The Edinburgh Etch: A Breakthrough in Non-toxic Mordants
(copied from Keith Howard's book "Non-toxic intaglio Printmaking")
The Edinburgh Etch made its official debut in Printmaking Today Vol.6, No.3. in 1997. My friend Friedhard Kiekeben, artist and researcher at the Edinburgh Printmakers Workshop and inventor of the Edinburgh Etch, has kindly provided the following updated information.

The New Etching Chemistry
In the acrylic resist etching system metal plates are etched in solutions based on
ferric chloride. This mordant which is a saturated solution of corrosive salt crystals has, over the centuries, been valued by etchers as the most accurately biting and controllable acid.
From a modern perspective it commends itself also because it gives off no toxic vapours, neither by itself nor during etching, causes little hazard during occasional skin contact, and cannot lead to violent reactions if it is accidentally spilled. The list of benefits almost looks too good to be true. But if etching with ferric gives the better etch and is so much safer than other methods why is it then that it has not always been used as the prevalent mordant in etching?
The answer simply lies in the speed and ease of biting. The way ferric was used in the past meant that etches of a reasonable depth, as are typically required in the intaglio medium, took a very long time to accomplish, and the methods employed often were cumbersome and inefficient.

However a number of ground-breaking innovations in working with ferric have now become available to intaglio printmaking which dramatically enhance the properties and the biting speed of this safe mordant; some of these have long been in practice in industry while others have resulted from my research into innovative mordants at the EPW. Due to technical limitations the use of ferric chloride has in the past been mainly restricted to etching copper plates. With the introduction of the Edinburgh etch method, ferric based mordants are now capable of eroding any kind of metal plate suitable for intagho printmaking. These encompass not only the most common metals copper zinc, and mild steel, but also aluminum and brass. In many cases the straightforward tray-etching method known from other acids suffices, while it is feasible for larger professional workshops to also provide so-called dip-tank facilities that can further enhance the biting process on copper and steel plates.

The problems traditionally associated with ferric chloride are to do with its peculiar biting chemistry: when metal plates are etched in ferric chloride normally a sediment is gradually building up inside the bitten intagho areas. If these crystalline residues are not continually removed by some means they eventually clog up the newly formed grooves and prevent the plate from etching any further. One way of avoiding this problem is by etching the plates in so-called dip-tanks - a reliable technology from the electronics industry which has first been introduced to printmaking by the Canadian/Australian Keith Howard.
The plates are inserted vertically in the corrosive solution so that any sediment particles can drop out of the etched grooves and sink to the bottom of the tank. A dip-tank should be equipped with an aeration facility which continually pumps air down to the bottom of the tank. This device agitates the ferric and makes it continually flow past the plate surface. Also the dip tank can be heated by simple means to accelerate the etch further. Especially copper plates can be etched with unrivaled speed and quality by using a tailored Edinburgh etch solution and dip-tank technology. Dip-tanks are also advisable for biting steel, but should not be employed for the more reactive zinc and aluminum which are always etched in trays.

The corrosive properties of ferric become apparent with a brief look at its chemical make up. A ferric molecule consists of one atom of iron which has three atoms of chlorine locked onto it. The bridge between iron and chlorine, which creates the chemical adhesion of the molecule, is made up of two electrons (negatively charged particles) for each chlorine atom. However, since the iron atom has to have eight electrons to be in a stable condition, but has only six in ferric chloride, it tends to attract atoms it comes in contact with, such as atoms of other metals, and reacts with them to gain the missing two electrons. A substance of this kind is not strictly speaking regarded as acid, but due to its similar corrosive properties is referred to as a Lewis acid by chemists.

Ferric Chloride - An Essential Ingredient for Etching
Ferric chloride is available from most chemical suppliers either as yellow granules or as a saturated solution, both of which are fairly inexpensive, especially if bought in larger quantities. If at all possible the ready made solution should be used, which for its industrial use comes in 25L or even larger containers at about 42 to 45 BE (Baume) density. The density/weight of ferric can be measured by immersing a so called
hydrometer in the solution. The Baume scale of describing the specific weight of liquids is not entirely accurate, and sometimes other specifications are given by manufacturers. The strength of the ferric solution can also be described as a percentage, in which case say 40% ferric chloride (or 40% FeCl3) simply denotes a ratio of 40% ferric crystals to 60% water. I find that 40% FeCl3 about equals a 42-45 Baume density.

