Canvas has been the classic painting support for oil and tempera since about the sixteenth century. It, too, needs priming before it is painted, to prevent the oil or other binder from soaking into the fibers of the cloth. The priming also provides a durable white surface, which is needed because even bleached canvas darkens considerably in contact with glue or oil. Other textiles of vegetable fiber, such as cotton or jute, can be used for painting, but none of them are in any way comparable to pure linen canvas.
Linen is woven from the spun fibers of the flax plant. It is sown in springtime and quickly sends up thick, light-green shoots which, in June, produce flowers of a color you would now, as a color specialist identify as light cobalt blue. The seed cases develop out of the flowers. The fibers come from the woody stalks, which grow more than a yard high. The wood content is dissolved by rotting (steeping) it. It is then broken up and hackled or combed away from the yellow-gray silky fibers. Linseed oil is extracted from the seeds. If painters were to adopt a plant as an emblem of their profession it would have to be the flax.
The spinning of the flax fibers first produces the single-strand yarn. Double or multiple yarn is then twisted from the single strands. The best texture of linen is obtained from untwisted yarn, for it best shows the characteristic thickenings arising from the constantly newly added fibers. They give the finished linen or canvas its typical surface texture. This texture shows to best advantage when the yarn is woven with a plain cloth weave (rather than the twill weave or its fancy variants) or the satin weave, which is generally used only for fine household linens. For these only the finest yarns are used. Coarser linens are used for painting.
All this, however, is, like the type of weave, a matter of taste; there are no differences in qual ity. The quality of the linen depends entirely on the flax used, and the fineness and length of its fibers. Linen with knobs in it, which is often recommended as painting canvas, still contains remnants of wood from the flax stalk; it is this which forms the lumps. Apart from being very noticeable in the surface of the picture, these knobs or lumps show that the linen is second rate.
Sail cloth can be used if a very coarse texture is desired. Gauguin was fond of painting on it, and the flat effect of his pictures is strongly emphasized by the coarse texture of his support.
If the canvas is held against the light it is easy to see if the warp (longitudinal threads) and weft (horizontal threads) run equally close together. Good machine weaves are always regular; handwoven linen is not. There is no particular advantage in handwoven linen as such, unless it comes from a good studio where handspun yarn is used, as this shows up most strongly the attractive, irregular thickenings caused by the spinning. Since every bleach injures the fibers somewhat, it is better to use unbleached linen, particularly as it will in any case be covered. by the ground.
Every linen receives a dressing in the factory. It makes a better "feel" and makes the cloth shinier when it is pressed. The dressing is usually tragacanth or starch and is harmless. Resulting folds and wrinkles, however, are tiresome, for they show through even the thickest priming. Some linens are as broad as 25 feet (theatricar canvas for scenery) and, of course, must be folded. The folds can be removed only by boiling the cloth and ironing it when it is still damp. The canvas can then be stretched on a frame, or, better still, stuck onto hardboard.