It is often advisable to make verdigris from a soluble copper salt; the process varies somewhat according to the salt employed. From copper sulphate verdigris is obtained by the action of calcium acetate. When solutions of these compounds are mixed, calcium sulphate separates as a white precipitate, whilst a solution of the easily soluble copper acetate is left. It is only necessary to mix the solutions in equivalent proportions, separate the liquid from the precipitated gypsum and evaporate. Gypsum is somewhat soluble in water, so that when the solution is evaporated a double salt separates out first. This consists of copper calcium acetate; it may be used as a pigment, but is of less value than verdigris. It is advisable to prevent the formation of this double salt, which is accomplished by adding copper sulphate to a slightly acid solution of calcium acetate until a precipitate is no longer produced. The solution of copper acetate is boiled for several hours to effect the separation of the dissolved gypsum; at the same time the iron contained as impurity in the copper sulphate is also precipitated. The purified solution of verdigris is concentrated and allowed to cool, when a further quantity of gypsum separates; the solution is then evaporated to crystallisation. If barium salts and lead acetate are cheap, barium acetate can be obtained by double decomposition. When copper sulphate is added to the solution of barium acetate so long as a precipitate is formed, a solution of verdigris is produced, which only requires evaporation to crystallise. The precipitate is barium sulphate, and can be used as enamel white. It obstinately retains a quantity of copper, which, though small, is sufficient to give it a greenish tint. By several washings with acetic acid this copper is removed; the acid can be used to obtain fresh quantities of verdigris, so that none is lost, and the copper retained in the precipitate is regained. The precipitate, after treatment with acetic acid, only requires thorough washing with water to produce permanent white, satisfactory in every respect. Ammonia may also be used in the manufacture of verdigris from copper sulphate. Strong ammonia is left in contact with copper sulphate in a covered vessel for several hours; gentle warming accelerates the formation of copper ammonium sulphate. The liquid has then a fine blue colour; it should not contain sufficient ammonia to be perceptible by the smell. It is then gently warmed and acetic acid gradually added. On continued heating small green crystals separate from the solution, which is boiled with continual additions of acetic acid in small quantities until small crystals appear on the surface, when it is allowed to cool and the crystals are strained off.