Return to Technical Information
by Nathan Zakheim
(Published originally in the MCLA Newsletter, v. 4, ns. 3,4, 1992)
One of the most common laments among artists goes something like this: "I wish the colors I paint would remain as vivid years from now as when I first painted them." Why don't they? Why do they appear to fade? Why do they turn dark and brownish? What happens to the beautiful colors once they are put on a wall?
To understand the nature of the result, the four main elements that constitute a completed painting must be examined:
· The wall preparation
· The choice of pigment
· The composition and use of the medium
· The coating(s) applied to the surface of the finished painting
Wall preparation. Acrylic paint is much more porous than oil, enamel, or alkyd. For this reason rust, mildew, algae, or migrating unbound pigments already on the wall can leach, or migrate through, when exposed to water or atmospheric dampness. This results in difficult, even impossible to remove stains. Thus, prior to applying paint to a wall it is important to clean it as meticuously as you would a skillet before cooking. Nothing should end up in your "dish" that is a possibly toxic residue of what was there before.
Pigment. Pigments are graded by international standards for color fastness. Eight is the most color fast, and anything over six will do quite well out-of-doors. As ancient Frescoes, sand paintings, petroglyphs and other pure-pigment art demonstrate, certain pigments can remain in direct sunlight for thousands of years without any indication of fading. These pigments are earth, metal, and chemical colors that are neither dyes nor tints. Dyes and tints, such as alizarin crimson, berry juice, etc. will bleach quickly due to ultraviolet exposure that occurs in direct or indirect sunlight. How do you find out the color fastness of the pigment in your favorite tube of paint? Ask the manufacturer, if available, consult their product data sheets. Today most paints are derived from much different sources than they were as recently as fifty years ago. Real cadmium, cobalt, copper oxide, to name a few, are no longer used. They are now formulated to appear similar to the traditional pigments. They may be extended out with white, resulting in an appearance that can differ with traditional counterparts. For the most part these new colors are very colorfast and without the addition of med-ium or varnish they will not fade when left in direct sunlight.
Medium. The medium of today's acrylic paint is the most vulnerable element in terms of color retention. It is the medium that absorbs and is stained by smog, mildew, or rusts from the wall. It is the medium that disintegrates in the ultraviolet light (UV) or direct sunlight. And if there is not enough medium in the paint mixture, the pigment will become exposed and the mural will come off the wall as a powder or in flakes.
Acrylic medium is quite vulnerable to UV, which I often see crystalized into a powder. The color of such murals appears "faded", but this is misleading. Using a spray device, one can apply xylene or toluene to the faded surface. Care should be taken to apply it in several rapid applications so that the solvant penetrates through all of the paint layers without dissolving them. At the molecular level, the acrylic is lubricated to re-form the paint film: The colors reappear as brilliant as when they were first applied, less whatever other erosion or damage may have taken place.
Protective coatings. Never apply anything to the surface of your mural that cannot be taken off without damaging the mural. A good protective coating will preserve and protect the paint beneath it. All too often, coating media is applied that obscures, discolors, or deteriorates the paint. I strongly recommend that use of soluvar varnish as the medium that offers the greatest protection and the least damage. The following commonly employed media should in fact be avoided by artists serious about protecting their work over time:
· Acrylic varnish (or acrylic medium used as a final varnish) forms a transparent layer on the surface, but UV from sunlight is reflected within the surface of the varnish. With time the varnish's molecular bond breaks down. The varnish gradually clouds, causing the mural to appear faded. Also, smog and other particulate matter in the air clings to the surface, gradually seeping into the pores of the varnish. The result is a gummy looking amber-tinted coating that alters the true colors beneath. This cannot be cleaned from the varnish.
· Urethane, spar varnish, damar varnish can only be removed with solvents--such as alcohol, toluene, xylene, or acetone--that are destructive to acrylic paint. These varnishes tend to discolor over time, and will partially loosen and peel away. Because of the solvent problem, once a mural is inhibited by this gummy barrier, clean removal of these varnishes is a difficult matter.
If an acrylic varnish is desired in order to give a mural a uniform appearance, it should be thinned with water and put on thin. This will minimize UV damage. A layer of soluvar should then go over it.
By considering each step in the process an artist can create a mural that will suit both aesthetic and technical requirements. The result should be murals that will last and retain all of their vibrant colors and beauty for a very long time.