Gypsum-bonded Investment and Dental Precision Casting: (I) Two Investments

The present study compared the TE of two gypsum-bonded dental casting investments, the oldest K and more recent G. They had almost identical composition, 70% refractory (cristobalite) and 30% binder, but had different recommended water/powder ratios, 0.40 (K) and 0.33 (G). The average TE was significantly less (1.19%) in K than in G (1.45%); the volume decrease accompanied by the phase change of gypsum was more pronounced in the less dense K. When the dehydration conditions around gypsum are similar to those prevailing in wet calcinations methods, an expansion is likely to occur in the mold due to the formation of dental stone by recrystallization. This additional expansion has not been detected in ordinary laboratory measurement but can effectively increase the actual TE of an investment. The present study has proved the formation of dental stone by rapid heating of an investment. Specific laboratory techniques may have been supplementing low TE by this mechanism.