Hot water can crack a cold glass because the inner surface warms and expands before the outer surface does. This uneven expansion creates thermal stress. If the stress exceeds the glass strength, often near a flaw or scratch, a crack can form. The risk depends on glass type, thickness, condition, and the temperature change.
A temperature difference creates stress
Glass does not instantly reach one uniform temperature. When hot water contacts a cold vessel, the inner surface warms first while the outer surface remains cooler. The difference creates stress across the glass. A crack may begin at an edge, scratch, or other flaw. This is thermal shock. [1]
Glass types respond differently
Composition matters. Borosilicate glass generally expands less with temperature than ordinary soda-lime glass, which can make it more resistant to thermal shock. But no label makes an item invulnerable: thickness, defects, heating rate, and product design matter. Follow the manufacturer’s temperature instructions. [2]
Do not test a glass’s breaking point
Do not pour boiling water into an unverified cold glass to see whether it breaks. Sudden failure can send hot liquid and sharp fragments outward. Use vessels rated for the temperature and avoid sudden changes. Related effects include ice cracking in a drink and metal feeling colder than wood.
Frequently asked questions
Frequently asked questions
Will every cold glass crack with hot water?
No. Breakage depends on glass type, thickness, flaws, and how large and rapid the temperature change is.
Is borosilicate glass resistant to thermal shock?
It generally expands less than common soda-lime glass, but it can still break. Follow the product instructions.
Is it safe to test at home?
No. Deliberately testing glass failure can cause burns and cuts.
Sources and evidence
- Corning Museum of Glass — Does Glass Break If It Gets Too Hot or Too Cold? ↗
Explains uneven heating and rapid temperature change in glass.
- Corning — Borosilicate Glass ↗
Describes borosilicate glass and lower thermal expansion.
- American Ceramic Society — Thermal Shock in Ceramics ↗
Explains thermal gradients and stress in brittle materials.
Sources consulted: 2026-10-18. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial editorial draft prepared 18 October 2026; claims are tied to linked sources and retain stated limitations.