VERDIGRISE, a kind of rust of copper, much used by painters as a green colour. It is chiefly manufactured at Montpellier; the vines of Languedoc being very convenient for this purpose.
The following process for making verdigrise is described by Mr Monet of the royal society of Montpellier, and is published among the memoirs of the academy for the years 1750 and 1753.
Vine-stalks well dried in the sun are steeped during eight days in strong wine, and afterwards drained. They are then put into earthen-pots, and upon them wine is poured. The pots are carefully covered. The wine undergoes the acetous fermentation, which in summer is finished in seven or eight days; but requires a longer time in winter, although this operation is always performed in cellars. When the fermentation is sufficiently advanced, which may be known by observing the inner surface of the lids of the pots, which during the progress of the fermentation is continually wetted by the moisture of the rising vapours, the stalks are then to be taken out of the pots. These stalks are by this method impregnated with all the acid of the wine, and the remaining liquor is but a very weak vinegar. The stalks are to be drained during some time in baskets, and layers of them are to be put into earthen-pots with plate of Swedish copper, so disposed that each plate shall rest upon and be covered with layers of stalks. The pots are to be covered with lids; and the copper is thus left exposed to the action of the vinegar, during three or four days or more, in which time the plates become covered with verdigrise. The plates are then to be taken out of the pots, and left in the cellar three or four days; at the end of which time they are to be moistened with water, or with the weak vinegar above-mentioned, and left to dry. When this moistening and drying of the plates has been thrice repeated, the verdigrise will be found to have considerably increased in quantity; and it may then be scraped off for sale.
A solution or erosion of copper, and consequently a verdigrise, may be prepared by employing ordinary vinegar instead of wine, as is directed in the above process. But it would not have the unctuousity of ordinary verdigrise, which quality is necessary in painting. Good verdigrise must be prepared by means of a vinous acid, or solvent, half acid and half spirituous. Accordingly, the success of the operation depends chiefly on the degree of fermentation to which the wine employed has been carried: for this fermentation must not have been so far advanced that
no sensibly vinous or spirituous parts remained in the liquor.
Verdigrise taken internally is a very violent emetic, and is reckoned among the poisons, though it is certainly the mildest of that pernicious tribe. It seems to act less as a corrosive than as a violent astringent, which renders it so disagreeable to the stomach that it is almost instantly evacuated by vomit.—In very small doses it has been given as an emetic, and in this intention it operates most powerfully and speedily. Many pernicious consequences have been attributed to the gradual introduction of small quantities of verdigrise or copper into the human body: but it is much to be doubted whether this would certainly be accounted the cause of them; since copper, either by itself or dissolved either in acid or alkali, when it does not prove emetic, always acts as a tonic.