Cobalt Blue
Thénard's blue is made by precipitating a soluble cobalt salt moving the cotton, supersaturate the liquid with a solution of alum, and heating the precipitate. When well made, it is a good permanent color, useful in oil and water. It can also be employed in fresco and silicious painting. It is, however, somewhat affected by light, losing its brilliancy slightly. with citric acid, and collect the precipitate, which must be repeatedly washed in cold water. For pink saucers the liquid is allowed to deposit in the saucers. Mixed with the scrapings of French chalk it constitutes rouge.
2684. Lakes are also obtained from Brazil-wood and madder, by adding alum to a concentrated decoction of the former, or to a cold infusion of the latter (made by triturating the madder, inclosed in a bag, with the water), to redness. A deep green powder is obtained.
2691. Elsner's Preparation of Zinc Green. Sprinkle with water a mixture of 5 parts oxide of zinc and 1 part of sulphate of cobalt, dry the pulp thus obtained, then heat and afterwards sufficient subcarbonate of pot- If 10 parts oxide of zinc, and 1 part sulphate ash or soda to throw down the alumina in com- of cobalt be employed, the product is grass bination with the coloring matter. The pre- green in color; the same color, only lighter, cipitate is to be washed and dried. A little is obtained when the latter proportion of zinc oxide is again doubled. These colors, especially the latter, may replace to advantage Schweinfurt green; they apply well on a coating of lime. solution of tin added with the alum improves the color. Lakes may be obtained from most vegetable coloring matters by means of alum and an alkaline carbonate. Yellow lake is made from French or Persian berries, by boil-