Page:The chemistry of paints and painting.djvu/46

12 Wood-pulp, esparto, and straw-pulp belong to this category.

Paper-making.—The technology of paper-making cannot be discussed here, but a few references to the chemicals employed in the process of manufacture may be usefully given at this point. Amongst these chemical substances, one or more of which will have been introduced into the fibrous basis of the paper or into the size may be named: caustic soda and caustic lime; chloride of lime, magnesium hypochlorite, moist chlorine gas, and sulphuric acid; alum, aluminium chloride, and aluminium sulphate; sodium sulphite; gelatin. Of course, it is possible to cleanse and bleach the higher class of rags without having recourse to any chemical treatment, but the 'souring' with sulphuric acid and the employment of some soda or sodium carbonate to remove grease are usual; while there is always a salt of aluminium present in the size. Indeed, in the best and purest drawing-papers, the alum, or its equivalent, is the one ingredient upon which the chemist interested in painting will look with suspicion. But the subject of the presence of chemicals, injurious or innocuous, in the finished product of the paper-mill may be relegated to the following paragraphs.

Paper-testing.—The simplest test of the suitability of any sample of drawing-paper for water-colour work consists in applying to its surface uniform and weak washes of a chosen set of sensitive pigments. A sound standard paper is taken for comparison ; this may be ' Whatman,' but it should be first swilled in cold distilled water for five minutes, and then hung up to dry. In applying this test, a strip of the sample to be tested and one of the standard paper should be laid side by side, and then the several colour washes, made with distilled water, carried