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 glass is heated or cooled unevenly, the part of the glass subject to the temperature change will expand (if heated) or contract (if cooled) while the glass at a constant temperature remains the same size. The glass that is changing size will break away from the unchanged piece. That is thermal shock.
There are three key attributes 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 less brittle materials.
Third, materials that conduct heat well are less likely to thermal shock because, by transfering the heat quickly throughout the material, they increase the likelyhood that the material will expand or contract evenly.
Glass tends to be 1) very brittle, 2) expand and contract quickly when subjected to temperature changes, and 3) is an insulator (when solid) and therefore does not readily conduct heat. That is why glass is highly susceptible to thermal shock. One exception is borosilicate glass (i.e. Pyrex). Though still brittle and an insulator, borosilicate glass is formulated to have a low expansion rate -- about 70% lower than typical art glass.
The best way to mitigate the risk of thermal shock is to heat and cool slowly. That not only reduces the risk of large temperature differentials from end to end but also from inside the glass to the surface.
