Sesquibasic cupric acetate is found in all the ver-digris of commerce. It is prepared by lixiviatingcommon verdigris with tepid water, and leaving thefiltrate to evaporate spontaneously. It is obtainedmore nearly pure by adding ammonia, little by little,to a boiling concentrated solution of the normalacetate, until the precipitate which is at first formedis redissolved. The new salt then crystallizes outas the liquor cools. The crystals are in pale bluescales; at 212° Falir. (100° C.) they lose 108 percent, of their water. Their aqueous solution is de-composed by boiling, acetic acid being given off, andthe black oxide of copper precipitated.
According to Berzelius , green verdigris is amixture of this salt with small quantities of all theother acetates of copper.
Dibasic cupric acetate constitutes the greater partof the blue variety of verdigris manufactured atMontpellier and other places in the south of France .It forms beautiful, delicate, silky, blue, crystallineneedles and scales, which, when ground, form a fineblue powder. When heated to 140° Falir. (60° C.)they lose 23"45 per cent, of water, and becometransformed into a beautiful green, a mixture com-posed of the neutral and tribasic acetates.
Dibasic Cupric Acetate. Normal Acetate Tribasic Cupric Acetate*
2Cu(C 2 H 3 0 2 ) 3 Cii0 = Cu(C 2 H 3 0 2 ) 2 + CutCjHaOJ^CuO.
By repeated exhaustion with water the dibasic isresolved into the insoluble tribasic salt, and a solu-tion of the normal and sesquibasic cupric acetates.
Tribasic cupric acetate is the most stable of any ofthe acetates of copper. It is prepared by boilingthe aqueous solution of the neutral acetate, byheating it with alcohol, by digesting its aqueoussolution with cupric hydrate, or by exhausting blueverdigris with water, as just mentioned. The firstmethods yield the salt in the form of a bluishpowder composed of needles and scales; the last asa bright green powder. This salt gives off all itswater at 352° Falir. (160° C.) ; at a higher tempera-ture it decomposes and evolves acetic acid. Boilingwater decomposes the solid tribasic acetate into abrown mixture of the same salt with cupric oxide.
Blue Verdigris. —-This salt is formed for commercialpurposes by exposing thin copper plates in a con-fined space, to the combined action of air and mois-ture, or by submitting copper plates to the actionof fermenting refuse from the wine factories (techni-cally marcs ); in the course of a few days a coat-ing, consisting for the most part of dibasic cupricacetate, forms on the plates, which may be scrapedoff, and the remaining part of the plate submitted toa fresh operation, till all the copper is converted intoverdigris.
The manufacture of verdigris on the large scale isconducted as follows:—In France the chief seats ofthis manufacture are at Grenoble and Montpellier ,where the operations are conducted in a rude buteffective way, and usually by women. Husks andrefuse of grapes from the wine factories, not entirelyexhausted of their juice, are spread loosely in casksuntil the acetous fermentation hikes place. The
casks or vessels are covered with matting to protectthem from dirt. The limit to which fermentation ofthe “marcs” should be carried, is known by intro-ducing a test sheet of copper into the mass fortwenty-four hours; if, on withdrawing it at the endof that time, it is found covered with a uniformgreen coating, the proper degree of fermentation isreached, otherwise the mass is allowed to remain aday or two longer. Alternate layers of sheets ofcopper of jh-th of an inch thick, and the fermentedmarcs are introduced into large casks, observing thatthe top and bottom layers are of the latter.
Sheets of copper are prepared by hammering barsof the metal to the above thickness—the more com-pact the copper sheets the better—and they are thencut out into pieces of C or 8 inches long by 3 to 4broad. These plates are immersed in a concentratedsolution of the verdigris, and dried over a charcoalfire; then heated to about 200° Falir., being held bya cloth in the hand, and packed in the vessels withlayers of the fermented husks, as above-mentioned.If the plates be not immersed in the solution of theacetate before packing in the casks with the fer-mented stalks and skins of the grape, they are liableto be covered with a black coating instead of thegreen acetate. The quantity of metal required tofill each vessel is between 30 and 40 lbs. Twentydays are sufficient to complete the corrosion of thecopper sheets, and induce their combination withthe acetic acid present in the marcs, but often six-teen or twelve days perfect the work. After thisperiod the upper layer of marcs will appear whitish,and if the whole has worked favourably the plateswill be covered with silky crystals of a green colour.The plates are then taken from the casks, and driedin the air for two or three days, after which they aremoistened with water and again placed to dry, bylaying them upright against each other for a week.This process of moistening with water is continuedat regular intervals of a week for six or eight times.By this mode of operation the plates swell andbecome encrusted with increasing Coatings of thecopper salt, which are detached from the remainderof the plates by a copper knife. The scraped platesare submitted to a fresh treatment, till the whole ofthe copper is converted into verdigris. After scrap-ing off the salt it is made into a thick consistentmass by kneading it with a little water, and in thisstate it is packed into leathern bags, which areplaced in the sun to dry, until the mass hardens andforms the tough substance which constitutes thecommercial article.
Green Verdigris. —At Grenoble the process isnearly the same as above, excepting that, instead ofmoistening the plates with acetous liquors, aceticacid is employed. In Germany, Sweden , and Eng-land, woollen cloths steeped in pyroligneous acidare used, which are placed alternately with thecopper plates in a square wooden box. The clothsare moistened with pyroligneous acid every threedays for twelve or fifteen days, when small crystalsbegin to form on the plates. So soon as thishappens the cloths are partly withdrawn, and a