SUMMARY
The presence of polluting elements in the raw materials to produce gold and silver alloys can involve a wide series of problems, including the appearance of superficial hard spots. These defects can be removed only by sending to refining all the material on disposal, which implies several production problems. Due to this, the choice of the raw materials to use turns out to be essential in the production of precious items.
PROBLEM
In this specific case was examined an item in yellow gold 18 kt which displayed on its surface the presence of hard spots hardly removable by polishing; after it they resulted even more projecting and left behind themselves some grazings commonly called comet effect (se the first pciture).
From the analysis made in our laboratory, it came out that these hard spots were due to the presence of some elements of the platinoid's family (in the specific case Osmium, Iridium and Ruthenium, as highlighted by S.E.M. analysis).
Platinum group metals tend to bond first among themselves and subsequently with other elements as well: it is hypothesized that during the melting process, the platinum group metals that were contaminating the gold joined those already present in the alloy. This excess agglomeration of platinum group metals, attracting other related elements present in the alloy, gave rise to hard spots.
The presence of hard spots composed of platinides cannot be attributed to the composition of the master alloy (in which any presence of platinides is calculated so as not to interfere with the performance of the alloy itself); it is hypothesized that it may be due to the use of gold with a fineness below 999.9 thousandths, or even to the use of gold not derived from electrolytic refining. Indeed, since platinides are elements very similar to gold in terms of chemical-physical characteristics, they are very difficult to separate from it during refining and, if the latter is not carried out perfectly, the gold may contain some traces of these elements. Hence the necessity of always using highly pure gold in order to avoid the appearance of hard spots on the surface of the finished piece.
SOLUTION
The use of pure raw materials is essential to obtain an object free of hard spots. It is therefore preferable to:
- Always ensure that there are no external elements in the gold; this can be achieved by refining the gold through electrolytic refining.
- Check that all tools are thoroughly cleaned, so as not to contaminate the pure material during processing.
- Always start with raw materials of high and guaranteed purity; never use ingots with a fineness lower than 999.9‰.