A green or bluish patina formed on copper, brass, or bronze surfaces exposed to the atmosphere for long periods of time , consisting principally of basic copper sulfate.
Verdigris is the lesser-known cousin of rust. When iron oxidizes, you get a reddish-brown crust called rust, as I’m sure you know. When copper or its alloys oxidize, you get a bluish-green coating. You may have seen this happen to old pennies. Many ancient artifacts and weapons are coated in verdigris.
The origin of the word is unclear. It appears to have come from the Old French vert-de-gris; vert, of course coming from the Latin verde (“green”). De gris means “of Greece.” So for some reason this was literally called “green of Greece.”
That a fight had taken place here earlier in the season was evident. Clips and empty cartridges, tarnished with verdigris, lay on the ground, which, while wet, had been torn up by the hooves of horses. Hard by the kitchen garden were graves, tagged and numbered. From the oak tree by the kitchen door, in tattered, weather-beaten garments, hung the bodies of two men.
– Jack London, “War” (1911)