Brass

Brass is copper alloyed with zinc, and the proportion changes it completely. Below about a third zinc it stays a single phase — ductile, easily drawn and pressed, the brass of cartridge cases and instrument tubing. Above that a second phase appears which is harder and more brittle, better for casting and hot forging and worse for cold work.

Its most useful trick is machinability. Adding a little lead gives free-cutting brass, which cuts faster and cleaner than almost any other metal and is why plumbing fittings, valve bodies and terminals have been made from it for a century. That lead is now the problem: drinking-water regulations have pushed the industry toward bismuth and silicon substitutes.

Brass is older than the ability to make it deliberately. Roman brass was produced by cementation — heating copper with zinc ore and charcoal in a sealed crucible, so zinc vapour was absorbed without anyone isolating the metal — and zinc was not recognised as an element in Europe until the eighteenth century.

That matters for interpreting objects. An early brass artefact is evidence of a process, not of a known metal, and the zinc content of Roman brass clusters near the limit cementation could reach rather than at a chosen composition.

Plumbing and valve bodies, electrical terminals, locks, gears and fasteners — anywhere a part must be machined accurately in quantity and resist corrosion in air.

Musical instruments are the visible use and the reason the orchestral section is named after it. Brass is also used for decorative work, where it polishes to a gold-like finish, and for non-sparking tools in explosive atmospheres. It performs poorly in seawater, where dezincification leaches the zinc and leaves a porous copper skeleton — which is why marine fittings are bronze.

Brass is made by melting copper and adding zinc, and the difficulty is that zinc boils at 907 degrees while copper melts at 1085. Added carelessly, the zinc simply leaves as white fumes. It is introduced below the surface of the melt, or as a copper-zinc master alloy that dissolves without ever being pure zinc at temperature.

What happens next depends on the composition. Single-phase brass is cold-worked — drawn into tube, pressed into sheet, deep-drawn into cartridge cases — with intermediate annealing, because it work-hardens quickly. Two-phase brass is hot-forged or cast instead, since cold work cracks it.