Page:The chemistry of paints and painting.djvu/45

Rh whitening, lead-white, baryta white or 'white dressing,' artificial gypsum or 'satin-dressing,' and a mixture of aluminium hydrate with magnesium carbonate or with calcium carbonate, known as 'satin-finish' or 'satin-white.' Other substances which increase the amount of ash left when a paper is burnt are blue colouring matters, introduced to counteract the natural yellow tint of the pulp. These include artificial ultramarine, smalt or cobalt blue, and Prussian blue.

Fibre.—What is put down as fibre in the analyses of paper previously cited, is a substance, or group of substances, to which the name of cellulose is given by chemists. Cellulose consists of the three elements—carbon, hydrogen, and oxygen; it is, when pure, entirely combustible, leaving no ash.

The source of this cellulose is by no means without influence on the durability, strength, and working quality of drawing-paper. The fibres of linen and of cotton present distinct differences of form and resistance to strain. When working on a paper with a knife so as to develop high lights, the water-colour painter soon discovers the weakness and fluffiness of abraded cotton, while the clear-cut surfaces of linen are equally obvious. Even in washing and in taking out lights from a drawing by sponging and rubbing, the superiority of linen-paper to cotton-paper is very marked; in fact, papers into which a high proportion of the latter fibre enters will not stand much worrying. The other fibrous materials commonly forming the basis of ordinary papers are, on one score or another, less desirable than cotton. Nearly all of them require, in order to fit them for paper-making, a very drastic treatment, which is liable to leave behind it traces of injurious chemicals, or to yield altered material of lessened strength and permanence.