Blue, Co′balt

Syn. Cobalt′ic Az′ure. This is commonly prepared by one or other of the following formulæ:— 1. Zaffre, 1 lb., is di olved in nitric acid (diluted with an equal weight of water), 3 ⁄ 4 lb., by digestion for some hours; the solution is evaporated nearly to dryne , and the residuum redi olved in warm water; to this solution, after filtration, a solution of phosphate of soda is added as long as a precipitate forms; this last is collected on a filter, washed with cold water, and mixed, whilst still moist, with 8 times its weight of fresh-precipitated hydrate of alumina; the paste is then dried, and exposed to a cherry-red heat in a crucible, after which the ma is cooled and reduced to a very fine powder. 2. A solution of nitrate of cobalt is precipitated with ammonia-alum, and the precipitate washed, dried, and exposed to a cherry-red heat, as before. The products of the above formulæ are very beautiful and permanent. See Cobalto-ultramarine .