Aluminum

Aluminum in watchmaking is not a single material: it's a family of solutions that differ by alloy, treatment, and function. Confusing them leads to wrong assessments, both at the bench and in the field.

The most common case is anodized aluminum in bezel inserts. Anodization is an electrochemical process that oxidizes the metal's surface, creating a hard, colorable, corrosion-resistant layer. We speak of standard anodization for most colored inserts — from black to blue to green — and oxalic anodization when particular shades are desired, like the hematite red Omega used on the Speedmaster X-33 Marstimer to evoke Martian dust. The limitation of anodized aluminum compared to ceramic is scratch resistance: over time the insert deteriorates, the dial fades. It's not a design flaw, it's the nature of the material. Rolex abandoned aluminum on the Submariner precisely for this reason, switching to Cerachrom. Other brands maintain it for aesthetic reasons: the matte surface of aluminum has a different character from glossy ceramic, and for certain vintage watch collectors it's preferable.

On the front of structural alloys, the distance between a bezel insert and an aerospace-grade aluminum case is considerable. F.P. Journe used aerospace-grade aluminum for the Centigraphe Sport, bringing the watch weight below 55 grams including the strap. Zenith developed Aeronith for the Defy Inventor bezel: it's aluminum foam filled with polymer, with a weight-to-strength ratio unprecedented in watchmaking. Texalium, instead, is a fiberglass structure with an aluminum surface: it originates in aeronautics, has a sparkling finish different from carbon fiber, and its working doesn't require the extreme pressures of carbon itself.

Another application is copper-aluminum alloy, which shouldn't be confused with phosphor bronze. Copper-aluminum alloy is harder, more corrosion-resistant, and develops a patina more slowly than classical bronze. IWC and Tudor use it in cases precisely for this reason.

Finally, a technical note that many overlook: synthetic sapphire is aluminum oxide (corundum Al₂O₃), Mohs hardness 9. Hublot's SAXEM composite material is built precisely on this base, with rare earth additions to obtain transparent colorations. The chemical bond between raw aluminum and the sapphire crystal covering the dial is more direct than it seems.

From the bench I can say that anodized aluminum ages differently from case to case: a bezel insert from the Sixties with still-vivid colors is worth more than a refinished one. Learning to recognize original anodization from refinished work is a skill acquired only with time and many pieces in hand.