Glass Devitrification – Look Closer

I want to chatter about glass devitrification but there’s far too much to say in a single posting. Therefore I’ll start the topic by talking about correctly identifying devitrification. I will leave further related topics for future postings.

As is usual, I’ll be walking you down a meandering path that others do not take.

In the Mood

I’ll start with a quote from my previous posting, Lubricate Your Cutter Wheel because it’s the motivation for this blog posting and several more to come:

A clean cutting oil is a simple hydrocarbon mixture which will cleanly evaporate or “burn off” within a kiln – but we are told this can cause devitrification. However the use of other “contaminants” such as Glastac or White Glue (PVA) are routinely used and also “burn off” – but we are told these are not a risk for devitrification. This all looks rather self-inconsistent to me. We want facts not superstition and hearsay!

I am also motivated by encountering internet forum postings that leave me with a suspicion that there are many people working with glass who don’t always identify devitrification correctly.

Looking and Touching

To use more than one of our senses, when it’s to our advantage, may seem obvious but try reading what people say in the internet forums and decide for yourself. Do we use more than one sense when describing what we think might be devitrification? I suspect not.

I therefore want to suggest that we must consider how the glass looks and feels before we decide whether a problem is devitrification or not. I say this because there are things you’ll see in glass that look like devitrification but do not feel like devitrification.

Descriptive Terms

We often see words like “glossy” or “shiny” in contrast to words like “scummy”, “white”, “cloudy”, “grey”, “hazy” or “misty” to describe whether glass has devitrified or not. I’m sure you could suggest other words as well, but now consider how often we use words like  “smooth”, “rough” or “gritty” or find them in descriptions within the Internet forums. I think that words that describe how glass feels to the touch are rarely mentioned.

Pass the Sugar

Perhaps the best way to understand what devitrification is, and is not, would be to use a more familiar material and relate that to the micro-structure of glass that is “good” and has “gone wonky”. And sometimes it’s the simplest of analogies that teach us the most and I hope what follow makes sense in your mind.

I’d like you to consider how individual grains of sugar look and feel and then consider how they look and feel when they are “bunched together” in the form of a sugar cube. Notice that I ask you to look and feel the sugar. Put your hand into your sugar bowl. Fiddle with a sugar lump when you next visit your local coffee shop. Study them closely.

Do you feel the roughness of individual grains of sugar as well as the rough surface of a whole sugar cube? The roughness of both comes from the “pointy ends” of the sugar crystals. It is the same kind of rough “pointy ends” of regular crystals (as opposed to amorphous crystals) that you are feeling when you run your fingers over a devitrified glass surface.

Did you see that individual grains of sugar are quite transparent and are quite shiny when you look at them individually (blow off the sugar dust if necessary!), but notice also that the sugar grains in a sugar cube tend to be opalescent, white, dull and cloudy? The clarity of a single sugar crystal is expected because light is not being scattered around and we can liken this single crystal of sugar to the single amorphous crystal of a nice “good” piece of glass. The loss of clarity that results in the white, dull and cloudy effect in the sugar cube is a consequence of light passing through many sugar crystals, scattered in many directions. This scattering of light in the sugar cube is exactly what you also see with devitrified glass because regular crystals have begun to form in devitrified glass.

So with our sugar analogy representing the way devitrification causes light scattering in mind, I now want you to explore another analogy for something that is sometimes mistaken for devitrification.

Extending the Analogy

I now want you to think about what’s between the crystals in a heap of sugar to extend the analogy we’ve been talking about. I think you’ll agree it will be lots of interconnected tiny pockets of air. If you’re not following me then think about the same situation at a larger scale – a pile of bricks or pebbles. Yes. Between the crystals of sugar are little pockets of air. It doesn’t matter how big or small those sugar crystals are, they always seem to have little pockets of air between them.

Now I want you to imagine the cooking process by which you might convert some granulated sugar into a boiled sweet. You’ll need to add a little water. But not too much food colouring or flavouring. After boiling, the sugar solution gets thicker and eventually the resulting sweets will be shiny and smooth on the outside and completely transparent inside. For the purposes of our analogy we used water to fill the gaps where air bubbles used to be. So you end up with boiled sweets that do not contain air bubbles and they’re crystal clear. By keeping bubbles of air out of our glass we keep it nice and clear.

I want you to now contrast the outcome of the boiled sweet cooking process with something you may have encountered before –  a bowl of sugar left for a long time in a damp environment. The same can happen with common salt as well. You should be able to imagine millions of little sugar crystals that have become damp and that the dampness allowed the crystals to stick together. Here we had enough water to connect the crystals together at some of their edges but not enough to “fuse” them together into a single solid block. We still have little pockets of air between the crystals yet it is a single block of sugar. In our fused-together heap of sugar we notice lots of light scattering that results in a white, dull and cloudy effect.

