Glass Thermal Stress - 'link'

This is a nuanced topic because (like thick slabs, blocks, or deep containers) behave very differently from thin sheets (like windows) under thermal stress.

Here is the direct technical answer for a deep piece of glass. In a thin sheet, temperature varies mainly through the thickness (surface vs. center). In a deep piece (e.g., a 6-inch thick block or a deep bowl), temperature varies in three dimensions : from surface to center, and from the hot outer edges to the cool inner core of the mass. glass thermal stress