At Heraeus, all the platinum melting and casting is done in a vacuum furnace. Our minimum melt size for jewelry alloys is 200 troy ounces, and this is cast into an ingot using the Durville Process. The Durville Process is a process where the mold is attached to the top of an induction furnace and the entire assembly is rotated, transferring the molten metal from the crucible to the mold with minimum turbulence. This process is carried out in a vacuum chamber. The platinum is then re-checked for purity. This is called assey. After assay, the cast bars are heated to 1200°C and hot rolled for 50% reduction. This process produces better quality sheet and wire. The bars are then machined to remove any surface contamination from the hot-rolling process. Finally, the machined bars are cold rolled and annealed. Annealing is the process of softening metal by realigning the molecular structure through heat. Cold working hardens metals, in some applications it is frequently necessary to soften the material by annealing to allow further processes to be carried out (e.g., deep drawing, wire drawing, etc.)

From research I did in the early 1970s, it was found that the amount of cold work will affect the temperature at which a metal re-crystallizes or softens. I chose 70% cold work for this paper because it represents the point where no significant increase in hardness takes place on further reduction. The effect of time at temperature on hardness has also been well researched.  Examples of this effect are shown when iron is annealed at 675°C. At this temperature, iron requires only eight minutes to attain a constant grain size, but at 550°C it requires 32 hours. I chose 30 minutes as an acceptable manufacturing norm.

In series of tests the hardness of different samples was determined at a variety of temperatures at 100°C intervals. For detailed information consult the original technical document. The Test results show the hardness falling rapidly after 800°C, and at 100°C there is little further reduction in hardness. It can be concluded that the optimum temperature is different for each alloy. In the 5% alloys, 1000°C is sufficient to fully anneal the metal. It is interesting to note that all the annealing was carried out in air and that the 5% copper alloy turned black at about 800°C, leaving an oxide that was difficult to remove. The hardening effect and the subsequent annealing is further examined in detail in the original paper.

In comparing the hardness results from the platinum alloys to karat gold there appears to be little difference and conclusions may be drawn that the two could be readily exchanged in design criteria. But this is not in fact the case, as the platinum alloys have a much higher elastic limit. Questions regarding the color of the different platinum alloys have been asked many times; I cannot see any startling difference in any of the alloys tested. Platinum is relatively new to the American mass market and in the production of platinum pieces. The unique properties of this metal should be taken into consideration and jewelry should be designed specifically to maximize these features. Lighter pieces can be produced, which could be more attractive to the market than merely using a design that has been successful in karat gold.

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Platinum Fabrication
Malcolm Warren
Heraeus

This is an abbreviated version of the original work. For full technical details, please consult the original paper.