How bronze is made

Bronze is copper hardened with about one-eighth tin, and the tin had to be traded between regions.

stations: 5
things upstream: 11
raw sources: 6
steps from the ground: 3

The short version

Bronze is copper alloyed with tin, typically about 88 percent copper and 12 percent tin. The copper is melted, the tin stirred in, and the molten alloy poured into a mold to make statues, bells, bearings or coins. Softer, lower-tin ingots can also be hammered into sheet.

Melt the copper

The copper goes into the crucible first. Copper-based alloys melt at lower temperatures than steel or iron, at about 950 °C for bronze, so a foundry can make them without a blast furnace.

Goes in
Copper
Stir in the tin

Tin is added to the melt, usually around 10 to 12 percent. It is much rarer than copper, and the two are seldom found together, so serious bronze work has always meant trade between regions.

Goes in
Tin
Pour into a mold

The molten bronze is poured into a mold. Tin bronze beat the earlier arsenic copper because its mix could be controlled more easily, and the result was stronger and easier to cast.
Hammer or roll sheet

For sheet, a softer bronze with only about 6 percent tin is cast into ingots and hammered flat. Bronze Age helmets and armor were made this way, while blades were cast from the harder bronze with about 10 percent tin.
Machine to size

Bronze parts are finished by turning and grinding. They are tough and have low friction against other metals, which is why they are used for bearings, bushings, springs and electrical connectors.

End of the lineBronze Next it goes into drum kit, guitar string, bronze statue.

Worth repeating

The earliest known tin-copper artifact dates to about 4650 BCE, from a site at Pločnik in Serbia.
The Bronze Age gave way to the Iron Age after population migrations around 1200 to 1100 BCE disrupted the tin trade.
Bronze corrodes only on the surface, because a layer of copper oxide forms and then protects the metal underneath.