Industry grade ferric solutions may also come under the name of P-grade ferric chloride. When ordering ferric I always found it easiest simply to ask for a saturated, ferric chloride solution without referring too much to density scales that the suppliers were unsure about. Remember that liquid ferric is a heavy solution of ferric chloride salt crystals in water, which could not go beyond a certain strength (i.e. 45 Baume) unless the crystals solidified. It is easy to dilute a strong solution with water to obtain a weaker strength, but impracticable to strengthen a ferric solution which is too weak for a good etch from the outset. Even though ferric chloride is fairly safe to use, eye protection and gloves must be worn when handling it. The saturated ferric solution of about 42-45 Baume is an ideal base ingredient for making up the various mordant needed in the etching workshop, and only in exceptional circumstances (i.e. very delicate etched photo-polymer work) would it be useful to obtain the (much more expensive) purer but weaker laboratory grade ferric, which tree actually etches less well than the impure industrial grade. Due to the variable strength of saturated ferric chloride certain adjustments to the mordant recipes given here may have to be made. If say a mix of one part ferric to three parts water does not etch as described in the recipe reduce or increase the water content until the strength matches that of the recipe. Inevitably this, as will fine tuning other variables, entails some experimentation, but finding a few things out for oneself is always half the fun of printmaking!

Making up Ferric from Granules
If only the granules (or powdered form) are available these should be handled very carefully as they are very corrosive and need some preparation before they can be used for etching. The crystals should be kept in sealed plastic containers to prevent any absorption of moisture from the surrounding atmosphere.
The etch solution is made up by dissolving about one part of ferric crystals in about two parts of warm water at approximately 40 degrees centigrade. If citric acid is also to be an ingredient of the mordant, this can be added to the ferric crystals at this stage. Once this mixture has gradually been added into the water content sufficient heat will be generated to aid the dissolution of both. The exothermic reaction may also generate some acidic gases, so adequate ventilation and protection are strongly recommended.

Etching Different Metal Plates
It is absolutely essential that different kinds of metal are always etched in different etching facilities, such as in separate tanks or trays. If a metal plate is accidentally placed in the wrong tank or tray this causes electrolytic processes, contaminates the ferric solution, and in the case of a zinc or aluminum plate put in a dip tank can even lead to violent chemical reactions.

Useful Tip: Taking the Sting Out of the Edinburgh Etch
Before a fresh etch solution is used, the initial sting should be taken off to prevent uncontrolled biting. This can either be done by inserting a blank piece of the kind of metal that is to be etched in the bath and waiting until that has completely been dissolved, or by adding a small amount of exhausted mordant to the fresh mix.

Discovery of the Edinburgh Etch
My aim in developing the new Edinburgh etch mordants was to harness the eroding power of ferric chloride fully. Artists in the past mainly resorted to mechanical devices enabling them to eliminate the sedimentation problem associated with the ferric bite. In its traditional form this meant etching plates in trays facing down, while rocking the bath continually - this method produced reasonable results on copper plates, but a good etch could take many hours to accomplish. Due to its generous filling with mordant and vigorous agitation a dip-tank represents the best facility for the mechanical removal of the crystalline sediments that ferric chloride generates as it etches metal.

In the course of my research I approached the problem from a new angle, searching for additives to ferric chloride which might be able to dissolve the sediment as it is produced. Already there was some evidence of this possibility: the known etch enhancing effect of a salt solution added to a ferric solution. This can be explained chemically: a ferric chloride solution consists of dissolved crystals; the dissolution process creates a weak bond between the ferric molecules and water known as the state of hydrolysis. This bond reduces the ability of the ferric to fill its electron gap by snatching metal ions out of the plate to be etched.
If other crystalline substances such as
sodium chloride are added to the solution the ferric may be activated -the hydrolysis bond with water is now partly taken over by this other substance, so that more ferric chloride molecules are free to bond with metal.

