What can happen if trees and castings are not properly cleaned before remelting and casting. ~ Stewart Grice
A customer asked us to look at an 18kt palladium white gold ring they were having problems with. This was a diamond channel-set cast ring and they were finding hard spots which were visible at the casting surface. The ring had been stone set and polished before the defects were visible. This was not our alloy so the exact composition of the casting was not known.
Examination of the ring under a stereo microscope confirmed the presence of hard spots on the shank surface that looked like PGM agglomerations (figure 1). We decided to take a microsection of the ring to see what may be going on. This revealed two different inclusion types present, the agglomerations and also an inclusion that was angular in shape (figure 2). EDX analysis confirmed the agglomerations to be iridium, and the angular inclusions to contain zinc and sulphur, so probably zinc sulphide.
The presence of sulphur suggests investment contamination, which often results from residual investment on recycled castings and trees. If all investment is not completely removed before re-melting and casting, contamination can occur. Our analysis showed that the alloy was grain refined with iridium. This can form agglomerations during casting, especially if there are impurities present, or it is also possible that the grain refiner did not fully disperse when originally made into grain.