Thermal and surface characterisation of CaSO4-SiO2 investment materials

Differential thermal analysis (DTA) and thermogravimetry (TG), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to study the thermal decomposition of calcium sulphate in the CaSO4 (25 wt%)-bonded silica investment which is the commonly used material for casting jewellery gold-based alloys. The thermal decomposition of CaSO4 generates sulphur dioxide, leads to gas porosity in the molten gold-based alloys and therefore, defective jewellery products. This latter reaction was studied as a function of the temperature and atmosphere (air, argon and argon-5% hydrogen), in order to simulate, as for as possible, different casting conditions used by the jewellery industry. Furthermore, special attention was given to the effect of the presence of ZnO, Cu2O, CuO and Ag2O on the thermal decomposition of CaSO4. DTG-TG results confirmed that the temperature of the thermal decomposition of CaSO4 bonded with silica was lower with respect to the nearly pure CaSO4. Unfortunately, it was very close to the casting temperature of some typical gold alloys. In addition, the temperature of the decomposition was further lowered when inert and reducing atmospheres are used, as well as in the presence of ZnO, Cu2O, CuO and Ag2O.