After experimenting with a variety of possible substances I stumbled across a new kind of non-toxic additive normally more associated with food rather than etching - citric acid. It turned out that a citric acid solution mixed at a certain ratio with a ferric chloride solution not only speeds up the bite of ferric (by freeing the ferric atoms from their weakening bond with water) but produces an entirely new kind of mordant with outstanding biting properties. Different kinds of metal require a different mix of this mordant now known as the Edinburgh etch. On zinc, copper, brass, and mild steel I found the unique property of the Edinburgh etch to be consistent throughout: the etch process takes place with the utmost precision. and without the build-up of sediment typically associated with unmodified ferric chloride. In chemical terms this is due to the fact that the individual molecules of the metal salts produced during biting are locked into the clamp-like carbon structure of the citric acid thus keeping them dissolved. They are no longer allowed to solidify as crystals which would impede biting and result in a coarser etch. So the main obstacle usually encountered with ferric chloride is literally being dissolved by the new mordant.

The crucial ingredient of the Edinburgh etch, citric acid, is widely available from larger chemical suppliers and suppliers to the food industry. Citric acid is a white powder which ordinarily finds its way into cakes or fizzy lemonade rather than into an acid bath. It should be obtained as anhydrous citric acid powder, and in terms of handling and storage it is about as non-toxic as any chemical could possibly be. Do however wear a dust mask and goggles when dispensing the fine powder.

Etching Zinc Plates
The erosion of zinc plates is hugely improved by the addition of citric acid to
ferric chloride.While unmodified ferric chloride typically produces nothing but a very coarse line the etched work obtained in the Edinburgh etch for zinc is nearly as accurate as the razor sharp intaglio grooves otherwise only known from copper plates. Typical biting times are as fast as those with nitric acid. As opposed to the nitric bite there is less chance of lateral or foul biting and no craters are formed by toxic bubbles that require constant feathering. The chemical reaction between ferric chloride and zinc is self-perpetuating, which makes it more vigorous than with copper and steel. For this reason the emphasis in etching this metal does not lie in activating and accelerating the etch but in moderating and controlling it.

WARNING: Etching Zinc Plates
Full strength ferric should not be used with zinc plates as the ferric eats into the metal too quickly and coarsely. In fact zinc plates with large open areas left unattended in full strength ferric can actually overheat and trigger an uncontrolled chemical reaction which destroys fine detail on the plate.

Edinburgh Etch for Zinc
The chemistry of biting the impure metal zinc in ferric or in the Edinburgh etch differs substantially from the one of biting copper and steel. As opposed to these metals zinc does give off some gas during biting, namely hydrogen gas. Despite its somewhat explosive reputation the hydrogen gas produced is easy to control, and does not provide any hazard as long as it is not artificially concentrated, for instance by trapping it under a lid while large quantities of metal are being etched. The atmosphere in a room would have to exceed a concentration of about 5% pure hydrogen to become flammable (*US HHMI safety sheet p2) - this is unlikely in a well ventilated environment.

To those who are used to biting with the previously used nitric acid hydrogen gas is no stranger, as half the fumes produced in that case are made up of harmless hydrogen gas as well; the other half consists of the odourless but highly ("toxic"??) nitrogen + nitrous gases dan which linger above the mordant and add to the danger of biting with nitric. Hydrogen gas which, as a matter of interest also is the most basic element in the material world has, by contrast, no toxic effect on the human body (it is classified as practically non-toxic by the US health and safety authority*), and quickly disperses in the atmosphere. For this reason it is best to ensure a good airflow during biting by opening windows adjacent to the ferric bath or by using ventilation if available.

The Edinburgh etch for zinc is ideal for those etchers to whose work the creative possibilities of the fairly inexpensive but versatile metal zinc are indispensable. The following mix produces a fast yet very controlled bite on zinc without a crystalline deposit or sediment building up in the bitten work. Plates should be bitten face up and do not require agitation. In fact agitated plates, or plates rinsed for inspection too frequently may turn out less well than those left to erode to the required depth by themselves. For this reason it is advisable to determine typical biting times on a small test piece before more ambitious plates are etched. The lacking need for agitation or feathering in this process is explained by the self-dispersing properties of the extremely light hydrogen bubbles. (In a nitric bite by contrast it is the weight of the toxic nitrogen dioxide bubbles produced which accounts for foul biting and the need for feathering.)

