Electroetch Articles

LEONARDO 25 #1 (1993)

Etching and Tone Creation Using Low-Voltage Anodic Electrolysis

Omri M. Behr and Marion R. Behr

(Reproduced by permission of the copyright owner, MIT Press)

Received 3 December 1990.

ABSTRACT

Etching of and tone creation on metals may be carried out by an ecologically sound method that avoids the use of acids and rosin. The method uses direct-current anodic etching, at very low voltages, in a bath of an aqueous electrolyte whose caption corresponds to the metal to be etched.

The technique of etching metal with acid is almost 500 years old. Iron was the original substrate; copper came into use in about 1520 and was first etched with nitrous acid, though only for a short time, since that reaction produced the poisonous gas nitrogen dioxide. Rembrandt etched with a mixture that contained mainly hydrochloric and tartaric acids. Nitric acid, which is still used to etch zinc, was introduced in the late eighteenth century, Dutch mordant, in the mid-nineteenth century. Ferric chloride, which some artists prefer, requires acid to prevent precipitates from forming. Interestingly however, no significant change has been made in the etchants used for printing in the past 150 years [1].

Governmental and other studies have shown that the strong acids presently used in etching are harmful both to workers and to the environment. The fumes from concentrated hydrochloric and nitric acids [2,3], as well as nitric oxide formed in metal etching do not have a strong smell [4] and therefore cause damage unnoticeably and in a cumulative fashion. Acids, as well as the chlorine generated by copper with Dutch mordant, are harmful to the lungs and epithelial tissues such as gums, nasal mucosa and bronchial passages [5,6]. When the tears that coat the eye absorb fumes, wearers of soft contact lenses [7] are at significant risk for corneal attachment.

Sensible laboratory handling methods can considerably reduce these dangers but will not eliminate them. Judicious handling of acid requires the availability of efficient ventilation systems, which are both expensive to install and to maintain. Furthermore, since acid corrosion is cumulative, pouring acid solutions into the sewer lines can ultimately damage 'in-house' lines as well as the operation of the general disposal systems. Repairs to damaged in-house lines represent just one example of additional costly expenditures to the traditional process.

It is not only acids and gases that are unhealthy---the asphaltum and rosin powders that are used in aquatinting are also dangerous to the lungs. The deposition of such powders onto the air-absorbing cells (pneumocytes) coats them and, ultimately, may block them permanently, leading to selective but irreversible loss of pulmonary function [8]. In this light, the use of acids and powders should be routinely avoided.

In traditional etching, a metal plate is coated with an acid-resistant material such as hard ground. Next the image is drawn into this ground with a sharp tool. This process removes the ground, exposing thin lines of the metal beneath. Acid is then applied (either directly with a brush or, more commonly, via submersion of the prepared plate into an acid bath), which dissolves the metal to leave grooves in the plate. The resultant acid etch solution contains dissolved ('ionic') metal. In contrast to the harmless 'metallic' metal, ionic metal should not be put into sewage systems because regular sewage processing will not remove it and its discharge is environmentally unacceptable.

The disadvantages of the traditional techniques can be overcome by using electrical methods. We have developed two related procedures with this in mind. The Electroetch ^® system produces lines etched into a plate. The related Microtint^® system provides a controlled roughened surface on a plate that will hold ink in a way similar to aquatint, but without the use of rosin. In both systems we use an electrically conductive, water-based electrolyte solution, and two plates submerged in the electrolyte and connected to a controlled-voltage direct-current source. The plate we wish to etch acts as the anode; the other as the cathode.

In this anodic etching technique, electrons are taken from the exposed metal of the etching plate at the anode, transforming the metal into water-soluble ions. These ions then travel through the electrolyte to the cathode where they regain the lost electrons and are reconverted into metal. The process is thus a closed circuit in which electricity moves metal from the anode to the cathode without the need to add or dispose of any chemical reagent or product. The amount of metal ions dissolved in the solution does not change, so the electrolyte bath is reusable. Microtint and Electroetch processes require no acid and, if properly done, do not result in the formation of gases.

