Bronze

Bronze is not a material, it's a family of alloys. In watchmaking this detail matters, because two cases both called «bronze» can behave completely differently over time.

The classic alloy is copper-tin. CuSn8 — 92% copper, 8% tin — is the same historically used in marine hardware and submarines: resistant to seawater corrosion, workable, but subject to intense oxidation if left without maintenance. It darkens rapidly, can turn blue-green, and with prolonged contact with sweaty skin leaves greenish marks. This is why many manufacturers mount a stainless steel caseback: not for structural reasons, but to protect the wrist from verdigris. Christopher Ward's CuSn6 behaves similarly but with a slightly more uniform patina, while Baltic's CuAl8 and the QAl9-4 used in some divers replace tin with aluminum, which forms stable, protective surface oxides. Result: darker, more controlled patina, virtually no risk of green stains.

Aluminum bronze — copper-aluminum — is the choice of manufacturers who want to minimize maintenance. Bremont uses CuAl7Si2 for the Terra Nova: hypoallergenic, harder than classic bronze, it ages slowly with golden tones and reddish hues. IWC and Tudor have turned to similar alloys for the same reasons. Omega's Bronze Gold (37.5% gold plus palladium and silver) is a special case: corrosion is almost absent, the patina is thin, and the alloy is designed to approach the aesthetic of rose gold without surrendering to the softness of that metal.

Patina is the center of the entire bronze discussion. It's not a defect or random phenomenon: it's the progressive oxidation of copper in contact with oxygen, humidity, and carbon dioxide. Each watch ages according to its own microclimate — the climate of the owner's residence, frequency of use, skin type, even diet. Two identical watches bought the same day develop different patinas after a year. Some collectors accelerate the process with domestic methods; I advise against it: the result tends to be uneven and artificial. Patina gained slowly has a depth that forced patina will never have.

A technical note that magazines overlook: sandblasted bronze patinas differently from polished bronze. The satiny surface has more exposed area, so oxidation starts earlier and advances more evenly. Polished bronze can retain its golden color longer in areas of contact with fingers, while shadowed areas darken first, creating marked contrasts. Neither one is superior: it depends on the effect you're after. PVD «bronze» applied over steel doesn't participate in any of these processes: it's a coating that mimics the color but doesn't age, and over time it wears away instead of evolving.