Artist Surfaces - Information and Advice

Information and Advice

How to Stretch Your Canvas

Before stretching raw canvas with a view to applying one or more coats of primer, it’s important to understand something about the nature of the materials involved. For example a wooden stretcher can become warped or twisted if exposed to drastic changes in temperature, or bowed if left upright at a severe angle for any length of time (e.g. overnight). The same problems can often be caused by canvas that has been stretched too tightly. Unlike pre-primed canvas, raw linen or cotton duck having no coating of size or primer to stiffen it, can be stretched much further before reaching its limit - and so can quite easily be stretched too tightly. After the primer has been applied and has soaked into the threads, it causes the canvas to shrink as it dries, which in turn causes the stretcher to bow badly enough so that it will not hang flush against a wall.

To avoid this and preserve the long term integrity of the stretcher, there are certain ways of stretching that may help.

When using un-primed cotton duck: firstly the canvas should be stretched just tight enough so that when lying flat and face up, the surface is smooth and not sagging in the middle, and so that by pressing on it, the frame beneath can easily be touched, particularly at the corners. This should ensure that after priming, when the canvas has dried, it will have tightened enough to give the desired tension, but not so much that it distorts the stretcher.

Just how tightly the canvas should be stretched can depend greatly on the size of the stretcher and the weight of the canvas. For example 15oz cotton duck, being the heaviest canvas with the thickest weave, needs to be stretched more loosely than 9.5oz which is much lighter and far less likely to cause a stretcher to bow. And naturally, the smaller the stretcher, the stouter and more rigid it is, whereas the larger it is, the more flexible it will be, making it far less able to resist the force of contracting canvas.

The main difference between un-primed cotton duck and linen, as far as stretching is concerned, is that unlike cotton duck, linen can only really be stretched with the weave; when going against the weave it has little or no give at all. This can often result in rippling along the sides, which becomes more prominent after priming. The best way to eliminate this is to stretch against the weave by hand, not with canvas pliers, and only so far that the linen is smooth and flush against the sides of the stretcher. Then when you come to stretch with the weave, any rippling along the already fastened sides can easily be stretched out.

Although linen is a much stronger material than cotton duck, even at its most coarse it is much lighter and so doesn’t need to be stretched as cautiously prior to priming. When using linen on large stretchers, it is often best to stretch as mentioned earlier, apply one or two good coats of primer, then once dry, re-stretch more tightly before adding further coats.

As far as very thin or fine cotton canvases, such as calico, are concerned, the material is so thin that it can be difficult to get the right amount of tension in it when stretching it over a large area. It also tends to dry rather inconsistently after being primed or painted on, often leaving lumps or dips that are isolated in the canvas making them nearly impossible to stretch out. For this reason calico should not really be used on stretchers over a metre square. For anything larger, if a very thin canvas is needed, then fine linen is a far better surface to work with.

The nature of the materials concerned means that none of these tips are guaranteed to prevent a stretcher from becoming bowed in the long term, but they should ensure that providing the stretcher was straight to begin with, once the surface is fully prepared it will still be straight.

Warping and Twisting

Wood is a natural material and despite our careful selecting, rigorous checks and quality control procedures, there is always a risk of warping or twisting. It’s possible to reduce the risk of this happening by ensuring that the stretcher bars are not exposed to extreme temperatures, damp or humidity – elements that are often difficult to avoid in urban artist studios and some galleries. It is important to store stretchers in the correct way, ie. try not to lean stretched canvases and paintings against the wall – instead keep them flat to the wall, especially if they are being stored for long periods of time. It is also a good idea to strap stretcher bars together and keep them weighted down.

If warping and twisting is a concern to you, please read our tips on stretching canvas. Alternatively you may wish to consider our aluminium stretchers.

Wooden Wedges

Timber, like stretched canvas and linen, can expand or contract when it comes into contact with moisture or is exposed to changes in temperature over short periods of time. The combination of these changes will cause the fabric to fatigue. To help remedy this, wooden wedges can be inserted into each corner of the stretcher so that the corners can be tapped out, expanding the stretcher itself and thus tightening the fabric. Although this is a quick and simple way to remedy the problem, it will change the dimension of your painting and weaken the corners, especially if it is tapped out too far. Therefore the best option is always to re-stretch your painting.
Wedges are also not an option on our circular or welded stretchers.

Priming

To prevent mould, especially on Claessens primed linens, the fabric is treated with a 0.3% to 0.5% solution of zinc chloride or a 0.2% solution of magnesium silicofluoride, which is a waterproofing, hardening compound. Some manufacturers add anti-mould treatments to their pre-primed canvas.

Sizing is a layering between the support and primer to protect it from oxidation. The traditional glue used for sizing is made of rabbit skin or hides. It has been used by artists for centuries and is purchased in a powdered form or prepared. When soaked in water and heated in a hot water bath it becomes a viscous gelatine that, when applied to canvas, protects the cloth from the acidic effects of the linseed oil found in oil paints and oil grounds.

Alternatively acrylic canvas sealer can be used, mostly on cotton surfaces. The sealers are non-hygroscopic, which means that they will not swell and shrink with changes in temperature and humidity. This size is recommended for a base coat on acrylic gesso primed canvases.

The basic ingredients of traditional gesso are rabbit skin glue and white chalk (whiting). Gesso is not a flexible ground and should not be used on a flexible support such as stretched canvas. It is perfect on wood panels, especially for creating a pure white ground. To make the gesso ground more white, Titanium White pigment can be added in proportions of 1:9 with the whiting or precipitated chalk. For a very smooth finish surface, clay can be added (red or white bole, China clay, or kaolin). For a rough surface with tooth, add powdered pumice, sand, silica or silex (powdered quartz), marble grit or limestone dust.

Acrylic gesso primers are not actually gesso at all, but a pigmented acrylic resin. As such, they are non-yellowing and non-cracking and give good adhesion to most surfaces without the need for a preliminary coat of glue size. They are extremely flexible and are appropriate on canvas for use with acrylic and oil paints. It is very common to find Acrylic gesso primers used on pre-primed canvases.

Traditionally oil primed linen cloth has been sized with rabbit skin glue and primed with layers of Lead white paint. Lead white is a flexible pigment that covers well, producing an ideal surface of good texture and absorption for oil paints. Lead as Flake or Cremnitz whites are available ground in cold-pressed linseed oil. The toxicity of lead has, in recent years, caused many manufacturers to replace it with combinations of Titanium and oil-modified alkyd resin. Modern oil primers or Alkyds are an alcohol and acid petrochemical resin that is less flexible than linseed oil, making it acceptable in grounds and under painting following the fat-over-lean rule.

Claessens produces only quality hand primed linen rolls that go through several processes to ensure quality. Starting with only the finest unprimed linen the canvas is inspected, pumiced with a stone, brushed clean and sized with traditional Rabbit skin glue. The canvas is then oil primed with a primer of zinc-white and linseed oil, allowed to dry 3 days and then coated with lead white.