Pt-Ir Alloy
Pt-Ir Alloy
Platinum-iridium for hard, springy prong and structural work
Pt-Ir alloy is a platinum-iridium formulation that combines platinum's nobility, white colour, and tarnish resistance with iridium's hardness and springiness, producing one of the workshop standard alloys for prongs, bezels, and other structural elements in fine jewellery. Iridium content typically ranges from 5 to 10 per cent, with Pt900Ir (90% platinum, 10% iridium) and Pt950Ir (95% platinum, 5% iridium) the two most common workshop grades.
Properties
Iridium is the densest of the platinum-group metals and one of the hardest pure metals known. Adding even a few per cent to platinum produces a marked increase in hardness and springiness over the pure metal, both as-cast and after work hardening. The alloy retains platinum's full white colour, its corrosion resistance, and its hypoallergenic character. Specific gravity is slightly higher than other Pt950 alloys due to iridium's exceptional density. Hardness in the as-cast state is significantly higher than Pt-Co or Pt-Pd, and Pt-Ir work-hardens further with bench finishing.
In the workshop
The trade-off for Pt-Ir's mechanical advantages is that the alloy is more difficult to work, weld, and solder than softer platinum alloys. Casting flow is poorer than Pt-Co, so Pt-Ir is more often fabricated from wire and sheet stock than cast in complex moulds. Soldering temperatures are higher, and the work-hardened material can be brittle if not properly annealed between forming operations. Setters who switch from softer alloys to Pt-Ir prongs find the metal less forgiving but more secure once set.
In the trade
Pt-Ir is the conventional choice for prong wire on fine jewellery, particularly for high-value diamond and coloured-stone work where the security of the setting is paramount. The springiness of the alloy means prongs hold their shape under wear and are less prone to splaying or deformation than softer platinum prongs. Pt900Ir is widely accepted in international markets, including the United Kingdom and most of Europe, where it meets the platinum hallmarking standard. For pieces marketed under the Pt950 standard, Pt950Ir with 5% iridium is the equivalent grade.