Sizing Painting Surfaces
(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Sizing is the application of a substance, such as hide glue or acrylic polymer, to a support to reduce the absorbency of the support and to keep the paint from coming in direct contact with the fibers that make up the support. The oil in oil paint will cause unprotected natural fibers to rot and become brittle. Polymer emulsion paints will not adversely affect natural fibers and can actually help preserve them. Although sizing is not needed to protect a support from acrylic or vinyl emulsion paints, it is often used to reduce the absorbency of the support because, if too much of the paint sinks into the support, the color loses its intensity and develops a dull appearance.
The sizing used by the manufacturer of paper and board to produce a specific working surface should not be confused with the sizing needed for paintings with oil or acrylics. Watercolor paper, for example, is sized to help keep the color on the surface, not to protect it from the paint or to make it more permanent. All such sizing incorporated in paper and boards during the manufacturing process is not adequate for the preparation of supports for oil, or polymer emulsion, paints.
Sizing Paper. The technology that makes it possible to use paper as a support is relatively new and is still considered experimental when compared to other traditional supports. When producing fine artwork, it is safe to use polymer emulsion paints directly on unsized paper or board, although better results are often obtained when sizing is used. Using oil paint directly on unsized paper or board will result in self-destruct artwork because the acid from linoleic acid of linseed oil in oil paint breaks down the cellulose of the paper, and the combination of the two provides an ideal environment for the growth of mold. To prevent this kind of damage to the paper fibers, they must be coated with a size.
The easiest way to size a paper support is to apply two coats of undiluted polymer emulsion medium (only the working side needs to be sized, but less buckling will result if both sides are coated). This produces a “plastic”-looking surface and tends to make thin paper translucent. There are several ways, discussed below, to reduce this plastic effect to a minimum if the natural surface appearance of the paper is desired. These methods are still considered experimental, despite the current positive results. Paper will tolerate only limited contact with oil, including linseed oil. The inks used in fine art printing contain oils. However, during the printing process only a small amount of ink and oil is actually left on the paper surface. Some deterioration does take place, but in most cases after decades of use it has not been considered significant. There is obviously some room for error, but only time will tell how much.
Diluted polymer media can be used effectively as a size for paper supports with less change in the natural surface appearance. When applied in dilute mixtures, both gloss and matte polymer media will appear matte on most paper surfaces, and since the gloss version dries clearer it is best to use it whenever possible. Adding more than 50 percent water to acrylic media should not be done if a colored support is used because the overthinning can result in a milky, or cloudy, appearance. It also lessens the ability of the acrylic polymer to form a strong paint film. When used as a size for white paper or raw canvas, however, the cloudiness cannot be seen and a strong paint film is not desired. A mixture of one part acrylic medium to four parts distilled water applied in two thin coats to both sides of a paper support appears to give good results. The mixture is easily applied with a wide squirrel-hair brush or with an inexpensive foam brush. The wet paper should be laid out smooth on a clean sheet of glass to dry. When dry it may be easily peeled off the glass.
Gelatin is a refined glue. It is considerably more expensive than other types of size because of its unique manufacturing process. When used as a size for a painting support, gelatin should be hardened with either formaldehyde or alum. Expense and inconvenience have kept painters away from using gelatin as a size. Gelatin is used primarily as a size for such papers as watercolor paper where only a dilute solution is needed and the quality of the size is important. Nevertheless, if you wish to use a gelatin size, Winsor & Newton offers prepared size in two-ounce bottles. It should be heated gently until liquid and applied directly with a soft brush. Two coats should give adequate protection without seriously altering the paper’s natural appearance.
Animal hide glues, such as rabbit-skin glue, are among the oldest and most reliable substances used for sizing paper and canvas; the preparation of and application to both supports are very similar. (For more information about sizing canvas, see below.) The only difference is that for paper a 3 percent solution (one part glue to thirty parts water) is used and two thin coats are applied. Recent investigation has shown that hide glues seem to be adversely affected by such air pollutants as sulfur dioxide; exposure has greatly accelerated the aging process so that the glue becomes brittle and tends to crystallize. This has caused a great unresolved debate about whether to continue using such natural sizing agents as hide glues, which have a track record of several hundred years, or to turn to synthetic polymers, which have only been used for about forty years. At the present time, acrylic polymers appear to be relatively inert to most pollutants; however, contrary to most beliefs, they are porous and do not actually seal the support as hide glues do. Therefore, acrylic polymers do not necessarily protect the support from pollutants and chemicals. What they seem to do is simply provide mechanical dimensional stability to hold a support together despite what reactions take place. At this time, the common wisdom seems to be to use the acrylic polymers for sizing and grounds and then to use an oil-based protective varnish over the completed painting regardless of whether it has been painted with acrylics or oils.
Sizing Canvas. Canvas, like paper, has to be protected with a size from the acidity and rotting effects of linseed oil. Acrylics are inert and can be applied directly to canvas without sizing; however, sizing would contribute to better control during subsequent application of grounds or paint. A mixture of half water and half acrylic polymer emulsion, gloss or matte, applied in two thin coats will act as a size for both oil and polymer emulsion grounds.
The traditional method of sizing a canvas for oil paintings involves the use of a hide glue, such as rabbit-skin glue. A hide glue can be prepared by making a 10 percent solution (one part glue to ten parts water). The glue is soaked in room-temperature water for twelve hours or until the crystals swell, at which point the mixture is heated in a double boiler to about 175°F (80°C). If the glue is heated too quickly or brought to a boil, it will become an unusable gelatinous mass. After the glue has liquefied, filter it through cheesecloth. Allow the glue to cool to about 140°F (60°C), then apply it with either a brush, spatula, or painting knife. Be sure to work the glue into the canvas. This will fill any pin holes.
The purpose of the sizing is to seal the fibers of the canvas, not to provide a continuous, solid paint film. If too much glue is applied, cracking will result. In addition, the ground will bond more to the glue than to the support, and as the humidity changes the glue will expand and contract more than either the ground or the support, which can cause the ground to detach from the support. A ground will adhere better if the sized support is rubbed with a pumice stone or lightly sanded. Rabbit-skin glue is still available from the Fredrix Artist Canvas Company and the Holbein Company.
Sizing Panels. Wood, or wood-product panels, such as plywood, particle board, and fiberboard, should be sealed with a wood sealer. Modern wood sealers reduce the wood’s absorbency and help to preserve and protect it-desirable qualities of a size. Consequently, a wood sealer can be used as a size for panel supports. In the past, hide glue was used for wood just as it is for paper. When a waterborne size, such as hide glue or polymer emulsion, is used the figure of the wood will swell and rise, requiring sanding to make the surface smooth again. Panels made of metals, such as the aluminum honeycomb panels, are not absorbent and do not have to be protected from the paint, hence sizing is not necessary.