough Glassline Paper tends to have colouring on one side, you will notice in the picture opposite that some papers have overspill on the reverse side.  I don’t suppose Glassline care about this overspill beyond recognising it means they’re using more colourant than they’d like to. For us, it means we have to buy papers that are neither single-sided nor double-sided. But bluntly, the backs are an untidy mess. I don’t like mess and it detracts from the quality of what we try to achieve with out work.

dscf3551-glassline-heart-2dscf3551-glassline-heart-1In these two pictures, on the left and right, you can see that it is possible to have one colour on one side and a different colour on the reverse. It may be a shabby example with sparse artistic merit but it does show us that there is no colour bleed from one side to the other other than my messy experimental workmanship.

dscf3549-glassline-paper-diyThe picture you now see offers a few examples that show we’re not limited to what Glassline have in their Glassline Paper product range. The grey-looking rectangle will fire to double-sided black. The narrow strip at the top has different colours on each side. The two examples on the right are single-sided and have two colours. The remaining example at the lower left is a single colour of brushed stippling.

And finally, do remember that when the thinfire paper has been fired, the Bullseye logo and associated markings will disappear.

I think you get the idea and hope these example inspire you to do your own experiments.

Thinking About Economics

It is worth considering the relative costs of buying ready-made Glassline Paper against making our own. What follows is a quick analysis that in turn will lead to some simple conclusions. If you’ve had your fill of mathematics then just skim read this section!

To begin with we need to know how much it costs to buy ready-made Glassline Paper. For those of you in the USA I offer you the Suede Glassline 5″x5″ Stone Paper kit as an example. For this product three sheets of 5 x 5inch costs $15.45. This equates to about $0.0319 per square cm.

By contrast, in the UK we might expect to pay £8.91 or thereabouts for a mix of 15 sheets of 6.5 x 6.5cm at Warm Glass UK. This is a similar quantity and equates to about £0.01406 per square cm.

We now need to compare this with the costs to make your own version of Glassline Paper.

Bullseye Thinfire paper costs vary depending on how much you buy and where you buy it. For the purposes of our calculations let’s take £19.47 for 10 sheets of 52x52cm quoted by Warm Glass UK. This turns out to be about £0.000720 per square cm. With modest bulk buying the cost of the Thinfire paper turns out to be a negligible cost when heading down the DIY route.

Glassline Pens cost around £7.25 or thereabouts from Warm Glass UK when bought in 2 ounce bottles. That’s about 56 grams for those of you who stopped using Avoirdupois units decades ago. Translating this mass into how much gets used per square centimetre is rather tricky as it depends on what we’re doing and how much water is in the mixture. A light wash of colour is going to use a lot less than a thick coating. All we know is that the colourant is costing us £0.1286 per gram used.

Let’s look at the problem in a different way instead. If we can limit our use of the Glassline colourant to less than £0.01334 per square centimetre we’re on a winning streak if we’re out to save money. We know this because we have calculated how much the finished Glassline Paper and raw Bullseye Thinfire papers cost per square centimetre.

But how much can we squirt out of a Glassline Pen before it costs us £0.01334? A quick calculation tells us it will be about 0.1037 grams per square centimetre. A tenth of a gram is a tiny amount, nothing more than a blob on the end of a skinny wooden coffee stirrer. However, even a tenth of a gram is enough to cover a couple of square centimetres of Bullseye thinfire paper with a heavy application and considerably more for a light coating.

But let’s not forget that we’re likely to find ourselves with mucky paintbrushes, palettes, airbrush equipment or whatever else we’ve been using as tools. Some of what was in our Glassline Pen will go down the drain as waste. Waste costs money too.

Practical Conclusions

If you have painterly skills then making your own versions of Glassline Papers is a viable proposition in many situations. Sometimes the cost to buy ready-made outweighs the time and effort needed to produce your own equivalent versions. Sometimes the uniqueness of what you make makes simple cost comparisons pointless. It’s a matter of judgement.

A generally applicable comment is to buy cheaply and minimise waste. This is common sense, but a failure to do this can negate your attempts to save money if that’s what you’re aiming for.

Heavy coatings of colourant laid on Bullseye Thinfire paper ought to be a financially viable alternative to buying ready-made Glassline papers but three factors should be borne in mind. If the thinfire paper is being purchased expensively in small quantities and if the results of the DIY activity result in a lot of waste colourant it is easily possible to make papers that are more expensive than could be purchased ready-made. Also realise that the time you take to make such papers adds to their cost.

