Pt-Co Alloy
Pt-Co Alloy
Platinum-cobalt alloy for high-detail casting work
Pt-Co alloy is a platinum-cobalt formulation used principally in jewellery casting, where its excellent fluidity at casting temperature and its ability to reproduce fine detail with low porosity make it a favourite of investment-cast workshops producing intricate designs. Cobalt content is typically in the range of 3 to 5 per cent; the balance is platinum, most commonly to a Pt950 fineness specification under the major international hallmarking standards.
Properties
The cobalt addition lowers the melting range modestly relative to pure platinum and improves castability, particularly for thin sections and complex filigree where coarser-flowing alloys produce porosity or incomplete fills. The alloy hardens through a combination of solid-solution hardening and a measure of precipitation hardening on heat treatment, giving good as-cast hardness without extensive cold work. Specific gravity remains close to that of pure platinum at approximately 21 g/cm3; colour is the characteristic platinum white, indistinguishable from other Pt950 alloys to the eye.
The principal idiosyncrasy of Pt-Co is that the alloy is weakly ferromagnetic at the cobalt levels used in jewellery, owing to cobalt's intrinsic magnetic character. The effect is slight but detectable with a small magnet and is occasionally noticed by jewellers using magnetic tools and by clients with sensitive medical or industrial equipment. The other consideration is the documented but uncommon possibility of cobalt skin sensitivity in a small minority of wearers.
In the workshop
Pt-Co is favoured for investment casting of Pt950 work where flow into intricate moulds is critical: cast filigree, openwork, micropavé settings with multiple small seats, and complex multi-element designs that would otherwise require extensive cleanup. The alloy is less common than Pt-Ir or Pt-Ru in standard prong-set jewellery production, partly because of the magnetic property and partly because Pt-Ir and Pt-Ru offer superior hardness for setting work that must hold a stone securely against years of wear. Refiners and casting houses publish guidance on Pt-Co handling, and most workshops that use it source the alloy in pre-mixed casting grain rather than alloying in-house.
In the trade
Pt-Co alloys are most often encountered in cast filigree, openwork, and detailed pavé settings where the casting demands push the limits of what coarser-flowing alloys can produce. For high-wear prong work, Pt-Ir and Pt-Ru are usually preferred for their superior hardness and lack of magnetic character. Many workshops mix-and-match: Pt-Co for the cast body of a piece, Pt-Ir or Pt-Ru wire for fabricated prongs added afterwards.