COPPER, ACETATES or—technically known as Verdigris. (FR., Vert-de-gris ; GER., Griinspan.)—= There are two principal acetates of copper, common verdigris— a sub-acetate—" bine " or " green,' obtained by exposing to the air plates of copper in cont,aet with the " mare " or refuse of grapes (i. e. the grapes after the juice has been expressed), and distilled verdigris, a neutral acetate, obtained by dissolving the common verdigris in hot acetic acid, and leaving the salt to crystallize out from the cooled solution. On account of the variable quality of the common verdigris, the distilled is often also manufactured frum a mixture of sulphate of eopper and acetate of lead ; sulphate of lead remains an insoluble precipitate, and the clear acetate of copper is filtered off, concentrated by evaporation, and allowed to crystallize.
The process for obtaining common verdigris is as follows :—The refuse from the wine-presses is thrown into casks, which are loosely covered over with matting to keep out dust and diit. It will be readily understood that the less severe the previous pressing of the grapes the better for the subse quent process. When'placed in the casks the material should be disintegrated as rnuch as possible, occupying at least double the space it did when compressed. Partial fermentation, with the generation of ae,:tic acid, speedily commences, and is allowed to go on for about four days, until a test sheet of copper, immersed in the mass for four-and-twenty hours, is covered with a green layer of acetate. In the meantime the sheets of copper to be operated upon have been subjected to a careful hand hammeting to ensure perfect consolidation, cut into pieeee about 8 in. long, 4 in. broad and iith of an inch thick, then immersed in a strung solution of verdigris and stacked up to dry, or, to kayo time, dried quickly over a charcoal fire. When it has been ascertain( d by the test sheet that acetous fermentation in the casks has gone far enough, the small sheets of copper nre bested to about 93° (200° F.) and sandwiched with the grape refose, taking eare to have a layer of the fer menting material both at the top and at the bottom of the cask. After a varying period of from twelve to twenty days the coverings are removed, and if the uppermost layer of material lias become white it is judgul that the opemtion is complete. The casks at e then emptied, the grape refuse thrown away, and the sheets of copper, which should now be evenly eovered with fine green crystals, set up to dry. After the lapse of about three days they are dipped in water (or, preferably, damaged wine, whence the trade terms of " one wine," " two wine," &c.), again set up to dry, and the verdigris formed scraped off with a knife. This process of dipping, drying, and removing the verdigrie occupies about eight days, and is repeated until the whole of the copper is con verted into verdig,ris. The damp.salt when scraped off is kneaded with a little water, packed
into leather bags (about 18 in. long by 10 in. in d'ameter, containing about 23 lb. weight), and exposed to the sun. When thoroughly dry it becomes a hard, tough mass, and is rcady for the market.
This process was formerly ahnost entirely carried on in France and Belgium, especially in the first-named country, upon the vine-farms, where it forms part of the regular domettie routine. Now, however, considerable quantities of verdigris are made in England, Germany, and Sweden, where cloths steeped in pyroligneous acid, and the eider refuse, are the materials chiefly employed in the corrosion of the sheets of copper. The cloths require to be damped afr(sh every three or four days until the plates are eovered with their even layer of crystals.
The distilled verdigris, obtained, as has been said, by crystallization from a hot solution of the common verdigris, or from a mixture of sulphate of copper and acetate of lead, forms dark-green crystals soluble in 14 parts of cold or 5 parts of hot water, and also in alcohol. It is extensively used in the manufacture of aromatic vinegar. as the source from which the aeetie acid is obtained. This process of distillation has been already described. It is also used as a mordant, but is feeble iu its action.
All the acetates of copper form valuable pigments; they are used in dyeing and calico-printing as "resists," (i. e. to prevent the indigo imparting a permanently blue colour to the cloth), and in various med:einal preparations. It should be noted that they are exceedingly poisonous. Commercial verdigris should not contain more than 4 per cent. of impur:ty--eltiefly insoluble matter. It is, however, often adulterated with chalk or sulphate of copper. The amount of insoluble matter ean be roughly estimated by the gritty feeling when the salt moistened and rublx:d in the band. Brightness of colour is a rough test of quality. Tl.e presence of chalk may be readily detected by pouting a little hydrochler:c acid over a sample of the salt. If chalk be present, effervescence will take pima ;—if pure, the verdigris will dissolve quietly in the aeid. The solution may be filtered off from the insoluble impurities, and chloride of barium add(d. If sulphate of copper be present, the white, heavy precipitate of barium sulphate will immediately form. The impurities from the solntion in hydrochloric acid, washed, dried, and weighed, should not ( xeeed, at the outside, 5 per cent. of the weight of the original sample.
A good sample of verdigris will test as follows : Oxide of copper .. . 43.50 Anhydrous acetic acid .. 29.30 Impurities 100.00 An ad v dorem duty of 10 per cent. was until 15,5;1 imposed upon the importation of verdigris. Since that time tho trade has been free.