Page:The chemistry of paints and painting.djvu/50

16 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.