The starting-point for the chromates is chrome iron ore, Fe(CrO₂)₂, a spinel (see p. 100). It is heated in a powdered state with a mixture of lime and potassium carbonate, in a reverberatory furnace, where the atmosphere is a strongly oxidising one. The product is a mixture of calcium and potassium chromates and ferric oxide : 2Fe(CrO₂)₂ + 4K₂CO₃ + 70 = Fe₂O₃ + 4K₂CrO₄ + 4CO₂. The fritted mass is treated with water, when the chromate dissolves, leaving the ferric oxide insoluble. On evaporation, potassium chromate crystallises out. If it is desired to produce " bichromate " or anhydrochro-mate of potassium, K₂Cr₂O₇, the solution of the chromate is treated with dilute sulphuric acid ; calcium sulphate is precipitated, and is removed by settling ; on evaporation, sparingly soluble sulphate of potassium crystallises out; and after removal of the crystals, on further evaporation," bi-chrome " crystallises. The conversion of the chromate into the anhydrochromate is represented by the equation: 2K₂CrO₄.Aq + H₂SO₄.Aq = K₂Cr₂O₇.Aq +K₂SO₄.Aq. This conversion is accompanied by a colour-change; for the ions of chromate, =CrO₄, are yellow, whereas those of anhydrochromate, =Cr₂O₇, are orange. On addition of potassium hydroxide to the bichromate, the opposite change takes place; the anhydrochromate ion is changed into the chromate ion :-