(1) The rouge used by machinists, watchmakers, and jewellers is a mineral substance. In its preparation crystals of sulphate of iron, commonly known as copperas, are heated in iron pots, by which the sulphuric acid is expelled and the oxide of iron remains. Those portions least calcined, when ground, are used for polishing gold and silver. These are of a bright crimson colour. The darker and more calcined portions are known as crocus, and are used for polishing brass and steel. For the finishing process of the specula of telescopes, usually made of iron or of steel, crocus is invaluable; it gives a splendid polish.

(2) Others prefer for the production of rouge the peroxide of iron precipitated by ammonia from a dilute solution of sulphate of iron, which is washed, compressed until dry, then exposed to a low red heat and ground to powder.

(3) A rouge suitable for fine work may be made by decomposing a solution of sulphate of iron with oxalic acid also in solution; a precipitate of oxalate of iron falls, which must be well washed and dried; when gently heated, the salt takes fire, leaving an impalpable powder of oxide of iron.