On Rinmann's Green.

The Chemical Gazette 336, 15.10.1856

By Prof. R. Wagner.

Under the name of Rinmann's green (cobalt-green) is known a colour obtained in the latter part of the last century by Rinmann, a Swede, by the calcination of a mixture of oxide of zinc and prot oxide of cobalt. The high price of the materials may be the principal reason why this colour has not come into general use. But since zinc-white has become a regular and cheap article of commerce, and protoxide of cobalt has also been brought into commerce at a low price and in a tolerably pure state, the conditions for the manufacture of cobalt-green have become more favourable than formerly, and the author has made experiments upon its preparation, of which the following are the results.

In the first place it is necessary to prepare a protoxide of cobalt as free as possible from foreign metals. For this purpose oxide of cobalt, such as is furnished by the Saxon blue manufactories, is employed; this is dissolved in 3 parts of concentrated muriatic acid, the solution is evaporated in dryness, the residue dissolved in 6 parts of water, and sulphuretted hydrogen gas is passed through the fluid as long as a precipitate is formed. The fluid fileted from the precipitated metallic sulphurets is again evaporated to dryness, and the residue is dissolved in so much water that the solution shall weigh 10 parts. A litre of the solution does not contain much less than 100 grms. of protoxide of cobalt, and 100 cub. centims, therefore contain 10 grms. The fluid is drawn off for use.

If this solution be precipitated with carbonate of soda, and the hydrated protocarbnate of cobalt produced be mixed whilst still moist after washing with oxide of zinc, a reddish-violet paste is obtained, which, when dried and long calcined, forms a green mass, the colour of which is intense in proportion to the quantity of solution of cobalt employed. Cobalt-green may be regarded as a mixture of zincate of protoxide of cobalt (corresponding to the aluminate of protoxide of cobalt in cobalt-ultramarine, or Thenard's blue) with oxide of zinc. From well-calcined cobalt-green ammonia first of all extracts oxide of zinc, and afterwards the cobalt compound dissolves. Glass fluxes, as might be expected, are coloured blue by cobalt-green. If the cobalt solution be employed in such quantity that there may be more than 1 equiv. of protoxide of cobalt to l equiv. of oxide of zinc, a dingy green or even black mass is obtained after calcination; this is probably a mixture of the compound ZnO, CoO with an excess of protoxide of cobalt. The most agree able greens are obtained when 1 to 1½ part of protoxide of cobalt is employed with 9 to 10 parts of oxide of zinc. The tints however never equal those of a bright copper-green, nor even those of the green ultramarine.

The Belgian chemist Louyet has ascertained that an addition of phosphoric or arsenic acid in the preparation of cobalt-ultramarine increases the beauty of the colour. If the addition of acids favours the combination of protoxide of cobalt with alumina, the presence of these acids must also have a favourable influence on the production of cobalt-green. If the above-mentioned solution of cobalt be precipitated by phosphate of soda or arseniate of potash, the phosphate or arseniate of protoxide of cobalt thus obtained possesses the property of communicating the green colour to oxide of zinc, even at a lower temperature than ordinary protoxide of cobalt. The protoxide of cobalt is also broken up and rendered more productive by these two acids, and the green colour is rendered purer and more brilliant. Alkaline arsenites behave like the arsenites and phosphates. If the mixed mass is obtained, to which the arsenious acid, which is partially volatilized, has given a loose spongy consistence, in consequence of which it is easily triturated.

Experiments with silicic and boracc acids and oxide of antimony did not give favourable results.

- Kunst- und Gewerbeblatt für Bayern, 1856, p. 83.