Bronze Gold is a patented alloy developed by Omega, classifiable as 9K gold for its 37.5% gold content, with copper around 50%, silver, gallium and palladium in minor percentages that the Maison keeps confidential. The distinction from traditional bronze is substantial: the latter alloys copper and tin, oxidizes to dark or greenish tones, and carries no gold hallmark. Bronze Gold bears the legal hallmark of gold and, thanks to palladium, inhibits the formation of verdigris.
The color sits tonally between Omega's Sedna Gold, which leans toward rosy-red, and Moonshine Gold, which is pale yellow. The result is a warm bronze with a rosy wash, noticeably more understated than yellow 18K gold but chromatically richer than new traditional bronze. The patina that forms over time is brown and uniform, not aggressive: this distinguishes it sharply from the verdigris of classic bronze.
The presence of gallium is technically interesting: it melts around 30°C and is used in metallurgy to lower melting points and improve the workability of alloys. Palladium belongs to the platinum group and contributes to the chemical stability of the surface. The absence of nickel and iron is confirmed and makes the alloy compatible with the skin of those with sensitivities to ferrous metals or nickel.
The 9K fineness allows legal hallmarks to be applied to cases and casebacks, something impossible with ordinary bronze. On the caseback of the Seamaster variant appears the engraving BG 859: BG stands for Bronze Gold, while 859 recalls the historical nomenclature of the case shape used in vintage technical sheets CK 859 and OT 859, a precise archival reference.
Blancpain has adopted a similar denomination for some editions of the Fifty Fathoms, but the chemical formulation is different: Bronze Gold as a generic term can indicate any hybrid bronze-gold alloy, while Omega's is specific for composition and patent. I do not have access to Blancpain technical sheets for a precise comparison of elements.