Development of strength in dental silver amalgam

ObjectivesTo characterize the development of strength during the setting process of dental silver amalgam in the context of ‘early strength’ measurements for standards compliance testing in relation to patient instructions, and demonstrate the applicability of the Hertzian ‘ball on disc’ method.Materials and methodsSixteen dental silver amalgam products were tested using the ‘ball on disc’ protocol at 1, 2, 3, 4 and 24 h after setting at 37 °C in air. The mixed materials were packed into a tapered steel disc mold (10 mm diameter, 3 mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10 min and placed immediately into an incubator at 37 °C. Testing was in Hertzian mode, using a 20 mm steel ball, with the specimen resting on a disc of glass-filled polyamide (E = 10 GPa) at a cross-head speed of 0.2 mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced.ResultsRadial cracking into 2–5 pieces, in a clinically-relevant (non-explosive) mode was observed in all cases. Considerable variation in setting rate between products, as indicated by the development of load at failure with time, was found. The distribution of normalized failure load values overall was lognormal (Weibull was excluded). The RMS coefficient of variation overall was 12.4%.SignificanceThe ball-on-disc test provides a facile, relevant measure of the strength of dental silver amalgam, and is viable as a standards compliance test. Early strength testing at a minimum of 2 h is suggested.