Correlation between the surface acid-base nature of solid metal oxides andtemperature of CaSO4 decomposition

By means of the combined use of scanning electron microscopy+energy dispers ive spectrometry (SEM+EDS), X-ray photoelectron spectroscopy (XPS), X-ray d iffraction (XRD), differential thermal analysis (DTA) and thermogravimetry (TG), the thermal decomposition of gypsum and gypsum bonded investment used for casting jewellery products has been studied in order to gain a further insight into the origin of the gas porosity in gold-based alloys produced via lost wax casting. The occurrence of the defect is related to the therma l decomposition of CaSO4 that constitutes with silica the investment materi al and the decomposition of which takes place at a temperature very close t o the casting temperature of some typical gold alloys. The decrease of the thermal decomposition temperature of gypsum is induced by the presence of s ilica and is related to the surface acid-base interaction between SiO2 and CaSO4. On the base of these results, the solid state thermal decomposition of calcium sulphate in the presence of other metal oxides characterised by different acid-base nature has been investigated and a correlation between the surface acid-base properties measured as isoelectric point of the solid surface (IEPS) and via XPS analysis and the temperature of CaSO4 thermal d ecomposition is observed.