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.