Crocus is an abrasive material used as a polishing medium for many metals. By one method of preparing it, a mixture of salt and sulphate of iron is put into a shallow crucible and exposed to a red heat; vapour escapes, and the mass fuses. When vapour ceases to be given off remove the crucible and allow it to cool. If the heat is too intense the oxide of iron produced will have a black colour. The mass, when cold, is pulverised and washed to separate the sulphate of soda. The crocus powder is then to be submitted to a process of careful elutriation, and the finer particlss reserved for the final stages of polishing processes. A somewhat similar method of preparing crocus is to heat sulphate of iron alone in an iron pan; constantly with an iron spatula after fusion until it is tie dry and drops into a pale yellow powder. This is powdered in a mortar and sifted, placed in a fresh crucible, and calcined. On cooling, the crocus appears as a red powder. The colour of the crocus varies from pale red to brownish red, blue, and violet, the colour being determined by the particular degree of heat to which it was raised during its manufacture; the greater. the heat the darker in colour and harder is the material; thus a pale red (rouge) is used for gold and silver, while violet, known as "steel red,' is employed for polishing steel.

To obtain the best results with crocus, it should be ground as tine as possible, and then washed with water. Three clean glasses are used for the latter purpose, one being filled with water; a quantity of crocus is well stirred in with a wooden stick, left to stand for about thirty seconds, and the fluid is then carefully decanted into the second glass, leaving a sediment at the bottom of the first; after two minuti the second glass the fluid is decanted into the third, where it is left for several hours to permit the complete settling of the powder. The sediment contained in the first glass is too coarse to be of use; that in the second is a crocus of a finer quality; while that in the third is of the best grade. Crocus of varying degrees of fineness maybe obtained on this principle. The material requires to dry slowly to be fit for use. It is advisable to moisten the dried powder with alcohol, and then to ignite it so that all traces of fat may be burnt. For this purpose the crocus should be contained in an iron pan. An excellent crocus powder for applying to razor strops can be made by igniting in a crucible a mixture of equal parts of well-dried green vitriol and common salt. Take care that the material does not boil over in a pasty state and be lost.

When well made, out of contact with the air, it has the lustre of freshly cut blacklead. After grinding, elutriating, and drying, a powder is produced that, by applying to a smooth buff-leather strap, may form a serviceable razor strop, or by being mixed with hog's lard or tallow may make a useful polishing paste for many kinds of metal.