Cracking of Glass

Glass expands as it heats up and shrinks as it cools. It has low thermal conductivity and it's hard and brittle. These three facts cause cracking glass when glass is heated and suddenly cooled. When glass is cooled the surface is trying to shrink, but can't, so it is forced into tension. The hot core is trying to stay the same volume, but the surface is squeezing in, so the core undergoes compression.