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1 University of Michigan, School of Dentistry, Ann Arbor, Michigan
A survey was made of the literature to date in this area. Analysis by x-ray diffraction was used in deriving the reactions occurring during setting and burnout. Crushing-strength tests at various temperatures confirmed these reactions. The reaction product of the alloy with the investment was shown to be MgCr2O4. It was proposed that with investment containing MgO, excessive heating of the alloy accelerates bonding of the Cr2O3 to the MgO grains, producing a scale which is very difficult to remove. Use of a non-MgO investment substantiated this conclusion.
The results for the three different reaction products showed that the thin gray scale formed immediately adjacent to the cast metal surface and contained most of the metal elements. Mn entirely and Cr partially diffused rapidly outward and were found in the green scale. To a lesser extent, Co occurred in the same manner, but not in so great a proportion as is found in the original alloy. The reddish-brown areas showed as a reaction of the easily diffusing Cr and Mn with another constituent of the investment.
Submitted on January 7, 1966
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