If you’re “on the ball” you’ll realise that the damp sugar analogy is exactly the same as the sugar lump analogy. What I want you to notice is that in the sugar lump analogy I was focusing your attention on sugar crystals in close proximity, but for the damp “caked” sugar analogy I was focusing your attention on the little air pockets between those same crystals.

There are two things we’ve learned that I want you to ponder over for a moment

  • A heap of glass crystals in close proximity will scatter light
  • A mass of tiny bubbles in close proximity inside glass will also scatter light

And, as we’ve learned the visual characteristics of devitrification seems to be the consequence of scattered light. So, why have I also be talking about little bubbles?

It is now time to apply the concepts of “crystals” and “bubbles” to real examples.

Wonderful Micro-bubbles

If you have never fused a heap of clear powder frit then now’s the time to try. At the same time you should also try fusing a heap of course clear frit. Even if you’ve never done this you’d not be surprised to find that the coarse frits fuse together to produce a clear lump of glass. What may be a surprise is that the clear powder frit fuses together to form a lump of glass that is white, dull and cloudy.

What’s happening with the clear powdered frit? It has been fused together and now resembles our analogy for a heap of caked sugar.

It may look like devitrification but what does it feel like? It feels smooth. The smooth surface tells us we’re touching a smooth amorphous glass surface which means it is not devitrification. The bubbles beneath are the causing of the white, dull and cloudy effect by light scattering.

If you have never used your glass grinder on the edge of a piece of glass and then fired the glass then now’s the time to try. If you have, you’ll have probably noticed a white, dull and cloudy effect that looks like devitrification. Indeed, I’ve seen many people describe this effect as devitrification on Internet forums, Facebook and even in conversations with me.

It  may look like devitrification but what does it feel like? It feels smooth. The smooth surface tells us we’re touching a smooth amorphous glass surface which means it is not devitrification. There are bubbles beneath that are the causing of the white, dull and cloudy effect by light scattering. The ground edge created a very roughened surface that would resemble a vast mountain range if you were a tiny microbe. When you fire the glass the mountains collapse, flop over and distort. As they do so, they trap millions of little bubbles of air.

DSCF3037 Grinder and FritLet me show you a picture of a “test piece” in evidence of what I’m saying.

On the top of the clear glass was a heap of clear powder frit that now looks white. You can’t tell, but I know it feels very smooth. The opacity is the consequence of micro-bubbles and is most definitely not devitrification. Imagine how a shallow heap of clear powder frit might behave. Would it be white or perhaps merely cloudy? Better still, do your own experiments!

On the side of the glass, nearest to you in the picture I used my (coarse) glass grinder. After firing you can see a cloudiness along the edge. You can’t tell, but I know it feels very smooth. Again the opacity is the consequence of micro-bubbles and is most definitely not devitrification. Imagine how a finer grit might behave (eg after using a diamond pad). Better still, do your own experiments!

DSCF3060 Xs OverglazeMy next example is a green opal blob made from scrap non-fusing glass. I made it especially for this blog.

This green opal blob suffered from a bad dose of devitrification along the line at the left and thin patchy devitrification here and there on the top on first firing. I knew it was devitrification because it looked and felt like devitrification. What I did next was quite deliberate, so that I could give you another example to consider.

I deliberately over-dosed the surface with an large excess of devit spray. What you need to know is that the devit spray I used contains very finely powdered clear glass and you’re likely to be using something similar. I know it contains very fine clear glass powder because the manufacturer’s MSDS safety data sheet tell me it does. The glass powder in devit spray is much finer than you encounter with powder frits.

Notice that the over-application of devit spray has produced an effect that looks like devitrification. It is similar to what we just saw with a heap of powder frit. The line down the left edge that really was devitrified now looks even worse. The top surface also looks worse. Although it might look like devitrification, I know from my sense of touch that the glass surface is now very smooth and glossy. It is not devitrification – it’s micro-bubbles again!

Nasty Roughness

DSCF3019 DevitI should now give you an example of real devitrification to complete the picture. You will see the dreary looking blob of dark opal glass in the picture.

You will immediately see the speckled effect on top. You will also notice that those patches are white, dull and cloudy. What you can’t tell is that the surface feels very rough. This is nasty dose of surface devitrification.

Something else to notice is that the devitrification is patchy with some of the patches being very small. This is a characteristic of devitrification. It starts very small and grows and grows.

DSCF3010 Spiky DevitAnd here’s another example of devitrification. It was made from a small stack of white scrap non-fusing glass. If nothing else, it will show you that devitrification is not always just a scummy surface defect.

Can you see the dull white surface? Can you also see some “spikes” sticking out from the surface? To the touch it is really rough and gritty. Those spikes are also rather sharp!

What’s happened here is that the non-fusing glass has quickly devitrified in the kiln, so quickly and to such an extent that the crystalline structure of the devitrification has stopped the glass changing completely into the more usual rounded shape. This is an amazing accidental one-off that I’ve never seen before or since. I doubt I’ll ever see it happen again so I will keep this little blob as a piece of “special treasure”.

