Page:The chemistry of paints and painting.djvu/48

14 distilled water. The united watery extracts, evaporated to dryness, should not amount to 8 grains.

3. If straw or esparto fibre be present in a paper, it will become red when immersed in a boiling 1 per cent, solution of aniline sulphate.

Attempts have been made to size paper with casein dissolved in ammonia, and also with ' viscose,' a modified cellulose made out of the substance of the paper itself by means of water, caustic soda, and carbon disulphide. At present, however, gelatin-sizing holds its own. The necessity of introducing alum, or an equivalent of some other aluminium salt, into this size is its chief drawback, although an animal product of the group to which gelatin belongs, being prone to decomposition and to the attacks of microscopic organisms, itself constitutes a source of danger. Alum is used not merely as an antiseptic, but because it exerts a peculiar liquefying effect upon the size. A little alum solution added to gelatin solution increases its stiffness, but further additions up to an easily ascertained point make the solution more mobile. It is absolutely necessary to keep the alum percentage low; I found in a batch of one well-known make of drawing-paper that exactly twice as much alum had been employed as was necessary. My remonstrance with the manufacturers had its due effect.

The roughness or smoothness of the surface of the paper, or cardboard, is not without influence on the permanence of water-colours. The pigments become less intimately associated with the smooth surface of a hot-pressed paper than with a comparatively rough natural surface. The rough surface is, however, liable to wider and more rapid fluctuations in the amount of hygroscopic moisture.

Some apparently sound papers deteriorate in strength and tint on being kept. Such changes may occur even when