Reaction With Copper
From Printmaking Today Vol 4 No.2
The solution of ferric chloride in water does not produce dangerous fumes, is odorless 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.
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.
The chemistry of the reaction
When ferric chloride
is dissolved in water the solution becomes strongly acidic as
a result of hydrolysis.
The chemical reactions, in words and formulae are:
Schematic representation of the chemical interactions occurring during etching copper in ferric chloride solution.
Basically, ferric chloride in water solution ionises to iron (ferric) and chloride ions;
whilst the water ionises to hydrogen and hydroxyl ions; The ferric ions will partially combine with the hydroxide ions to form ferric hydroxide, a compound which is only slightly soluble and precipitates from solution as a brown solid.
This precipitation can be regarded as removing hydroxyl ions from the solu-tion, leaving a relative excess of hydro-gen ions and it is this excess that makes the solution acidic. Because of a property of metals known as electro-negativity, copper replace iron from the solution and form a mixture of ferrous and cupric ions. Effectively the copper dissolves without producing any gas;
Ferrous ions become increasingly stable as the solution becomes more acidic and this helps the copper dissolution to proceed more easily whilst in basic (less acidic) solutions the ten-dency is for ferrous ions to convert back to ferric. As more copper dissolves and the solubility limit is exceeded, cupric chloride precipitates 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 when it is usually supplied in a strength of 45º Baume. This mea-surement 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 measure-ments and often profess complete igno-rance of any form of the others! To form the basic stock etching solu-tion, water is added in the ratio 2 parts water to 1 part ferric chloride solution resulting in a solution strength of 42º Baume.
Preparing the solution
At this stage the strongly acidic ferric 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 10 cc of the stock solution and add 10 cc of 9% household ammonia solution. This results in a sludge of ferric hydrox-ide which is allowed to settle before pouring off the liquid and adding the precipitate to 1 litre of the stock solu-tion. 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 42º Baume solution, in the proportion 1 part spent liquor to 10 parts 42º Baume solution.
Colour in use
The optimum etching conditions usu-ally occur after some etching has been done (this reflecting an 'incubation' period for the chemical reaction to 'settle down') but as etching proceeds the solution becomes less effective. The progress of this exhaustion can be oh served 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 effi-cient, and finally to a dark green solu-tion and black precipitate (cupric salts in solution and precipitated with the hydroxides). At this stage the solution is exhausted and requires disposal. This can be effected by the slow addition of either calcium carbonate (whiting) or sodium carbonate (washing soda crystals) - until effervescence ceases. At this point the solution is neutralised and may be disposed of safely.
The myth upside down
The normal recommendation is to bite copper plates in ferric chloride in-verted or on edge to avoid precipitated 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 min then washed in cold running water to remove the pre-cipitate 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. On completion of any biting, or if the plate is to stand between bites for any length of time, it should always be cleaned thoroughly with running water, since residual ferric chloride in bitten lines will continue to react.
Ease of use
To summarise, 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 precautions necessary are to use goggles and gloves, 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 42º Baume solution at its most ef-fective is very good for a deep line bite. When diluted to approximately 32º 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 re-duces foul biting. Due to the low depth of bite necessary in aquatint, the strength of solution does not greatly affect time taken to create a black, therefore strength of solution tends to be a personal taste. The minus side is the length of time taken to bite a plate: half an hour in stock solution for a black aquatint, and an hour for an aver-age etched line. Very deeply bitten lines may take several hours.
Notes 1 To speed the process, some sources recommend agitation of the solution by bubbling air through an upright tank. Whilst this undoubtedly works there is a risk of hydrogen production by reac-tion with hydrochloric acid. Therefore either extraction or a constant and ade-quate flow of air is required to prevent hydrogen build up. 2 It is preferable to use industrial grade 45º Baume ferric for both cost and speed (obtainable in the UK from Ellis and Everard) these less refined solutions often bite faster and it is the safest way t&) buy the salt. Merck chemi-cals sell small quantities for laboratory use but this is the Analar pure grade at a 60% weight per volume ratio. This needs greater dilution to 42º Baume and tends to be a slower acting form of the salt.
Dangerous properties of industrial materials,
1984. ISBN 0442-27373-8. R.J. Lewis.
The rapid guide
to hazardous chemicals in the workplace.
ISBN 0442- 01759-6.
Contact: Steve Hoskins, University of the West of England,
Clanage Road, Bower Ashton,
Bristol BS3 2JT, UK