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How does gypsum become plaster of paris when heated?

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A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

Plaster of paris is produced through the process of calcination, which involves the partial dehydration of the mineral gypsum (calcium sulfate dihydrate, ${CaSO}_{4} \cdot 2 H_{2} O$ ).12

When gypsum is heated to temperatures between 120–180 °C (248–356 °F), it loses a portion of its chemically combined water to become calcium sulfate hemihydrate ( ${CaSO}_{4} \cdot \frac{1}{2} H_{2} O$ ). This resulting fine white powder is known as plaster of paris.32

The material is valued for its ability to set and harden quickly when mixed with water, a process that occurs through the chemical recombination of the hemihydrate with water to reform the original gypsum structure. While partial dehydration creates standard plaster of paris, complete dehydration at higher temperatures is used to manufacture specialized, harder-finish plasters.412