Bronze is an alloy made mostly of copper, usually with about 12 percent tin. Watchmakers use it for cases. It matters because it ages: the copper oxidises and builds up a patina. Aluminium bronze also resists seawater corrosion, which suits dive watches.
Why does a bronze watch case age?
The copper in the case oxidises. As it does, it builds up a patina on the case. Cases with more copper in the alloy show this ageing more. The change stays mostly on the outside. The copper oxide layer that forms shields the metal beneath it, so bronze usually corrodes only at the surface. The metal below stays protected.
What types of bronze are used in watches?
Tin bronze is mostly copper with tin added. When a small amount of phosphorus goes into the mix, it is sometimes called phosphor bronze. Aluminium bronze contains 5 to 12 percent aluminium. It is stronger than tin bronze. It is also a hard alloy that resists wear, and in industry it is used for bearings. It stands up to seawater corrosion and is described as anti-magnetic. These two traits make it a good fit for dive watches.
How is bronze different from brass?
Bronze and brass differ in the main metal added. Bronze takes tin as its main addition. Brass takes zinc. Names do not always follow this rule. Some alloys called ‘commercial’ bronzes are really brasses, because zinc, not tin, is their main addition. So the name alone does not tell you which metal was added.
Where it comes from
English borrowed the word ‘bronze’ from French in the 18th century. The French word is recorded from 1511. So the word was in French writing for well over a century before English took it up. In watchmaking, the designer Gérald Genta came up with the idea of a bronze watch case. Panerai has used bronze in its watches since 2011.
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