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 across both strips by means of a broad brush. The pigments used may be French ultramarine, chrome yellow, and carmine. Unless they are employed in very dilute admixture, the changes produced by alum and other chemicals will not be perceptible. There should be no bleaching of the ultramarine or the carmine, or any blueing of the latter, and no dulling of the chrome, even after the lapse of a week from the date of the experiment. Washes of tincture of azolitmin from litmus, tincture of dahlia flowers, and tincture of methyl-orange may be similarly applied to paper-strips; in this case it will probably be found that the two former tests will show an acid reaction, and the methyl-orange a basic or alkaline reaction.

This seemingly strange result has been found to arise from the presence of a derivative of the alum in the size, namely, an aluminium sulphate which is acid to some tests and basic to others. This point has been established by the experiments* of Messrs. Cross and Bevan, Mr. C. Beadle, and Drs. P. N. Evans and Quirin Wirtz, who have proved that all the drawing-papers of well-known makers which they have examined contained no free sulphuric acid. Of course, the question remains, 'How far, if at all, is the basic aluminium sulphate in drawing-paper injurious to sensitive pigments?' This inquiry can, I think, be answered by applying the colour-tests already described, not only to the suspected papers themselves, but also to extracts from them made with cold distilled water and also with hot. Other useful tests are the following:

1. Burn 100 grains of paper to a white ash; not more than 1.5 grains of incombustible residue should be found.

2. Extract 100 grains of paper repeatedly with boiling distilled water. The united watery extracts, evaporated to dryness, should not amount to 8 grains.

* Journal of the Society of Chemical Industry (1892), pp. 212, 213, 261.

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 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 ckartarum).

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 accidents of this sort by previously scraping and colouring a corner of the sheet to be used. The peculiarity is generally owing to the too prolonged and slow drying of the sheets of paper after they have been removed from the warm sizing-bath and pressed. The solution of size is brought to the surfaces from the interior of the sheet, and remains there. Moreover, in very slow drying, the size is apt to decompose with loss of its glutinous character and, possibly, the formation of mildew. A good drawing-paper will indeed have rather more size at the surface than in the interior, this result being secured by a rate of drying which is neither too rapid nor too slow. Let us add that the strength of paper when completely wetted and in the presence of free water, is very low.

If, however, it has been gelatin-sized and afterwards sprayed with a 40 per cent. solution of formalin to coagulate the gelatin it becomes appreciably stronger.

As to vellum, parchment, and ivory, little need be said. All three contain the characteristic ingredient ossein, an insoluble nitrogenous organic substance, which by long boiling with water is converted into gelatin: a solution of gelatin constitutes ordinary size. Water-colour paints placed upon any of these materials sink either very slightly, or not at all into their substance - a very few, such as aureolin, strontia-yellow, and madder carmine, stain the superficial layer. The old method of preparing vellum for the reception of water-colours consisted in rubbing the surface with very finely-ground bone-ash, or with pulverized sandarac. Pumice-stone or cuttle-fish, reduced to a minutely divided state by pounding, grinding, and sifting, may be used for this purpose; the infusorial earth known as polishing silica, or kieselguhr, may also be employed.

Ivory which has become yellowish through age and seclusion from light may be safely bleached by contact with an ethereal solution of hydrogen peroxide. The treatment is best carried out in a wide-mouthed stoppered bottle, care being taken to immerse the sheets of ivory wholly in the liquid, and not to allow them to touch each other.

Much care is necessary in selecting tinted and coarse coloured papers for water-colour work. The tints of the former are often obtained by the introduction of fugitive pigments into the pulp; the latter are often made of inferior and mixed fibres, and sometimes contain lead-white and other injurious fillings. 'Turner' paper, for example, owes its grey-blue tint to the presence of indigo, while 'Varley' paper contains about 20 per cent. of 'mechanical' wood-pulp, a material which steadily darkens into brown after but a short exposure to light. 'Sugar' paper, whatever its hue, should be avoided. Mill-board is often made of wood-pulp, oakum and straw-pulp: its surface is primed for oil-painting in the same way as canvas.