Copper as a Painting Support
|I’ve heard a lot of stories about defeated attempts to paint on copper along with the comment that the lack of porosity in metals makes the bond with a ground uncertain; and that there is no reliable known method to overcome this problem I find this comment debatable. The finished work of art painted on copper provides outstanding results that maintains it’s beauty almost indefinitely as proven by many of the Dutch masters. I’m uncertain when the photo was taken but the painting to your left is now 400 yrs. old. I can only wish that I look that good at that age.|
Copper panels can stand alone if they are smaller pieces intended to be matted and glazed like paper. It is not recommended to allow large pieces to stand alone since flexing can cause cracks in the paint. It is not recommended to use thicker sheets for larger paintings due to the weight. Even the smaller sheets should have some form of backing when painting and framing.
Thinner gage copper sheets should be laminated to plywood with a polyurethane adhesive such as Elmer’s Ultimate or Franklin Titebond to make them extra rigid. These can be purchased at almost any hardware or home improvement center. Rough sand the underside of the copper before gluing to provide bonding tooth. Because poly-glues use moisture to cure be sure to dampen the wood surface with a sponge before applying adhesive.
The key to painting on copper is your initial steps in preparation and if they are missed you’ll be headed for serious disappointment. The primary concern is the green or bluish copper carbonates that form on copper, brass, or bronze surfaces. If not sealed off from the elements (no matter how well controlled) this coating of verdigris, which naturally develops as a form of oxidation when exposed to air, will block the bond of your ground eventually causing peeling.
The Dutch masters method was to first clean all trace of grease or dirt with an alcohol solvent then sand the surface with pumice powder “in circular motion” and repeating this cleaning process with solvent to deep clean and provide tooth for the ground. Grain or denatured alcohol is good for this step. Since sandpaper had not been invented as of yet pumice powder was used but, a good fine grade emery cloth will work better. The surface should be absolutely clean and bright. Any solvent residue or grease left on the surface including finger oils from handling are contaminants that will block adhesion. Your final cleaning should be done with a fine grade sandpaper to remove any solvent residue that may remains then wiped clean only with a clean dry rag and/or vacuum.
Next the surface gets rubbed with cloves of garlic. One of the active ingredients in garlic is a cysteine-based sulfur rich amino acid compound called allicin. When crushing the clove a garlic bulb, a protein-based enzyme called alliinase is released converting the compound into a sulfenic acid and is almost spontaneously condensed down to form Allicin, which bears the typical odor of garlic. Highly unstable Allicin rapidly converts to other sulfur-compounds such as Ajoene capable of slowing or preventing oxidation. Allicin and Ajoene are the active ingredients for use with copper preparation. Allicin rich Cysteine residues play a valuable role by cross linking proteins with the blue copper proteins found in verdigris on the surface of copper, where as the proteins containing cysteine will bind with metals such as lead found in your primer. Acid preparation to a surface to aid bonding of paint is known in today’s painting industry as etching. Use nothing but garlic after this step. Keep in mind that every chemical you apply at this stage is reactive so any spirits you apply after the fact may turn part of the surface black and/or interfere with the paint bond.
You can crush the cloves in a juicer to a liquid state that could be brushed on sparingly and/or rubbed with a clean rag and allowed to dry. You may also find cold pressed garlic juice in the grocery store in the spice section. A little goes a long way here. This treatment is sort of a damp wiping to treat the surface. Sort of the way you apply wood stain. This is a surface treatment and may feel slightly tacky or textured when dry. However, it should not appear as a built up coat of paint. Garlic contains sulfur compounds which react with copper to form copper sulfate, a deep blue or blue-green compound. The garlic juice will also turn green (develop chlorophyll) if exposed to a change in temperature or if it is exposed to sunlight. Most metals when etched with acidic compounds will produce a surface color change. The stains created indicate that you have been successful with the etching process.
Use of metals as support with glue gesso type grounds will not generally be successful. Acrylic gessoes as well will not adhere properly to any metal surface. This is why with copper an oil based white lead pigment primer should be used. There are hazards to using lead paints so take all safety precautions necessary. Otherwise it is a safe paint to use. Because all metals start to oxidize once exposed to air don’t allow too much time between the dry etched copper and the application of your white lead primer. If you are familiar with the process of sweating copper tubing, think of the garlic step as the flux used . The lead in your primer is the solder. So in order to get a strong bond cleanliness and pace are essential. Apply in multiple thinned layers with a hake or roller in a cross hatched pattern to avoid sanding. I prefer to skip use of a traditional gesso since the surface is already smooth and adhesive enough to paint on if primed right but this is my personal choice. You can built a gesso ground if you feel necessary. I’ve also been working with shellac as a base sealer to paint directly onto copper revealing a transparency effect over the copper on the painting. You can view the preparation of a WIP here.
From here it would be safe to say that you could paint freely with any oil based paint or by using a pre-mixed Maroger’s Medium. Similar oil mediums work well at this point. I favor copal varnish resins blended into my oil medium which will be discussed at another time. Keep in mind that your panel is intended for traditional thin layered techniques to take advantage of the smooth surface of copper and that heavy impasto techniques are contradictory to the use of this support.
Note: In an alternate and modern method of painting, copper is also an excellent support for painting with lacquers. Although it is not wise to venture into this medium unless you are willing mix them from scratch and invest in the proper ventilation equipment, I have entered this information in case there was interest. This method would require you to start from bare metal to use a Self-Etching Primer and then apply one or two coats of Urethane Primer. The reason for the urethane primer is that lacquers will dissolve in solvents when dry. You need a cured undercoat in order for it to prevent the lacquers solvent from penetrating into it. Urethane Primer has a high solids content so it cures fast and it is easy to sand to build a durable smooth ground. This painting would have to be sealed again with a urethane clear coat once completed.
| Copper as Canvas: Two Centuries of Masterpiece Paintings on Copper, 1575-1775 (Paperback)
by Phoenix Art Museum (Author)