Metal Salt Etching, Resists and Resist Removal
This article summarizes my experiences with these exciting methods for metal salt etching and removing resists. I have also included my own observations as well as an overview of the etching process, resists, equipment and materials necessary.
Metalsmiths and enamelists have used ferric chloride for several years as a safer alternative to nitric acid for copper and brass etching purposes. Although it is generally thought of as etching with an acid, ferric chloride is actually a salt.
Etching and resist removal solutions recently developed by Friedhard Kiekeben have greatly improved the process and final product, as well as safety, cost and simplicity. Information in this article has been freely taken from F. Kiekeben’s “Innovative Intaglio Fact Sheet 2004” with discussion of his creation, the “Edinburg Etch” ( © F. KIEKEBEN 1997) , developed for the metal plate needs of intaglio printmakers. A more detailed discussion can be found in the source article.
The Edinburg Etch
There are several important advantages to the “Edinburg Etch”. The resulting etch will be faster, easier, cleaner and will allow the solution to be used numerous times before it is spent. Lines, textures, and open areas are cut into the metal as with a razor blade, and even the finest detail registers more accurately on the bitten plate.
In a ferric-based etching solution fortified with citric acid, a non-toxic additive, metal plates can even be etched face-up in a tray. The addition of citric acid allows the sediment in the solution to be dissolved as it is formed, as there is no crystalline sedimentation to impede the biting process. This solution will also erode metal faster than pure ferric chloride and will stay active for a much longer period of time. These improvements can be attributed to two causes:
- The citric acid will lower the pH of the bath (making it more acidic).
- The sediments released from the metal tend to form a complex with the citric acid as it is released from the surface of the metal, also increasing its solubility.
Kiekeben maintains that this solution, used daily and occasionally topped off to compensate for evaporation, has been known to remain active for several years without a significant drop in its biting properties. Eventually the solution will acquire a deep olive color signifying that is spent and ready for neutralization and disposal.
Note: I have been using the initial solutions in my workshop for several months with consistently good results.
Although the tray method can be safely used in an artist’s studio, goggles and gloves are recommended and there should be adequate ventilation in place.
Edinburgh Etch for Copper and Brass
- 4/5 saturated ferric chloride solution (40%)
- + 1/5 citric acid solution: ¾ hot tap water to ¼ anhydrous citric acid
- 16 oz ferric chloride
- 3 oz H 2 O (hot tap water)
- 1 oz citric acid powder (anhydrous)
Ferric chloride is available from most chemical suppliers. Anhydrous citric acid, which simply means “powdered citric acid”, is generally found as an ingredient in food products and can generally be found in health food stores and from chemical or food industry supply companies.
Start with hot tap water. If your water supply is hard, soften the water by boiling before using it. When to temperature of hot tap water, proceed with the formula.
Gradually add the citric acid powder to the hot tap water while stirring continuously until it has fully dissolved. Slowly add the ferric chloride to the citric acid solution and keep stirring until you have produced a uniform liquid.
Please note: Although the etchant solution is the same for both copper and brass, separate solutions should be maintained for each.
Preparation of Metal and Application of Resists
Make sure the metal surface is clean, degreased and dry before applying any resist. Before a metal plate can be etched, the back of the plate has to be covered with an acid resist unless erosion on both sides is desired. Plates etched without this protection erode from the back, and the resists applied to the front may lift off. A very quick and reliable way to cover the back of an etching plate is by applying sheets of self-adhesive film or strips of packing tape or other strong adhesive tape. In addition to protecting the back of the plate, a resist should cover the metal’s edges.
I am always experimenting with new and old resists both from the standpoint of endurance and ease of removal without toxins, e.g., acetone or turpentine. Some resists that work well for me are listed below. In general, “red” markers and acrylics seem to have greater endurance rates than other colors or black. All resist products must also be completely dry before submerging the metal into the solution.
- uniPAINT (red) fine line PX-21, medium line PX-20 and broad line PX-30
- Staedtler Lumocolor (red) 318-2 (F), 313-2 (S) and 317
- Acrylic paints – (red)
- Strips or cutouts of packaging or other strong adhering tapes
There are numerous products that can be used effectively and many articles have been written on the subject. The best way to find out what works is to experiment.
My current etching projects can be accommodated in a deep-sided Rubbermaid dishpan. Plastic containers of all sorts can be recycled from their prior lives depending on your needs. A method of retrieval needs to be designed ahead of time so that the submerged plate can be easily lifted to check progress and for final retrieval. Because this method allows the metal to lie face-up at the bottom of the tank or pan, I have devised a plastic inner basket punctured with several holes. The basket can easily be submerged and retrieved. A simple link of tape affixed to the backside and retrievable from outside the solution is also an excellent option. Retrieval systems should always be made of non-corrosive materials.
Creating a situation in which agitation takes place further enhances the non-sedimenting properties of the etch and results in speedier biting times. This can be accomplished by use of a fish aquarium aeration pump (found in most pet supply stores) with the plastic tubing line taped or clipped to an inner wall of the etching container, submerged to the bottom with the air circulating upward. Warning: Only the plastic tubing is submerged. The electrical pump unit must remain outside of the container and solution .
A controlled environment within the solution of 68ºF – 86ºF, will also improve biting time and the overall responsiveness of the solution. I place my dishpan atop an old food-warming tray purchased at a local thrift shop. A meat thermometer covered with plastic tape allows me to test the bath temperature and assure that it does not get too hot.
Several variables will determine the length of time for any given etch. Factors such as gauge of metal and desired depth of cut or effect will dictate time. Maintaining a log of personal experiences with various resist techniques and type and gauge of metal will assist greatly. Removing the plate briefly to check the depth at intervals and logging the overall time is recommended.
Even during etching, containers can be covered with a lid to prevent evaporation.
Solutions should be covered up or placed in sealed containers when not in use. This minimizes cross-contamination between different solutions as well as evaporation, thus extending their usable life.
Safe Metal Neutralization & Resist Removal
Upon removal from the etch solution, the metal plate should be neutralized (de-oxidized) in a solution of salt and vinegar in water. I use a ratio of 1 part s., 1 part v. to 6 parts water. Drying with hot air from a blow dryer further minimizes oxidation. This step is useful if there will be a second stage of resists applied to the metal. If the resist is ready to be removed, the plate can go immediately into the resist removal solution without drying.
Resist Removal Solution
The main ingredient of the stripping solution is sodium carbonate, also known as washing soda or soda ash. A commonly found form of this is Arm & Hammer’s Super Washing Soda found in the laundry products section of most grocery stores.
Making the sodium carbonate solution:
- Mix warm water with the washing soda crystals at a ratio of about 2 to 3 parts water to 1part crystals while continually stirring until dissolved. Replace often to ensure fast stripping times.
- Place the metal in the resist removal solution for 15 to 20 minutes. The period of time may vary, but generally even a very thick deposit of resist will lift off quite easily after this amount of time with little more than rinsing with a brush and hot water or a bit of pumice slurry (ground pumice, water and dish soap).
Neutralization and Disposal of “Edinburg Etch”
The best way to dispose of spent etching solutions, after neutralizing, is to take them to a chemical disposal company or municipal toxic waste collection center. Details on the neutralization process can be found at the website referenced at the beginning of this article.