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.
Colour.—The colour or tint of a pigment can only be estimated by comparing it with a standard sample; this is done as follows:—A sheet of black paper, with, a dead surface, is spread out on a table in front of a window, a small heap of the standard is placed on the paper, and next to it a similar heap of the sample to be compared; by means of a palette knife the surface of the two heaps is flattened out; on now carefully looking at the two heaps, the one which has the purest colour can readily be picked out. The heaps should be looked at from several points of view before a final judgment is arrived at. If the pigment is dark-coloured, it should be spread on white paper, taking care that the same kind of paper is used for the two samples.
Durability.—Durability is not a difficult point to test, but it takes some time to make a complete test. The best method is to mix a small quantity of the pigment in question with raw linseed oil, cover a piece of glass with the mixture, and expose it outside to the action of the sun and air for some time, noting at intervals how it behaves. It is well for the sake of comparison to coat a second piece of glass with the mixture, and keep this in a dark place. The difference between the two from time to time will show how the pigment behaves under the influence of light and air. As the durability of pigments is decidedly different according as they are used in oil or water-colour painting, the oil in the former case acting as a protective agent, it is well to use a similar test, using a little gum water as a vehicle to mix the pigment with. It will take from two to three months at least to properly test the durability of a pigment in summer, while in winter the time will be increased considerably. Glass is the best substance to use, as it is quite neutral, and does not of itself introduce into the test any injurious element, as wood or paper might do, although these bodies may be used if thought desirable.
CHAPTER XII.
VEHICLES AND DRYERS.
Paint consists essentially of two parts, the pigment (see [Chapters I. to VIII.]), and the vehicle or medium. In the case of oil paints, a third substance termed a dryer becomes necessary, to facilitate the “drying,” or solidification of the vehicle.