Thursday, August 20, 2009

Q + A with Professor John Sarra!

You have painting questions? We have painting answers. Today's answers are courtesy of John Sarra, painter and professor extraordinaire. 

FB: How does one put a layer of water-based paint on top of oil? 

JS: The rule to follow is "oil over water, but not water over oil". When we talk about oil paints "drying", we are actually talking about a process called polymerization in which the molecules continually cross-link, making the surface more and more brittle over time. Water based paints, on the other hand, are usually fully cured in less than two weeks. The surface has a tendency to continue to breathe, allowing water vapor to move through the paint film. This works fine when the flexible, breathing layer joins the substrate of fabric or wood, which are also mobile as moisture levels go up and down. Problems happen when the flexible and mobile layer is adhering directly to an oil film. The result, whether it takes a long or short time, will be a degradation of the painting's surface.

FB:  How does one put a layer of oil-based paint on top of paper?

JS:  The problem with oil on paper is that the linoleic acid, which is found in many of the oils used in oil paints, destroys natural fibers over time. The principle to observe, then, is that a coat of sizing must be applied to the paper to isolate the fibers from the oil. There are a number of options for sizing including shellac, natural or synthetic glues (especially PVA glue), or acrylic polymer (in the form of a medium, acrylic paint, or acrylic gesso). I have also used left-over latex house paint (better brands are fortified with acrylic-- they don't actually have natural latex rubber in them), which behaves differently depending on the specified sheen. Matte colors remain more absorbent. Your choice of sizing depends on whether you wish to preserve the original look of the paper or if you wish to have a white or colored ground.

FB: "I don't have access to an industrial sink. How do I wash my brushes?"

JS:  I know many artists who wash their brushes with soap and water either regularly or occasionally, but I never do it. Instead I give them a good cleaning with rags and solvent, and this is just fine for natural bristle brushes. If you are using soft-hair brushes, this probably won't work for long-- the hairs will gum up and ruin the brush. The thing to do is to clean the brush thoroughly with solvent until it wipes "clean" on a rag. Then wash using soap and water. Most people don't have sinks with filtration devices for the waste water. The concern here is that heavy metals can be washed down the drain and into the environment. By the time you reach the sink, you should not be washing out any significant amount of pigment-- it should only be a trace amount. But you should recognize that these are cumulative poisons, and even trace amounts are bad amounts. Another strategy would be to eliminate heavy metals from your palette (lead, cadmium, and cobalt are three of the big nasties), replacing them with other pigments. I suppose that you could keep containers of rinse water in your studio, allowing solids to settle and collect as the water evaporates. Disposal of the residue will be covered by the final question, below.

FB: "What's the safest way to dispose of my chemicals and paint rags?"

JS:  The best way to manage waste is to minimize it. Solvent disposal is the biggest issue for most painters. In my view the best practice is to use as little solvent as possible, and to use it responsibly. I don't let my brushes soak in solvent. Instead, I clean them during or at the end of each painting session. This is my procedure for brush cleaning: (1) wipe excess paint off of the brush onto the palette, (2) wipe excess paint off of the brush onto a rag, (3) dip the brush into solvent, then "paint" the slurry onto a rag until the brush starts to wipe clean (4) submerge the bristles in the solvent and twist them against the SIDE of the jar, releasing any final particles (5) wipe the brush dry on a clean cloth. By cleaning the brush against the side of the jar in step 4, you allow the sediment on the bottom of the jar to remain undisturbed. Occasionally you can allow a jar of used solvent to settle out and then pour off the good solvent, collecting the sludge for later disposal. Rags should be allowed to dry thoroughly. If you are using strong solvents or fast-drying oils with your paint, be sure to spread the rags flat in a well ventilated area. Some materials may spontaneously combust if left crumpled. This is not a concern with artist-grade linseed oil. Once rags are dry, they can usually be disposed of with your regular garbage. Many communities have annual collections of hazardous liquid and solid wastes - check with your local government. I don't recommend that you stockpile the used rags because they present a fire hazard.

*bolding by F***art

1 comment:

  1. Thank you for the info! I belonged to the barbaric school of soaking paintbrushes in turpentine until they rinse clean, so it's particularly helpful to someone like me.