Why Does Plaster of Paris Get Hot?

Plaster is also used for passive fire protection.
Plaster is also used for passive fire protection. (Image: Fire image by Luke Haverkamp from Fotolia.com)

Plaster of Paris is created when gypsum is heated to 150? C. At this temperature, the mineral partially dehydrates, with 75% of the water content escaping as water vapor. This is an endothermic reaction. When water is re-added to plaster of Paris, it resets itself as a gypsum crystal lattice and undergoes an exothermic reaction, which creates heat.

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Plaster of Paris is otherwise known by its chemical name calcium sulfate hemihydrate and has the chemical formula CaSO4·½H2O.


There are several gypsum quarries in the Montmartre district in Paris, and these have long been established providing burnt gypsum for a variety of purposes. This is from where plaster of Paris inherits its name.


Plaster is used in architecture for interior detailing and also on movie sets to simulate wood or stone surfaces. Plaster may also be used in casts to support broken bones and to fireproof products such as doors.

Time Frame

Plaster of Paris exists in three types: fast-setting, regular-setting, and slow-setting. These have an average setting time of 17, 36 and 110 minutes respectively.


The exothermic reaction caused by rehydrating plaster of Paris can be powerful enough to cause third-degree burns on the skin.


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