Page:The chemistry of paints and painting.djvu/70

36 the dimensions of the canvas vary. As the contraction on drying and the expansion on taking up moisture are not the same in the direction of the warp as in that of the woof, there is an unequal strain upon the layers of paint upon the surface. These may, therefore, become irregularly fissured, and even loosened. The importance of selecting a canvas so woven as to expand nearly equally in both directions is evident, but the maintenance of a uniform temperature, and of a suitable degree of moisture in the atmosphere where pictures are hung, is also obvious. The absorption of moisture by canvas occurs through the back, unless that be also protected by paint. With the moisture deleterious gases may also be absorbed, and these may easily pass through and affect the priming, even the picture. Canvas protected by panel behind, or coated at the back with a layer of white lead which has been ground up with starch paste, escapes this injury in great measure, as the sulphuretted hydrogen, etc., are then intercepted. The colouring-matter of the fibre and size of the canvas may move towards the front and discolour the priming and even the picture. An excess of damp and a high temperature are the chief causes of this movement. When the first priming coat contains size, though it may adhere firmly to the sized canvas, it may not hold the subsequent oil-painting quite so tenaciously. Canvas is liable to accidental injuries from mechanical causes: a double canvas mitigates the evil. The elasticity of the priming may not suffice, when the canvas is rolled up, to prevent cracking. A small addition of a non-drying oil, such as almond or olive oil, to the linseed oil used in the priming coats, proves useful, but such addition