(1) By Thénard’s original process, roasted cobalt, from Tunaberg, Sweden, is dissolved under heat in an excess of nitric acid. The solution, evaporated nearly to dryness, is boiled in water, and the deposit of arseniate of iron is filtered out. Into the filtrate is poured a solution of basic phosphate of soda, which throws down a precipitate of basic phosphate of cobalt, varying in hue from violet to pink. This precipitate is washed on a filter, and, while still gelatinous, 1 lb. of it is intimately mixed with 8 lb. of hydrated alumina, recently precipitated by ammonia from a solution of potash-alum. The mass is first dried to brittleness and then calcined in a covered clay vessel for half an hour at a cherry-red heat. The resultant blue pigment is stored in glass receptacles. The preparation of the gelatinous alumina is conducted as follows. The potash-alum is dissolved in at least three times as much water as is necessary, and is then precipitated by an abundant excess of ammonia, with frequent stirring. When settled, the supernatant liquor is siphoned off, and the precipitate is thoroughly washed several times on a filter. Thénard blue will vary in tint according to the proportions of alumina used. A pure colour is obtained with 4-5 parts alumina to 1 of phosphate of cobalt; a greenish hue with equal parts of alumina and cobalt salt; and almost any intermediate tone by varying the proportions between these limits. (2) To 16 parts of gelatinous alumina add 1 part of arseniate of cobalt, obtained by precipitating the solution of cobalt by arseniate of potash. (3) A richer and more velvety blue is got by using oxide of cobalt and substituting phosphate of lime for the alumina (Boullai-Marillac).