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. It is used in making casts for statues, toys, and more. 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 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. Uses of gypsum plaster 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. (see Plaster In decorative architecture) for making toys, decorative materials, cheap ornaments, cosmetics, black-board, chalk and casts for statue. a fire-proofing material. (see Plaster in Fire protection) an orthopedic cast is used in hospitals for setting fractured bones in the right position to ensure correct healing and avoid nonunion. It keeps the fractured bone straight. It is used in this way, because when plaster of Paris is mixed with a proper quantity of water and applied around the fractured limb, it sets into a hard mass, thereby keeping the bones in a fixed position. It is also used for making casts in dentistry. (see Plaster in Medicine) chemistry laboratory for sealing air-gaps in apparatus when air-tight arrangement is required. Lime plaster Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry. To make lime plaster, limestone (calcium carbonate) is heated above approximately 850 °C (1600°F) to produce