Blue gold is neither a coating nor a paint: it is a metal alloy in which gold — at 21 or 24 carats depending on the source — is combined with iron elements and then fired in an oven at controlled temperature. The thermal reaction produces an oxidation layer that shifts toward blue, with shades ranging from petroleum blue to blue-grey to cognac, according to the exact temperature, firing duration, and oven atmosphere. No two pieces emerge identical: variability is structural, not a defect.
The term BlueGold is associated almost exclusively with Andersen Genève, which developed it as a distinctive element of its production. Svend Andersen uses it on dials and rotors, often pairing it with hand-executed guilloché work — wave patterns, grain d'orge, magic lozenge, scales — that require different machines depending on the pattern. The technique of guilloché on blue gold is particularly challenging: the material must be engraved before or after the heat treatment according to a precise sequence, because heat following turning can alter the crests of the pattern.
From a metallurgical standpoint, the color is not superficial. Unlike PVD or other galvanic treatments, the oxidation penetrates the material and does not flake off over time like a deposited film. This is the technical fact that Andersen Genève uses to distinguish its blue gold from any coating-based imitation. That said, I do not have access to independent spectroscopic analysis that confirms the exact thickness of the oxidized layer, so I limit myself to reporting what the maison declares and what is consistent with the metallurgy of ferriferous oxides on gold.
The material appears on automatic winding rotors — visible from the transparent caseback — and on peripheral rings engraved with edition numbers, as well as on entire dials as in the Communication 45, where the alloy in cognac tone hosts a world map in guilloché filled with liquid gold lacquer. Each application leverages the same logic: color as a direct consequence of a metallurgical process, not as a decorative effect applied afterwards.