dental02.jpg (11342 bytes)Fundamentals of
Dental Materials

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Gypsum Products, Dental Waxes, and Impression Materials

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3-18. EFFECT ON GYPSUM PRODUCTS

Some agar-type hydrocolloid impression materials retard the setting of gypsum products. The surface of a cast in contact with the gel may harden very slowly or not at all. The cast will also absorb water from the gel. As a result, the surface will be soft and rough. The hardness of the cast can be improved by "fixing" the impression with a hardening solution. For this purpose, a 2 percent solution of potassium sulfate is recommended. The impression is immersed in the solution for 5 to 10 minutes. Immersion for longer time may affect the dimensional stability of the hydrocolloid gel. The stone cast or die should be left in the impression for at least 30 minutes--preferably for 60 minutes--before the impression is separated from the cast. This is recommended because the setting time of the gypsum product in contact with the impression material will be lengthened even though a proper hardening solution is used.

David L. Heiserman, Editor

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Revised: May 22, 2017