Brunswick Green, Green Verditer, is a mixture of copper arsenite, copper hydroxide and gypsum. It is prepared by dissolving 100 parts of copper sulphate in water, adding a solution of ¼ part of arsenious acid and 10 parts of anhydrous potassium carbonate, and finally precipitating with milk of lime made from 20 parts of quicklime. The pale to bluish-green precipitate is washed and pressed into flat cakes, which are brought into the market and used as lime colours. This pigment is not adapted for use in oil; when applied in that medium it darkens considerably.
Neuwied Green is made in almost exactly the same manner as Brunswick green, the chief difference being that a larger quantity of arsenic is used, and thus a product obtained inclining more to green than to blue. As a rule 2 to 2½ parts of arsenious acid are used to 100 parts of copper sulphate. The colours found in commerce under this name very often consist of mixtures of emerald green and finely ground barytes.
Copper Oxychloride was at one time largely used. At present it is scarcely employed; it has been replaced by handsomer and cheaper pigments. Copper oxychloride is made by exposing a wet mixture of copper, copper sulphate and common salt to the action of the air. To 111 parts of copper sulphate 112·5 parts of copper and the same quantity of common salt are employed. The mixture is made into heaps, which are wetted and from time to time shovelled about, so that the air may come in contact with all portions of the mass, and the copper chloride be converted into basic chloride. It is advisable to allow the heaps to dry frequently after they have been turned over, so that the air may more readily penetrate into the interior. Copper oxychloride has a pale green colour of little brilliance. On this account it is hardly ever used now; it forms the raw material for the preparation of Bremen green.
CHAPTER XXIX.
EMERALD GREEN.
There are many versions as to the discovery of this pigment. According to one it was first made in Schweinfurt by Russ and Sattler; another version gives it that Mitis of Vienna first made it on the large scale. It is possible, as has been the case with many chemical products, that this pigment was simultaneously discovered by both. The method of making the pigment was at first a secret possessed by few. Not until the publication of a method by Liebig in 1822 was the preparation of emerald green generally known. In recent times the industry has supplied other colours which are less poisonous. The time should not be far distant in which this handsome but highly poisonous pigment shall no longer be found in commerce.
The circumstance that the poisonous nature of the pigment and cautions as to its use were given great publicity, brought about the discovery of the countless names under which it has been sold. It was endeavoured by re-naming the pigment to pass it off as a different substance. All the pigments which have been sold under the names of Mitis, moss, patent, new, king’s green, etc., are either pure emerald green or mixtures of it with barytes or gypsum, which additions were intended to alter the shade of the green and make it appear to the buyer as a new pigment.
In chemical composition emerald green is a compound of copper arsenite and copper acetate. It has the following formula:—
3(CuO.As₂O₃) + Cu(C₂H₃O₂)₂.
This compound rarely comes into commerce in the pure state; it is generally mixed with chrome yellow or lead sulphate, which are added with the object of producing shades between yellowish-green and dark green. Emerald green is a compound of constant composition. When made by different processes it has the same properties. Under the microscope it is seen to consist of crystals; when the pure substance is ground it becomes paler, the intensity of the colour being diminished by the breakage of the crystals. The colour of emerald green is not altered by artificial light. This is a most valuable property; the majority of green pigments appear yellowish-green in artificial light.
Emerald green can be made on the large scale by two processes; these differ according to the copper salt used, which is either the acetate (verdigris) or the sulphate. The latter is by far cheaper than verdigris, which is now seldom used for this purpose.