”Distilled” verdigris forms deep bluish-green crystals, which effloresce slightly in the air; they dissolve in 5 parts of boiling water and in 13·4 parts at 20° C. When the solution is boiled for some time the tribasic acetate separates; the liquid becomes brown and acquires an acid reaction. =German Verdigris= does not differ essentially from ordinary verdigris. It is a compound of the following basic acetates: 2Cu(C₂H₃O₂)₂.CuO and Cu(C₂H₃O₂)₂.2CuO. The method is similar to that in which grape residues are used; where acetic acid is cheap the process is especially appropriate. In Sweden, where pyroligneous acid is made in large quantities and copper is also cheap, verdigris is made by arranging copper plates and flannel in alternate layers. The plates are frequently turned over and the flannel kept saturated with acetic acid. The same chemical reactions occur as in the process in which grape residues are used. When a layer of verdigris has formed on the copper plates they are taken apart, exposed to the air, and frequently wetted. The formation of the verdigris is thus completed. The difference between this process and that adopted in wine-producing countries lies simply in the use of pure acetic acid; the product is free from the mechanical impurities introduced by the grape residues. In properties German or Swedish verdigris is completely identical with blue verdigris. CHAPTER XXXI. CHROMIUM OXIDE. =Chrome Green.=—Although very different substances are commercially known as chrome green, this name is usually applied to the very valuable pigment which consists of pure chromium oxide. There is hardly another pigment for the preparation of which such varied directions have been given and whose shade varies so much according to the manner of production. The cheapest process is to ignite potassium bichromate with sulphur, extract the mass with very dilute sulphuric acid and wash the residue. The chromic acid is reduced by the sulphur; when the mass is treated with sulphuric acid sulphur dioxide is evolved, potassium sulphide and sulphate are dissolved, whilst pure chromium oxide remains. The larger the quantity of sulphur used the paler is the chromium oxide. The purity of the potassium bichromate has considerable influence on the shade of the pigment; if iron is present in any quantity a discoloured product always results. Convenient proportions are: 19 parts of potassium bichromate and 4 parts of sulphur, which produce 9·33 parts of chromium oxide. If potassium bichromate free from iron cannot be obtained, the shade of the product is somewhat improved by treatment with dilute hydrochloric acid, in which ferric oxide dissolves more easily than chromium oxide. When the latter has been strongly ignited it is soluble with very great difficulty. There are many formulæ for the preparation of chromium oxide by means of sulphur: in all the above statement holds good, that the more sulphur the paler the product.