The general lay opinion seems to be that a picture has no technical worth unless it is painted on canvas held by the proper stretcher bars. This is, of course, a fallacy, a survival of old fashioned techniques as outdated as painting with fatty oils of unknown constitution. The stretcher is made of four pieces of wood fitted together with double mortices at the mitered corners. For a large stretcher, cross pieces are fitted to keep the frame rigid against the pull of the canvas. The wood is supplied ready-prepared to standard lengths graded to inches and is obtainable in any art shop.
To make the stretcher, the corners of the wood bars are joined as tightly as possible and adjusted with a set square. A piece of thin wood may be lightly nailed across the corner to keep the correct angle until the stretching is completed. The canvas should be at least an
inch and a half larger on all sides than the stretcher, and care must be taken to keep the lines of the canvas weave parallel to the sides of the stretcher, for a slanting weave upsets the painter's sense of the perpendicular when he is working.
The canvas is first fixed to the stretcher provisionally with large architects' drawing pins, and thus temporarily stretched, it is given its preliminary two coats: a glue size solution first, which is allowed to dry, and then the first coat of the primer, such as gesso. Once the latter has dried, the canvas can be restretched. The pins should be removed in the same order in which they were put in and replaced by special rust less pins.
Stretching large canvases requires at least two helpers and is even easier with four. The work is always begun from the center of each side, pulling the cloth across from the middle of the opposite side. For heavy canvas and large pictures special canvas pliers are used.
Next, the priming of the canvas has to be completed. The first coats were given before the stretching because, unlike paper and wood, linen crumples and shrinks as soon as it is damp. The fibers swell when they are wet and shorten and stretch again when they are dry.
Thus, if the linen were placed untreated on the frame the excessive stretching of the fibers when it was wet would make it very slack after the first coat of priming had dried. But if the first coat of glue hardens in the fibers before the cloth is stretched, it prevents it from swelling and shrinking much afterwards; although in fact the canvas always gives a little when it is primed again and has to be given a final stretching before the actual painting begins.
The stretcher is so made that the edges can be forced apart by wedges. The wedges are placed in pairs into the corners; the laths holding the angles are removed, and the wedges carefully hammered in as far as necessary, while keeping the frame rectangular, to make the canvas taut.
A stretched canvas is always sensitive to knocks, and the priming makes it difficult to remove dents once they have been made; so it is much better not to stretch canvas but to stick' it down. This gives the most solid painting support while keeping the attractive woven texture and hiding the unnatural smooth surface of the board. Even a very thin canvas has a texture that looks like something that has grown naturally, and in this it is superior even to paper.
It is best to hand over the mounting of the canvas to a competent framer or bookbinder, although the process is less difficult than mounting paper. The board should be free of grease ,and slightly roughened; the paste must be of a good quality which can be used cold, like starch paste_or dextrin. Carpenter's glue works equally well, except that the work has to be done very quickly and in a warm room, and the board and canvas both have to be warmed beforehand or the glue will not take.
A cold glue allows more time to lay rhe canvas carefully according to the markings, and if a mistake has been made the canvas can be lifted off and stuck again.
Both board and cloth are covered with paste. Once the cloth is laid correctly on the board it is rubbed over with a straight-edged, rounded block; the movement is always from the center towards the edges, following the lines of the thread. Lastly, the overhanging edges of the canvas are bent under and stuck to the back of the board. The whole back is covered with cheap muslin or brown paper, concealing the turned edges of
The painting board prepared in this way is - better in quality and appearance than a canvas pn a stretcher and is more easily handled, both for painting and framing. It cannot go slack and is virtually invulnerable.
You already know that there are many reasons for priming: it prevents the binder from soaking into the cloth fibers, glue and filler being less absorbent, and with its white pr colored pigments it gives a durable background which is unaffected by light or binder.
Only paper is given just a single light priming; boards and canvas must have several coats. Generally, three applications are needed:
1. Size without filler-one coat.
2. Priming with filler-two or three coats, which will darken, however, especially in contact with oil binders thus necessitating:
3. Pigment priming-one or two coats, which can be white or colored, and will not darken with oil.
The second and third categories need more than one coat to make the covering
quite uniform. A single thick coat will always spread unevenly, causing the ground to absorb the paint unevenly. Moreover, a thick coat of priming hides the texture of the underlay too completely. A ground painted on in thin and frequent coats is more uniformly absorbent than one thick coat.
After each coat is dry, it is rubbed over to remove the skin of glue that forms, which would prevent the following coat from holding well. Smooth surfaces can be rubbed with a piece of sandpaper mounted on a wooden block; for canvases it is best to use a hard nylon or soft wire brush. Brushing penetrates the hollows in the cloth, whereas sandpaper would begin to smooth down the canvas itself. To ensure even distribution, the priming is always put on with the brush strokes running at right angles to those used on the previous coat.