How to Prepare a Canvas the Safe Traditional Way
The following describes the materials and techniques used in the construction of paintings by Damon Denys and Bryan Larsen. The methods described have been chosen by the artists for the purpose of ensuring the highest possible archival quality of the final product, in keeping with the most long-standing and time tested findings by chemical and material scientists and researchers working in the field of conservation and preservation of paintings. Whenever possible, the exact technical names and ingredients have been listed so the readers may verify for themselves the soundness and durability of our process.
Construction of the Stretcher Frame
The stretcher frame is the wooden, rectangular frame of wood on which the linen (called the "support") is stretched to create a flat surface for painting. The stretchers we use are usually made by the Frederix art company. Frederix supplies stretchers in more than one kind of wood, but those most often used by us are of pine. We are also trying out some stretchers made by Upper Canada Stretchers, so far they are working very well.
There are two different weights of stretcher bars available in most art stores. The heavier type is approximately 1.5 inches thick and 2.25 inches wide and is suitable for medium to large canvases over about 25 inches in their longest dimension. This weight is also suitable for smaller canvases, but is more expensive than the lighter weight stretchers and as such is usually used only for larger canvases that require the extra strength of the heavy bars to maintain their shape after the canvas has been stretched. The lighter weight bars available are about half the size of the heavy ones, and are usually suitable only for canvases under 25 inches in their longest dimension.
Stretcher frames consist of 4 separate pieces of wood (one for each side of the rectangle), which fit together at the corners by interlocking mitred joints. The mitred joints allow the pieces to stay firmly together, without actually having to secure them together with nails, staples, glue, or any other means. The joint also allows the insertion of wooden wedges called "keys", with which the stretcher can be expanded in order to take up any slack in the linen due to changes in humidity or to fatigue caused by age. "Strainer frames"--which are the same as stretcher frames but are firmly secured at the corners--don’t allow for expansion of the frame, which can lead to damage of the painting surface if significant sagging takes place in the linen support.
On canvases of larger dimensions, one or more crossbars may be necessary to prevent the bowing-in of the frame under the pressure of the stretched canvas. If a crossbar is not used, the stretcher bars themselves may not be strong enough to keep straight against the strong pull of the tightly stretched linen, resulting in a bowing toward the center of the canvas, which can lead to framing problems and an uneven--and potentially damaging--distribution of stress on the final paint layer.
Appropriately-sized lengths of wood for use as cross bars can sometimes be bought alongside stretcher bars in art stores, but unfortunately are not ready to use. The wood is available in the correct width (about 3/4 inch), but must be cut to the correct length to fit the interior of the frame. The crossbar should be of the same kind of wood as the stretcher bars to ensure an even rate of expansion during atmospheric changes in temperature and humidity. It's also important that the cross bar is not secured to the sides with glue or nails, but is instead fitted to allow the frame to be expanded. A simple way of doing this is by drilling two equally spaced holes (about 5/16 to 1/4 inch size drill bit, and about 1/2 inch deep) in the ends of the cross bar, with two matching, alligned holes in either side of the inside of the stretcher frame. From a wooden dowel, which is the same width as the drill bit, are then cut four segments about 1 inch in length, which are inserted into each of the four aligned holes and used to keep the cross bar in place without the use of glue, staples, etc.
The Linen Support
The term "canvas" in painting is a general term to refer to practically any kind of fabric that is used as a support. "Cotton duck" and "linen" are two distinct fabrics made of two different kinds of plant fibers. Duck--the most commonly available and least expensive form of canvas--is a white fabric made of cotton fiber. Linen--a much more expensive and less commonly used (in modern times) canvas--is a light brown fabric made from the fibers of the flax plant, which is the same plant from which linseed oil--the most commonly used oil in oil painting--derives.
The most ideal canvas for oil painting is raw, unprimed, fine-weave linen. Our linen is also usually purchased from the Frederix art company, and measures about 35 threads per inch. Linen is superior to cotton canvas for use in painting because it’s less flexible and expands and contracts less with changes in humidity and temperature, thus creating a more stable support for the brittle layer of paint upon it. The higher volatility of cotton can result in cracking and damage to the paint layer that can be avoided with the use of linen. Linen is universally accepted by conservation scientists as being superior to cotton. Despite this fact, most artists--being unaware of the technical issues of archival painting--continue to use cotton duck canvas, mainly because it is so much less expensive.
