Gypsum Plaster (Plaster of Paris)

Gypsum Plaster (Plaster of Paris)

Gypsum plaster, gypsum powder, or plaster of Paris, or P.O.P. consists of white powder of calcium sulphate hemihydrate. The chemical formula is given as CaSO4. 1/2H2O

Etymology

The name “plaster of Paris” came from the fact that it was first of all made by heating gypsum which was mainly found in Paris. A large gypsum deposit at Montmartre in Paris led “calcined gypsum” (roasted gypsum or gypsum plaster) to be commonly known as “plaster of Paris.

Chemistry

Gypsum plaster, gypsum powder, or plaster of Paris, is produced by heating gypsum to about 120–180 °C (248–356 °F) in a kiln.

Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water.

Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time.

When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume due to which it is used in making casts for statues, toys, etc. The initial matrix consists mostly of orthorhombic crystals – the kinetic product. Over the next 72 hours the rhombic crystals give way to an interlocking mass of monoclinic crystal needles and the plaster increases in hardness and gains strength. If plaster or gypsum is heated to between 130 °C, (266 °F) and 180 °C (350°F), hemihydrate is formed, which will also re-form as gypsum if mixed with water.

On heating to 180 °C (350°F), the nearly water-free form, called γ-anhydrite (CaSO4·nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C (480°F), the completely anhydrous form called β-anhydrite or dead burned plaster is formed.

 Applications:

Plaster of Paris is used in hospitals for setting fractured bones in the right position to ensure correct healing. It keeps the fractured bone straight. This use is based on the fact that when plaster of Paris is mixed with a proper quantity of water and applied around the fractured limbs it sets into a hard mass, In this way it keeps the bone joints in a fixed position. It is also used for making casts in dentistry.

In addition, dentures (false teeth) are made by first taking a dental impression using a soft, pliable material that can be removed from around the teeth and gums without loss of fidelity and using the impression to creating a wax model of the teeth and gums. The model is used to create a plaster mold (which is heated so the wax melts and flows out) and the denture materials are injected into the mold. After a curing period, the mold is opened and the dentures are cleaned up and polished.

for making toys, decorative materials, cheap ornaments, cosmetics, black-board, chalk and casts for statue and a fire-proofing material and fire-proofing material.

chemistry laboratory for sealing air-gaps in apparatus when air-tight arrangement is required.

for making surfaces like the walls of a house smooth before painting them and for making ornamental designs on the ceilings of houses and other buildings.