In North America, five platinum alloys are primarily used in jewelry manufacturing:
Pt950 / Ruthenium (Pt950/Ru)
Pt900 / Iridium (Pt900/Ir)
Pt950 / Cobalt (Pt950/Co)
Pt950 / Iridium (Pt950/Ir)
Pt950 / Heat treatable.

A PLATINUM FOR ALL SEASONS
Embraced by master jewelers, Pt950/Ru is a universal alloy and used for all methods of manufacturing. When made into tubing, it creates a very dense molecular metal, exhibiting great hardness, which is preferable for wedding bands since they are meant to last a lifetime. As sheet and wire stock, Pt950/Ru is used for hand fabrication and a viable choice for three-stone platinum rings.

Pt950/Ru has an annealed hardness of 130 HV and can work-harden to 190 to 210 HV, creating beautiful castings. Featuring good white colour, this alloy is 100 per cent precious. Given its hardness and ease of fabrication, it is popular with jewelers. While casting is a little more difficult and fine detail often requires extra effort to fill, Pt950/Ru can be cast with relative ease once casting parameters have been established. (Since parameters are different for every casting machine and environment, they must be established to be consistent.)

Pt950/Ru is the most commonly used machining alloy in Canada and the United States, and is mainly used for wedding bands, watches, and other jewelry requiring lathe or machining. It has a high ductility and therefore is also used for platinum wire- and chain-making.

Featuring a tight grain structure, Pt950/Ru produces hard castings, which are strong and hold stones securely.

A CLOSER LOOK AT PT900/IR
Although it was known as iridio-platinum in the 1920s and 30s, Pt900/Ir is the platinum alloy with which most U.S. jewelers are familiar. There was even a specific stamp used for quality marking of this alloy-IRIDPLAT. Today, it is marked Pt900 or 900Pt and is still very popular, especially in the Boston area, where most platinum jewelry made there is for domestic consumption.

Traditionally, Pt900/Ir was the alloy everyone was familiar with and few saw a reason to change to 950 alloys when they were introduced. Since it does not contain 95 per cent platinum, this alloy cannot be quality marked as PLATINUM, as the marking must reflect purity in parts per 1000. It is also rarely exported, as most countries prefer Pt950 alloys.

Pt900/Ir is a universal alloy, which means it can be used for all phases of manufacturing. With a Vickers hardness of 110 HV (annealed) and 185 HV (work-hardened), this alloy is especially suitable for fabrication.

Pt900/Ir is 100 per cent precious, since iridium is one of the PGMs. This alloy features a beautiful, natural white colour, is very malleable and ductile, and seams remain shiny when welded. When properly polished, it can take on a very high lustre. Also, once polished, it can be welded without losing its lustre, which is beneficial when making jewelry where light reflection is a concern. Pt900/Ir can be rolled down to make sheet and wire with reductions of more than 65 per cent without annealing. Doing this results in work hardening, which is an important benefit. As is the case with all platinum alloys, Pt900/Ir develops a beautiful patina over time.

CASTING WITH A COBALT ALLOY
Pt950/Co was specifically designed for casting. Cobalt acts as a grain refiner, which makes this alloy fine-grained and capable of producing fine and detailed castings. Pt950/Co is slightly magnetic and also oxidizes, which is unique for a platinum alloy. As such, many bench jewelers resist working with it.

Ninety-five per cent precious, Pt950/Co has an annealed hardness of 135 HV and can work-harden to 185 HV. This alloy has good mechanical properties and fluidity for fill of fine sections, which allows for hard and scratch-resistant castings. Pt950/Co has few or no metal-to-mould reactions, polishes easily due to its hardness, and its small grain sizes makes for dense castings. Since it oxidizes, this alloy is not suited for hand fabrication. It develops patina in time through wear and handling

FABRICATING WITH PT950/IRIDIUM
Pt950/Ir's work-hardening attributes make it is a great fabrication alloy, as it contains 50 parts iridium. This mix is enough to make it a viable alloy, but not enough so that it can used for casting. The low hardness causes castings to bend, dent, and deform in a very short time.

Pt950/Ir's low hardness of 80 HV in the annealed state is close to the hardness and wear behaviour of 22-karat gold. Hardness increases to more than 185 HV through work hardening.
As with Pt900/Ir, Pt950/Ir is 100 per cent precious, which is very malleable, ductile, and offers great white colour. These characteristics make it suitable and recommended for hand fabrication. Given its softness, Pt950/Ir does not machine well when annealed. However, when extruded to a tube, this alloy becomes very hard, which is conducive to machining wedding bands.

Although this alloy is good for welding, it is not very suitable for castings requiring no additional working steps. Given its ability to work harden rapidly, Pt950/Ir is a good choice for die striking. Beautiful patina is also achieved through wear and handling. 

PT950/HEAT TREATABLE
In general, platinum alloys do not have metal memory, which means that once a specimen is bent in position, it tends to stay and doesn't spring back. This can be a problem for clasps, Omega clips, tension rings, earring nuts, and other similar items.

In the early 1970s, one specialty chemicals company addressed that issue, developing heat-treatable platinum alloys that could hold springiness and be used for findings or other applications requiring this characteristic.

In the 1990s, a smelting and refining company produced a very fine heat-treatable alloy and today, most findings needing spring action are made from this material. While the exact content of these alloys is proprietary knowledge, this material can be easily purchased in all forms of mill products ranging from sheet to wire to casting grain. It is important to keep in mind this alloy has a much lower melting point than others, which can be an issue when one is unaware of it.

The hardness of these specialty alloys can range from 160 to 330 HV. Pt950/heat treatable alloys are 95 per cent precious, hard, and ductile. Exhibiting good mechanical properties, these materials are specialty casting alloys, which can be used for findings and other applications where extra hardness is required. When heat-treated, great hardness can be obtained, making these alloys suitable for tension settings. Their melting range varies according to alloy combination. 

The above is an excerpt from "Platinum--It's not difficult, just different" by Jurgen J. Maerz. Reprinted with permission from Jewellery Business, Canada's jewellery magazine, August 2008.