Mounting the Linen on the Stretcher Frame
The process of mounting the linen on the stretcher frame is called "stretching the canvas". When stretching the canvas, it's important that the threads of the linen are exactly perpendicular to the frame. This ensures even tension and stress when the linen is stretched tight to produce a firm, flat surface. Uneven stretching of the canvas can cause uneven tension on the paint layer and result in eventual damage and warping.
The linen is secured into place with steel framing tacks. The tacks we use most often are Frederix "Artist Canvas Tacks", no. 6. They’re made of copper-plated hardened steel and measure about one half inch in length. The copper plating ensures more longevity than simple steel, which tends to rust easily and can cause damage to the linen. Mounting canvas with staples, nails or uncoated screws is a common practice, but is not preferable due to their potential of rusting. Copper coated tacks are readily available from art supply stores and are relatively inexpensive, making the use of staples and nails especially unnecessary.
The tacks are spaced about 1 inch apart down the length of the stretcher, and are placed along its thin, outer edge. This spacing is to prevent warping of the stretched linen. If the tacks are spaced too far apart, warped "draws" can form in the linen between them. These curved draws present areas of unequal tension on the canvas surface, which should be avoided.
An important consideration in stretching the canvas is to keep the stretcher in a solid rectangle. The tension applied on the frame during the stretching process can easily work the frame out of place, resulting in an uneven frame. To keep the frame in place during the stretching process, four triangular pieces of sturdy cardboard are stapled into the corners of the frame. This maintains the integrity of the frame without the necessity of nailing or gluing the corners together. After the canvas has been fully stretched, the triangles can be easily removed, and the pressure of the canvas itself will do the job of keeping the frame in the right position.
The easiest way to stretch the canvas while keeping it perpendicular to the frame is to start by securing it temporarily (staples are the easiest way of doing this) in the middle of each side of the frame, creating a fairly firm center area on the canvas. This holds the canvas in place while the rest of the fabric is stretched and secured with the tacks. Place 3 tacks on each side of the center staple on one side of the stretcher frame (this is the "first" side), then do the same on the side exactly opposite (this is the "stretching" side), pulling the fabric taut before each tack is set. Rotate the canvas, and do the same for the other opposing sides. Rotate again, and set the next 3 tacks on either side of each stretcher bar and continue alternating until the job is complete. It works best if you keep track of which sides are the "first" sides, and which sides are the "stretching" sides. Otherwise, it's much easier to end up with an uneven stretch. Don't forget to replace the temporary staples with tacks before you're done.
After being stretched, an extra couple inches of linen are left on each side to allow re-stretching of the canvas, if necessary, in the future. A painting executed with good archival techniques will outlive its stretcher bars and its fabric support. Both will eventually have to be replaced. The process is not difficult, assuming the artist took steps to allow for it. Leaving an extra few inches of fabric on each side of the frame makes it much easier for conservators to replace an old frame. This extra fabric should be folded over to prevent frayed edges from getting out of hand, and secured to the rear of the frame with more tacks. Neglecting this simple step can cost a patron hundreds of dollars in future repairs.
A final note about stretching is to mind the initial tension of the stretched canvas. It isn't necessary to stretch the canvas too tightly as this stage. The linen may be stretched just tightly enough to take up the slack in the surface. Once the sizing glue is applied, the canvas will shrink and become much more taut. Over-stretching the canvas at this stage can result in extreme tension of the final support, which can lead to many undesireable problems, including warping of the stretcher frame and tearing of the support fabric. The common stretching pliers available in art stores are generally unnecessary for stretching most canvases, which can be adequately stretched with one's hands, but are appropriate for those who wish to avoid sore thumbs. Just be careful not to get too carried away with the extra leverage the pliers provide.
Coating the Rear of the Linen with Formaldehyde
It isn't absolutely necessary to take the following precaution, but doing so will increase the lifespan of the fabric support, which in turn will reduce the overall cost necessary to care for the painting in the long run:
After stretching the linen, the rear side is sprayed with a solution of 4% formaldehyde, 96% water and allowed to dry completely. This thin coating is an added means of preventing early deterioration of the fabric by making it repellent to many organisms. Care is taken to make sure that the strips of the fabric that are underneath the stetcher frame bars get sprayed as well. It's not necessary to soak the fabric. A simple misting will do. Bottles of formaldehyde are usually available from pharmacies or lab supply shops, but will need to be diluted to the proper 4% strength. If the lab won't do it for you, you may need to purchase a graduated cylinder. Formaldehyde is poisonous, so be careful when you handle it and store it safely.
