SUMMARY
Calcination is a fundamental phase in obtaining a plaster with the best possible characteristics. If this phase does not take place correctly, the gypsum may not achieve the desired chemical-physical properties. This leads to a high probability of finding defects in the piece such as porosity from embedded gypsum or porosity from gas due to the metal-gypsum reaction.
PROBLEM
In the case in question (figure 1) a piece had numerous inclusions of oxides, located in the areas near the bezels, which made it impossible to properly seat the stones. Mechanical removal treatments have also been carried out without satisfactory results.
The electron microscope analysis (see enlargements in figures 2 and 3) carried out on the areas affected by the defect, revealed areas with evident inclusions in the surface of the object.
More in-depth analyses (table in figure 4) made it possible to discover that these inclusions were made up of Carbon, Sodium, Sulfur and Silicon. The presence of these elements is most likely due to reactions that took place between the metal and the plaster during the casting phase. We therefore hypothesize that this phenomenon was triggered by a calcination cycle performed incorrectly. In this specific case, the cause is to be found in a calcination phase that took place at a temperature that was too low, which produced a gypsum with characteristics that were not optimal for use in casting.
SOLUTION
To avoid reactions between plaster and metal during the casting phase, it is advisable to properly carry out the calcination of the plaster that will be used for the molds. It is important to ensure:
- that the correct temperature is reached during the calcination cycle, approximately 740°C,
- that all areas of the cylinder reach the same temperature (to achieve this, it is recommended to maintain the cylinders at the maximum calcination temperature for at least three hours),
- that the instructions provided by the plaster manufacturer are carefully followed.