Vellum, Parchment, and Ivory

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 percent 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.