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:

• First, is expansion rate. Materials that expand and contract quickly due to temperature change are more likely to thermal shock than materials with low expansion rates.
• Second, materials that are brittle (i.e. do not stretch or bend far without breaking) are also more likely to thermal shock than flexible materials.
• Third, materials that conduct heat well are less likely to thermal shock because, by transferring the heat quickly throughout the material, they increase the likelihood that the material will expand or contract evenly.

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