dental02.jpg (11342 bytes)Fundamentals of
Dental Materials

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Gypsum Products, Dental Waxes, and Impression Materials

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a. Uses. Plaster of Paris is used for pouring casts, making matrices for prosthodontic restorations, for attaching casts to articulators, and general use in the dental laboratory where strength is not important. The crushing strength for plaster of Paris is 2,600 psi.

b. Mixing. Water-powder ratios must be used as stated by the manufacturer. Before mixing, the can containing the material should be agitated to evenly disperse all elements in the powder. A clean, dry rubber bowl and plastic spatula are used to manipulate the materials. First, the water is measured and poured into the rubber bowl. The powder is weighed and sifted into the water to avoid trapping air bubbles. Then, with a spatula, the mix is stirred (spatulated) for 30 to 60 seconds in a knifing or stirring motion, making sure to include all powder from the sides of the bowl. (Whipping the mix will entrap air and should be avoided.) Before the mixed material is poured, it should be vibrated to remove any trapped air bubbles.

c. Setting Time. The initial setting time for plaster of Paris is 5 to 10 minutes. In this stage, the plaster loses its glossy appearance and is hard enough to hold for carving. The final setting time is approximately 45 minutes. In this stage, the plaster achieves a dry, hard condition. The setting of plaster can be hastened by using less water, by mixing longer, by using chemical accelerators, or by using warm water (up to 85o F (29o C)). Reversing these processes or using chemical retarders lengthens the setting time. The most satisfactory results will be obtained by following the manufacturer's directions.

David L. Heiserman, Editor

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Revised: May 22, 2017