PREVIOUS EFFORTS

These electrochemical steps are not new. In 1912, Schwuchow and Johnston [9] obtained a U.S. patent for a process that made use of zinc plates, coated with anodic photo-resist layers, as anodes in an electrolytic cell for the purpose of making half-tone plates. The underlying principle has been used for many different applications since that times [10--14]. Anodic etching, however, has not gained acceptance or recognition by artists. The most recent books on printmaking, which exhaustively discuss the important methods of intaglio printing, make no mention of it whatsoever [15,16]. Nik Semenoff and Christine Christos have recently reported using anodic etching for line etching on intaglio plates [17]. Their work was done at higher voltages and with different electrolytes than are used in our method. We believe that, contrary to their experience, anodes and cathodes should be of identical metals and that it is preferable to use an electrolyte having a cation of that same metal. Furthermore, the work of Semenoff and Christos suggests that the use of traditional aquatint methods is necessary to retain ink on large (i.e. non-line) areas. We have not found this to be the case.

Previous anodic etching applications have used voltages at the 5--10 volt levels. What happens at the anode or the cathode depends on several factors including voltage, and the composition, concentration and acidity of the electrolyte. The reason for this complexity is that various ions compete with each other based on their relative electrical `positiveness' or `negativeness'. At this 5--10 volt level, the most common of the `successful' ions in a water-based solution forms hydrogen at the cathode and oxygen at the anode. Unfortunately these two gases are highly explosive in combination and can be ignited in the presence of a single spark.

The concentration of the metal ions in the solution is one of the factors that determine the amount of current flow and therefore the amount of metal dissolved. Low concentrations lead to very slow etching. Concentrations near the saturation level of the electrolyte can even cause the electrolyte to form a thin but solid coating at the anode, thus reducing the current flow. Even at the 5--10 volt levels, ion concentrations of the order of 25 gm. per 100 ml. of copper or zinc sulfate encourage oxygen generation at the anode in the form of metal oxide. If the anode is copper, then oxygen generation will rapidly stop further etching, because copper oxide does not conduct electricity. If the anode is zinc, however, the entire bath environment will alter but slowly, since zinc hydroxide is basic and will cause the pH to rise ultimately causing the bath to gel. Because of these factors, previous anodic etching systems have required the use of acid in order to counteract these effects.

OUR METHODS

Explanation of Technique

As with traditional etching methods, the use of the Electroetch and Microtint processes requires that areas of the plate be covered with a resist. The resist is then removed from the areas to be etched. Traditional resists, used with certain precautions, are suitable for our method.

The type of etching to be done and the time of exposure contemplated determine the choice of resist. The back of the plate should be blocked from the acid, with the traditional paint or adhesive plastic sheeting. Hard ground is a good resist for exposure times of less than 2 hrs.

Even at very low voltages this method can etch in 5 minutes, a printable line into a copper plate which has been covered with hard ground and has been drawn into. Curiously, the etch lines we obtained electrolytically showed a 'glitter' not observed in acid-etch lines. We have found that the ink retention of these lines is superior to that of the acid-formed lines. Low power magnification (about 6 X), reveals a crystalline surface structure which does not appear at higher voltages (e.m. 5--10 volts).

Scanning Electron Micrographs confirmed this difference. Comparatively, the acid-etched surface has a far more regular appearance than that of the electrolytically etched. The latter clearly shows rather strikingly, the different erosion levels of the crystalline substructure of the metal.

Crystal masses react more quickly at intersections of surfaces such as fault lines or edges, than at the surfaces of the crystals themselves. This means that the metal dissolves faster at such intersections. The gentle nature of low voltage etching accentuates this difference, one that does not occur in acid etching. This explains the rough qualities of the interior of the electrolytic etch and, therefore, it’s more efficient retention of ink.

There is a further advantage of our method. It has been reported that in acid etching, after the initial groove has been cut, that eventually erosion will occur horizontally under the resist layer [18]. Such undercutting leads to a weakening of the top layer of the metal and subsequent widening of the groove. In the Electroetch process such undercutting has not been found, although in the creation of embossments, discussed below, it occurs to a negligible extent after many hours of exposure.

