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The usual, and probably the best fibre for the manufacture of canvas for painting is unbleached flax - that is, linen; hemp and cotton are decidedly inferior. The material is woven in different ways, and with strands of different degrees of fineness, so as to produce cloths of various degrees of thickness and fineness, and having several kinds of texture and surface.
The canvas is first treated with size or a solution of glue; this should be as free from colour as possible: the addition of honey to the size is undesirable. The priming consists of two coats, the first containing whitening and size, the second lead white and linseed oil. Fuller primings are often given where it is not desired to allow the texture of the canvas to remain evident. Such primings are put on alternately in directions at right angles to one another, and are treated in the same way as the primings of panel -If before the last priming be dry it be dusted with zinc white, or if a very thin final priming of zinc white and drying oil (free from lead) be given, the usual discoloration of the canvas which occurs on keeping it, especially in the dark, will be avoided. But such discoloration can always be removed by leaving in contact with the priming a piece of blotting-paper saturated with a solution of 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 percent 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 1 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 the dimensions of the canvas vary. As the contraction on drying1 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 should not exceed 1 part of non-drying oil to 20 of drying oil.
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