Thermal shock

In detail

When a solid is heated it expands, and when part of it is cooled abruptly that part contracts while the material around it has not yet caught up. The result is internal stress, and where the stress exceeds what the material can carry, it cracks. Glass is particularly exposed to this because it is stiff and brittle and cannot relieve stress by deforming. Glass-ceramic of the kind used in stove doors and ceramic hobs is engineered specifically to resist it, by having a very low coefficient of thermal expansion so that a temperature difference produces very little dimensional difference — which is precisely why it can sit between a fire and a room. Engineered resistance is not immunity, and the manufacturers do not treat it as such: the instruction to let the glass become cool to the touch before cleaning, and the specific warning against spraying water onto hot glass because it could crack or shatter, appear in both the trade body's guidance and the manufacturers' own. The same principle explains a set of otherwise unconnected domestic accidents, from cold water into a hot pan to a hot dish onto a cold stone worktop.

Why

• Stove glass is explained by Thermal shock
• Microwave turntable is explained by Thermal shock

  Why the timing is the instruction rather than the water.

• Fridge shelf is explained by Thermal shock

  Thermal shock is the same mechanism described for stove glass, running the other way: there the glass is hot and the water cold, here the glass is cold and the water hot.

• Ironing is explained by Thermal shock
• Waxed wood is explained by Thermal shock
• Laminate worktop is explained by Thermal shock
• Hob surface is explained by Thermal shock
• Fridge shelf is explained by Thermal shock