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 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 locally changes the micro-morphological features and the thermal stability of the CaSO4-bonded investment, varying the shape and size of the needle-like gypsum crystals at the contact surface between the investment and wax where gas porosity defects could be generated. The role of the dry dewaxing atmosphere has also been investigated and the results show the positive effect of a forced ventilation and of an oxidizing atmosphere, even though the steam dewaxing should be used wherever possible.

Author: Dr. G.M. Ingo