"The life size statue of Piupi in the Cairo Museum belongs to this dynasty ; and with it there is also one of his son.
As the authorities decided not to clean the statue, the surface remains crusted with a thick coating of oxy-chloride and carbonate of copper"
This statue is either bronze or copper.
"Whilst the bronze was in good condition, metallic, and only slightly coated with a green crust [...]
Antique metals containing copper, however, are often rather more difficult to etch [...] These difficulties are best overcome by removing as much as possible of the oxidised crust from the surfaces of the specimen not required to be etched, or by covering them with a layer of wax."
Ancient Egyptian Metallurgyhttps://archive.org/stream/ancientegyptianm00garl/ancientegy...This is on a Roman lead plaque:
"The piece offered here has a light brown patina with a thin oxidized crust over the outer surface"
http://www.trocadero.com/stores/apoloniaancientart/catalog/q...