When neutral nitrate of copper is decomposed by an insufficiency of a potash carbonate solution, the flocculent precipitate of copper carbonate formed at first is gradually changed into a subnitrate of copper which is precipitated as a heavy green powder. In practice the operation is conducted as follows:—Copper scales are calcined in a reverberatory or muffle furnace, till all the suboxide is converted into protoxide, or until a sample dissolves in nitric acid without evolution of red nitrous vapours. The copper nitrate solution is heated and decomposed by a clear solution of potash carbonate, and when the effervescence subsides, small doses of potash carbonate solution are added, till but little undecomposed copper remains in the solution. To recover this last portion, the clear liquor is decanted, and the green precipitate is washed several times with small quantities of water. All the liquors are collected, and the remaining copper is precipitated by potash solution. The green carbonate of copper is introduced into a new solution of copper nitrate, in which it is transformed into a basic salt. The previous liquors are evaporated till they afford crystals of nitrate of potash, which is a valuable secondary product.
Brighton Green.—The following recipe has been published for making this pigment. Dissolve separately 7 lb. sulphate of copper and 3 lb. sugar of lead, each in 5 pints of water; mix the solutions, stir in 24 lb. of whiting, and when the mass is dry grind to powder.
Brunswick Green.—(a) Old process.
The Brunswick green of former days was closely allied to Bremen green, essentially consisting of a basic chloride or oxychloride of copper, and possessing all the faults incidental to that class of copper salt. While having fairly good covering power, and capable of being used either as a water colour or an oil colour, it was tedious and therefore expensive to prepare, and not thoroughly durable under exposure to air and sunlight. Nevertheless it was a useful bluish-green pigment. Following are some of the many methods by which it has been prepared:—
(1) Poor oxidised copper ores are moistened with hydrochloric acid, and spread out exposed to the air. The metal is thus rendered very susceptible to the action of chlorine, and is even attacked by solutions of ammonium chloride and of common salt. The sub-chloride produced is rapidly transformed into oxy-chloride, and forms a fine light-green pigment.
(2) Place 2 parts by weight of copper-filings in a vessel capable of being tightly closed, and over them pour 3 parts by weight of salammoniac in the form of a saturated aqueous solution. Keep the mixture in a warm place for some weeks and thoroughly agitate it occasionally. In due time the newly formed oxychloride is removed from the vessel, and separated from the non-oxidised copper by washing on a sieve. This washing must be continued until all traces of alkali have been destroyed, when the pigment is drained, and very slowly dried at a low temperature to avoid decomposition.
(3) Copper scrap is covered with a concentrated solution of chloride of copper and allowed to remain until the chloride has undergone conversion into basic chloride. The latter is then subjected to the straining, washing, and drying treatment prescribed in (2).
(4) In a lead-lined vessel, place a quantity of copper filings or waste, and add to it two-thirds of its weight of common salt, and one-third of its weight of concentrated sulphuric acid, the latter being however first diluted by admixture with three times its volume of water. The mass is left to stand, with occasional stirring till all the copper has been transformed into oxychloride, when it is strained, washed, and dried as in (2).
(5) A modification of (4) is to put the copper scrap into a wooden vessel, and cover it with an equal weight of common salt and an equal weight of sulphate of potash dissolved in water. After standing and agitation as before, the oxychloride is formed, and the straining, washing, and drying are repeated.
(6) A solution of crude carbonate of ammonia is added to a mixed solution of alum and blue vitriol so long as any reaction takes place. When it is completed, the precipitate is collected, washed, and dried as in the other cases.