=Cobalt Green=, which is also known as Rinmann’s green or zinc green, is a compound of the oxides of cobalt and zinc. It is always produced when a cobalt compound is ignited with zinc oxide. Rinmann’s green has not so deep a colour as the poisonous emerald green, but it is distinguished by extraordinary durability, and deserves to be more largely used than it is at present. This pigment is most simply made by precipitating the mixed solutions of a zinc salt and a cobalt salt. It is also formed by moistening pure zinc oxide with a cobalt solution and igniting. In precipitating the mixed solutions of zinc and cobalt salts products of different shades are obtained, according to the proportions in which the salts are used. If equivalent quantities are used, an almost black product is obtained, quite useless as an artists’ colour. The best result is obtained by mixing intimately pure precipitated cobalt carbonate with zinc oxide and igniting the mixture; a mixture of 9 to 10 parts of zinc oxide with 1 to 1·5 part of cobalt carbonate gives colours between pale green and dark green. Especially fine Rinmann’s green is obtained by igniting cobalt arsenate with zinc oxide and arsenious acid. The addition of the last-named substance can only have the object of preventing the temperature from rising too high, which might injure the beauty of the colour; arsenious acid is volatile at a fairly low temperature. Cobalt green is also obtained by evaporating a solution of cobalt nitrate and zinc nitrate and igniting the residue, or by igniting a mixture of the sulphates. In the latter case a tolerably high temperature is necessary in order that the sulphates may be decomposed. The author has found that a particularly handsome product is obtained by mixing pure zinc oxide with a dilute solution of cobalt chloride, drying and then slowly heating to redness in a crucible with a well-fitting lid. Towards the end of the operation the heat is considerably increased and maintained for a short time, after which the mass is rapidly cooled. CHAPTER XXXIV. GREEN MANGANESE PIGMENTS. =Manganese Green, Rosenstiehl’s Green.=—This pigment, which is handsome but difficult to produce, consists of barium manganate. It can be obtained by several methods, all of which produce a good green, but with very unequal properties. Manganese green made from barium nitrate has little durability; when made from barium hydrate it is more costly, but very durable. Barium manganate is most easily obtained by precipitating a boiling solution of potassium manganate by barium chloride. The almost blue precipitate becomes nearly white when washed and dried, but when it is gradually heated on a porcelain plate to a dark red heat it acquires a fine green colour. The heating must be carefully conducted; if the temperature rises too high, the colour changes to a dirty greyish brown, owing to the reduction of the manganic acid.