The mordant is suitable both for fine and delicate work as well as for deeply bitten work on any acrylic etch ground. Especially the deep etch, which can be accomplished with surprising speed, reveals the astounding accuracy of this mordant as it penetrates the metal along the edges of the acid resistant marks without - lateral biting. At room temperature (20 C) fine line work on a hard ground etches within a matter of a minute or two, while a medium deep groove is eroded in 10 to 15 minutes and deeply embossed intaglio 3 work (of 0,5 mm depth or more) comes about in 40 to 60 minutes. As with all etching these figures are merely benchmarks and are likely to be iniluenced by variables of your etching set up.

In a busy print workshop such as the EPW the mordant once made up in a large tray and used daily, consistently etches zinc plates for up to 10 - 14 days before it needs replacing. The tray should be covered at night to minimise evaporation.

The Edinburgh etch indicates the end of its usable life by acquiring a less liquid consistency while turning a es dark brown. Depending on the specific ingredients er used, the Edinburgh etch may in some cases leave a er loose, thin layer of grey powder on open areas; this does, however, not impede biting and comes off when an the plate is rinsed.

Use the mixture below as a standard multi-purpose a mordant for zinc.

Regular Edinburgh Etch for Zinc

1 Litre

saturated ferric chloride solution (40%)

+ 500g

citric acid powder (can be reduced to 300g)

+7 Litres

tap water (See Local Water Variations.)

Amounts can be adapted given that the general ratio is retained. Say, if you only need half this quantity add 0.5 liter of ferric to 250g citric to 3.5 litres of water.

Some acrylics have shown a tendency to be undercut by an Edinburgh etch with a high citric acid content. If you encounter such problems choose a lower concentration of citric (i.e. 300g).

Other Materials:
1. Acid resistant tray placed in designated well ventilated area (photo tray suffices for shorter use)
2. Goggles/visor
3. Acid resistant gloves.

Fill a bucket with 2 Litres of hot water Gradually add the citric acid powder content while stirring continually. Once this has fully dissolved pour the liquid into the tray. Add the remaining water content (5 liters; now use cold water). Gradually pour in the ferric solution while gently rocking the tray, until you have produced a uniform orange-coloured liquid. After taking the sting off the bath of Edinburgh etch for zinc it is ready for use. Once a plate is immersed, biting in progress is indicated by hydrogen rising in discreet bubbles. The mordant is transparent so it allows you to assess the bitten depth while the plate is submerged.

Tip and Trouble Shooting
If you do not see bubbles forming after several minutes in, say, a drawn line you may have to redraw the line to expose the metal; grease sitting on the plate surface may also impede biting.

Note: Local Water Variations
Please note that all formulations of the Edinburgh etch given here have been developed and tested using the soft Scottish water supplies. In areas with hard water, i.e. with water that is more alkaline and contains lime, certain precautions must be taken to ensure consistent results. Otherwise the non- sedimenting effect of the citric acid may be weakened, as it is known to absorb lime. If your water supply is hard either soften the water by boiling before using it in mordant mixes or increase the citric content to your requirements.

Edinburgh Etch for Zinc in Special Conditions
Aquatinted areas on zinc plates etch very quickly in the regular Edinburgh etch. The large amount of metal exposed accelerates the biting action so that a dot structure, deep enough to print a rich black tone, can already result from an etch not longer than 8 minutes or so. Consequently, the whole spectrum of grey tones lies within this spectrum: Light grey tones are already etched into the plate within seconds, while the successive medium greys are a matter of a few minutes. Owing to its speed, this process requires some experience from test plates for perfect and predictable results. Especially the blacks are easily over bitten, as some undercutting of the aquatint may start after a 10 minute immersion.

Weak Edinburgh Etch for Zinc
If you do a lot of work with fine aquatint or other delicate etch grounds you may consider setting up a tray of weak Edinburgh etch for zinc alongside the regular one. The less vigorous biting action of this solution allows for finely stepped aquatint gradations (or indeed of other very delicate grounds) to be etched over a longer time span without much risk of over biting. A typical aquatint scale from light grey to black would be spaced out over a period of up to one hour.

