Additives are used to alter the setting time three variations are available: (1) Extra fast setting takes 2 to 4 minutes, (2) fast setting takes 5 to 6 minutes, and (3) slow setting takes 10 to 18 minutes.ī. Plaster is available as bandage rolls in widths of 8 in, 6 in, 3 in, and 2 in and splints in 5- × 45-in, 5- × 30-in, 4- × 15-in, and 3- × 15-in sizes.Thick casts take longer to dry than thin ones. The evaporation from the cast surface is influenced by air temperature, humidity, and circulation about the cast. Cast drying occurs by the evaporation of the water not required for crystallization. The interlocking of crystals (the critical setting period) begins, when the plaster reaches the thick creamy stage, becomes a little rubbery, and starts losing its wet, shiny appearance. Motion during the critical setting period interferes with this interlocking process and reduces the ultimate strength by as much as 77%. The interlocking of the crystals formed is essential to the strength and rigidity of the cast. 3, 4 The more plaster and the hotter the water, the more heat is generated. The amount of heat given off is determined by the amount of plaster applied and the temperature of the water.1, 2 When this material is dipped into water, the powdery plaster of paris is transformed into a solid crystalline form of gypsum, and heat is given off: Plaster bandages and splints are made by impregnating crinoline with plaster of paris.
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