In extreme contrast to heavy coatings, an airbrush can be used to disperse small quantities of watered-down colourant over a large area as speckles. This method ought to be the least costly because it uses the least quantity of colourant. Do remember that airbrushing speckles will only be financially viable if you already own an airbrush.

Related to speckled effects would be brushed effects, drawn effects or sponged effects, especially when watered-down colourants are being used. Again we should expect relatively low costs that are viable compared to ready-made papers.

Some of the Glassline papers will be harder to replicate and will use a lot of colourant. Examples of the crinkled, granite and sandstone papers where Glassline use two colours and/or the thinfire paper that has been “crumpled” and flattened-out before applying the colourants. Unless there is a compelling reason to make your own I reckon it’s not worth the trouble to make these at yourself, other than as experiments for the learning experience. Not only will it be a time-consuming task but the amount of colourant and waste colourant are likely to make it more expensive than ready-made papers.

A final conclusion is that we now realise that Glassline papers with overspill on their backs can be “finished off” to make them double-sided rather than “single-sided with mess on the back”. It may not matter to Glassline, but I try to consider how things look from the back of what I make.

In the final analysis, the most compelling case for producing your own papers is when you want to colour both sides of the thinfire paper and when you require colours or patterning that can not be obtained ready-made from Glassline.

Posted in Experiment, Glassline Paper, Glassline Pen, Inclusions, Money-saving ideas | Tagged , , , , , , , | 4 Comments

Secrets of the Microwave Kiln

I’ve just bought my third Hot Pot Maxi microwave kiln. But, why buy yet another “toy” kiln when I already have a “proper” glass kiln of a distinctly robust and modern design?

I hope to answer that question by talking about the distinctive nature of firing glass in a microwave kiln, a little about the economics of using them, and a little about how they work and how they deteriorate.

I think I need to do all of this because I don’t see anyone else talking much about it.

Experiences With a Microwave Kiln

I still use a microwave kiln because I can melt a small arrangement of glass and have it back out and in my hands, fused and shiny, within about two hours. My “proper” kiln makes me wait a whole day (and night). So, speed and convenience is one reason.

The barely-controllable ferocious heating in a microwave kiln results in a greater risk of glass cracking at it heats up and the lack of processing temperature control means it’s not uncommon to find a mutant distorted blob of glass is the unexpected result of a firing. And of course the small firing chamber means we don’t get to make anything larger than a brooch or pendant. Sometimes this is not a problem.

The down-side of the almost uncontrollable heating, as I’ve just mentioned, is that the shape and form of the resulting glass masterpiece is rather unpredictable. This becomes a particular problem when the microwave kiln gets older and heats less evenly. I’ll be coming back to the “getting older” aspect of microwave kilns later as it seems to be a widely neglected topic!

Another characteristic of microwave kilns is the incredibly rapid cooling inside the microwave kiln. It has scant regard for “proper annealing”. You might think that this must lead to problems but in truth it very rarely does.

We are repeatedly told that it is important to properly anneal our work. From this we might suppose that the rapid cooling in a microwave kiln without “proper annealing” might cause us significant problems. In truth I find that breakages caused by poor annealing are very rare. If this is a surprise to you then consider the size of items being produced and realise there’s only so much stress and strain that can be built up and “stored” in such a small piece of glass. This is particularly the case for simple shapes like a blob of glass, a little decorated tile or a simple pendant – in other words, exactly the kinds of things that you’d use a microwave kiln for.

You can, of course, pop your microwave kiln masterpieces into a “proper” kiln to “properly anneal” them though we can take paranoia too far sometimes.

Another aspect of the rapid heating and cooling in a microwave kiln that I have not seen mentioned anywhere relates to devitrification. With a microwave kiln the processing time is so short that glass that is susceptible to devitrification rarely has time to devitrify. A practical consequence is that I am able to reliably produce recycle my otherwise unusable scraps of “ordinary” non-fusing glass into blobs with little risk of devitrification. You can see real examples in my Recycling Scraps of Stained Glass blog and you should bear in mind that every single glob you see in the picture is not fusing glass. For the lazy amongst you, and because it’s colourful, I’ll re-post the picture from that blog:

DSCF1857 Recycled Glass Globs

And here’s and interesting example that proves the opposite situation from my last blog. This little wonderous spiky blob of glass devitrified before it melted completely:

DSCF3010 Spiky Devit

The big surprise is that my “proper” kiln fails to produce shiny globs with “ordinary” non-fusing glass. Devitrification is always a problem. Processing time is important when dealing with glass that was not designed to be re-fired.