Mixing Edinburgh Etch for Zinc

I Litre

saturated ferric chloride solution (40%)

+ 300g

citric acid powder

+ 14 Litres

tap water

Make up solution as before.

Strong Edinburgh Etch for Zinc
This extremely fast acting solution is an ideal mordant for the cleanest possible open- and deep etch. The heat generating reaction must be carefully monitored, and due to the high temperatures involved acidic fumes may be generated by evaporation. For this reason its use is only rycommended under controlled conditions, i.e. in a ventilated booth and with an inorganic respirator and goggles. A deep bite, which will be an exact intagho reproduction of the acrylic resist marks applied to the plate surface will, on a l mm / 18 gauge zinc plate, be etched within 10 minutes or less. Even the finest detail, which may eventually lift during long immersions in weaker solutions, is retained by this short burst method.

Mixing Strong Edinburgh Etch for Zinc

1 Litre

saturated ferric chloride solution (40%)


citric acid powder

+ 3 Litres

tap water

Make up solution as before.

Irregular Etch on Zinc
Zinc plates can also be etched in an unmodified ferric solution. This provides an interesting alternative to the clean bite of the Edinburgh etch as the coarser etch of ferric produces jagged lines and textures which make interesting pictorial tools. However, a deposit of zinc oxide crystals will build up in the bitten work. Plates then require a certain degree of scrubbing to reveal the full extent of the etched intaglio and with increasing length of biting the etch becomes more and more irregular. The plate should be frequently agitated during etching and it should be taken out of the tray, rinsed and inspected at regular intervals. For a good, coarse biting solution follow the basic recipe of the Edinburgh etch for zinc, but cut out the citric acid content.

Edinburgh Etch for Copper
Intaglio work carried out with acrylic grounds on copper plates and etched in an Edinburgh etch is of the best possible quality: Lines, textures, and open areas are cut into the metal as with a razor blade, and even the finest detail registers accurately on the bitten plate which in turn can produce the crispest possible intagho print.

Copper is a less reactive metal than zinc. If a ferric based etch solution fortified with citric acid is used plates can now be etched face up in a tray. The Edinburgh etch erodes copper about twice as fast as pure ferric, and the hindrance of sedimentation does not occur. The very controlled etch process, which neither involves the generation of heat nor of hydrogen bubbles, is somewhat slower than the one of zinc, and a fairly concentrated solution is normally used. The tray method can safely be employed even in an artist's studio lacking ventilation if basic precautions such as the use of goggles and gloves are followed. Frequent rocking of the tray is not crucial to the quality of the bitten intaglio, but can speed up the biting action. If the non-sedimenting properties of the Edinburgh etch are combined with a dip-tank set up in which agitation takes place automatically, very speedy biting times can also be obtained for copper plates. For instance a black aquatint, a crisp line, or a well developed open bite ridge are already sufficiently etched at about 20°C after a 30 - 40 minute immersion in a dip tank aerated with an aquarium pump. The Edinburgh etch mixture given below is a universal mordant both to be used in flat trays or upright tanks.

If no citric acid is available a saturated ferric chloride solution can also be used, but the etch will be slower and somewhat less precise. The mixture is based on the overall ratio of;


saturated ferric chloride solution (40%)

+ 1/5

citric acid solution, which consists of 3/4 tap water


citric acid powder (anhydrous)

In actual quantities this works out, for example at:

6 Litres

saturated ferric chloride solution (40%)

+1.2 Litres

tap water


citric acid powder (by volume) which equals 400g powder

Mixing: Edinburgh Etch for Copper
Fill a bucket with 1.2 litre of hot water. Gradually add the citric acid powder content while stirring continually. Once this has fully dissolved, gradually pour this into the ferric solution and keep stirring until you have produced a uniform liquid. Pour this into the etching tank or tray and take the
sting off ; the bath of Edinburgh etch for copper is now ready for use. Try to maintain a reasonable temperature in your etching facility for copper. Good results are ensured at 18 to 20°C, but higher temperatures of up to 30°C can further improve biting times as well as the overall responsiveness of the mordant. This mordant is exceptionally long lasting; a tank filling used daily at the EPW, occasionally topped up to compensate for evaporation loss, has been known to remain active for up to one year without a significant drop in its biting properties. When eventually the mordant acquires a deep olive colour it becomes less active and is then ready for replacement and disposal.

