=Potassium Nitrate= (Saltpetre), KNO₃ = 101, consists of large crystals, which quickly dissolve in water. On heating it readily gives up oxygen, and thus finds use as an oxidising agent. In former times, when the colour manufacturer was compelled to make his own materials, saltpetre was of great importance in colour making; at present, when such materials are to be bought at low prices and no colour maker prepares his own, saltpetre is little used. =Potassium Bitartrate=, C₄H₅KO₆ = 188.—This salt, known as tartar in large crystals, and as cream of tartar in the form of meal, is occasionally used in colour making. It is little soluble in cold water, but more easily in hot. The hot solution is generally used. =Potassium Bichromate= (Bichromate of Potash), K₂Cr₂O⁷ = 295.—This salt is made in special works, by melting chrome iron ore with saltpetre and extracting the mass with water, when a yellow solution of potassium chromate is obtained; to this sulphuric acid is added, which unites with half the potassium, thus leaving potassium bichromate, which is obtained by evaporation of the solution in fine red crystals. These are purified by recrystallisation. At present, in place of the above method, calcium chromate is formed by roasting chrome iron ore with lime; the calcium chromate is then decomposed by a soluble potassium salt, thus forming potassium chromate. Potassium bichromate is unaltered in air; it dissolves easily in water, and is of great importance in the preparation of many colours, in particular chromium oxide and the lead pigments. The commercial salt generally contains potassium sulphate, with which at times it is intentionally adulterated. The adulteration is detected by dissolving in water, adding half the volume of pure hydrochloric acid, and cautiously and carefully dropping in spirits of wine. A rapid action takes place, which is only assisted by warming when necessary. The red liquid changes to emerald green. If barium chloride be now added, a white precipitate is obtained in the presence of potassium sulphate. =Potassium Sodium Chromate=, KNaCrO₄ = 279, is also used in colour making. Its solution is made by adding soda to a solution of potassium bichromate so long as an effervescence of carbonic acid occurs, and until the liquid turns red litmus paper blue; the solution of the double salt is yellow. =Chrome Alum=, KCr(SO₄)₂.12H₂O = 499.—This salt occurs in commerce as beautiful violet crystals. It is obtained as a by-product in the manufacture of aniline and anthracene dyes, and may often be bought at lower prices than other chromium salts; 100 parts of water dissolve approximately 20 parts of chrome alum.