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Q. I can’t find a definitive answer to the question of whether it’s archivally sound to do an oil painting over acrylic gesso. I’m going to be teaching some classes soon and I want to give my students the correct information. Can you help?
A. Your question brings up one of the toughest issues facing artists and conservation scientists today. If done correctly, acrylic gesso is acceptable as a ground for oil paints, although the final word has yet to be clearly articulated by researchers. Recent studies at the Smithsonian Institution by Dr. Marion Mecklenburg and his colleagues give very good marks for acrylic gesso in this role, but the technique for applying the gesso is critical to its success.
First, there are a few basic problems with acrylic gesso grounds that make them risky to use with oil paints. If applied directly to the support, gesso can draw water-soluble components from the support as the gesso dries, causing it to discolor. Second, if the surface of the gesso is sanded too smoothly the oil paint might not make a secure bond. Third, acrylic gesso grounds absorb oil from the paint layer, which can saturate the gesso and later darken.
However, while problems have been reported with adhesion between oil paint layers and acrylic gesso grounds, most examples of failure with this pairing involve a problem in the application technique of either the acrylic ground or the paint layers. Here are a few guidelines you can follow to avoid these problems and enhance the archival quality of your painting:
- When applied to unprimed linen, cotton or hardboards such as Masonite, acrylic gesso will darken—an occurrence known as Support Induced Discoloration (SID). Research by Golden Artists’ Colors has produced conclusive evidence of this phenomenon. (One of the best sources I’ve found for more information on acrylic gesso is Golden’s Web site at www.goldenpaints.com.) To prevent SID, seal the canvas or hardboard with a thin layer of clear acrylic medium or with a sizing of acid-free PVA. To reduce movement of the canvas and prevent the oil from migrating into the fabric, coat both the front and back of the canvas.
- Allow the first coat of medium or sizing to dry, then apply a second coat to the front.
- Once the sized canvas has thoroughly dried, sand it very lightly.
- With a stiff brush, work a coat of acrylic gesso into the front of the canvas, then level the coat by drawing the brush across the entire canvas in one direction.
- After the gesso is thoroughly dry, lightly sand this first coat.
- Again with a stiff brush, apply another coat of gesso just as in Step 4, but this time level the surface by drawing the brush in a direction perpendicular to the first coat. Golden recommends using a total of four coats of acrylic gesso to prevent SID.
- Allow a drying time of at least two or three days.
- Before you begin painting, wipe the surface with a cloth dampened with water, mineral spirits or turpentine. Acrylic gesso contains additives to enhance its performance, and some of these materials will migrate to the surface as the water evaporates. Wiping the surface should remove these additives. Let the cleaned surface dry before starting to paint on it.
I recommend applying at least three coats of acrylic gesso to your support to keep the gesso from becoming saturated and darkened by the oil, but don’t over-sand it. Oil paint doesn’t form a chemical bond to acrylic gesso; the bond is entirely mechanical, and sanding the gesso to a glass-like smoothness reduces the surface area for bonding with the oil paint, thereby reducing the adhesion. Also, any tone or imprimatura applied to the surface of the gesso must be the fastest drying layer in the oil paint film, so let it dry thoroughly before painting over it.
Finally, don’t underpaint your oils with acrylic paint. Acrylic gesso is formulated to have an “open” surface to accept the paint layer, but acrylic paint has a “closed” surface to protect the paint film from stains, grime and other pollutants. The closed surface is nonabsorbent and provides a very poor bond for oil paints.
Loraine Crouch is assistant editor for Magazine.