Why and how devitrification happens will be the subject of another blog posting but in the meantime you might like to read up about nucleation.

Look Closer

There are two tools I found useful when I began exploring devitrification. One is a cheap microscope and the other is a dental tool. The remaining tools were, of course, my eyes and fingers.

DSCF3058 Dental ToolThe dental tool, shown in the picture, is useful because the sharp point can be used to hunt down little defects on a glass surface. In this sense it is nothing more than a miniature version of a finger. You can also use it to hear the roughness of a surface when scraping.

DSCF3054 Microscope2.pngThe microscope I used was very useful, even though it is sold as a child’s toy. It has enough magnification to easily distinguish “scaly” crystalline devitrification from “bubbly” effects that are not devitrification. There are many other designs that are as cheap and equally good but you’ll see what I used in the picture.

Raiding a child’s toy box is perhaps the quickest and cheapest way to find a “toy” microscope. Giving your child a similar “toy” microscope for their next birthday would also be a good idea because then you can “borrow it” after they’ve lost interest in it.

This particular microscope is small, lightweight and fits into a pocket. It has a “zoom” that offers magnifications from 20x to 40x which is perfectly adequate for our purposes. The focus can be varied and it has an optional white LED light to illuminate what you’re looking at.

The black plastic attachment converts this microscope from being a “dissecting microscope” into a “slide microscope”. For our purposes we want a “dissecting microscope” so this slide attachment was not useful for my glass investigation work.

I did try to use a 10x magnifying glass of the type field biologists commonly use but the magnification wasn’t sufficient in my opinion. But do try one if you happen to have access to one – 10x is better than nothing more than 1x magnification!

Further Study

For a very nice summary of what devitrification all about, have a look at the Encyclopædia Brittanica site here .

You can read more about what an “amorphous solid” is here  and you might like to read up about the nucleation  process that triggers devitrification.

Why and how glass devitrifies, and what you can do about it, will be the subjects of future blog posting.

Do it Yourself

This posting has been rather practical in its outlook so it must be time for me to prompt you into action. Here’s your homework:

  • First, find a nice little oblong scrap of clear glass that’s suitable for use in a kiln, making sure that it is clean and has has no ground edges or signs of devitrification.
  • Next, grind three of the sides of the scrap of glass. Use a coarse grinder along one edge, then use two other grades of diamond grit pads (if you have them) on the next two edges and leave the final edge un-ground.
  • To one side on the top, add a small heap of clear powder frit.
  • To the other side on the top, add a big glob of undiluted PVA and allow it to dry.
  • Fire on a full fuse schedule (or a profile fuse) when you’re next using your kiln.
  • Inspect the top and sides of the fused glass, remembering to look and feel.
  • Confirm in your own mind whether or not I have not been talking rubbish.
  • Think about how your discoveries might change your glass working practises.

As a budding scientist you should be able to predict what will happen even before you even try the experiment:

  • The excess of PVA will burn off but will cause pitting on top of the scrap glass. It will be rough to the touch and will look “cloudy” or worse.
  • The heap of clear powder frit will fuse together but trap micro-bubbles of air. It will be smooth to the touch and will look “cloudy” or worse.
  • The three ground sides will show differing degrees of micro-bubbles that relate to how coarsely the edges were ground. They will be smooth to the touch and to varying degrees they will look “cloudy”.
  • The underside of the scrap of glass as well as the fourth side will be shiny and smooth to the touch and you will see no problems. These are your “controls” and should not devitrify on their first firing, nor should there be any micro-bubbles.

Remember to investigate how the glass looks and feels to understand the difference between devitrification and micro-bubbles. If have an opportunity to look at the top and edges with a microscope you will instantly recognise the difference.

You may reach a conclusion that the pitting caused by the PVA is merely “damage” but not devitrification. The answer to this conclusion is to subject the piece of glass to further firings and observe what happens to the pitted surface. If it continues to be rough to the touch then it was devitrified and will therefore remain devitrified. If it becomes smooth to the touch then it gets “repaired” which suggests the PVA did not cause devitrification.

Next Time

I’m not finished with devitrification so I promise I’ll address another devitrification topics in future postings. Exactly what that topic is I’m not sure. All I hope is that you’ll find them all useful.

I wish you a prosperous and happy 2016 and thank you for following my blob.


About chatterglass

Maker of stained glass frippery.
This entry was posted in Devitrification, Experiment, Frit and tagged , , . Bookmark the permalink.

3 Responses to Glass Devitrification – Look Closer

  1. I really like your “blob” and am now a follower. I too, don’t believe things just because they’re in print. I appreciate all the information that you put in your posts. I am trying to read them all and have learned quite a bit. Thank you!

  2. Pingback: Secrets of the Microwave Kiln | Chatter Glass

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