Page:The chemistry of paints and painting.djvu/49

Rh pure linen rags have been used for the pulp; they may be generally traced to the disintegrating action on the fibre of the chemical bleaching agents employed. The development of rust-spots, when not due to the mount or backing of a drawing, arises from the presence of small particles of metallic iron from the machinery having become embedded in the pulp. These particles appear grey, brown, or black; they may be detected by placing a drop of oxalic acid solution on the suspected spot, allowing it to dry, and then moistening the place with a drop of a freshly-prepared solution of tannin. If the particle be iron an ink-stain will be produced. However, some dark spots consist of blackened grease, or of tar, or of the paper-fungus (Myxotrichum chavtanim).

Naturally, there is a small quantity of oil or fat in paper; it varies from 3 to 5 parts in a thousand. The difficulty experienced in immediately wetting a surface of paper, caused by the presence of this trace of oil, may be overcome by first washing the surface with distilled water to which a drop or two of caustic ammonia has been added. A solution of the natural mixture of alkaline organic salts, known as oxgall, effects the same purpose. The use of borax had better be avoided. It is always advisable to wet the whole surface of the paper before beginning a water-colour drawing. Thus any abrasions or defects of the surface will become apparent.

As drawing-papers are sized in the sheet they occasionally show a peculiar defect arising from the irregular distribution of the size. In such cases, when the surface is scraped off, an absorbent layer of imperfectly sized pulp is revealed beneath. When such paper is used for water-colour painting the sinking-in and running of the pigments produce disastrous results; but it is easy to guard against