The copper sulphate solution as an etchant for zinc.
Edinburgh Etch as an etchant for copper
In the printmaking world there is a need to limit working with nitric acid as much as possible. In the PWE (Printmaking Workshop Edinburgh) copper, iron and zinc are etched with ferric chloride to which citric acid has been added, the so-called Edinburgh Etch. The addition of citric acid prevents the well-known black precipitation with ferric chloride.
When working with Edinburgh Etch on copper, no toxic fumes are (probably) released, but a small amount of hydrogen gas is released; the liquid etches nice, smooth grooves. Disadvantages are that hydrochloric acid fumes are released when preparing the etching bath and can irritate the skin, eyes and lungs when the crystals are sprayed. In addition, the solution is very acidic and gives stains that cannot be removed. For academies and printmaking workshops, however, it is (for the time being) a useful way to work on copper, because no toxic fumes are released during the etching itself.
However, copper is too expensive for most graphic artists, so they use zinc. Our experiences etching zinc with Edinburgh Etch are not positive. It should give nice smooth grooves, but it doesn't: it bites more irregularly than nitric acid. A bigger disadvantage is that toxic fumes are released. If many zinc plates are etched in one bath at the same time, or if a plate with many open structures is bitten, a hydrochloric acid vapor rises from the bath. We absolutely did not want to do that to the artists, students and ourselves. Friedhard Kiekeben now also advises against the use of Edinburgh Etch on zinc. He says: "Soon after the publication of Keith Howard's book Non-Toxic-Intaglio Printmaking, I realized the possibility of an uncontrolled chemical reaction in zinc etching, a lot of heat can be released and even chlorine fumes can be released"
Bordeaux Etch as an etchant for zinc
"I have called this Bordeaux Etch, because a copper sulphate solution is better known to vine growers, farmers and gardeners as Bordeaux mixture (Bouillie Bordelaise, La Morsure Bordelaise), very widely used as a spray against mildew." The English printmaking artist Cedric Green published an article and a booklet on electrolytic etching in 1998. Green describes another technique: etching zinc plates in a solution of copper sulphate (CuSO4). The technique is convincingly simple: mix copper sulphate with water and wait five minutes for the crystals to dissolve. If you now put the plate in the bath, a black deposit immediately forms on the exposed rink. That means the chemical reaction occurs. The big advantage over etching with nitric acid or iron chloride is that no toxic fumes are released. When you etch zinc with copper sulfate, two major reactions occur. First, the acidic copper sulfate attacks the zinc, releasing hydrogen gas and dissolving the zinc. Second, the zinc reacts with the copper ions, dissolving the zinc and precipitating the copper (this is the dark brown to black sediment you see forming). The two reactions proceed simultaneously and through each other.
New development: In the spring of 2003, Friedhard Kiekeben (the "inventor" of the Edinburgh Etch) developed the "Saline Sulphate Etch". We call it the "salt sulfate etch" although the name is a bit misleading. This has significantly accelerated and improved etching in copper sulphate. Click on Salt-Sulphate Etching for more information.
You can prepare the etching plate as you normally do. Put the plate in the bath, the lines turn black instantly. Wipe the pleat with a feather or brush to remove this deposit. If there is a lot of precipitation floating around in the bath, it makes it unclear to see the lines. It is useful to put a plastic grid in the bath under which the sediment sinks. Once the plate has been etched long enough, remove it from the bath (use gloves). Drain and rinse with water. Individual artists, etching small amounts of zinc, can dispose of the rinsing water through the sewer. We recommend that graphic studios and academies use a settling tank. If the bath becomes too weak, you can add a solution of copper sulfate two or three times before it becomes unusable, then everything must be drained.
The simplest and most expensive way is to dispose of the spent liquid with the sediment in its entirety as chemical waste. The slow way is to empty the container into a large plastic bucket and let it sit for a few weeks until the water has evaporated and all the salts in the solution form a thick hard crust. Place the bucket upside down in a plastic bag and hit on the bottom. The crust falls into pieces in the bag and you can dispose of it as small chemical waste.
Health and Safety
When etching with copper sulfate, the solution does not heat up, no toxic fumes are formed, the etching process is easy to control and the grooves etched in the zinc are smoother and more regular than with nitric acid. Because no toxic gases are released, you can hang with your face over the bath and keep watching what happens to the plate. A freshly made solution is Mediterranean blue and about as acidic as kitchen vinegar. When the bath weakens, it whitens and becomes clear as water; the solution also becomes slightly alkaline. The chemical reaction releases hydrogen gas, which can be seen as fine bubbles rising from the metal. Since hydrogen gas is highly flammable, the area should be well ventilated and the etching tanks kept open. In principle, etching with copper sulphate is not dangerous, as long as the hydrogen gas cannot accumulate. In studio practice, this means that etching does not necessarily have to take place in a fume cupboard or that an extraction system is required. Opening windows and doors opposite each other is usually sufficient. If you spill with the used solution you will see white spots, this is zinc sulphate. Be as careful with that as you were with the copper sulfate. Put on gloves and wipe off the white spot with a wet mop. Pregnant women should avoid ingesting copper sulfate (e.g. as a substance), because copper sulfate can be harmful to the unborn fetus.
Zinc is the only common metal that is easy to etch with copper sulfate. Copper in copper sulfate does not work, no reaction occurs. Iron is affected in the beginning, but a layer of pure copper soon deposits on the exposed iron and the reaction stops: only large, open areas are etched over time. Aluminum is not or hardly affected by copper sulfate, unless additional substances are added. Semenoff and Bader explored this further. The trick is to add table salt to the copper sulfate and the reaction will start instantly. You see more bubbles appear than with zinc etching, more hydrogen gas is released, and a thick red-brown layer of copper forms on the lines. You don't necessarily have to sweep away this sediment, so much gas is released that the lines are kept open. A gel can also form, this is aluminum hydroxide. It makes the solution cloudy, so you can't really see what's happening. You can prevent the formation of the gel by making the solution more acidic. This can be done by; for example, adding a few drops of sulfuric acid to the solution or adding the salt NaHSO4. The reaction is violent and the temperature of the bath rises. Lines in the aluminum become coarse when etching and open surfaces so pockmarked that you don't need aquatint. Zinc plates can be etched in the same mixture at the same time as aluminum plates.
Copper sulphate etches zinc in a regular manner, the reaction is not violent and no heat is released. The proportions below are a guideline for making a mixture for certain techniques.
100 grams of copper sulphate on 1 liter of water etches as nitric acid of approx. 12% (suitable for wild biting and line etching)
50 grams of copper sulphate on 1 liter of water etches as nitric acid of approx. 8% (suitable for fine line etching, varnish mou and coarse aquatint)
25 grams of copper sulfate on 1 liter of water etches as nitric acid of approx. 5% (suitable for fine aquatint)