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the chemistry of ferric chloride etching

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From Printmaking Today Vol 4 No.2

The chemistry of ferric chloride
Or how to get one up on your Safety Officer
by Stephen Hoskins and Roy Pearce

Since Rembrandt, etchers have used nitric acid and Dutch mordant to etch copper. At the turn of the century ferric chloride solution was introduced in gravure work, principally because of its interaction with gum bichromated gelatine. With the increased emphasis on health and safety at work, we have had to reappraise these and the other chemicals we use.

The salt, ferric chloride, has none of the disadvantages associated with the other two acid 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 strong 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.

The chemistry of the (etching) 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 chloridesolution.

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 dis-solution 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 45� 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 42� 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 42� Baume solution, in the proportion 1 part spent liquor to 10 parts 42� 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 42� Baume solution at its most ef-fective 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 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.


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 45� 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 42� Baume and tends to be a slower acting form of the salt.

Further reading

N.I. Sax.
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


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“Years ago I recognized my kinship with all living things, and I made up my mind that I was not one bit better than the meanest on the earth. I said then and I say now, that while there is a lower class, I am in it; while there is a criminal element, I am of it; while there is a soul in prison, I am not free.” E. V. Debs

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i confess i was a bit confused by this after Bob Kramer mentioned that ferric chloride was "a base... on the other side of the acidic scale" in the recent episode of Raw Craft with Anthony Bourdain.


i often add a dribble of it to lemon juice to boost the acidity (when etching steel for hamon) and for a second there had to check to see if i was doing the opposite.

(see 6:37)
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yeah, I saw that too. That video made me want to get a good resource for all of us. That, and a conversation I had with someone who called my home after reading some forum posts about ferric and ferrous chloride. The person was troubled because they had read that ferrous chloride is actually what does the etching, and they could not buy ferrous chloride. Worse, something I had written had confused them, because I pointed out that it was the lower-oxidation number iron group that etched. I may have set that poor guy back by days of confusion because of what I wrote. So, I am trying to help.

Ferrous chloride seems to do the etching, but at room temperature, and especially dry, we are dealing with ferric chloride, and it ionizes and reacts with water, and we get ferrous chloride. No one need worry about buying ferrous chloride!

please visit my website http://www.professorsforge.com/


“Years ago I recognized my kinship with all living things, and I made up my mind that I was not one bit better than the meanest on the earth. I said then and I say now, that while there is a lower class, I am in it; while there is a criminal element, I am of it; while there is a soul in prison, I am not free.” E. V. Debs

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Here is what I get from your post. (thank you, it is very interesting )

The posted article states , a water solution of ferric chloride is an acid. The article also states it is the Ferric ion reacting with the copper that is creating the etch.

I think Bob Cramer had a slip of the tongue.


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It has always been my understanding that ferric was not an acid any more as it had been hydrochloric acid that had been "killed " with iron ..

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In one case, the etching is done by the acid, as in HCl and a little Ferric probably, in the other case the etching is done by the Ferric Ion (Fe+3) and there is a little acid present as well . So the water solution of Ferric has some acid in solution as well and is acidic by nature ( not a strong acid). The source of the acid in the second case is from the disassociation of water into H+ and OH-. The OH- is being tied up with some of the Ferric Ion leaving an excess of H+ ions which makes the solution somewhat acidic.


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Yes, Jan said it very well. Each time ferric hydroxide forms and precipitates out of solution, it leaves hydrogen ions. Hydrogen ions in solution are an acid (same as HCL, only HCL leaves a maximum amount of hydrogen ions available, while this reaction only generates a few at a time, but as the reaction continues, there are more to liberate).


Matt, hydrochloric acid plus iron = ferrous chloride. To make ferric chloride, add h202 (hydrogen peroxide).


However, ferrous chloride, if all of the hcl has been reacted, should be pretty dead as an etchant.



please visit my website http://www.professorsforge.com/


“Years ago I recognized my kinship with all living things, and I made up my mind that I was not one bit better than the meanest on the earth. I said then and I say now, that while there is a lower class, I am in it; while there is a criminal element, I am of it; while there is a soul in prison, I am not free.” E. V. Debs

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Chemistry... Not my strong suit.. Thank you both

Edited by Matthew Parkinson
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