In other experiments, using a “proper” kiln, I find that most kinds of “ordinary” non-fusing glass can barely cope with slumping without devitrifying at least to some degree. Someday I’ll do a blog about this but I’ve not finished messing about yet!

Reasons to Use a Microwave Kiln

A consequence of the foregoing chatter is that I continue to use a microwave kiln in four very particular situations:

  1. I can quickly and cheaply perform a simple glass-related experiment in a microwave kiln. Firing-up a big kiln and waiting a whole day to find out what happened can be too long to wait sometimes.
  2. Children visiting for a “smashing time” can arrive in the morning to make something small and simple in a microwave kiln then take it home that same afternoon. While they wait for the microwave kiln too cool down they can also make something bigger and more “special” that later will go into the big kiln. Immediacy is important for kids, as is the excitement of seeing seething red-hot glass when they “peek”.
  3. I can recycle scraps of non-fusing glass into blobs without devitrification problems and in turn it means I throw very little waste glass away.
  4. I can quickly make small quantities of frit balls (and other similar little things) when I run out of them which means I don’t have to suspend my project work for a long time. That they’re badly annealed doesn’t matter here because they will be fired again!

You may be surprised to learn that point (3) is what my microwave kiln gets used for most of the time. Let me explain…

No matter how hard we try to make use of smaller pieces of glass we end up with small scraps that are unusable. Where possible unusable scraps get melted into globs. It makes environmental sense through I doubt the time and effort to make them is commercially viable.

Some of the smaller globs (under 6 grams) I use in my own copper-foiled work or give away to other crafters when we meet at events. They might end up as a glass highlight in a wooden decoration for example.

Larger globs (typically 6-10 grams) are supposed to be sold though I tend to give away most of them. My rule is simple – kids who show an interest in my work can have one free but horrible kids have to pay for them. There has to be a reward for being “nice”.

Microwave Kilns Deteriorate

I’ve already mentioned that I’m now on my third Hot Pot Maxi microwave kiln. What happened to the other two?

As battered and bruised old-timers the old microwave kilns have been retired. They now live in landfill. The blunt truth is that they’re fragile, get damaged easily and really do get old and tired.

I should now explain how a microwave kiln works (in brief) and then pull-in information to explain how and why they deteriorate and get old.

The body of a microwave kiln is made of a light and brittle ceramic material. Considering how light and thin the ceramic material is, it performs remarkably well as a thermal insulator.

With a new microwave kiln we can expect the grey heating material to heat up reasonably evenly. The relatively small degree of uneven heating will be caused by subtle differences in the mixture of materials and their thickness. With time the degree of uneven heating gets worse for reasons that follow…

Repetitive heating and cooling causes repetitive expansion and contraction which will result in hairline cracks. The brittle nature of the ceramic material (and the inside coating) of a microwave kiln means it starts rather soon and gets progressively worse the more you use the microwave kiln.

Exactly where the hairline cracks appear depends on the unavoidable “defects” of manufacture and some basic physics. That the cracks always seem to run from top to bottom is purely down to the combination of geometry and coefficients of expansion – the inside gets hottest so wants to expand proportionately more than the outside. The reverse happens when cooling. This difference causes stresses and strains which result in hairline cracks appearing. So, don’t be unduly concerned by hairline cracks because they’re a natural consequence of the heating and cooling and the materials being used.

We now need to remember some high school physics. Do you remember that heat can be transferred by any combination of conduction, convection or radiation?

The hairline cracks will cause uneven heating because areas that heat up fastest can not conduct some of their heat to cooler areas because of the barrier caused by the cracks. So, any minor differences in one area heating up faster than another due to original manufacturing “defects” is made more pronounced when hairline cracks come into play. As the size of the firing chamber is so small we can assume there is no heat transfer by convection. There will however be some heat transfer by radiation because that’s what we’re using to heat up the glass in the firing chamber.