In conventional aquatinting, rosin powder is deposited on the entire plate. The plate is heated so that the particles of powder just melt, but do not coalesce. The areas where no tone is desired are covered with hard ground and the plate immersed in acid. Tiny pits are formed in the metal surface between the rosin or asphaltum particles of the ground. These pits hold the ink. Just as low-voltage anodic etching creates a rough crystalline surface inside the etched lines, it can create this rough texture on the surface of a plate. We have found that our Microtint method can create very successful tones.

The application of low voltage to a clean metal surface provides, predictably and reproducibly, beautiful, substantially uniform grained surfaces. We have been able to print shades ranging from the palest gray to deep black as shown in Figure 1. Unlike aquatinting, Microtint does not carry the potential danger of 'whiting out' or undercutting the rosin particles so that ink is no longer retained by that part of the plate. The longer the current passes through the plate, the darker the shades will print.

Microtint can also be utilized with conventional procedures such as soft ground and sugar lift without the need for rosin coating on the plate. One can also draw the resist onto the plate using, for instance, a lithograph pencil, permanent marker or India ink. The imperfections of these resists leads to attractive 'soft' effects, since they do not have the strength of hard ground or of stop-out varnish. After a time, they will allow the current to pass.

Complex designs of different tones may be obtained by initially covering the surface of the plate with an adhesive-coated plastic sheet. The sought after designs are drawn on to the sheet and the appropriate portions of the sheet then cut out to expose the metal. This approach can be utilized to obtain an ordinary Microtint surface, to obtain an embossment with a Microtint surface, or to cut through the metal of the plate completely. This cut through technique can also be used to shape the external circumference of the plate. An exposed area where the erosion does not continue long enough to penetrate the plate completely will create an embossment; however it will differ from conventional embossments formed by acid in that its 'base' is rough and will retain ink.

TECHNICAL CONTROLS

Success with the Microtint process lies in the simple but strict control of a number of factors. All the conventional techniques of plate preparation such as cleaning, polishing and avoidance of grease and fingerprints still apply. Since the new process is a gentle one, a misplaced thumbprint will be highly visible on a surface treated, for example, to print pale gray.

There is a matter of procedure in our method that requires more attention than in traditional etching. The ground, of course, must be clean and, when applied, free from bubbles. In acid etching, minor leaks in the ground are usually not serious since the flow of acid through them is minimal. In the Electroetch process, the metal is removed from inside the plate as soon as a passage for current is established. If the coating of ground is too heavy but nevertheless a pinhole is present or develops, substantial erosion will proceed unnoticed over longer exposure times. When long exposure times are needed and no drawing required a special electrolytic resist such as Miccroshield, should be used. Such coating yields a flexible film that cannot be drawn into.

The level of the bath temperature is important. The passage of electric current as is well known, releases heat. This effect is negligible during the etching of small areas but can become significant when cutting or embossing large plates, particularly when the temperature of the studio is high. While elevated temperatures encourage current flow, they may also lead to softening of the resist's adhesion to the plate. As in conventional etching, a good seal must exist between the resist layer and the plate. A rise in bath temperature to over about 90 ^oF should be avoided to prevent electrolyte from leaking between the metal and the resist layer, as this causes contamination of the printing face.

In contrast to an acid bath, an electrolyte bath can hold plates for short periods when the current is switched off without causing any corrosion to them. Over longer times, however, the electrolyte solution can creep up the plates by surface tension and can corrode the contacts. We recommend removing the plates, washing them with water and storing them when not in use. Once the plate has been etched, we proceed with printing according to the standard methods.

