1911 Encyclopædia Britannica/Verdigris
VERDIGRIS, a pigment, consisting of basic copper carbonates, made by acting upon copper plates with pyroligneous acid soaked up in cloths, exposing the plates to air, then dipping in water, and finally scraping off the greenish cruet; the plate is re-exposed and the operation repeated till it is used up. Another method consists in exposing thin copper sheets to the acid vapours rising from the residues or "marcs" of wine factories, the product being scraped off, and the plate re-exposed. Both processes require several weeks. The pigment appears with several shades of blue and green; blue verdigris is chiefly CuO-Cu(C2H3O2)2-6H2O, while light blue and green verdigris contain 2CuO-Cu(C2H3O2)2-2H2O. Besides being used as a paint it is employed in dyeing and calico-printing, and also in the manufacture of other paints, e.g. Schweinfurt green, which is a double salt of the acetate and arsenite. A liniment or ointment is also used in medicine as a cure for warts. It is an irritant poison (hence the need that acid substances should never be cooked in copper utensils); the best antidote is white of egg and milk.