Transparent Ground for Copper

Maurice — Sat, 02 Jul 2022 22:31:02 +0000

For some time now I have received numerous inquiries about an appropriate or archive quality ‘clear’ ground when using copper support. I must honestly admit that in the past I only appreciated copper for it’s smooth surface texture failing to recognize the advantages of it’s metallic properties. Although I have heard of direct painting onto copper with linseed oil paint, I am (as well as many other artists interested in using this support) very skeptical about the results. The most obvious is that with copper as with all metals continues to corrode when exposed to air. This is why it must be sealed as soon as it is cleaned as explained in my page Copper as Canvas.

From reports I have received from conservators about the use of urethane as sealer for wood panels I decided to experiment with urethane and epoxy without much personal satisfaction. The bond I was trying to create with the linseed oil (in my opinion) was just not adequate. Plastics used as sealants such as urethane have been known to cause de-laminating of the upper linseed oil paint layers after a short period of time. I was just not willing to allow my creative time to be wasted on a piece that would start peeling off with in my lifetime. The component that I believe eventually became the most obvious was de-waxed blonde shellac.

With all modern choices I could make today, pure de-waxed shellac is still an excellent paint primer. Used for centuries to seal and protect many wood surfaces, the advantage of it’s use is, when shellac is mixed with pigment and fillers it seals stains that normally bleed through other primers. The real advantage is it’s superior adhesion. It bonds to many slick surfaces such as metals and high gloss paint. Its durability against abrasives and some common solvents has not been reported as very good and there are claims of cracking on aged flexible supports when used as a finish but, shellac provides an excellent barrier against water vapor penetration. Shellac’s excellent adhesion properties provides a superior bond between both the support surface and the paint layers. Although it does get hazy in contact with water, when dry it resists water-vapor very well. Tests done on the moisture-inhibiting effectiveness of wood finishes found that shellac rates above all other finishes such as polyurethane, alkyd varnish and lacquers.

Shellac is a safe finish. Non-yellowing, quick drying Shellac is actually edible and it is FDA approved for use as a glazing agent for pharmaceuticals and candies. Dry flaked Shellac is dissolved with ethanol or denatured alcohol but is not effected by turpentine making it an excellent choice for use with oil paints. Denatured alcohol contains ethanol or grain alcohol blended with small quantities of methanol which is very toxic (making it unsuitable for human consumption) restricting it’s use for industrial purposes. Grain alcohol works well for cutting shellac making the mixture a completely non hazardous sealer to work with provided you can avoid drinking the thinner during the process. In both cases the smell of alcohol is present when applying but it quickly dissipates when drying.

Laminating copper sheet to panel

I have begun this project trimming a rough cut piece of .010″/ 30 gauge Copper Sheet 1/2″ larger than the birch panel to be used. Caution: the edges of copper are very sharp. Use caution and/or protect your hands from harm.

Because this layer of copper will conform to whatever surface it is glued to, it is important to have a smooth side of the panel to glue to. It should be sanded smooth to about 80 – 100 grit with no imperfections. If there are any knot holes, creases, dents, chips, etc. you must remedy these issues with filler or steam and sand out before moving forward.

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The copper was sanded with 60 -80 grit paper to provide tooth to the glue side and cleaned with acetone and a Scotch-Brite pad (synthetic steel wool). Do not use Real steel wool. It will leave unwanted iron deposits. Be sure there are no bends or folds. If there are they can be rolled flat with a wooden roller on a flat surface.

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Then using a small short nap roller this time I applied TC-20 Copper Adhesive to the birch panel and glue side of the copper and placed the copper on the panel. Securing one edge with tape to prevent movement and using wood blocks/scrap wood to span the panel I clamped it around the parameter

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After allowing over night to set the clamps are removed and the edges were trimmed with a utility knife. Make sure all sharp edges are filed smooth and that there are no bubbles in the lamination. If there are, small areas can be pulled up and re-glued and clamped until you get the lamination to the panel perfected.

Copper panel preparation

Conservators have all agreed that the de-lamination of paint on copper is the mainly caused from improper cleaning preparation of the copper surface, so now that we have the panel made up we begin cleaning.

