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Alloy Development; The development of the optimum alloy can be a lengthy and time consuming task. In formulating a new alloy it is important to keep in mind the needs and the skills of the bench trained jeweller. As the majority of craftsman jewellers in the world are small workshop operators. My research is initially carried out and tested in and for these conditions. The steps we take to develop a new alloy are;
The assessment involving testing for any particular aspects of jewellery fabrication, hand working, casting and any methods of machine working by which the new alloy is to be processed. Once proven it then goes into trials in one of Australia's largest custom casting and fabricating factories, ultimately going into production with several large scale precious metal suppliers throughout the world. As an example of the evolution of an alloy, I use here the master alloy for our lower carat rated yellow gold alloys. This master alloy is used for gold alloys with carat ratings up to and including 14 ct.
In 1964 I began my business and needed to make casting alloys. I asked around my trade associates for a formula with which to start. I was given a formula for a 9ct yellow gold alloy. Realising it needed a little modification, I adjusted the silver, copper and zinc until it looked fairly evenly balanced. I had a place to start. The 9ct which alloy I made, although possibly a good metal for wrought product, gave me problems with porosity when cast. A friend who was neither jeweller nor metallurgist suggested the use of silicon to 'aid the fluidity', as he had observed the use of silicon in brazing rods. Not being hampered with the knowledge that silicon and gold are not readily compatible, I set out on a course of discovery which has led me to seek further improvements in precious metal alloys. The first silicon-containing alloy I made included 5% silicon in the form of a 1 0% Si / Cu alloy, which was added to a master alloy of Cu, Ag and Zn. This appeared to have some beneficial results in that the castings were bright, smooth and cleaner looking, and did not bear the oxide coating the non- silicon bearing alloys had always produced. The problem was that the castings virtually crumbled in my hand. Nonetheless I was satisfied that silicon was a beneficial additive, certainly worthy of further research. It only remained to establish the optimum quantity. As a result I lowered the silicon content 50% and found this showed a marked improvement. I continued to halve the amount of silicon addition and eventually established 0.32% silicon content as a workable alloy. The metal no longer cracked and now produced bright porosity free castings. Further research proved, due to the de-oxidising properties of silicon, zinc oxide is not able to form, surface tension is increased and the resultant alloy when cast is free of the porosity previously caused by inclusions of zinc oxide. Our customers, happy to be rid of the problems of porosity they had previously endured, embraced the new alloy with enthusiasm. Over a period of 2 months, further trials established a lower parameter for silicon. There were some slight modifications to the formula in the following 3-4 years, but the current formula has been in use for 25 years'. I was so pleased with the discovery of the effect of silicon on gold alloys I immediately tried it in 18ct yellow gold. The initial result was, the rings fell off the tree, and crumbled in my hand. I tried lesser and lesser amounts of silicon until I was convinced the amount of silicon 18ct gold could tolerate was so insignificant as to be of little, if any benefit. Although others have found some benefit with silicon in the higher carat rated alloys, my opinion has not changed. When I espoused the use of silicon in gold alloys to the Worshipful Company of Goldsmiths in 1968, it was received with astonishment. I gave them an alloy formula, which was trialed in the trade by the two largest precious metal suppliers in that country at the time. By the time of my next visit three years later, silicon containing gold alloys, especially in lower carat ratings, had become a commercial reality. I'm pleased to say silicon is now a standard alloying constituent in the lower carat rated gold alloys and accepted worldwide. |
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