Manufacture of Emerald Green from Verdigris.—By boiling for several hours, 70 parts of ordinary verdigris are dissolved in water; in a second vessel 100 parts of arsenious acid are dissolved in 1,500 parts of hot water; the verdigris solution is filtered through a cloth into a larger wooden vessel and mixed with two-thirds of the arsenic solution. The mixture is well stirred and allowed to stand for some hours. A precipitate of dull green colour is formed. This consists of copper arsenite. After three to four hours the remaining one-third of the arsenic solution is added; the precipitate then changes into the double salt of which emerald green consists.
The use of such dilute solutions as are given in these directions is essential when a product of particularly good colour is required. Emerald green is a crystalline substance which is deeper in colour in proportion to the size of its crystals. Large crystals can only be formed from very dilute solutions; when strong solutions are used, the emerald green has never the deep colour possessed by the product of dilute solutions. In the place of ordinary (basic verdigris) the so-called distilled verdigris (neutral copper acetate) can be used, of which seventy-six parts are required for the quantity of arsenious acid given. The emerald green made from ”distilled” verdigris is of a yet deeper colour than that made from ordinary verdigris, but is considerably dearer. Such emerald green is commercially described, very incorrectly, as distilled emerald green.
The method given by Liebig is as follows: 4 parts of basic verdigris and 3 parts of arsenious acid are separately dissolved in boiling vinegar; the liquids are mixed and evaporated until a precipitate of emerald green separates.
According to Ehrmann and Kastner, 70 parts of verdigris and 56 to 63 parts of arsenious acid are separately dissolved, the boiling solutions mixed and boiled until the dirty green precipitate changes into a bright green. In consequence of the rapid formation of the precipitate due to the boiling, the colour is pale green; if a dark-coloured product is required, the solutions are allowed to cool before mixing. After several days a very dark-coloured precipitate separates from the liquid; it consists of comparatively very large crystals of emerald green.
Manufacture of Emerald Green from Copper Sulphate.—There are two processes. Either the solution of copper sulphate is mixed with an acetate, by which it is converted into copper acetate, and then with arsenious acid, or copper arsenite is precipitated from the solution of copper sulphate and then converted into emerald green by treatment with acetic acid. If acetic acid can be cheaply obtained, the latter process is to be recommended.
Braconnet gives the following directions: 3 parts of copper sulphate are dissolved in a little boiling water, the hot solution is mixed with a hot solution of 4 parts of arsenious acid and four parts of potassium carbonate; the precipitate is then treated with 3 parts of pyroligneous acid. The dull-coloured precipitate at first formed is changed by the acetic acid to a bright green. The colour is more quickly produced when the liquid is heated nearly to boiling for several hours. When the desired shade appears the boiling is stopped and the precipitate at once separated from the liquid, by long contact with which it would give up arsenious acid and thus lose in shade.
According to Fuchs, 5 kilogrammes of lime and 25 kilogrammes of copper sulphate are dissolved in acetic acid, then a boiling solution of 25 kilogrammes of arsenious acid is added. The precipitate is at once formed; it is immediately separated from the liquid, washed, and dried; the top liquor, with the addition of arsenious acid, can be again used to precipitate the copper lime solution.
Emerald green has an extremely bright colour which surpasses other green mineral pigments in beauty. The larger are the microscopic crystals of which it consists the deeper is the colour. In order to obtain the desired deep shade of emerald green it is necessary to use very dilute solutions; from strong solutions the precipitate is instantaneously produced, in which case it is impossible to obtain large crystals. The covering power of emerald green, in consequence of its crystalline nature, is less the deeper the shade. It cannot be regarded as a permanent colour: very dilute alkalis and acids attack it. By the action of sulphuretted hydrogen it is discoloured, owing to the formation of black copper sulphide. It ought not to be used upon lime, which withdraws acetic acid and produces yellowish-green copper arsenite.
This pigment should not be used for distempering or for colouring wall papers. By the action of the atmosphere on the arsenic it contains, arseniuretted hydrogen is formed—an extremely poisonous gas, very small quantities of which are sufficient to cause serious illness in the case of susceptible people. In spite of its poisonous nature, emerald green is still largely used. All possible shades of green can be produced from it, yellowish-green or pale green, by admixtures of yellow pigments, such as chrome yellow, or white pigments, such as barytes or white lead. Many qualities of green known as palette green, Basle green, etc., are generally composed of such mixtures.
Mitis Green or Vienna Green is obtained by dissolving verdigris in acetic acid and adding a boiling solution of white arsenic in water; acetic acid is then added until the precipitate dissolves. On boiling the clear solution a green precipitate forms, which, when dry, is a deep bluish-green of characteristic shade. This pigment is now practically disused because of its poisonous nature, which is equal to that of emerald green.