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To speed up my cleaning I use a palm sander with 220 grit paper to the painting side of the copper usually directly after rubbing it down with mineral spirits or citrus degreaser. Sand the surface with fine grit sandpaper in a circular motion, until the surface appears matt.

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Do NOT touch the cleaned copper surface, or your fingerprints will show up through your paint layers! The surface must be absolutely clean and bright. Solvent residue or grease left on the surface including finger oils from handling are contaminants that will block adhesion.

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It should be wiped down again with denatured alcohol and then sanded as the final step in circular motion by hand again with a fine grit sandpaper. Then wiped clean with only a clean dry rag. Keep wiping it down until no residue or dirt is recovered from a clean rag or paper towel.

The Etching Process

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After cleaning, the surface is prepared to aid bonding of paint by etching it. Whole cloves sliced at the wider end where traditionally rubbed on the surface to etch the copper. I will use the juice of crushed garlic cloves rubbed on with a clean rag and allowed to dry. Some times the etching will produce color reactions during this step. This project did not.

At this point some artists believe you can start painting on copper directly. I have a problem with doing that personally because the metal needs to be sealed from oxidation and contaminates immediately. If not corrosion will eventually cause de-laminating of the paint layers.

Applying Shellac Ground

Normally this step would be started with thin coats of white lead primer but today we will use shellac. For the best results, I have prepared my own shellac from dry flakes. This is because a sealed container of dry shellac flakes will store much longer in a cool dry place. However, de-waxed shellacs do not keep long once they have been cut so, you should only make up enough for projects you have within say a six month period.

Making the cut

The ratio of dry shellac flakes dissolved in alcohol is known as the cut. It refers to the amount in pounds of dry shellac flakes dissolved in 1 gallon of alcohol. A 3 lb. cut would be 3 pounds of shellac dissolved in 1 gallon of alcohol. A 1 lb. cut would be 1 pound of shellac dissolved in a gallon an so on. Since a gallon is a large amount for most finishing tasks, you can factor down the ration to suit your needs. For example ¼ lb flakes dissolved in 1 pint of alcohol will yield a 2 lb. cut. It’s best to weigh the shellac flakes – small inexpensive food portion scales calibrated in ounces are available at house ware supply stores.

Shellac can be mixed with any pigment, other resins, calcium carbonate, zinc sulfide, aluminum oxide and/or bone ash but, for this project I will be using it just to seal the panel for a transparent ground.

Note: If shellac flakes go bad you will know because it will be very difficult to dissolve into liquid form. Regardless of what you may hear, shellac flakes won’t store forever. Given enough time, especially under high heat, and humidity, dry shellac reacts with itself creating polymers that will not dissolve in alcohol. If you produce a jelly consistency try to cut the shellac it is no longer usable and should be disposed of.

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Applying the shellac

Shellac can be applied by practically any method — brushing, padding, or spraying. My favorite method is padding, which many woodworkers may or may not have heard of. This technique takes advantage of shellac’s rapid drying and allows the finisher to apply a finish in a much shorter time than varnish, polyurethane or oil finishes.

It works best on flat surfaces, and in a nutshell, here’s how it’s done. Use a 2 lb. cut shellac and some padding cloth, which is available from finishing suppliers as trace cloth or French polishing cloth. It should be as lint-free as possible. Do not use cotton T-shirt type cloth or cheesecloth. Apply thin diluted layers quickly in circular patterns and allow to dry between coats.

Always wet-sand, but use oil instead of water. Don’t use alcohol as it will cut into the finish. Wipe the surface dry and clean with a rag.

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Discussion on supports and ground preparations for Canvas, Wood, Papers, Copper and related materials.

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Transparent Ground for Copper

Tools

Have a new or interesting tool to share? Place your comment about artist tools you like/dislike and information you want to share.

Oil

Targeting Linseed (flax or hemp) oil based binders and additives, use and technique.

Water Based Paints

Watercolor, Gouache and Acrylic mediums are posted here

Mixed Medium

Post your experimental use of different mediums in a single work of art or blend of unusual materials to create paint.

Fresco

Encaustic

Drawing

Covering the use dry mediums such as Metal-point, graphite, colored pencil, Oil & Chalk Pastels.

Supports

Discussion on supports and ground preparations for Canvas, Wood, Papers, Copper and related materials.

Egg Tempera

Painting with Egg Tempera?

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