The Edinburgh Etch: A Breakthrough in Non-toxic Mordants The New Etching Chemistry 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 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 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 WARNING: Useful Tip: Taking the Sting Out of the Edinburgh Etch Discovery of the Edinburgh Etch 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. 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 WARNING: Etching Zinc Plates Edinburgh Etch for Zinc 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: 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 Note: Local Water Variations Edinburgh Etch for Zinc in Special Conditions Weak Edinburgh Etch for Zinc 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 Mixing Strong Edinburgh Etch for Zinc 1 Litre saturated ferric chloride solution (40%) +500g citric acid powder + 3 Litres tap water Make up solution as before. Irregular Etch on Zinc Edinburgh Etch for Copper 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;
4/5 saturated ferric chloride solution (40%) + 1/5 citric acid solution, which consists of 3/4 tap water +1/4 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 +400ml citric acid powder (by volume) which equals 400g powder Mixing: Edinburgh Etch for Copper Tip: Evaporation Self-Texturing Etch on Copper Self Texturing Edinburgh Etch Mixture for Copper Plates Edinburgh Etch for Steel 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 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. |