Why Can Hot Glass Crack When It Is Cooled Suddenly?

A very hot ordinary glass can sometimes crack if it is placed on an icy surface or immediately rinsed with cold water. The cold does not simply “freeze the glass apart”. The problem is that different parts of the glass change temperature at different rates.

Glass expands when heated and contracts when cooled. During sudden cooling, the surface loses heat and tries to contract while the interior remains warmer and temporarily larger. Because the layers are joined, they cannot change size independently, so thermal stress develops. Glass generally tolerates compression better than tension. A chipped rim, scratch or microscopic flaw concentrates tensile stress. If the stress at the crack tip exceeds what the material can bear, the crack can spread rapidly.

This is thermal shock, but sudden cooling does not guarantee breakage. The outcome also depends on the temperature difference, cooling rate, wall thickness and shape, existing damage, and the glass’s expansion and strength. Borosilicate glass usually handles temperature differences better than ordinary soda-lime glass because it expands less, but heat-resistant does not mean immune to every abrupt change. Follow the vessel’s instructions, allow temperatures to change gradually, and do not use chipped or cracked glass for a hot–cold experiment.