Gypsum is generally used as the soft white mineral forming the base of the coloured mass. It is much better to use steatite (soap-stone), which is not much dearer, and has many advantages over gypsum. The preference given to steatite is based upon a comparison of the properties of the two minerals; gypsum is crystalline, steatite is non-crystalline. Powdered gypsum has essentially a dry character, whilst powdered soap-stone is of a peculiar greasy nature, and consequently can be readily smeared upon a flat surface; it also imparts a pleasing lustre to the colours with which it is mixed. The manufacture of crayons consists in preparing and moulding the coloured mass. The process is commenced by mixing the powdered colour and soap-stone in a closed rotating cylinder. The rotation is continued until the mixture is uniform in colour. The quantity of colour to be mixed with the soap-stone depends upon the shade to be produced. Manufacturers who make a speciality of crayons prepare a number of shades of each colour. The artists who use crayons, however, usually require only those of a pure shade, since they themselves can produce the intermediate shades from the essential colours. On this account it is advisable to make crayons of distinct and deep colours. The mineral pigments are the most suitable: for yellow, deep chrome yellow; for red, vermilion or deep madder lake; for green, one of the pure green pigments, such as chrome green; for blue, Chinese blue or ultramarine; for brown, burnt sienna or manganese bistre. For white and black crayons levigated chalk is used, alone or mixed with a sufficient quantity of fine vine black or some other good black. Either gum Arabic or size may be used as binding material. When the former is employed the crayon becomes so brittle when dry that it breaks when pointed with the knife, in spite of the greatest care. Size produces a less brittle crayon, and is also much cheaper, so that it is preferred. The paste from which the crayons are to be formed is made by mixing thin size with the colouring matter and steatite to a soft pulp, which is then kneaded to secure uniformity. The process then proceeds in a different manner according as the crayon is to be pressed or sawn out.