Platinum as a Metallic Material
Melting of Platinum and Alloys
Pure platinum is manufactured by melting chemically precipitated sponge or powder in an induction furnace. Because of the high oxidation resistance of platinum, it is advantageous to conduct the melting process under the ambient air atmosphere, as the less noble impurities oxidize and are removed from the platinum in the form of a slag. Despite the high melting point of platinum, quartz or fused silica crucibles can be used. During the melting process, process recycle materials (e.g. sheet offcuts from the manufacture of semifinished products) can be added to the freshly refined and precipitated metal. Before casting the ingot, it is usual to cast a sample for chemical analysis.
Alloys of platinum with gold can be made by an analogous method as they have a somewhat lower melting temperature than platinum and are similarly resistant to oxidation.
Alloys of platinum with rhodium or iridium tend to absorb oxygen from the air during melting which can lead to the formation of pores in the cast ingot. For this reason they are normally melted under a protective atmosphere (nitrogen or argon) or vacuum. Furthermore, they have a higher melting temperature than platinum. Under these conditions, it is prudent not to use quartz crucibles because the material firstly become too soft and secondly can be reduced to silicon in contact with the alloy. Here, it is usual to use zirconia crucibles.
The melt is normally cast to ingots in copper molds, with or without water cooling.