So, uneven heating becomes an unavoidable and noticeable problem once the microwave kiln starts to develop hairline cracks. This in turn adds to the unpredictability of what you can produce in a microwave kiln. A partial answer to this uneven heating is to pause the firing mid-way, have a peek, rotate the lid by half a turn, then continue to the firing. With practise and good timing this can almost negate the effects of uneven heating.

Glass slippage is another problem because. It is very easy to accidentally nudge the lid of the microwave kiln and cause the glass pieces inside to slip. A microwave oven platen that rotates badly (wobbling or shuddering) can also cause glass to slip. Heating too rapidly may cause glass to crack and move, so is another form of slipping. Any of these (and other) mishaps may result in hot glass “gluing” itself onto the base or the sides of the microwave kiln. You can also achieve the same effect by over-cooking the glass such that it becomes very fluid and “runs” to the side of the kiln to glue the top and base together.  Yes folks, I confess. I’ve experienced all these mishaps.

The trouble with glass fused onto the ceramic material is that you will find yourself gouging a big hole into the base or sides of the microwave kiln in your attempt to remove the glass. It is rarely possible to remove the glass without damaging the ceramic material, even if you have use kiln wash to protect the kiln base. Such mishaps tend to shorten the life of a microwave kiln, either because you find yourself with a kiln base that resembles the aftermath of World War I trench warfare, or sides where big chunks of the grey heating material are missing.

Using kiln wash and fibre paper can help deal with some of the problems some of the time but in my experience they will only reduce the rate of kiln destruction!

Another aspect of the deterioration relates to the heating ability of the dull grey gritty substance on the inner surface of the microwave kiln’s lid. It’s the heating element. The dull grey gritty substance is something I’ll talk about in more detail at the end of this blog so for the moment just accept that it is chosen for its ability to absorb microwave energy and re-emit that energy as heat. In other words, a microwave kiln works because of a peculiar characteristic of the grey material.

I am not sure why, but the effectiveness of the grey “heating” material seem to deteriorate over time, partly because of minor mechanical defects such as hairline cracks, but also because it seems to take longer and longer to heat up as the kiln is used more and more. This is something I noticed with my first microwave kiln but I hadn’t been keeping any records.

The fact it takes longer and longer for the microwave kiln to heat up with age implies there is some form of chemical deterioration in the “heating element” part of the microwave kiln. Anything that’s hot and in air tends to get oxidised as a matter of routine. This is perhaps most familiar to you if you’ve ever put some lovely salmon-pink shiny copper elements in your kiln-fired work and was disappointed to discover they came our red, purple or even black as heat and oxygen progressively turned the copper to copper oxide. This is what heat and oxygen routinely do to most things around us. This is what I suspect is happening to the “grey stuff” in the microwave kiln. But I suspect there are two other possibilties.

One of the alternative possibilities is that metals in coloured glass are “firing off” and reacting with the heating element. The other possibility is that the mixture of materials in the heating element react with each other causing chemical changes.

Whatever the cause, the effect is that the heating element becomes less susceptible to microwave energy so is not able to re-emit heat so effectively.

Firing History

My first microwave kiln told me that there was deterioration. So, for my second microwave kiln, I kept a record of each firing. Not much more than the date, what kind of task and how long it was “cooked” in the microwave. What you see in the graph below is the result of my nerdy record-keeping. Have a look at the graph then I’ll explain what it all means.


The graph shows that my second microwave kiln didn’t quite make it to 300 firings before I felt it was time to throw it away. The exact number of times was 283.

You can also see from the graph that the jagged curve runs from the lower left (the first few firings) to the upper right (the end-of-life firings). Notice also that the curve is steeply upwards on the left and goes shallow on the right. This curve tells us that a new microwave kiln is much quicker than an old one and that the super-duper performance of a new microwave kiln doesn’t last long.

Notice that I’ve scaled the processing time so that 100% represents how quick the new kiln was. This means that when I threw it away it was taking almost twice as long to do exactly the same job – over 180% of the original firing times. Notice also that the graph shows us that the rate of deterioration slows down and seems to be levelling out at around 180%.

There are consequences for this “deterioration”. One is that it takes more time and energy with an older microwave kiln when compared to a new one. The other is that there’s no point in relying on detailed accurate firing records with a microwave kiln because its behaviour changes over time.

