Monday, February 5, 2018

335: Preventing Paintings from Cracking

–IN time nearly every oil painting will crack. But the challenge is to postpone that inevitability as long as possible. If you observe the paintings of Van Gogh, and other paintings of that era, you will see significant cracks in the lightest parts of the paintings. I attended a Maxfield Parrish retrospective a few years ago, and every painting was severely cracked. The painting surfaces looked like crazed porcelain plates–an unfortunate circumstance, especially since some of the paintings were less than 50 years old.

So how do we prevent paintings from cracking, especially with thick impasto passages that present additional challenges? There are several factors that lead to cracking. One of the easiest to control is the flexibility of the painting surface. Images painted on cradled wooden panels from the middle ages (some over 500 years old) show no signs of cracking. Many contemporary artists are painting on cradled aluminum panels that are extremely resistant to flexing. This is the current gold standard in painting surfaces. As you can imagine they are quite expensive. The solution, until I can afford aluminum panels, is to prepare stretched canvas on stretcher bars using acrylic technology.

The goal of canvas preparation is to make the surface stiff and impermeable to moisture. To make the surface less flexible, use the thickest canvas possible–somewhere between a 7 and 10 ounce is best. Stretch the canvas as tightly as your stretcher bars will permit. Use canvas pliers and heavy-duty stretcher bars if necessary. Paint the front of the stretched, raw canvas with a coat of matte acrylic medium. This will inhibit moisture from passing through the canvas and stop delamination of the oil paint from the gessoed surface. Some suggest priming the back of the canvas with an acrylic medium, but this can cause puckering of the back surface which can create surface distortions on the front of the canvas. Such stress will result in cracking. Never adhere anything to the back of the canvas (even a certificate of provenance).

To seal my canvases I apply Golden Colors Fluid Matte Medium to the front. Golden is a reputable company, and I prefer to use their mediums to ensure all the acrylic products will successfully cross-link in the preparation process. Cross-linking is a process where a chemical bond is created by the proximity of two layers of acrylic. When a plastic bag is left on a plastic surface, and you return later to find that the two substances have bonded together, that is cross-linking. It is a very powerful type of chemical bonding, and you want to foster such links in every layer of your acrylic applications.

The reason you apply a layer of acrylic matte medium first is you want the canvas to be impervious to humidity and to completely isolate the canvas from the layer of oil paint that will be applied later. The matte medium provides such an impermeable layer, both from the subsequent oil on top and humidity from below, while creating a surface the next layer of acrylic gesso can cross-link to. If the Fluid Matte Medium doesn't soak into the canvas easily, add acrylic wetting release.

Over the top of the dry acrylic matte medium apply two to three layers of acrylic gesso. The reason you don't paint oils directly onto the dried layer of Fluid Matte Medium is that oil will not chemically cross-link to acrylic. A physical, not a chemical bond, must take place between acrylic and oil. Gesso contains calcium carbonate (marble dust) to facilitate physical bonding. The marble dust additive makes the acrylic gesso medium porous. The porosity allows the oil to seep into the gessoed surface creating microscopic interlocking nodules. These nodules lock together like a zipper or velcro and form the mechanism of the acrylic/oil adhesion. Anything that inhibits this interlocking action, like adding a non-porous layer of acrylic medium or acrylic paint, will cause the oil to eventually delaminate from the acrylic surface.

Once my canvases are sealed and gessoed, it is possible to tint the surface with a thin acrylic wash. I prefer using Golden Colors' High Flow Acrylics because I can get a dark tint without having to use excessive amounts. You don't want to add too many washes as it will clog the porous nature of the gesso necessary for proper adhesion. One or two thin washes of the highly potent High Flow Acrylics will provide a deep color without clogging the gessoed surface.

I highly suggest sealing and gessoing canvases in this manner even if your canvases are commercially prepared (unless they are oil primed. Acrylic over oil will not adhere properly). Many formerly reputable companies are off-shoring their products, and the gessoed surfaces are often of dubious quality and will lead to cracking–sometimes within years, not decades.

This might seem like an elaborate process, but it is one that will prevent paintings from cracking well into the next century.

Brad Teare –February 2018

Above: Desert Journey, 20" x 24", oil on canvas, available at Anthony's Fine Art

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