A technical dictionary of printmaking, André Béguin.

The Chemistry of Ferric Chloride
Since Rembrandt , etchers have used nitric acid and Dutch mordant to etch copper. At th eturn of the century ferric chloride solution was introduced in gravure work principally because of its interaction with gum bichromate gelatine. With the increased emphasis on health and safety at work, we have had to reappraise these and other chemicals we use.
The salt, ferric chloride, has none of the disadvantages associated with the other two etching solutions: it does not produce dangerous fumes, is odourless and, though corrosive, is not absorbed through the skin.
Ferric chloride solutions in water are strongly acidic but in their reaction with copper no significant fumes or gases are produced. Simply, except in the presence of free oxygen, copper will not react to any significant extent with hydrochloric acid which is the acid produced with ferric chloride solutions.
When ferric chloride is dissolved in water the solution becomes strongly acidic as a result of hydrolysis. The chemical reaction, in words and formulae are:
basically, ferric chloride in water solution ionizes to iron (ferric) and chloride ions; whilst the water ionizes to hydrogen and hydroxyl ions;

FeCL³		<====>	Fe³ + 3CL		(1)
H²O		<====>	H³ + OH		(2)

The ferric ions will partially combine with the hydrogen and hydroxyl ions to form ferric hydroxide, a compound which is only slightly soluble and precipitates from solution as a brown solid.

Fe³ + 3OH

Fe(OH)³

(3)

This precipitation can be regarded as removing hydroxyl ions from the solution, leaving a relative excess of hydrogen ions and it is this access of hydrogen ions that makes the solution acidic. Because of a property of metals known as electro- negativity, copper replaces iron from the solution and forms a mixture of ferrous and cupric irons. Effectively this copper dissolves without producing any gas;

2Fe³

+

Cu(solid)

<===>

2Fe³

+

Cu²

(4)

Ferrous oins become unceasingly stable as the solution becomes acidic and this helps the copper dissolution to proceed more easily, whilst in basic (less acidic) solutions the tendency is for ferrous ions to convert back to ferric. As more copper dissolves and the solubility limit is exceeded, cupric chloride precipates from the solution, as anything from a green to a blue solid.

Strengths and Mixing
Ferric chloride can be obtained either as a solid (not recommended as it can give off highly toxic hydrogen chloride fumes when mixed with water) or in liquid form which is usually supplied in a strength of 45° Baume. This measurement equates to a specific gravity of approximately 1.43 or a weight to volume ratio of 39-41%. Purveyors of ferric chloride can use any of these measurements and often profess complete ignirance of any form of the others!

To form the basic stock etching solution, water is added in the ratio 2 parts water to 1 part ferric chloride solution in a solution strength of 42° Baume.

Preparing the solution
At this stage the strongly acidic feeric chloride requires further treatment to remove free acid and to condition the solution. Either small quantities of copper may be added and allowed to dissolve or (a more effective way) take 10cc of stock solution and add 10cc of 9% household ammonia solution. This results in a sludge of ferric hydroxide which is allowed to settle before pouring off the liquid and adding the precipitate to 1 litre of the stock solution. Once an etching cycle using ferric chloride has been initiated, the third method is probably easier. Here, spent ferric chloride liquor (containing ferric hydroxide) is added to the new stock 42° Baume solution, in the proportion 1 part spent liquor to 10 parts 42° Baume solution.

Colour in Use
The optimum etching conditions usually occur after some etching has been done (this reflecting an "incubation" period for the chemical reaction to settle down) but as etching reaction proceeds the solution becomes less effective. The progress of this exhaustion can be observed by the colour changes - from an initial red brown (ferric hydroxide) through a turbid (muddy) brown (ferric and ferrous hydroxide) when the solution is working at its most efficient, and finally to a dark green solution and black precipitate (cupric salts in solution and precipitated with hydroxides. At this stage the solution is exhausted and requires disposal. This can be effected by the slow addition of either calcium (washing soda crystals or soda ash) - until effervescence ceases (and a ph of 7 is reached). At this point the solution is neutralized and may be disposed of safely.

The Upside Down Myth
The normal recommendation is to bite copper plates in ferric chloride inverted or on edge to avoid precipitate ferric hydroxide clogging the etched line, thus halting etching. However, there is little need for this if the plate is bitten face up and removed from the solution every 20 minutes then washed in cold running water to remove the precipitate before continuing to bite. This procedure does not present the hazard of other etchants and has the advantage of checking progress. Extremely deep bites or plates for relief etching are still bitten upside down. In completion of any biting, or if the plate is to stand between bites for any length of time, ir should always be cleaned thorougly with running water, since reidual ferric chloride in bitten lines will continue to react.

Ease of Use
To summarize, ferric chloride is a chemical and should be handled with respect. However, it does not present the level of hazard associated with nitric acid and Dutch mordant; indeed the only precoautions necessary are to keep the solution in a plastic bath and wash off any liquid on the skin with water. Extraction and fume cupboards are not necessary.

A 42° Baume solution at its most effective is very good for a deep line bite. When diluted to approximately 32° Baume by the addition of an equal part of water, it becomes very effective for soft-ground and delicate work without a noticeable loss of biting time, and reduces foul biting. Due to the low depth of bite necessary in aquatint, the strength of the solution tends to be a personal taste (some acrylic resist techniques require a certain strength of ferric chloride solution). The minus side is the length of time taken to bite a plate.