Study of gypsum-bonded casting investments. Part 1

In order to develop analytical methods for identifying plaster or dental stone (calcium sulphate hemihydrate) and quantifying its content in gypsum-bonded dental casting investments, six dental gypsum products were studied by means of simultaneous differential thermal analysis-thermogravimetry (DTA-TG). Under the same analytical conditions employed in the present study, a phase change in calcium sulphate anhydrite (III-CaSO₄ to ll-CaSO₄) was shown to occur at about 220°C for four dental stones manufactured by various wet methods and at about 350°C for two plasters manufactured by dry methods. This confirms the general view that the two types of calcium sulphate hemihydrate manufactured by dry or wet method could be identified by the DTA method. A reasonable estimate of chemically bound water could be made from the TG results at 300°C. The present DTA-TG method can be used to identify the plaster or dental stone and calculate its content in gypsum-bonded casting investments.