In Classical Deception (page 54) Wayne Sayles mentions the excellent patinas on Sicilian fake coins starting in the 1990s. After his book was published I noticed copies of the coin Sayles used as an example appearing in the printed catalogs of several dealers.
I was wondering recently if hand-held XRF guns could be used to identify real and false patinas and a web search lead me to a five-year-old post on FORVM by Lloyd Taylor referencing a 2009 paper in Mediterranean Archaeology and Archaeometry. The paper is “Ancient Coins and their Modern Fakes: an Attempt of Physico-Chemical Unmasking” by A. M. Mezzasalma, G. Mondio, T. Serafino, G. De Fulvio, M. Romeo, and A. Salici.
They analyze five fake bronze coins. This is the recipe the forger confessed to:
“…one of the most used recipes for fakes artificial ageing utilizes a solution composed by penthydrated CuSO4 [Copper Sulfate] together with HNO3 [Nitric Acid] and NH4Cl [Ammonium Chloride] in distilled H2O [Water]. In this solution, warmed up until 80‐90°C, the just produced fakes (at about 100 °C) are immersed. Such a bath gives them an artificial patina with an “antique green” colour and a deceptive antiquity appearance. Drops of the above CuSO4 solution on clean bronze surfaces may form dendritic structures with the same characteristics of those observed in our fakes.”
Note: the fake BR02, which the authors couldn't identify, is a Himera gorgoneion / pellets.