Gypsum-bonded investment and dental precision casting (III) composition of investment for the quick casting technique

A simultaneous differential thermal analysis and thermogravimetry method previously established was used to estimate the composition of gypsum-bonded investment marketed for the quick casting technique. Three commercial investments of this type were heated to 700degreesC at 10degreesC/min and the hemihydrate content was estimated by the mass decrease reached at 300degreesC after subtracting the mass decrease at 100degreesC as moisture content. The hemihydrate contents were between 25% and 30%, which appears to be the range also chosen for the conventional gypsum-bonded investment of cristobalite type over 70 years by the industry. However, the new type of investment contained both cristobalite and quartz. The small sample size is a disadvantage of the present method but this can be overcome by more frequent use of the method by investigators.