Surface properties of dental amalgam: roughness produced by setting reaction

During the setting of dental amalgam, silver-mercury (gamma1) and tin mercury (gamma2) intermetallic compounds (phases) are formed. Since these compounds have distinct crystalline forms, they may be discerned on the surface of a setting sample. Visual identification of these phases on polished and etched samples is often difficult and, hence, surface morphology studies have an advantage in following the stages of the setting reaction. The purpose of this investigation was to use high-resolution light microscopy to follow the course of the setting reaction. Amalgam specimens were triturated and condensed against glass plates under pressure. The sample surfaces were then photographed under a high-resolution metallographic microscope using oblique illumination. The roughness of four samples for each alloy type was determined with a surface profile analyzer at times between ten minutes and 23 hours after trituration. An electron microprobe was used to identify the gamma1 and gamma2 phases. The gamma1 phase had a polyhedramorphology, and the gamma2 crystals formed intermeshed smooth areas. During the setting reaction, the average times for the roughness to reach 95% of the final value were 189, 227, 96, 65, and 116 minutes after trituration for the various amalgam types studied. All alloys tested required polishing several hours after trituration.