[The forming phase and various properties of Au, Ag, Cu and Ga mixture in metal fired crowns].

A new time-saving method has been developed to produce artificial crowns without using the casting process. Plastic mixtures of gallium and other metal particles are kneaded into desired shape and then heated for hardening. By this method, the time required for hardening and producing restorative materials has been shortened greatly. In the present experiment, gallium was triturated with powdered gold, silver and copper to make binary alloy samples. The dimensional change was measured between heat treatment. After heat treatment, the test piece was examined for compressive strength, compressive shrinkage, hardness, tarnishing and difference in phase. Non-heated and heated alloy specimens (Au-Ga, Ag-Ga, Cu-Ga) expanded to form the new phase. The ability of Au-Ga samples to bear compressive strength, when heated at 300 degrees C or more (AuGa2----AuGa), became 2.6 times greater than that of non-heat-treated specimens. The compressive strength of Ag-Ga samples dropped briefly at 350 degrees C (Ag0.72Ga0.28----Ag3Ga) but increased at 450 degrees C (Ag3Ga----AgGa). The strength of Cu-Ga pieces fell by half at 475 degrees C and upward (CuGa2----unknown phase). A compression test showed that the contraction percentage of Au and Ag specimens became large as a result of heat treatment, while that of Cu alloys remained almost unchanged. The results of a hardness test (HV) were comparable to those of the compressive strength test. The Au-Ga alloys increased in hardness after high-temperature treatment. In the Ag-Ga alloys, hardness declined at 350 degrees C and increased at 450 degrees C. There was no difference in hardness between Cu specimens after heat treatment and those allowed to stand at room temperature. A tarnishing test revealed that Au-Ga samples turned slightly yellowish. In the case of Ag-Ga samples, the reflectivity Y (%) dipped slightly but discoloration was not recognizable. However, the Cu-Ga samples which were heated at temperatures of up to 280 degrees C showed a slight drop in reflectivity, but those heated at temperatures higher than 280 degrees C decreased to 50-66% in reflectivity and turned black.