Phosphor Bronze
Phosphor Bronze
A copper-tin-phosphorus alloy valued for spring temper in jewellery findings
Phosphor bronze is a copper-tin-phosphorus alloy used in jewellery principally for findings — clasps, pin stems, hinge pins, and the small mechanical components that require spring temper, fatigue resistance, and corrosion resistance. The most common composition for jewellery applications is approximately 90 per cent copper, 9 per cent tin, and around 1 per cent phosphorus, although the family of phosphor bronzes runs across a range of tin contents from about 0.5 to 11 per cent. The phosphorus addition during melting deoxidises the bronze and, more importantly for the application, increases the alloy's hardness, elasticity, and fatigue strength.
Properties
Phosphor bronze takes a much higher temper than tin bronze of comparable composition without phosphorus, allowing the production of fine wire and sheet in spring tempers that recover their shape after repeated flexing. The alloy resists fatigue cracking and corrosion in normal jewellery service environments, and its mechanical reliability over years of repeated opening and closing is the principal reason it has displaced softer alloys for high-quality findings.
Colour is a warm reddish-brown to medium brown, distinguishing phosphor bronze visually from yellow brass and from the cooler-toned nickel silver. The alloy patinas over time to a darker brown surface, which can be polished back to brightness or allowed to mellow.
Use in findings
Phosphor bronze is used principally for the springs and mechanical components of jewellery clasps — box clasps, fold-over clasps, lobster claws, and the spring mechanisms in many bracelet catches — where the alloy's recovery from repeated flexing is essential. It is also used for the pin stems of brooches, where a stem that holds spring temper without snapping is the difference between a serviceable brooch and a frustrating one. Hinge pins and small pivot components in articulated jewellery are routinely phosphor bronze.
The alloy is also called gunmetal in some contexts, a name carrying over from its historical use in firearms components.
In the trade
Buyers and dealers will not normally encounter phosphor bronze as a feature material in jewellery — it is too pedestrian a metal for that — but they will encounter it constantly in the findings of any well-made piece. Recognising phosphor bronze components is part of the basic vocabulary of jewellery condition assessment: a brooch with a phosphor bronze pin stem holds its temper indefinitely, while a similar piece with a soft yellow-metal stem fatigues and bends.
Repair of phosphor bronze components requires bronze brazing rather than the soldering techniques used for the precious-metal portions of a piece, and is normally a job for a specialist findings restorer rather than a general jewellery bench.