A very fine blue pigment, extensively used in the arts. It is composed of prussiate of iron, and the earth precipitated from alum or pure alumine. It is commonly obtained by calcining blood or other animal substances, as hoofs, horns, parings of leather, etc. by which a black coaly residuum is obtained. Three parts of this are added, at intervals, to four parts of potash, kept in a state of fusion in a stout iron vessel, the mixture being constantly stirred during the process. At first a reddish flame appears upon the surface of the mass; this afterwards changes to a bluish tinge, denoting the formation of prussiate of potash, when the whole is to be removed as speedily as possible into a large vessel of boiling water, and stirred, to promote the dissolution of the prussiate of potash. After allowing the dregs to settle, the clear liquor is drawn off, and fresh quantities of water boiled upon the residuum, until it ceases to impart much taste to the water; and the whole of the liquor thus obtained being mixed together, a solution of alum and green vitriol is added, when a precipitate of Prussian blue is immediately formed, which is washed repeatedly to free it from the sulphate of potash; after which it is put into bags and pressed, and then exposed to the air to dry, during which process it assumes a deeper colour, and acquires a hard stony consistence.
Blue Powder or Blue Stone, used in washing linen, is the same with smalt, either in the lump or powder. When the smalt is taken from the pot, it is thrown into a large vessel of cold water; this makes it more tractable, and more easily powdered. When examined after cooling, it is found to be mixed with a greyish matter resembling ashes, which must be separated by washing and then the blue substance being powdered and sifted through fine sieves, forms what is called powder blue.
The best Saxon blue may be prepared as follows: - mix l oz. of the best powdered indigo with 4 oz. of sulphuric acid in a glass bottle or matrass, and digest it for one hour in a water bath, shaking the mixture at different times; then add 12 oz. of water to it, stir the whole well, and when cold, filter it.