Tip: Evaporation
When not in use, Edinburgh etch solutions used in trays should be either poured back into plastic containers or covered with a lid to prevent evaporation.

Self-Texturing Etch on Copper
Previously etched copper plates were never known to display anything but a smooth surface in the eroded areas. The research into the Edinburgh etch has also yielded a mordant which can bring out a self-texturing tonal quality in copper, which is similar to a very fine aquatint. Tonal ranges or black tones can be produced simply by step-etching open areas of a plate in successive stages, while no added aquatint has to be applied. The dot structure appears in all intaglio areas, thus enhancing the ink retention of the plate and the richness of the resulting print. This mordant, which contains twice as much citric acid than the standard recipe, works best in an aerated dip tank.

Self Texturing Edinburgh Etch Mixture for Copper Plates
Mix 60% ferric chloride solution (40%) with 40% citric acid solution, consisting of; 1/3 citric acid powder, mixed with 2/3 tap water.

Edinburgh Etch for Steel
Mild steel etches best in the following Edinburgh etch in a well aerated dip-tank. Tray etching of steel plates in ferric chloride or the tailored Edinburgh etch is only satisfactory if the bath is kept warm and frequently agitated. The process using the Edinburgh etch brings out a self-texturing property in steel, which causes open areas of the plate to acquire an aquatint-like roughness. Open bite, if etched for long enough, has the typical key on the surface of the bitten area that is so desirable for dense intaglio printing. The process using unmodified ferric etches about 50% slower and open areas appear less textured. The Edinburgh etch also remains usable for much longer (for about 6 months in a dip tank) and, unlike ferric, it stays active even at temperatures below 18°C. Both mordants benefit hugely from a temperature raised up to 30°C.

Mixing: Edinburgh Etch for Steel

8 Litres

saturated ferric chloride solution (40%)

+ 3 Litres

tap water

+ 500 ml

citric acid powder (by volume) which equals 500 g powder

Mix ingredients as described for Edinburgh etch for copper.

Other Metals
Occasionally two other kinds of metal may be employed for etching purposes namely brass and aluminum. With respect to the quality of the bitten intaglio these materials are not a great extension of the possibilities provided by copper, zinc, and steel, but if etching is regarded as a sculptural as much as an illusionistic medium it is certainly worth experimenting with the fairly inexpensive silvery-grey aluminum and the gold coloured and fittingly costly brass.

Aluminum, which like zinc reacts vigorously with saturated ferric, etches well in trays of a medium dilute ferric chloride solution - normally one part saturated ferric chloride solution is mixed with 3 to 4 parts water. A small amount of citric acid can be added to this, but the non-sedimenting property is in this instance weaker than with other metals. The mordant exhausts itself quickly after a fairly vigorous biting reaction which once again involves the emission of hydrogen bubbles, and the generation of a certain degree of heat. Despite its fizzing nature the process remains controlled at this solution strength, and an open bite is etched within 40 minutes to 1 1/2 hours (note that the process accelerates with time). Aluminum plates always require thorough scrubbing with a stiff brush after etching to free the bitten intaglio structure from crystalline residues.

Due to its softness aluminum lends itself to mechanical work (i.e. drypoint etc.) but is less suitable for editioning as it is not resistant to the pressure involved in repeated printing. Brass by contrast, which is a very hard alloy of zinc and copper, can be etched in similar conditions as copper (see Edinburgh etch for copper) and is a very serviceable material for intaglio printing. It combines the graceful delicacy of copper intaglio with the robustness of etched steel - like steel its open surfaces also display a self texturing effect and plates do not due to their hardness, suffer from wear in large editions. However, due to the fact that it is more than twice as expensive as copper its use seems more justified when the plate is used not only for its printing properties, but also as an object in its own right.