Origin of the gas porosity in gold-based alloys cast in calcium sulphate bonded investment and influence of metal oxides acid-base properties on calcium sulphate thermal stability

The origin of gas porosity in gold-based alloys produced via lost wax casting in CaSO4 bonded investment has been identified by means of the combined use of microanalytical and thermal techniques,. The occurrence of the gas porosity is related to the thermal decomposition of CaSO4 which together with silica constitutes the investment material and decomposes at a temperature very close to the casting temperature of some typical gold alloys used for jewellery production. The thermal reaction generates sulphur dioxide, leading to gas porosity and therefore, to defective products. Furthermore, the results show the detrimental effect played on the thermal decomposition by the presence of ZnO, Cu2O, CuO, NiO and Ag2O formed on the surface of the gold-based alloy during the air melting or casting. Therefore, the solid state thermal decomposition of calcium sulphate in the presence of other ceramic oxides has been investigated and found to be related to their surface acid-base properties, measured as isoelectric point of the solid surface (IEPS).