Page:The chemistry of paints and painting.djvu/51

Rh 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.