Sizing the Linen with Glue
The process of coating the painting surface with glue is called "sizing", and the glue itself is often called "size" or "glue sizing". When the formaldehyde solution has completely dried, a solution of water and glue is prepared. The glue we use is Frederix brand dry granulated animal skin glue, which comes in a tub and is a brownish, powdery substance that smells vaguely like a wet dog.
To make the sizing, you'll need to double boil it. A double boiler consists of a container of heated water, into which is placed a second container that holds the glue solution. By heating the water in the first container, the contents of the second container can be carefully heated without running the risk of scalding. For the first container, we use a common electric tea pot, which can be plugged into any wall socket, filled with water, and comes equipped with a dial to control the temperature of the water. These are available at many kitchen supply stores. The second container can be any pot that fits into the first one. It's useful to have something large enough to allow larger batches of glue to be made. A container large enough to carry 1/2 quart (about 500 ml) of glue solution should suffice for most canvases that are under 6 feet (about 180 cm) in their longest dimension.
In the double boiler, the glue is mixed with water at a ratio of 12 parts water, 1 part glue granules. This mixture is double boiled until the glue is completely dissolved, taking care not to let the glue mixture itself ever reach the boiling point. If the glue is allowed to boil, it will undergo changes that will make it unsuitable as sizing. This is why it's important to be able to control the termperature of the water in your boiler's first container. Stir the glue frequently while it dissolves. You can always test to see if your sizing is the proper strength by allowing it to cool for several hours. If it cools to become something that highly resembles and acts like Jell-O, then it's strong enough. This cooled Jell-O glue can be re-heated and applied to the canvas as usual, but shouldn't be allowed to sit around for more than 24 hours.
A coat of the melted glue is applied to the front painting surface of the linen and to the thin sides in which the tacks have been driven. The object is to soak the linen thoroughly and let it dry completely. The first coat takes the longest to dry because it has penetrated deep into the fibers. Subsequent coats will have less fabric to penetrate, and will dry more quickly. A second coat is applied to ensure that the linen fibers are completely sealed off on the front side of the painting. Additional coats are then applied, making sure that each is thoroughly dried before a new one is added.
A spare piece of linen is used between coats to scrub down bumps and smooth the surface. Never use sandpaper in constructing a canvas! The sharp teeth of the sand particles are too abrasive and will cut through the glue sizing layer, re-exposing the raw fabric. The whole point of sizing a canvas is to seal the fibers off completely from any contact with the oil in the painting medium. Sandpaper is the surest way of undermining all your careful preparations.
Between 3 and 6 total coats are used on a canvas, and the amount of canvas texture that is evident on the final canvas can be controlled somewhat by regulating the layers of glue applied. When you apply the first coat, you will notice that the linen becomes much darker in color. As it dries, it will return to its natural color. As more coats are applied, more of the fabric will become completely sealed off, and fewer areas will change color when a coat is applied. When you apply a coat and no color changes are visible, you can be sure that the entire canvas has been successfully sealed. This usually happens on the 3rd to 5th coat. Do not apply too many coats, as the sizing layer will become too thick and will be prone to cracking. This will also happen if your glue solution contains too high an amount of glue particles. To be safe, 6 coats are about the maximum you should risk, and every canvas should have at least 2 solid coats.
After each coat is applied, you will notice that the linen becomes quite lax on the stretcher frame. This is normal and is because the fabric is expanding due to exposure to the water in the glue. As the water evaporates, the fabric will become taut again. You can test to see if a coat has dried completely by touching it. If it's cool to the touch, that's a sign that evaporation is still taking place. If it seems to be at room temperature, that's a good sign that it's ready for another coat. The areas that usually take the longest to dry are those closest to the stretcher bars. It's important to wait until the entire surface has dried before applying the next coat. After the final coat has dried completely, the surface should be quite taut. Tapping it should produce a sound rather like a drum.
