Page:The chemistry of paints and painting.djvu/44

10 ranges of temperature. Under these conditions the moisture of the paper is first partly turned into vapour, then condensed on the glass, and, lastly, is re-absorbed by the paper, and, for a time, especially by the pigments lying on its surface. This temporary condensation of an excess of moisture upon the coloured surface does much injury before hygroscopic equilibrium is once more re-established. Much less harm would accrue were the vapourized water allowed to escape.

Size.—The size must be considered next. It may be applied to the pulp or to the sheet, and may consist of gelatine with a little alum, of colophony or rosin dissolved in soda-lye, followed by treatment with alum or alum-cake. Sometimes starch is used along with alum or alum-cake. From good drawing-papers, which are sized in the sheet with animal size, the greater part of the size may be extracted by means of boiling distilled water, the solution being usually neutral or faintly acid, sometimes faintly alkaline, to test-papers. Gelatine and starch, to the extent of about 5 per cent, of the weight of the paper, are the safest sizing materials.

Ash.—The ash or mineral matter in paper may be derived from three sources, namely, traces of the original mineral substances taken up by the flax plant from the soil, and still remaining associated with the felted pulp; the mineral matters, such as soda and alum, introduced with the size; and, lastly, the mineral compounds used to whiten, to weight, or to finish the paper, or in bleaching the fibre and as 'antichlors.' In common and adulterated papers the ash greatly exceeds i per cent., twelve parts per hundred of paper being no unusual proportion. This 'filling' may contain or consist of the following substances: kaolin or china-clay, silicate of lime or 'pearl-hardening,' chalk or