Resistivity of Silver-Tin Amalgams

¹ Department of Dental Materials, Indiana University School of Dentistry, Indianapolis, Indiana

The resistance of amalgam specimens of known size and composition was measured at different time intervals and under several different conditions. Variables studied included temperature, mercury content, alloy composition and surface area, effect of elevated temperature and addition of mercury to the set amalgam.

The pure metals, silver and tin, as well as the various phases of the Ag-Sn system, were mixed with mercury to form the amalgams. An attempt was made to prepare amalgams representing the phases gamma-1, gamma-2 and beta-1 (60 per cent Hg-40 per cent Ag).

Following trituration of a silver-tin alloy with mercury, the resistivity decreases, reaching a minimum between 6 hours and 5 days. The time period, rate, and amount of decrease depend primarily upon the alloy composition, although the residual mercury content also affects the time-resistivity curve. A gradual increase in resisitivity then follows. When pure silver is mixed with mercury, the resistivity of the specimen increases with time. Tin plus mercury specimens changed very little in resistivity. All of the amalgams formed had a positive thermal coefficient of resistivity.

Annealing of the specimen at 60° to 65° C. produced a sharp increase in resistivity. Addition of mercury to a hardened amalgam resulted in a decrease in resistivity, particularly when the specimen had a low mercury or a high silver content.

Specimens which would correspond in composition to beta-1 (Ag and Hg) had a higher resistivity than gamma-2 specimens, which in turn had a higher resistivity than specimens of gamma-1 composition. Specimens corresponding in composition to a combination of beta-1, gamma-2 and gamma-1 (or perhaps just gamma-1 and gamma-2) had a lower resistivity than gamma-1.