SUMMARY
The effect originates from hard spots that can be caused by impurities present or introduced into the alloy.
PROBLEM
The comet effect can be noticed both on pieces made by investment casting and mechanical working. It is pretty much all the times visible only after the finishing process; the quality of the surface before finishing tends to hide it. It is a surface defect and has the typical shape of a comet tail.
SOLUTION
In this particular case, the operator has mixed two different alloys, which contain elements that cannot be added together in the gold alloy. These are Silicon and Iridium which, if added to the same alloy, must comply with very fixed and precise concentration ratios. The analysis with the SEM (Scanning Electron Microscope) in the last picture, confirmed this hypothesis since the composition of the plates that can be seen in the first photo, and more enlarged in the second photo, includes both Iridium and Silicon. The alloy that the operator usually used does not contain Iridium and, after some exchange of information, we received confirmation that two alloys had been mixed: one containing Silicon and one containing Iridium. This operation is strongly discouraged unless you have consulted ProGold technicians