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All Pigments Are NOT Made Equal: Recommendation for exterior mural use.
02/25/08 Blog
The Philadelphia mural tour
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Seth Turner (Philadelphia Mural Art Program) and Amanda Norbutus examining the parachute cloth adhesion to the wall on Dr. J, Kent Twitchell, Philadelphia, 1990. (Photo by Joyce Hill Stoner) |
The power of murals to infuse city and community pride clearly resonates in Philadelphia. Mural Arts Program (MAP) artist David McShane put it best when he said, mural art is “art for anybody; anyone who walks past, owns it.” On a recent tour of the murals with Seth Turner from MAP, I was taking a picture of Josh Sarantitis’s Metamorphosis when a local passerby recommended that I walk a few blocks south to see another great mural. Another walker, coming from the other direction, seconded his suggestion. They obviously wanted to share with me the murals that they enjoy in their daily lives. (By the way, the mural “down a few blocks” turned out to be Kent Twitchell’s Dr. J).
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Pedal Thru (Bikin' in the O-Zone), Paul Santoleri, Philadelphia, 2008, This mural has been coated with Golden's MSA varnish. (Photo by Amanda Norbutus) |
The mural program in Philadelphia has recently become more selective in its projects, choosing walls that do not require extensive prep work and commissioning fewer murals so that each budget includes the cost of a protective anti-graffiti coating (in MAP’s case, Golden’s MSA varnish). Mural programs are starting to realize that when you spend the time and money in the beginning, you will not have to repeatedly repaint, repoint, or restore a wall.
By selecting relatively undamaged walls and preparing the surface by sanding, stabilizing, and sealing, less flaking will occur, paint will adhere to the surface better, and the mural will last longer. Preparation of the wall should include the application of priming layers to present a smooth surface and to prevent moisture from seeping into the mural from the rear. Create a clean, level surface using gesso, a paint primer, or masonry sealer before painting.1 During our tour, Seth Turner mentioned that recent restorations have included building up the gesso in certain areas to direct water away from the central image.
While the sealers and primers are drying, look into the new paints and pigments available for exterior murals. The use of more stable, permanent pigments and paints can be more expensive, but the cost will likely be recouped in the future, as the mural will not need to be repainted or retouched as often. Finally, the mural should be sealed with a protective coating that inhibits UV, is anti-graffiti, and removable (e.g., Golden’s MSA varnish). The application of primers and final coatings prevents water from coming into contact with the paint. Water can degrade the color of certain pigments, such as Cadmium yellow, especially when mixed with titanium and/or zinc white.
The life expectancy of a mural will increase with the use of any or all of these tips.
Recommended Pigments for Exterior Use
After the tour, we discussed the use of bright colors in murals. One artist was quoted as saying that he uses the brightest color paint available so that his murals will last longer; however, bright paint does not last longer, it just exhibits a duller version of that color upon aging. However, it is true that high key colors that are not mixed with white would fade less than the same color mixed with white. One approach might be to use high key colors for bright areas and the more stable, permanent low-key pigments (such as yellow ochre or burnt sienna) for the rest of the mural.
We recommend that the use of fugitive high key colors be restricted only to the areas you want to be bright. Do not cut unstable bright colors for use on the rest of the mural ~ there are much more stable pigments available to give you low key color (see chart below). By restricting the use of high key colors to specific areas, repainting in later years will only be needed in select areas.
As many artists know, several colors that work well indoors cannot be used in exterior murals, at least not if the artist wants the color to last! Exterior colors must be able to withstand continuous exposure to ultraviolet light, fluctuations in the temperature and humidity, and various physical stresses such as excessive moisture, windborne grit and debris, defacement, and pollution. For this reason, paint scientists at Golden and Novacolor have separated their available exterior paint colors into two groupings; the first is a group of the stable pigments best suited for exterior, and the second is a select group of stable pigments that are lightfast but should not be used for thin layers or glazes, as the color may shift minimally.2 Use of isolation coats and protective varnishes, as mentioned before, will also extend the life expectancy of the pigments for exterior murals.
The lightfastness of the following pigments are all rated 1 according to ASTM D5098. The pigments are separated by their permanency and durability ratings, since all of the pigments listed here have a lightfastness of 1. I have italicized the pigments that are lightfast yet not quite as durable as the others.
Golden Recommendations
RedPyrrole Red Light |
OrangeOrange Oxide |
YellowTitanate Yellow |
GreenCobalt Green |
BlueCobalt Blue |
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Earth tone, Oxide, BrownBurnt Sienna |
White, Black, GreyTitanium White |
NovaColor Recommendations
RedPyrrole Red |
OrangeOrganic Pyrrole Orange |
YellowBismuth Yellow |
GreenChromium Oxide Green |
BlueCobalt Blue |
Magenta/PurpleQuinacridone Magenta |
Earth Tone, Oxide, BrownBurnt Sienna |
White, Black, GreyTitanium White |
*Novacolor advises that the cadmiums (orange, red, and particularly, yellow) and ultramarine blue should not be used for exterior mural paint, as these pigments degrade in moisture and alkaline conditions, respectively.
Weather Advisory
Remember that when on deadline, or behind deadline during winter, it is not advisable to paint outdoors at temperatures below 45°F. The best option for wintertime painting would be a very sunny day, with temperature above 45°F, mid-morning to early afternoon. The sun should shine on the wall’s surface to heat up the brick, wood, or stucco, because you should never paint on a frozen wall! If a wall is frozen, the paint cannot properly adhere to the wall, which can lead to a greater likelihood of flaking and delamination over time (and at a faster rate). When the wall defrosts, it can introduce moisture to the paint from the back of the mural; this will cause degradation and may increase the rate of fading with some pigments.
As for the paint itself, the paint is not going to form a film or dry below 40°F. If the paint freezes before forming a film, it can ruin the paint and the paint will never form a film, no matter how long you allow it to dry. It also goes to say that you should only paint mid-morning to early afternoon, because with approaching darkness, the temps will fall and the paint can freeze before fully drying, and you are back to the point where the film can never truly form once the paint has frozen.
A recommended cut-off would be 45°F in winter, and only on sunny days with no chance of rain or snow. However, the best cut-off would be between 50–60°F.
1Isolating layers such as Sherwin Williams’ Pro-Cryl primer or SealKrete masonry sealer, or NovaColor gesso.
2 Golden, “GOLDEN Suggested Color List for Exterior Murals.” Just Paint, Issue 10, April 2002, 11. Novacolor, “How to Paint a Mural: Tips for Painting an Exterior Mural Using Acrylic Paint,” December 2008.