Page:The chemistry of paints and painting.djvu/69

Rh hydrogen peroxide: a slight warmth greatly hastens the bleaching process.

Some painters in oil have employed with success a tempera-priming on their canvases. This priming may be prepared with a mixture of a strong, though elastic, size, with whitening. A good composition of this sort may be made by taking equal weights of fine whitening and of fine plaster of Paris, which has been slaked in and soaked with abundance of clean water, or of the preparation called satin-finish, an artificial gypsum, used by paper-makers: the warm size is incorporated with this mixture. When the priming coats are dry the surface is dressed with a layer of pure size, and allowed to harden thoroughly before the picture is begun.

An ordinary primed canvas was examined with the following results. The amount of moisture present was 5*5 per cent, of its weight, the priming 25 per cent., and the dry substance of the size 15. The dry fibre which constituted the remaining constituent would weigh, therefore, about 54 parts. It was further found, with the same canvas, in a dry heat of 100° C. (212° F.) continued for twenty minutes, that a strip 20 inches long became shorter by a quarter of an inch, changing in colour from a creamy white to a pale buff. After immersion in boiling water for twenty minutes a piece of this canvas 20 inches square was found to have shrunk more than i inch in one direction, and in the other direction rather more than half an inch. The piece was somewhat crinkled, and had become yellow in patches.

A few remarks as to the bearing of the above observations on some of the phenomena presented by oil-paintings on canvas may be here introduced. The water present in canvas varies with the temperature, and in consequence