DISPOSAL

Sound environmental practice precludes discharge of the copper electrolyte into the sewage system (zinc sulfate, on the other hand, is not harmful). Copper ions are easily converted into harmless and readily disposable metallic copper by contact with metallic iron, preferably in finely divided form. Plates removed from the copper sulfate bath should first be rinsed in a water tank containing a stainless steel pad, the sort commonly used for scrubbing metal pans. Aluminum turnings can also be used. The iron in the pad will not rust but will convert the copper sulfate that washes off into copper. The copper sulfate bath can probably be used indefinitely. If it must be disposed of, it should be treated with iron, steel or aluminum shavings, which can be obtained as scrap from local machine shops. The conversion process works more quickly if the shavings are first washed with strong detergent to remove residual machine oil. We transfer the copper sulfate electrolyte into a holding tank, optimally one with a drain at the bottom. Then we add the shavings to the tank and stir occasionally. The conversion is complete when the deep blue of the copper sulfate is replaced by a substantially clear, pale yellow solution and a dark orange sludge. When aluminum is used in place of iron, slight acidification with acetic acid (white vinegar) is advisable. The change in appearance is the same. It is safe to pour these solutions and sludge into the sewer.

Once the plate has been etched, printing should proceed according to the standard methods.

SPECIAL CASES

The Electroetch and Microtint methods are also applicable to the etching of solid objects such as jewelry. For this type of three-dimensional object, it is better to use 'wrap-around' cathodes to get an even distribution of current around the anode.

To use Electroetch or Microtint on large plates, we recommend large, ideally transparent, vertical tanks. Transparent tanks

Allow for the observations of unexpected occurrences, such as plate disorientation, or changes in electrolyte level due to the inadvertent opening of a valve. Using electronic controls to monitor and set the voltage, current, and temperature levels, as well as the timing of the current [19] achieve efficient control of the process in such tanks. It is also helpful to circulate the electrolyte through a filter that removes the metal debris that forms during the etching. This procedure maintains the cleanliness of the electrolyte and the tank.

HEALTHIER PRINTMAKING

Artists need not sacrifice their health for the sake of their art or vice versa. By developing tools and methods that are not detrimental to the body, both can be satisfied. Many artists do not etch at home because they fear that strong acids will harm young children. The dangers involved in the traditional etching process have excluded the widespread introduction of etching at the secondary-school level. We believe that our new process will make life safer and healthier for artists and the environment. It may also open up a beautiful art form to many that could not enjoy it until now.

Note: Some very minor variations from the printed text have been made for website purposes.

Acknowledgment

We thank Mohammad Khalil, Master Printer, New York, NY for editioning Fig. 1.

Omri M. Behr (patent attorney, chemist), 325 Pierson Ave., Edison, NJ 08837-3123, U.S.A.

Marion R. Behr (artist, printmaker, author), 325 Pierson Ave., Edison, NJ 08837-3123, U.S.A.

References

1. D. Saff and D. Sacilotto, Printmaking, History and Process, (New York: Holt, Rinehart & Winston, 1978) p.119.

2. Occupational Exposure to Nitric Acid, Department of Health, Education and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, May 1972.

3. Occupational Exposure to Hydrochloric Acid, Department of Health, Education and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, March 1972.

4. "Nitric Oxide," in G. Weiss, ed., Hazardous Chemicals Data Book (Park Ridge, NJ: Noyes Data Corp., 1980) p. 961.

5. K. O. Schmidt, "Pathological Findings on Late Sequelae After Inhalation of Nitrogenous Fumes", Pneumonologie > (in German), Vol. 150 (1974) pp. 133--137.

6. See Entry #2095 (chlorine) in the Merck Index, 11th Ed. (Rahway, NJ: Merck, 1989).

7. Personal communication from William Phipps, Hoboken, NJ, U.S.A. 1991

8. Personal communication from Donna Schulman, Brooklyn, NY, U.S.A. 1991

9. E. G. Schwuchow and G. F. Johnston, U. S. Patent 1,047,995. 1912

10. L. E. Holland, U. S. Patent 2,074,221. 1937

11. G. F. Johnstone, U. S. Patent 2,110,487. 1938

12. L. B. Corbett, U. S. Patent 2,536,912. 1951

13. S. Raviv et al., U. S. Patent 3,635,805. 1972

14. H. H. Nee et al., U. S. Patent 4,729,940. 1988

15. Saff and Sacilotto [1].

16. J. Ross and C. Romano, The Complete Printmaker (New York: Free Press, 1991).

17. Nik Semenoff and Christine Christos, "Using Dry-Copier Toners and Electro-etching on Intaglio  Plates", Leonardo , No. 4, 389--394, 1991.

