The Salt-Sulfate-Etching for Zinc.

Cedric Green developed the etching of zinc in a copper sulfate solution in the late 1990s. It is not known whether he was the first user of this system. Copper sulfate is used by vintners to combat a disease in growing grapes. Cedric Green lives in France and has therefore called this technique the "Bordeaux etch".
Zinc etching in a copper sulphate solution is environmentally friendly and no harmful substances are released. The only drawback is that it is a rather slow process.

Friedhard Kiekeben developed the Edinburgh-Etch at the time; he added powdered citric acid to ferric chloride, which accelerated the etching process considerably. Friedhard has recently been investigating whether etching in copper sulphate could also be accelerated in a similar way. That turned out to be possible, simply by adding.... kitchen salt to the copper sulphate. (Semenoff and Bader were also on this track when etching aluminum). He calls this process the "Saline Sulphate Etch for Zinc". In the Dutch translation of the following article (from Friedhard) this process is called "Salt sulfate etching". (chemists probably won't like this, because the "copper" of the copper sulphate is no longer present)

(from "The Contemporary Printmaker" by Keith Howard, article written by Friedhard Kiekeben, Spring 2003)

Salt sulphate etching.

Salt-sulphate etching is an ideal process for graphic artists who want to make optimal use of the possibilities of zinc. Some report that zinc has the disadvantage that it is relatively soft and that an undesirable bond can form between the material zinc and some inks. Others believe that this is more than offset by the special properties of zinc as a material.

The salt-sulfate etching for zinc works very well in a normal horizontal acid tank and does not require heating or additional aeration. The copper sulfate solution was (as far as I know) first presented by Cedric Green. Cedric Green sought a solution to health/environmental problems associated with etching in nitric acid or ferric chloride.
After three years of research I had to conclude that a copper sulphate solution is indeed the best solution for zinc etching, both in terms of safety and possibilities in combination with acrylic-based etching grounds.

A "regular" copper sulfate solution works well for zinc, but the process is rather slow and the solution wears off fairly quickly. So I started looking for ways to better exploit the potential of copper sulfate. In the case of ferric chloride, a deposit is normally formed which causes the etching in ferric chloride to proceed slowly. For iron chloride, citric acid is therefore added, so that the deposits remain in solution. Something similar had to be found with copper sulphate.

Tests showed the following; A solution of copper sulfate with equal parts of table salt (sodium chloride) is three times more active than normal copper sulfate solution (Bordeaux-Etch). The lines are bitten very sharply, without the deposition of salts and the roughness of the surface that is characteristic of the Bordeaux etch.
During etching, metal oxides (combined with hydrogen) come to the surface, so that the etching process is not hindered. The etching can be improved slightly by brushing over the plate with a soft brush. However, weak areas, such as an aquatint or a soft etching ground, should not be touched due to their vulnerability. The etching solution works even better if the metal oxides on the surface are regularly removed. This prevents the bath from becoming alkaline.

Making a salt-sulphate solution.

The solution is made by dissolving copper sulfate crystals and table salt in water. It is better not to use the copper sulfate used in agriculture here because it contains many impurities. Use the so-called "industrial grade". There is also a "laboratory quality" but this quality is much more expensive and unnecessary.
Wear gloves, a dust mask and goggles when working with these crystals to prevent inhalation of dust particles. Once the solution is ready, it is completely safe to use.

1.  75 grams of copper sulfate
 
2.  50-75 grams of sodium chloride (table salt)
 
3. 1 liter of water
 
(for larger quantities, mix proportionately).