Thermal and microchemical characterisations of CaSO4-SiO2 investment materials for casting jewellery alloys

Differential thermal analysis (DTA) and thermogravimetry (TG), X-ray photoe lectron spectroscopy (XPS), and scanning electron microscopy (SEM) and ener gy dispersive spectrometry (EDS) were used to study the thermal decompositi on 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 por osity in the molten gold-based alloys and therefore, defective jewellery pr oducts. This latter reaction was studied as a function of the temperature a nd atmosphere (air, argon and argon-5% hydrogen). in order to simulate, as for as possible, different casting conditions used by the jewellery industr y. Furthermore, special attention was given to the effect of the presence o f Zn, Cu2O. CuO and Ag2O on the thermal decomposition of CaSO4. DTG-TG resu lts confirmed that the temperature of the thermal decomposition of CaSO4 bo nded with silica was lower with respect to the nearly pure CaSO4. Unfortuna tely, it was very close to the casting temperature of some typical gold all oys. 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. (C) 1998 Elsevier Science B.V.