Verdigris is a crude acetate of copper, employed in the arts as a pigment; see Painting. It is usually obtained by moistening the surfaces of copper plates with vinegar, and exposing them to the action of the atmosphere; a bluish green rust, or fine salt, thereby forms upon the surface, which is verdigris. According to Mr. Phillips, the constituents of English and French verdigris are as follow:-
The French verdigris has been usually considered the best, but the English of late years has been so much improved in the manufacture, as to be rendered equal to the foreign in the opinion of many. In a manufactory established at Deptford, about twenty years ago, the process which we saw in operation (and which we believe is continued without any essential variation) was as follows: Thin plates of copper, of which there were an immense number, about a foot square each, were folded up in coarse woollen cloths, saturated with pyroligneous acid, (distilled on the premises;) a dozen or more such plates, with the moist cloths between them, forming one pile, were placed to the number of several thousands upon stout wooden racks, built up in an extensive cellar, through which the air had free access; but the underground situation having the effect of preserving the air in a moist state, which we understood was favourable to the process. The plates, after remaining a few days in this state, were taken down, the cloths unfolded, and the green saline matter upon the surfaces of the plates was scraped off by instruments calculated not to remove any portion of the metal; the plates were afterwards folded anew, in the moistened acid cloths, and the process was thus continually repeated, until the copper plates were by imperceptibly slow degrees worn away.
The quality of the verdigris thus produced was in great estimation. The manufacture was conducted under a patent-right, which has now expired.