A Better Shade of Black: Effects of Manufacturing Parameters on the Development of Ancient Black Bronzes*

Copper alloys with controlled contents of Sn, Ag and Au were treated with methods and solutions that mimic those used in ancient and traditional procedures for the production of black bronze. Examination of the resulting patinas with colorimetry, SEM–EDS, TEM, Raman spectroscopy and XRD revealed no systematic differences according to the fabrication method, whereas patina homogeneity and colour in those alloys that did not contain precious metal were affected by superficial work. Patina microstructure determines adherence and coverage, and depends upon the solution used irrespective of the alloy composition, but the colour of the patina is mainly related to the alloy composition. Gold promotes the development of cuprite, slowing the oxidation to tenorite in the corrosion process. Gold additions produce a more uniform patina as well as a desirable blue–black tone, which is likely to have been a factor in alloy selection.