title: The effects of alginate impressions on the surface of cast gypsum. I. The physical and chemical structure of the cast surface.
abstract: The surface of cast gypsum which has set against alginate impression material has been shown to vary according to the specific impression material used. The variation is both physical and chemical. Poor cast surfaces have been shown to consist completely of unreacted calcium sulphate hemihydrate to a depth of at least 80 micrometer. The best cast surfaces consist of considerable amounts of syngenite (potassium calcium sulphate), along with a small amount of gypsum and unreacted hemihydrate. Cast surfaces with a lower syngenite content were intermediate between these two extremes. No cast surfaces which had set against any alginate impression exhibited complete conversion of hemihydrate to gypsum, which characterized the surface formed when the same gypsum products set against Teflon. Although dental stones may contain a small amount of a potassium salt as an accelerator, it was shown that this amount does not significantly alter gypsum surfaces cast against alginates. The potassium ions involved in the setting reaction to form syngenite clearly enter the cast at its interface with the impression, presumably by way of an exudate from the gelled impression.
mesh_terms: D000464:Alginates; D002133:Calcium Sulfate; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003461:Crystallography; D003734:Dental Casting Investment; D003760:Dental Impression Materials; D006801:Humans; D066298:In Vitro Techniques; D011138:Polytetrafluoroethylene; D011188:Potassium; D013499:Surface Properties
chemical_list: D000464:Alginates; D003734:Dental Casting Investment; D003760:Dental Impression Materials; D011138:Polytetrafluoroethylene; D011188:Potassium; D002133:Calcium Sulfate