The effect of wax removal by steam and dry process on the micro-chemical structure and morphology of CaSO4 bonded investment

The effect of wax elimination by steam and dry process on the micro-chemical and structural features of CaSO4 bonded investment and on the thermal stability of gypsum has been studied by means of the combined use of different analytical techniques such as SEM-EDS, XRD, OM and DTA-TG. The results show that the dewaxing process changes locally the micro-morphological features and the thermal stability of the CaSO4 bonded investment varying the shape and size of the acicular gypsum crystals and inducing the formation of carbonaceous residues at the contact surface between the investment and wax where gas porosity could be generated. The role of the dewaxing atmosphere has been also investigated and results evidence the positive effect of a forced ventilation and of an oxidising atmosphere, even though the steam dewaxing should be used wherever possible.