I’ll now reinforce that last paragraph in a different way. If you use a firing time from an old microwave kiln to guide to what you should do with a new microwave kiln you will likely “double blast” your glass. It will be “double processed” and you may end up producing an very runny pool of molten glass. And runny molten glass flows rather well if a surface is not exactly level. This is how I managed to “glue” the inside of the lid of a microwave kiln onto its base using molten glass. Don’t be as stupid as I can be. Consider yourself warned!

Now that we have some evidence about how microwave kilns deteriorate, and why, lets look at the economics of using a microwave kiln.

Microwave Kiln Running Costs

There are different brands of microwave kiln and some brands come in different sizes. The kind I’m using has a firing chamber that is about 10cm in diameter and cost about 50 GBP. Knowing that your 50 pound investment will deteriorate and may be ready for landfill after about 250-300 firings is something to think about. So is the ever increasing cost of the electricity, the kiln wash, fibre paper, currency exchange rates etc.

So, how much does it really cost to fire-up a microwave kiln? Lets find out…

I’ve already mentioned 50 pounds Sterling (notice it’s “Pounds Sterling”, not “English Pounds”) as the purchase price of my new microwave kiln and that I got 283 firings out of my second microwave kiln. So, that’s about 17.6 pence per firing due to the kiln cost.

But electricity also costs money. I am using an old 650W microwave oven. The 650W measure is the microwave output, not the electricity consumed. From the technical information at the back of the microwave’s manual I see it consumes 1.1kW per hour. So that’s about 60% efficient. My electricity costs around 16 pence per kWh and I’ve factored-in a proportion of the standing charge. We end up with just a few pence of electricity per firing which I can now plot on a graph.


I see that the cost starts somewhere between 3 or 4 pence, quickly rises to nearly 5 pence, then slowly drifts upwards to a little over 6 pence per firing. If you compare this graph with the previous one you’ll see exactly the same shape but a different Y-axis scale. This is because we’re doing nothing more complicated by converting a Y-axis in units of time into units of pence by multiplying by a constant value. For full marks in a mathematics exam I should have perhaps chosen a Y-axis starting at 3p rather than zero to make better use of the space.

Other Running Costs

There are other running costs that were not included in the previous graph. We tend to use some kiln wash to protect the base of the microwave kiln. A tiny fraction of a penny per firing for kiln wash is negligible compared to the cost of your time and the other costs associated with running a microwave kiln.

You, like me, might also use Bullseye’s thinfire paper between the glass and the kiln surface. It’s expensive and it can’t usually be used more than once. But how expensive is it?

If you’re lazy you’ll buy ready-cut 10cm squares at around 11 pence per firing, such as from here at Glass Studio Supplies in the UK but if you compare the price for buying 100 big sheets, such as from here from Warm Glass in the UK, you find you could instead be paying around 6 pence for the same amount of thinfire paper. All it takes is the will to buy in bulk, a pair of scissors and a few minutes of your time.

And finally, we need to remember to allocate a portion of the cost of buying the microwave kiln to each firing as well as the electricity cost, both of which were calculated in the previous section.

Overall Running Costs

My second microwave kiln tells me to expect a lifetime of about 250-300 firings, or maybe more if I treat the microwave kiln with more respect and care. As most of my use of a microwave kiln is to produce circular blobs of glass, we’re talking “full-fuse-plus”. We might therefore reasonably expect a longer life for the kiln with profile fused work.

Record-keeping may be boring and nerdy but it clearly has its uses. I now know the lifespan for my second microwave kiln and how it has behaved from new until the time I threw it away. Combining all the information at current (2016) UK prices tells me that the total per firing will be somewhere in the region of 25 to 35 pence, depending on how old the microwave kiln is, the kind of work being done, and whether or not I am prepared to buy raw materials in bulk.

You might like to think about how costly it is to fire-up your “big kiln”. The same ideas and methods apply, but the numbers will be bigger.

How Microwave Kilns Work

If you’ve got this far and have an urge to find out more about how microwave kilns work, and would also like to know how you can make your own, you’re in luck. I’ve gathered together a few links below which I’ll pad out with some commentary.

When you hunt around the Internet you’ll maybe find some misleading information about “the grey stuff” in a microwave kiln. The grey material is not granite, nor is it graphite. It is a mixture of silicon carbide and sodium silicate. Notice I say silicon and not silicone. Silicon is a shiny silvery metal. Silicone is a kind of plastic used for waterproofing products, breast implants and more besides. Silicon and silicone are not the same things.

