Phosphor bronze: the springy bronze (C521).

Tin bronze, stiffened with phosphorus

C521 — the workhorse phosphor bronze — is roughly 90% copper and 8% tin, with a small dose of phosphorus (a fraction of a percent). The phosphorus deoxidizes the melt and, more importantly for sellers, makes the finished metal springy: bend it and it snaps back.

That springiness plus excellent wear resistance is why phosphor bronze lives in springs, switch and relay contacts, bellows, wear plates, and bushings. It also solders and forms well, which is why stamping shops buy it as strip and coil.

Warm reddish-gold, often as strip

Phosphor bronze runs a warm reddish-gold — between brass's yellow and tin bronze's brown. Sellers meet it most often as strip and coil: flat rolled bands, sometimes still on the coil, in widths from a finger to a hand. Offcuts from stamping runs have clean sheared edges and sometimes progressive-die notches.

Springs and electrical contacts are the other common form: small, precise, and often stamped with the alloy number. If a strip is stamped C521 or C510, that's phosphor bronze — keep it with the bronze, not the brass.

Stamping shops, switchgear, and springs

Look to metal-stamping and spring shops for strip and coil offcuts; to electrical equipment and switchgear for contacts and springs; and to machine shops for bushings and wear plates. Bellows from instruments and regulators are phosphor bronze too.

Cymbals are a different bronze (bell bronze, much higher tin) — musical, but not what this page covers. Keep phosphor bronze strip separate from mixed bronze solids; the strip is the cleaner, more identifiable item.

What counts as clean phosphor bronze

Clean phosphor bronze is solid alloy, free of steel, plating, and plastic. Strip should be free of heavy oil — a light rolling-oil film is normal on coil stock, but dripping cutting oil is not. Keep stamped C521/C510 strip in its own pile; it's one of the easiest bronzes to identify correctly, and correct ID protects its grade.

Keep reading the brass & bronze dictionary.

Brass & Bronze Dictionary — the full hub of alloy-by-alloy guides. Bronze — the copper-plus-tin family. Brass & Bronze Alloy Numbers — C521 and the rest in plain English. How Brass & Bronze Get Graded — the four value levers.

Phosphor bronze questions, answered.