Popular U.S. Platinum Jewelry Alloys
Kieran Conlon, Johnson Matthey
The following is a brief description of the general properties and applications of the most popular Pt alloys currently in use in the U.S. for jewelry manufacturing.
95.2% Pt/4.8% Ru
Ruthenium is a grain refiner, meaning it gives the alloy a tight grain structure. In its wrought state, it has an annealed hardness of 120 Hv (Vicker's hardness rating), and a steady work hardening rate, topping out at a full hardness of approximately 220 Hv. The alloy Also has a high tensile strength. These three properties of Pt/Ru combine to give it particularly good machining and polishing characteristics. It Is therefore an excellent alloy for use In manufacturing rings from Pt/Ru tubing. However, by comparison to other available alloys, it is not the most appropriate alloy for casting due to metal-mold incompatibility, producing surface toughness. Fine section Items are also difficult to fill due to Ruthenium's affinity for oxygen, resulting in porosity.
90% Pt/10% Ir
Ten percent Iridium Is a medium hard alloy (110 Hv - annealed state) with fairly low ductility, and is thus workable for most manufacturing processes. Traditionally the most popular casting alloy in the U.S., 10% Ir does not form an oxide film when molten (unlike Pt/Ru), thereby allowing for fine detail pieces to be quite easily reproduced. Iridium has the highest melting point of all of the platinum group metals (2,443° C vs. 1,768° C for Pt). The very high temperature required to cast Pt/10% Ir can result in very slight surface erosion due to metal-mold reaction. Of all the Pt alloys, Pt/Ir is the brightest/whitest alloy available, thus making it ideal for setting gemstones and diamond. Because it is quite soft in its wrought state, 10% Ir does not exhibit particularly good mechanical properties- its low ductility makes it feel sticky and unmalleeable when machining or polishing; however its softness does make it easier to handwork than the 95% alloys.
95.2% Pt/4.8% Ir
While 5% Ir exhibits many of the same properties as its sister 10% Ir alloy, in terms of its brightness and ability to fill fine sections, the higher Pt content renders it the softest of all the Pt alloys currently in use in the U.S. (80 Hv annealed state). For manufacturers mindful of overseas hallmarking requirements, the alloy's main attraction is its minimum 95% Pt content, while from a casting standpoint its ability to reduce porosity and eliminate metal-mold reaction has contributed significantly to its growing popularity in the U.S.
95.2% Pt/4.8% Co
Long the Pt casting alloy of choice In Europe, 5% cobalt has recently become very popular with U.S. casters. Like 5% Ru it has good mechanical properties with very similar hardness and tensile strength characteristics, but unlike 5% Ru, cobalt suppresses oxygen in the melt, thereby eliminating metal-mold reaction while improving fluidity and castability, and giving a good final hardness (200 Hv). Consequently, it is an excellent alloy for use in intricate, hard castings. A very slight gray-blue color may appear on the metal surface during casting. This can be removed during subsequent joining operations by coating the piece In Boric Acid and then raising it to an orange heat. As this alloy is slightly magnetic, extra care needs to be exercised when working with it; i.e. a magnet cannot be used to separate Pt/Co fillings from broken saw blades.
Hard Spring Pt
JM has recently developed a 96% Pt "Hard Spring" alloy for use in springs, clasps and catches. It has a degree of work hardening that suits applications where rigidity, strength, and spring are required. The formability of the alloy in its softened state allows it to be used in designs requiring good workability, and if the material is correctly annealed, in cold water, considerable softening can be achieved. This will allow the alloy to be reduced tip to 60% before it requires reannealing. A fully work-hardened hardness of 360 Hv can be achieved, which is a suitable hardness for findings production.
Editor's Note: This article is reprinted with permission from Platinum Times, April 1998, Johnson Matthey, Precious Metals Marketing Division, 608 Fifth Avenue, New York, NY 10020, (212) 245-6790
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