You will be familiar with silicon carbide as an abrasive if you’ve ever tumbled rocks and minerals. You will also be familiar with sodium silicate though it’s unlikely that you realise it. Both are inexpensive chemicals that you can buy on eBay and I’ll give you a couple of links later that tell you more about both of them.

Once upon a time I found a reference to both these materials when I was reading something about LVR Products’ Micro-Kiln EZ-5 and Micro-Kiln No 9. I made a note that in their parts list it said there was a ‘Repair Solution Set’ which consisted of Silicon carbide (solution A), Sodium silicate (solution B) and a Brush. I forgot to make a note of the URL and I can’t find with Google any more, so I’m sorry I can’t give you a link to this evidence. But not to worry. I have more sources of information, as you will see later, that should reassure you I’m not talking out of my backside.

Silicon carbide is used as the heating element because it has the interesting property of absorbing microwaves and re-emitting the energy as heat. You can find out more about this grey “heating” chemical at Wikipedia’s entry for Silicon Carbide (especially in the Heating Elements section). You will also find silicon carbide mentioned in some of the links listed below.

To “glue” the silicon carbide to the microwave kiln lid requires a binding agent and although there are several possibilities, you will you find that the commercial repair kits seem to use sodium silicate. Find out more about this “binding” chemical at Wikipedia under Sodium Silicate (especially in the Refactory Use section).

Over at Paragon you will see repair instruction that mention a silicon carbide layer. Actually, this is a very useful little instruction manual for any microwave kiln user, not just the Paragon MagicFuse microwave kiln.

You can get a really good insight into how microwave kilns are made by watching a YouTube video called How to make a microwave kiln (Furnace) from scratch for £5 but the link is now private and inaccessible to us mere mortals. The audio is not good but it is worth the struggle. Not only will you see a microwave kiln being made but you discover silicon carbide is just one of many “susceptor” chemicals that can be used as a heating product and that there are different binders, not just sodium silicate. Also interesting in the narrative is an explanation of how the same heating method is used to cook microwave chips.

With the “old” link now inaccessible I suggest you watch the YouTube video called Diy Microwave Kiln | Melt Glass in the Microwave as well as another called How to make a Microwave Kiln – Easy and Cheap DIY Glass Fusing & Melting for a less comprehensive insight. If nothing else they confirm the use of silicon carbide and sodium silicate as appropriate materials.

You can find out more information about microwave absorbers here though in a completely different context.

If this isn’t enough for you then there is an old technical reference about “self heating” ceramic crucibles for microwave melting of metals and nuclear waste glass  at the Office of Scientific and Technology Information in the USA which is not as irrelevant as you might initially suspect. Vitrification has for many years been considered as a “safe” method of disposal for nuclear waste materials.

For the fearless amongst you, I have found some rather technical references. I can promise you an especially dreary read with this patent. If it is too much for you, I suggest try the readable article here because they’re both about the same thing.

Are We Being Ripped-Off?

And finally, we should give some thought to whether microwave kilns are good value or not.  The same applies to repair kits that you might encounter.

For almost the cost of buying a replacement microwave kiln you can buy a microwave kiln repair kit. One example is here. I am always suspicious of spare parts and repair kits that cost almost as much as the original item.

If you have a look in eBay (or elsewhere) you’ll discover just how cheap silicon carbide and sodium silicate really are. This should make you wonder why there’s such a big difference between the price of these raw materials and the price of a commercial kit or a microwave kiln.

If you understand the instruction in the YouTube video I mentioned in the previous section you’ll begin to understand that 50 GBP is ten times the cost of making your own. Again, this should make you wonder why there’s such a big difference between the price of the raw materials and the price of a commercial microwave kiln.

Yes folks. Information is power. The power to exploit. And now you know their secrets they can’t exploit you so easily. But you can exploit what you know. You too can make a microwave kiln. You too can buy the materials you need to make your own repair kit.

If you enjoy making things and you don’t have a microwave kiln then making one is surely a candidate for the top of your “Things to Make” list.

Bye for now. Tomorrow I’m going to make some rainbows. How about you?

Posted in Devitrification, kiln schedule, Melting Glass, Microwave kiln, temperature curve | Tagged , , , , , , | 31 Comments