Impurities deriving from the raw materials and possible consequences

SUMMARY

A determining element for the success of a casting is certainly the raw material quality. In addition to the use of a high quality master alloy, it is also important to use gold as pure as possible. In fact, non excellent quality gold may involve the formation of defects very difficult to remove, such as hard spots.

PROBLEM

An 18ct yellow gold specimen displayed many small bulges on its surface after casting.

We hypothesized that it was a matter of hard spots - i.e. intermetallic compounds peculiar for their high hardness. For this reason, we decided to carry out an analysis by means of the scanning electron microscope to confirm our thesis and individuate the defect's cause.

In this way we found out that those hard spots, squared and with sharp edges, were randomly spread all over the item's surface (see the next five photos).

Then we carried out a series of microanalysis on the intermetallic compounds, determining that they were mainly composed of osmium, ruthenium and iridium. Then we understood that the problem was due to the quality of the gold used - we were noted that it was 999.7 millesimals purity.

Buying non-excellent quality gold coming from refining, you risk to buy 'polluted' material, that is containing some particular elements (in the case in point, belonging to the platinoids family), which often cause the formation of intermetallic compounds during casting. For this reason, it is good rule to choose only 999.9 millesimals gold coming from electrolytic refining, as this allows reducing the probability the that gold contains 'dangerous' elements for the casting outcome.

SOLUTION

To avoid the formation of the above described intermetallic compounds, it is necessary to always buy high purity gold (999.9 millesimals). In case you had already faced the hard spots problem, to avoid polluting the new high purity material we suggest to:

1. send to refining all the material that outlined the above cited defect;
2. use gold 999.9 millesimals pure only;
3. evaluate if to replace the materials which had come into contact with the polluted metal (i.e. crucibles, stoppers, mixers etc.) would be in order.