The use of ferric chloride in "classical" graphic techniques
Ferric chloride is not an acid, but a salt and it is one of the most commonly used salts in printmaking (certainly for aquatint), but also in photoengraving and to apply an oxide skin to a metal. It gives a dark brown solution in water. The chemical reaction with copper proceeds according to the formula 3Cu + 2 FeCl3 ==> 3CuCl2 + 2Fe where the released iron oxidizes and precipitates as Fe2O3.
Ferric chloride is available in solid and liquid form. In solid form, they are crystals that are extremely sensitive to moisture and must remain closed off from the outside air as much as possible. In the past, engravers made it themselves to ensure a good result. The chemist Bonnet, author of a treatise on photogravure, gave the following recipe: "Put some iron nails in a large earthenware pot a mixture of hydrochloric acid and nitric acid. (1 part nitric acid to 3 parts hydrochloric acid, also called as "aqua reqia" ). The work must be done in the open air because after a few seconds of contact between the acids and the iron, a lot of vapors (nitric acid / hydrochloric acid) are produced which are very harmful. The mixture must be stirred every now and then. If desired the reaction time can be shortened by heating the mixture.The result of this reaction is a brownish-yellow liquid that is quite thick. The content is largely iron chloride with some iron nitrate and a little free acid. It is an excellent mix for photoengraving and although less pure than the ready-made product, it is still better".
In our modern times, however, the ready-to-use iron chloride is of excellent quality. Fresh ferric chloride must be "tempered" by, for example, adding 2 grams of copper per 100cm3 of solution, or by mixing the new solution with 5 to 10% of the old solution. A fresh "undempered" solution of ferric chloride bites too quickly and too superficially. The etchant is at its best when it is brown, that is the stage between yellow/light brown slightly transparent and black (when it is completely worked out.). As soon as metal dissolves, the weight increases. In the case of copper, for every 10 grams of copper dissolved in a liter of ferric chloride, the "weight" will increase by 1° Beamé. At the same time, the concentration of ferric chloride will decrease by 10%. An etchant that has worn out will etch slowly and erratically, which is a sign that it needs to be replaced.
Ferric chloride was used much less in the past than nitric acid. Nitric acid had the advantage that the process could be followed much more clearly. Ferric chloride obscures the process because of its brown color. However, it has a favorable property that it mainly etches in depth. "Undercutting" of lines will therefore occur less with ferric chloride. Another reason for using it less was the deposit in the lines that forms during etching and hinders the continuation of the process. Therefore, normally, a copper plate in ferric chloride is etched in reverse, i.e. face down, so that the deposit falls out. From time to time the plate is taken out of the tray to see what has happened. The remaining precipitate can be removed with a weak solution of acetic acid (add table salt if necessary). Now it is possible to measure exactly how far the etching process has progressed.
Today, there is another solution for reducing deposits; the so-called "Edinburgh Etch" (adding citric acid to the ferric chloride). It seems that 10% hydrochloric acid was added to the iron chloride in the past, also to reduce the deposit. Zinc plates can also be etched in iron chloride, although modern methods such as "Bordeaux-Etch" or "Saline Sulphate Etch" are more suitable for this.The "standard" concentration that Polymetaal has in stock is 40%. (About 45° Beamé). It can be used at this strength, but for general work 25-30% is more suitable. (A matter of adding some water)