Stretching a Canvas Using Unprimed Linen is Explained
Canvas has become the generic term applied to the coarse-fibered, closely woven materials used by artists as flexible supports for painting. The most common types of canvas used are linen and cotton. Two advantages which make cotton desirable are its affordable price and its stretching ease. It is possible to stretch cotton tighter than linen, without straining its wooden support, because cotton fibers stretch more easily than linen fibers. Although not as strong as linen, a heavy grade cotton fabric can compensate for its lesser fiber strength by its weight.
Linen, although expensive, is traditionally the painter's preferred fabric. There are four reasons for this. One, linen is the most durable of all fabrics for painting. This is because the warp and weft threads are equal in weight, making linen less susceptible to expansion and contraction problems from moisture. Two, linen retains its natural oils over time, which preserves fiber flexibility and decreases embrittlement with age. Three, linen is very receptive to sizing and priming films. And four, linen is characterized by a pronounced weave- less uniform and mechanical than that of cotton- and thus more interesting to paint on. Available in a variety of textures from smooth to rough, and in weights from light to heavy, linen maintains its distinctive weave even through layers of paint.
While linen is a more durable painting fabric than cotton, stretching raw linen requires more care and delicacy than does cotton and is far more time consuming. If you have never stretched a canvas, raw linen is not a good practice material. It is far too costly and finicky to attempt to stretch linen without experience. If you want experience, stretch cotton, which is available in inexpensive student grades. Imperfections can be smoothed with a pumice and these canvases provide a great surface for all kinds of painting.
While primed linen (as well as primed or unprimed cotton) can be stretched tightly on its support bars with canvas pliers, raw linen must be stretched loosely to its frame, preferably with #6 carpet tacks. This should be done by hand, rather than with pliers, since the nature of the fabric will change substantially after the canvas bas been primed. Because linen has a more pronounced weave than cotton, it is important that the weave of the fabric is either parallel or at right angles to the stretcher bars. Linen must be stretched with equal tension on all sides. These precautions will prevent the linen from damaging the stretcher bars as it tightens after priming. If an acrylic gesso is used for priming, you can expect the linen to tighten up considerably when dry. The extent to which it tightens up also depends on environmental conditions. If you live in Arizona, for example, you may have to stretch looser than we would here in Seattle.
If an oil primer is used, the outcome is somewhat less predictable. Following an application of rabbit skin glue sizing, the canvas will most likely tighten. After an oil primer is applied, linen has been known to either tighten up or loosen itself and sag, depending on the grade and weight of linen, the consistency of the primer and the environmental conditions.
What all this means is that you have to experiment with stretching unprimed linen because there are so many variables. To determine how loosely the fabric should be stretched comes from feel and from experience. If a particularly heavy grade of raw linen is stretched too tightly, its strength could sufficiently warp or even split the stretcher bars or joints after an application of rabbit skin glue and primer. Therefore, it is safer to stretch raw linen too loosely, and restretch after priming, than to risk severe damage to the stretchers. Heavy duty stretcher bars should always be used with linen canvas, and sizes over 48" should be braced, as well. If, after priming, the linen is only slightly loose, restretching can often be avoided by the insertion of corner keys.
Some artists stretch large pieces of raw linen, for sizing and priming, on a heavy frame made of 2" x 4"s. When dry, these pieces are then cut to the desired size and restretched on permanent stretcher bars. Although restretching is time consuming, it is a good method of insuring that if the linen either tightens up or sags after priming, it can still be transferred to a final support without damage.