Origin of gas porosity in gold-based alloys cast in calcium sulfate-bondedinvestment and influence of metal oxide acid-base properties on calcium sulfate thermal stability

The origin of gas-porosity in gold-based alloys produced via lost-wax casti ng in CaSO4-bonded investment has been identified using a combination of mi croanalytical and thermal techniques. The occurrence of gas-porosity is rel ated to the thermal decomposition of CaSO4, which, with SiO2, constitutes t he investment material and decomposes at a temperature very close to the ca sting temperature of some typical gold alloys used for jewelry production. The thermal reaction generates SO,, leading to gas-porosity and, therefore, to defective products. Furthermore, the results show the detrimental effec t of thermal decomposition caused by the presence of ZnO, Cu2O, CuO, NiO, a nd Ag2O formed on the surface of the gold-based alloy during air melting or casting. Therefore, the solid-state thermal decomposition of CaSO4 in the presence of other ceramic oxides has been investigated and found to be rela ted to their surface acid-base properties, measured as isoelectric points o n the solid surface.