_Grey_: 45 parts of the varnish are mixed with 6 parts prepared barium sulphate, 6 parts prepared calcium carbonate, 0·5 part ultramarine blue, 6·5 parts grey zinc sulphide. _Yellowish-brown_: 48 parts varnish, 10 parts precipitated barium sulphate, 8 parts auripigment, and 34 parts luminous calcium sulphide. Luminous colours for artists’ use are prepared by using pure East India poppy oil, in the same quantity, instead of the varnish, and taking particular pains to grind the materials as fine as possible. For luminous oil-colour paints, equal quantities of pure linseed oil are used in the place of the varnish. The linseed oil must be cold-pressed and thickened by heat. All the above luminous paints can be used in the manufacture of coloured papers, &c., if the varnish is altogether omitted, and the dry mixtures are ground to a paste with water. The luminous paints can also be used as wax colours for painting on glass and similar objects, by adding, instead of the varnish, 10 per cent. more of Japanese wax and one-fourth the quantity of the latter of olive oil. The wax colours prepared in this way may also be used for painting upon porcelain, and are then carefully burned without access of air. Paintings of this kind can also be treated with water glass. CHAPTER XI. EXAMINATION OF PIGMENTS. Besides the chemical tests for purity and adulteration, which necessarily must vary with each pigment, there are certain other examinations which partake rather of a mechanical nature, and which are applicable to practically all pigments without any modification. They are directed chiefly to ascertaining fineness, body, colour, and durability. FINENESS.--Fineness may be tested for as follows:--A tall glass cylinder is filled with clean water, and about ½ oz. of the pigment under examination is well shaken in the water; the glass is placed on one side to settle out, and the length of time taken to settle may be noted for future reference. The finer the sample, the longer the time it takes to settle out; and the time in seconds may be taken as an approximate estimate of the fineness of the sample. BODY OR COVERING POWER.--An equal and exact quantity, say 50 gr., of the sample under examination and of a standard sample of the same degree of fineness is weighed out, and placed on two separate sheets of paper. To each sample is added an equal quantity, say 15 gr., of vegetable black or of very finely ground barytes, according as the pigment is a light or a dark-tinted kind. The ingredients of each sample are most intimately and completely mixed, and the tints of the two mixtures are compared by observation with the naked eye. The pigment which most nearly retains its own colour, possesses the greatest body or covering power.