Nanoparticles used to darken the patina of antic black bronze: TEM observations on new experimentations

Some studies that started in the nineties [1] proved that some antic artefacts or parts were intentionally patinated using a chemical treatment. In particular one type of patina was characterized on artefacts coming either from Egyptian civilization, Mycenaean period or Roman Empire. This patina has been identified as a precious material mentioned in ancient Egyptian and Roman texts. This particular material is named, depending on the artefacts’ origin, black bronze (hmty km), or corinthium aes. This patina is made on copper alloys containing gold and/or silver and is composed mainly of cuprite. It was compared with a Japanese patina which appears in medieval times but which is still in use and known under the name of shakudo. Shakudo are copper gold alloys and they are treated chemically by means of different recipes named nikomi-chakushoku which developed a black layer of cuprite on the surface.
An important study of antic artefacts coming from the collections of French museums has been carried out since the beginning of the 2000’s. An experimental protocol based on non-invasive analysis was developed to analyse these very precious objects and to identify and characterize this type of patina in function of the provenance and age of the artefacts [2, 3].
However, some questions about this particular surface layers could not be resolved using this experimental protocol due to the limitation of non-invasive analytical techniques: In particular the question of the formation of the oxide layer, the colouring mechanism of this black cuprite (copper oxide which is red under natural form) and the important adherence properties.
We developed a program of experimentation to make some black patina. These experimentations were based on the utilisation of the Japanese recipes, and we tried to differentiate the effect of the alloying element (Au, Ag, As) and the effects of the chemical treatment.
These patinas were studied by means of various analytical methods and in particular we used TEM to characterize the fine structure of the oxide layer. The use of transmission electron microscopy (TEM) allows to evidence a hypothesis already mentioned in previous studies [4]: the presence of nanoparticles of gold in the cuprite layers and their role on the coloration of the patina making the black bronze the very first applications of nanoparticles in technical history.