Understand and Avoid Thermal Shock

If you've ever had a piece of work break in the kiln then you might have been the victim of thermal shock.

In this tutorial we explain what thermal shock is, what causes it, and – most importantly – how to avoid it.

To navigate this tutorial, click a link in the list below to jump directly to that section OR navigate forward and backwards using the links in the blue bar below.

What is Thermal Shock?

Most glass artists know that glass that breaks due to rapid, uneven temperature change is said to have suffered thermal shock. Why this happens may not be obvious.
When solid glass is heated or cooled unevenly, the part of the glass that is heated will expand (or contract if cooled). The glass that isn’t changing temperature stays the same size. The uneven expansion creates a lot of stress inside the glass. If the stress is strong enough the glass will break. That is thermal shock.

There are three key characteristics that determine a material's potential to thermal shock:

As you'll see, this all adds up as bad news for glass.


Why Glass Thermal Shocks so Easily

Glass tends to:

  1. Be very brittle – not much bending or stretching.
  2. Have a relatively high expansion rate compared to the other material we most often put in a kiln – ceramic
  3. Is an insulator (when solid) so it doesn’t conduct heat. That is why fiber glass is used as insulation (and why beer stays cold longer in bottles than in cans).

Put that all together and you understand why glass is very vulnerable to thermal shock in a kiln.

One exception is borosilicate glass. Like most glass, borosilicate is brittle and is excellent insulator. Borosilicate, though, is formulated to have a low expansion rate -- about 70% lower than the soda-lime glass used for most fusing (and every day windows). The low expansion rate makes borosilicate excellent for the oven. While most people might not have heard of borosilicate, just about everybody knows trade-name “Pyrex” – the original borosilicate cookware.

A Quick Look at Heat in the Kiln

Heating elements do not create small amounts of heat by trickling the electricity through to create a small amount of heat. To make a small amount of heat the elements turn on completely but for a short burst of time. Those little heat bombs then disperse into the kiln. That means, no matter how you program your kiln there will be moments that the air close to the elements is a lot hotter than what your controller reports.

If there is a piece of glass close to the elements it is going to take these burst head on.

This is one of the reasons that lid elements are preferred when firing glass: the edge of your glass may be closer to the side of the kiln – but it is exactly the same distance from the lid as the center of your glass.

Top heat vs side heat

Will uneven heating in your kiln break your glass?  To answer that question you need to understand what characteristics of glass put it at increased risk.

Understanding when Your Glass is at Increased Risk

Some glass is more likely to thermal shock than others:

All of these challenges can be overcome. The first step is to understand that you have a challenge and the generic firing schedule that you have isn’t going to work. Once you recognize this you are ready to take steps to minimize the risk.


How to Reduce the Chance of Thermal Shock in the Kiln

There are a number of things we can do to ensure more even heating – and lower chance of breaking – of glass in a kiln.

Here are some of the easiest and most effective methods.

Slow down:

The slower the kiln heats, the more time the heat has to disperse evenly throughout the glass. This is always the first and best line of defense against thermal shock.

Slow down:

Yes, I’ve listed this one twice because it is really, really important.

Use a kiln with lid elements:

Heating the glass from above tends to heat more evenly because the entire surface is (usually) the same distance from the elements. Kilns with just side elements (like ceramic kilns) present some real challenges for kilnforming glass artists.

Position the glass for even heating:

The best place to put your work is the center of the kiln shelf. The kiln shelf should be as low as possible while still leaving space between it and the floor of the kiln for air circulation.

If you are firing multiple pieces at one, consider which one(s) are most likely to thermal shock and place those near the center.|

Baffle side heat:

Sometimes it isn’t possible to put as much space as you would like between the glass and the side heating elements. When that’s the case, you can baffle the heat with kiln furniture, fiber board, strips cut from an old kiln shelf or fire brick.

Baffling heat is easy – simply position the kiln furniture (or whatever you are using) around the glass on the shelf like this:

heat damn in kiln


Here’s a close-up:

heat dam

How does this help? The heat from the nearby elements now needs to travel farther to reach the glass. The farther the heat travels the more time it has to disperse and the more evenly it hits the glass.

One note on this technique – it is important that there are no gaps in your baffle since that can contribute to the uneven heating.

What to Remember from this Tutorial

Here's a summary of the most important points from this tutorial:

And don't forget to slow down :)