18. R. Mayer; S. Sheehan, Ed. The Artist's Handbook of Materials and Techniques (New York: Viking, 5th Ed., 1991) p.588.

19. U. S. patents 5102520 and 5112435 have been granted for the apparatus and the process, and further U. S. and foreign patents have been applied for (WO 92/07978).

Glossary

anode--- an electrode connected to a positive terminal of a direct current source.

asphaltum--- fine particles composed of heavy mineral or coal-tar residues. Asphaltum has a melting point of approximately 200 ^oF.

cathode--- an electrode connected to a negative terminal of a direct-current source.

chlorine--- (Cl₂ )A corrosive, strongly oxidizing gas.

Dutch mordant--- an acidic etchant used with copper, made up from hydrochloric acid and potassium chlorate.

ferric chloride--- (FeCl₃) a somewhat water-soluble salt of iron and hydrochloric acid that has a tendency to become basic.

electrode--- a conductor connected to a positive or negative terminal of a current source.

electrolyte--- a solution, usually aqueous, that contains a water-soluble compound that facilitates the conduction of electricity. It may be an acid, a base or the salt of a metal or similar ion.

electrolytic cell--- a bath or tank containing an electrolyte and two electrodes.

hard ground--- a complex wax used in the form of a molten solid or dissolved in a slowly-evaporating polymerizable solvent to make metal plates resistant to acidic and electrolytic erosion.

hydrochloric acid--- (HCl) this acid, also known as muriatic acid, when concentrated gives off irritating fumes.

intaglio--- printing method in which the artist creates lines or areas below the regular surface level of a plate. The plate is then inked and printed onto dampened paper, usually by passage through a press. This is the most common method of etching.

ion--- an electrically charged modification of an atom or group of atoms. Ions are usually water soluble and are attracted to electrodes of opposite charge. Metal ions become metals on contact with a cathode.

Microshield--- a proprietary, quick-drying solution of polymers which, when dry, is highly resistant to the passage of electrical current.

nitric acid--- (HNO₃) an exceedingly strong, oxidizing acid. When mixed with hydrochloric acid, nitric acid will dissolve gold.

nitric oxide--- (NO) a colorless gas having an acute inhalation and irritation toxic-hazard rating of 3.

nitrogen dioxide--- (NO₂) a dark brown, toxic, oxidative gas.

nitrous acid--- (HNO₂) an unstable acid formed by the action of strong acids on nitrites. Rapidly decomposes into nitrogen dioxide and nitric oxide.

potassium chlorate--- (KClO₃) A strongly oxidizing salt.

pH--- a measure of the acidity of a solution. pH 7 is neutral, lower numbers are more acidic, higher numbers are more basic.

rosin--- a dried gum derived from turpentine residues, rosin has a melting point around 200 ^oF. It is used in finely powdered form in aquatinting.

scanning electron micrographs--- photographs obtained from images projected from samples placed in the beam of a scanning electron microscope. Typical magnifications are in the range of 50 X to 10,000 X.

soft ground--- a substance similar to hard ground, but which additionally contains tallow, and therefore has a lower melting point and greater sensitivity to displacement by pressure.

sugar lift--- technique in which an aquatint (i.e. rosin) layer is put down on the plate. An image is painted on a plate with a mixture of corn syrup, tempera paint and soap or detergent. The mixture is allowed to dry and the entire plate covered with hard ground. The plate is then immersed in water, preferably warm water and brushed gently to remove those areas previously coated with the syrup mixture. The exposed areas are then, traditionally, aquatinted by immersion in acid. The Microtint process, of course, eliminates the need for the aquatinting steps.

tartaric acid--- a very mild acid derived from grape lees.

FIGURE CAPTIONS

Fig. 1. Marion Behr, FREE SPIRIT Microtint on copper, 5" x 5", 1991. In FREE SPIRIT .The artist varied exposures of plate segments via the successive removal of adhesive film sections over a period of 2 hours. ©Marion Behr