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
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
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.
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.
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!