The effects of variable factors on crushing strengths of dental amalgams.

DENTAL amalgams constitute a most important and very high proportion of individual tooth restorations. The physical properties of finished amalgams, upon which the restorations depend for satisfactory service, vary with the alloy formulas, manufacturing methods, mixing procedures, and condensation technics. Previous studies of amalgams have established the effects of many variables and the American Dental Association and Federal Specifications for Dental Amalgams have established performance standards which are generally accepted. In the earlier tentative drafts of these specifications, the physical qualities governed by detailed requirements included setting expansion, flow, and crushing strength. In later revisions the crushing strength requirements were eliminated primarily because those amalgams which met the flow requirements also met the crushing strength requirements. Notwithstanding the discontinuance of this early and rather low requirement, the crushing strength of amalgams under many conditions have been reported in dental literature." 2, 3 In considering some problems involved in the selection of testing methods for gypsum products it became apparent that some characteristics of these materials could best be determined by studying their elastic properties and crushing strengths under loads very slowly applied. As data available on the crushing strengths of amalgams gave no definite indication of the loading rates employed, it appeared of interest to observe the effects of variations in loading rates on amalgams. An introductory series of tests to determine crushing strengths of amalgams under slow loading indicated that it would be possible to determine both elastic and plastic characteristics of these materials, and so gain some new basic information which could be related to the usefulness of these alloys in service. After considerations of many of the variables involved in amalgam technics, series of tests were outlined which would show the effects of several individual factors. Those factors which can be controlled by the dentist were given major consideration. In recent years the increasing use of mechanical amalgamators has made their effects of interest; similarly, the effects of mechanical condensing devices are of interest to the dental profession, so these were also included. In practice, amalgam mixes seldom contain more than twelve grains of alloy plus the required amount of mercury; and as the current amalgamators are