CaSO4 BONDED INVESTMENT FOR CASTING OF GOLD-BASED ALLOYS : STUDY OF THE THERMAL DECOMPOSITION

Differential thermal analysis (DTA) and thermogravimetry (TG) have been used to study the thermal decomposition of calcium sulphate in the CaSO4 bonded investment using also X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy + energy dispersive spectrometry (SEM+EDS) as ancillary techniques. The thermal decomposition of CaSO4 generates sulphur dioxide, leading to gas porosity in the gold-based alloys and therefore, to defective jewellery products. The thermal decomposition has been studied as a function of the temperature and atmosphere (air, argon and argon-5 % H2), in order to simulate as better as possible, different casting conditions of the jewellery industry. DTA-TG results have confirmed that the temperature of thermal decomposition of CaSO4 bonded investment is lower with respect to the nearly pure CaSO4 and is also influenced by the atmosphere. Unfortunately, it is very close to the casting temperature of some typical gold alloys. Furthermore, special attention has been given to the effect of the presence of ZnO, Cu2O, CuO and Ag2O on the CaSO4 thermal decomposition and DTA-TG results show that the temperature of the decomposition is furtherly lowered in the presence of these latter oxides and when inert and reducing atmospheres are used.