It’s extremely important to prepare the linen with glue. Any oil that comes into contact with the actual linen fibers will greatly increase the speed at which the fabric deteriorates. Also, oil that seeps into the fabric and dries essentially secures the paint layer to the fabric. This means that when the fabric eventually decays (as it must), the paint layer itself may be unsalvageable. If the paint layer is separated completely from the fabric with a layer of glue, the old canvas can be removed and a new canvas can be re-attached quite easily to the paint layer by a professional restoration lab, thus ensuring that the life of the paint layer itself is independent of its fabric support. Again, it’s important not to use sand paper when smoothing the surface of the fabric during this process. The grains of the sand paper can scratch through the glue layer and leave the fibers exposed and unprotected. By using a piece of linen to smooth down the surface, we ensure that the glue layer will remain unharmed.
Priming the Linen with Lead White
The process of adding initial fast-drying layers of paint or gesso to the surface of the support before it’s painted on is called "priming". True "gesso" is composed of a mixture of powdered calcium carbonate (a lot like chalk dust) mixed into animal skin glue. Gesso should never be used for priming fabric supports! Gesso becomes extremely brittle when it dries and will crack easily as the flexible fabric expands and contracts. True gesso (the kind Cennini describes in his famous 15th century book on painting technique) is only used when painting on firm supports like wood or walls.
There’s a common priming product available by most art suppliers that’s labeled "gesso", but it is not true gesso. Usually, it's actually a form of white acrylic paint. Although this product isn’t brittle like true gesso, acrylic itself is a medium that hasn’t been used in art for enough years for artists or conservationists to be certain of its stability or longevity. Instead of either of these products, fabric oil supports should be primed with a mixture of lead white oil paint and turpentine.
Lead white--often called "flake white", or "crementz white"--is lead carbonate mixed with linseed oil. We use Old Holland brand lead white, which uses cold pressed natural linseed oil. Many products labeled "flake white" and "crementz white", or "foundation white", "underpainting white", etc. contain lead, but also contain zinc and/or titanium. These are not as desireable as pure lead paint because zinc and titanium are slower-drying and more brittle than lead when they dry. Both of these qualities can lead to cracking. Linseed oil is preferable to other kinds of oil for the same reason. A pure lead-based paint using exclusively linseed oil is most desireable for use as a priming coat.
To three parts of lead white is added one part pure spirits of turpentine. This is true turpentine, not an artificial "turpenoid" product containing harmful petroleum components. True turpentine can be identified by its smell, which is a pleasant pine tree odor, unlike the artificial turpentine products that have a strong petroleum smell.
We typically coat each canvas with 2 layers of the lead mixture (called "double-priming"), allowing the first to dry completely before the second is added. This ensures full and complete coverage of the surface at a reasonable thickness. Any extremely thin layers of paint applied to the surface after this will thereby not be in danger when the fabric support must be replaced down the road because they’ll be firmly secured to a reasonable buffer layer of white paint. At this stage you can control the amount of canvas texture present in your final canvas. It's a good idea to apply the paint with a brush to ensure an even coat, but after the coat is applied it can be smoothed out with a pallette knife or by use of squeegee-like paint-sculpting tools, which are often available in art stores.
Laying the Design to the Surface
It's a good idea to allow your primed canvas to dry for a minimum of 1 week. Ideally, a canvas would be allowed to dry for at least a month, or up to six months if possible. If the priming layer has not dried completely before it's worked on, it will still be slightly malleable. This can lead to a very slight scoring of the surface. If, for example, a design is drawn upon a semi-dry canvas with a pencil, the pencil will leave very subtle grooves in the surface which, after the painting has been completed, can be visible to the naked eye if light hits the surface from the right direction. The result is a distracting ghost image underneath the paint. This is, of course, something to avoid.
Once the priming is dry, the design for the painting may be drawn out on the surface first with a charcoal, graphite or pastel pencil, or be painted on using a thin, inky mixture of umber and turpentine. This mixture dries within a matter of minutes and can be cleanly wiped away, allowing for ease of use in applying the design. Whichever method is used, paint may be applied directly on to the design, or the design itself may be "set" by coating it with a thin mixture of linseed oil and turpentine. Setting the design ensures that it will remain unchanged during the painting process and prevents color from the graphite or pencil from getting picked up and "dirtying" the paint. If charcoal is used for the design, this can be a big problem. If a pencil is used for the design, a soft lead is preferred so less pressure needs to be applied to the canvas while working, and the lines are more easily erased. A soft kneaded eraser is usually used for this purpose.
