Etching Copper and Brass:
The Edinburgh Etch
A new etching solution for copper and brass
special thanks to Keith Howard, Michael McCann, and Robert Adam who helped revitalize ferric chloride-based etching
(Above) Friedhard Kiekeben, Floral, brass frieze, 1997
Plate etched in Edinburgh Etch.
Made in collaboration with Eduardo Paolozzi, 1997
note of caution: all safer etching methods involve more or less hazardous chemicals and by-products, they are not ‘nontoxic’. Safe handling, informed training, and various protective measures are essential requirements in metal etching.
The History of Etching with Ferric
A solution of ferric chloride crystals has long been known to etchers as a highly controllable etchant with the most accurate bite for copper plates. It doesn't give off toxic vapors and is of low risk if it comes into occasional contact with the skin - although this should of course be avoided. While a bottle of nitric acid can generate explosive and fuming reactions when dropped, ferric is not readily volatile. In contrast, ferric spillages can be diluted, neutralized with soda ash, and disposed of safely and easily.
Why, then, hasn't ferric been used as the predominant mordant in etching?
The answer lies in the speed and ease of biting. The way that ferric was used in the past meant that etches of a reasonable depth took a very long time. When metal plates are etched in ferric chloride, sediment gradually builds up in the bitten intaglio areas. If the crystalline residue is not continually removed by some means, it eventually clogs up the grooves and prevents the plate from etching further. Old handbooks recommended placing plates face down while rocking the bath: a slow, cumbersome and inefficient method.
This problem with ferric was eventually addressed by etching plates in vertical dip tanks, a reliable technology borrowed from the electronics industry and first applied to printmaking by Keith Howard. Plates are suspended in the upright position in the solution, allowing sediment particles to drop out of the etched grooves. Agitating and heating the heavy iron salt solution also accelerates the bite and improves efficiency.
Dip Tanks for Etching
Due to its generous volume of solution and vigorous agitation, a dip tank represents the best facility for the mechanical removal of the crystalline sediments that ferric chloride generates as it etches. Larger tanks can be custom made by most acid unit manufacturers (or plastics fabricating companies), preferably from welded polypropylene. For home use and smaller scale work, even a square bucket or plastic crate will make a very serviceable etching tank, especially if fitted with an agitation pump.
Dip tank at RIT: Many print studios have customized dip tanks for etching.Aeration
To agitate the etch solution; one or several tubes of fish tank airline can be fitted to a dip tank. The air outlets should point upward and be connected to a powerful fish tank aeration pump. The stream of bubbles rising on the sides of the tank produces a circular flow within the solution which activates the etch. Aquarium shops also supply small valves that regulate the air and therefore the strength of the flow. Some delicate resists such as aquatint can be etched with reduced flow. Ensure good ventilation if using fish tank aeration. Ideally use a pump or propeller system to agitate the solution mechanically as this minimizes corrosive fumes coming off the tank.
The Discovery of the Edinburgh Etch
The innovations outlined above dramatically improved the performance of ferric chloride, but I was convinced that the eroding power of this safer mordant could be extended still further. This thought motivated me to develop the Edinburgh Etch solution - a chemically enhanced formula in which the corrosive potential of ferric is fully realized as a superb etchant for copper and brass.
The Chemistry of Ferric Chloride
The corrosive properties of ferric become clear when you look at its chemical composition. A ferric chloride molecule consists of one atom of iron with three atoms of chlorine hooked onto it.
The bridge between iron and chlorine, which creates the chemical adhesion of the molecule, is made up of two electrons (negatively charged particles) for each chlorine atom.
Since the iron atom needs eight electrons to be in a stable condition, but has only six, it wants to react (electrochemically
) with the copper atoms of the etching plate to gain the missing two electrons.
Citric Acid: A New Catalyst
In the course of my research, I approached the issue of how to activate the ferric etch from a new angle, searching for additives to ferric chloride which might be capable of dissolving the sediment as it is produced. After systematically experimenting with a variety of possible substances (initially with mixed results) I tried a new kind of additive normally associated with the soft drinks industry rather than with etching: citric acid. It turned out that a citric acid solution mixed at a certain ratio with a ferric chloride solution not only speeds up the bite of ferric but also produces an entirely new kind of mordant which I christened The Edinburgh Etch.
On copper and brass plates the etch process takes place with the utmost precision and without the build-up of sediment typically associated with unmodified ferric chloride. In chemical terms, this is because the individual molecules of the metal salts are locked into the clamp-like carbon rings of citric acid atoms, thus keeping them dissolved. They are no longer allowed to solidify as the crystals which impede biting and result in a coarser etch. The main obstacle encountered with ferric chloride is literally being dissolved by the new mordant.
The white citric acid powder is synthetic lemon juice.
It acts as a catalyst when added to ferric chloride.
Inspecting a brass panel etched using Edinburgh Etch.
Edinburgh Etch Ingredients
CITRIC ACID (anhydrous) is now available from most major printmaking suppliers; it can also be obtained from suppliers to the food industry. A dust mask and goggles should be worn when dispensing the fine powder.
FERRIC CHLORIDE is available from most chemical suppliers either as yellow granules or as a saturated solution. If at all possible, the ready-made solution should be used which for its industrial applications comes in 25 liter or 5 gallon containers of about 42 to 48 BE (Baume) density; increasingly printmaking suppliers also stock smaller bottles. The density or weight of ferric can be measured with a hydrometer; but the Baume scale describing the specific weight of liquids is only a rough guide, and sometimes manufacturers give other specifications. The best thing to do is to ask the chemical supplier for a strong or a saturated ferric chloride solution, technical grade.
Remember that liquid ferric is a heavy solution of ferric (iron) chloride salt crystals in water. The solution would not normally go beyond a certain strength (about 48 BE) otherwise the crystals solidify. It is easy to dilute a strong solution with water to obtain a weaker strength but impracticable to strengthen a ferric solution which is too weak for a good etch from the start.
Even though ferric chloride is relatively safe to use, respect it as a corrosive chemical; eye protection and gloves must always be worn when handling or using the solution.