Plaster of Paris

Also called POP, chemically it is 2CaSO4.H2O or CaSO4.½H2O (calcium sulphate hemi hydrate).

Manufacture

Raw Material

Gypsum, (CaSO4.2H2O) is used as the raw material.

Process

The only difference between gypsum and Plaster of Paris is in the less amount of water of crystallization. When gypsum is heated at about 100° (373 K) temperature, it loses a part of its water of crystallization.

CaSO4.2H2O + heat (100°C / 373 K) → CaSO4.½H2O + 3/2 H2O

The temperature is not allowed to rise beyond 100°C otherwise whole of water of crystallization is lost and anhydrous calcium sulphate is produced which is called dead burnt as it does not have the property to set after mixing with water.

Uses

  1. In making casts for manufacture of toys and statues.

  2. In medicine for making plaster casts to hold fractured bones in place while they set. It is also used for making casts in dentistry.

  3. For making the surface of walls and ceiling smooth.

  4. For making decorative designs on ceilings, walls and pillars.

  5. For making chalk for writing on blackboard.

  6. For making fire proof materials.