Jewelry Plating -- Need hard gold not soft gold

A. You should be able to have the quality of plating you require produced in Australia. There are a number of high quality platers who can offer the finish you require. From your description you seem to be describing hard gold plating over a bronze undercoat.

Regards

A. It seems to me that you are using soft gold. This, I suspect, is very pure, so it will be both economically and technically beneficial if you use a hard gold bath. This will increase the wear resistance of the gold overlay and reduce the amount of expensive gold being deposited.

Alternatively you can coat the chain in titanium nitride; this is a very hard (typical 2,200Hv) gold coloured material that also offers good wear resistance. It is deposited by vacuum techniques. The titanium nitride can then be plated with gold or a gold alloy to give the gold surface. The beauty of this is that the titanium nitride is a (relatively) rough deposit and the gold infills the valleys in the TiN. As the gold wears off the outer surface, the peaks of the TiN are exposed, thereby reducing the wear rate and maintaining the gold colouration.

One other thing strikes me; as you are probably aware, it is now illegal to sell jewellery in Europe that allows the release of nickel at a rate of more than 0.5ug/cm2/week. This is because of the high rates of dermatitis and allergic sensitivity caused by nickel. By using nickel you are limiting your potential export market by a population factor greater than the USA. Europe is not the only economic zone concerned about nickel allergies, so it is probable that this nickel ban will become more widespread. Why not bite the bullet now and seek an alternative to nickel as a barrier layer? One recommended layer is copper-tin (bronze) and by choosing the right alloy mix, you should be able to get a "goldish looking" layer. However, the alloy composition of copper-tin varies with the applied current density, so you may suffer from colour variation of the Cu-Sn alloy. It also has another down side; I believe the only commercial solutions are based on cyanide. If anyone knows of a non-cyanide Cu-Sn bath, I would be very interested in further details.

A. Hi cousin Aaed. Hard gold is often obtained by alloying cobalt or other metals with the gold in cyanide baths. But if you need high purity acid gold, alum and hydrazine sulphate have been reported as hardening agents but may function simply by reducing grain size according to Goldie. .

Strikes often contain a low gold concentration in order to discourage immersion deposits. Goldie reports a strike from the 1930's with sodium gold cyanide of 6.25 g/l, sodium cyanide of 50 g/l, and HCl of 450 ml/l, but cautions how dangerous this is to prepare and that it must only be done under a fume hood. We appreciate your inquiry but there are two problems with it ...

The first is that in this country virtually no plating shops formulate their own gold plating solutions; we almost always buy from specialized suppliers who have determined over the course of decades of development what the most effective formulations are. These formulations are held as proprietary trade secret, and people cannot reveal them. If someone unknown to us were to answer you, we would not know whether their knowledge was legitimate or they are a disgruntled employee posting with a fictitious name and revealing trade secrets. So we are limited to very general info and telling you about books and published articles containing those formulations.

The second problem is that "strike" may hold a different meaning for different readers. To some it simply means a flash, i.e., a very thin plating; to others it means a low concentration bath designed to minimize immersion deposits. That's one reason why we ask people to please post the full details of their own situation rather than abstract questions. What are you applying a hard gold strike to and why? Exactly what do you mean by a strike? Thanks!

Regards,