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The New Etching Chemistry        
Friedhard Kiekeben

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A variety of marks printed from a brass plate, using the Edinburgh Etch process.
Using as a resist: Sharpie, Crisco, wax crayon, dry brushed acrylic, acrylic wash etc.


Etching Without Acid: Metal Salt Etching & Electro Etching

New developments in etching chemistry have made it possible for metal plates to be etched in metal salt solutions rather than acid. The new and safer method, Metal Salt Etching,
can and has effectively replaced the traditional acid etching approach in many shops, studios and schools.

Since the first publication of the Edinburgh Etch process in 1997, I have been able to further develop, test, and refine a comprehensive range of new metal salt etching processes for all metals used in intaglio printmaking and for sculptural plate making. The system comprises specific metal salt solutions for the fast and accurate erosion of copper, brass, zinc, steel and aluminum. All of these etching processes are compatible with common resists, including the whole range of mark making methods, such as HARD GROUND, stop out or AQUATINT within ACRYLIC RESIST ETCHING.

The benefits of this new etching methodology over the traditional acid etch approach are compelling, both in terms of safety and in regard to the quality of the bitten work. In 2003, the safety benefits of Metal Salt Etching were scientifically validated by the RIT chemists Dr Paul Craig and Dr Paul Rosenberg (CLICK FOR ARTICLE) in the context of my contribution to Keith Howard's book The Contemporary Printmaker. Key researchers, safety experts and scientists also endorse the use of metal salts for etching. There are several variants of processes, but all are essentially electro-chemical in nature and share many of their basic chemical properties. Metal Salt Etching is becoming accepted as the generic name of salt based etching. I am especially indebted to the essential work of Keith Howard, Nik Semenoff, and Cedric Green that made my own etching developments possible. 
This web site will be updated regularly to give visitors the latest developments and information on Metal Salt Etching.

The safest and most eco friendly method of etching is electro etching. The method has recently been perfected and popularized by Alfonso Crujera. See: Electro-Etching Made Easy. Although more technically demanding than chemical etching in metal salts it is expected that this method will be adopted by many professional print studios in years to come, especially in places that need to etch a lot of small plates on a regular basis. For medium to large scale projects electro etching becomes overly cumbersome. For more ambitious projects metal salt etching using chemical solutions in trays and tanks is the recommended medium of choice, following the various safety and recycling guidelines that are recommended.

The Science of Metal Salt Etching

I recommend a system of Metal Salt Etching that comprises two basic kinds of process:


(ferric chloride based etch)                                           (copper sulfate based etch)                 


(from left) Edinburgh Etch - Eduardo Paolozzi, etched brass plate / print with open bite marks / print taken from aluminum plate etched in Saline Sulfate Etch

The Edinburgh Etch (© F.K. 1997), suitable for copper, brass, and steel consists of a specific solution of ferric chloride with the addition of citric acid as a chelating agent or catalyst. This process represents a new invention in etching methodology. The use of agitated dip tanks for fine art etching was pioneered by Keith Howard.

The Saline Sulfate Etch (© F.K. 2002) is designed for etching zinc, aluminum, and mild steel and consists of a copper sulfate and sodium chloride mixture in equal parts. Due to their lower hazard rating, the metal salt etching methods are both suitable for use in a professional printmaking environment as well as in an artist's personal studio. This process follows on from ground work done in the late 1980s by both Nik Semenoff and Cedric Green in the use of copper sulfate mordant.

Both salts, ferric chloride and copper sulfate, have been used for centuries but their potential for etching is not fully harnessed when used without a catalyst. In fact, I would argue that this knowledge remained unexplored because metal salts were judged as if they were acids. But metal salts do not corrode metal through the destructive and harmful processes that typify acid etching; by contrast they owe their etching properties to electrical attraction in which atoms of the metal plate are elegantly removed by other metal compounds that are dissolved in a salt solution.

Today we know that this is an electrical kind of chemistry, more akin to the workings of a battery than the corrosive action of strong acids. Think of the rabbit in the Duracell adverts: a battery on a full charge will give plenty of electrical energy so the rabbit runs at full tilt, whilst a weak or depleted battery will soon stop the toy rabbit from moving at all. The difference lies in the strength of the electrical charge: the rabbit with the biggest charge wins. The salt based etching bath with the right kind of addition of a catalyst equally works as if on a full charge. In the case of electro etching an external current is added to metal salt solutions (this called the electrolyte), thus making the method even more effective.



Health and Safety 
Nitric Acid and its Toxicity

Some unpleasant facts about Etching with Nitric Acid
A potent mixture of toxic nitrous oxide fumes is produced in traditional etching with nitric acid. The plate feathering method (shown above) practiced by artists such as Whistler, and recommended in most traditional manuals, directly exposes users to these fumes. In contact with chlorine-based cleaning products, highly toxic nerve gas (nitrogen mustard gas) can even be created. The concentrated nitric acid is a strong oxidizer which may react uncontrollably with other substances (in an exothermic reaction). Nitric acid is a key ingredient of explosives manufacture.
The Journal of Occupational and Environmental Medicine warned in 1966; "In past years, critical illness and death have resulted from accute exposure to nitrous fumes...Nitric oxide and nitrogen dioxide with its polymer, nitrogen tetroxide [all of which are produced during etching] are highly toxic". Merle Spandorfer wrote in 1993 (Making Art Safely): "Nitrogen dioxide, the gas that evaporates from nitric acid used in etching and lithography, is highly toxic if inhaled in large quantities. Symptoms include fever, chills, cough, shortness of breath, headache, vomiting, and rapid heart beat. Chronic exposure may cause pulmonary dysfunction and symptoms of emphysema". 


'An Etcher's Studio', from the classic 'Treatise' on etching and engraving by Abraham Bosse, 1645. The book is widely regarded as the most influential etcher's manual. 
In order to gain faster etching speeds, acid solutions were poured over copper plates that had been propped upright in a shallow trough. The etcher was in immediate contact with the process and its reactive chemistry and very likely to inhale significant amounts of toxic fumes. 

(above) quoted from a nitric acid msds sheet

I experienced the hazards of working with strong acids at first hand. During the making of the large-scale etched installation Project David (pictured below) in 1988, whilst still using nitric acid, I contracted immune system failure and a severe infection in both eyes (conjunctivitis). I was wearing a respirator with acid fume cartridge at all times but still the acid and its brownish nitrogen dioxide fumes that are visibly produced during etching steel, had damaged my health. This despite the fact that the print studio had local fume extraction and good safety precautions. Thanks to a good doctor and antibiotics I made a full recovery, but the incident made me determined to find alternatives to strong acids for etching.

It is worth noting that the hazards listed here, although discussed in terms of etching, are equally relevant in the medium of stone lithography where nitric acid and its reaction products are often applied directly to a stone surface; often with little protection.

In industry today, strict regulations prohibit the use of nitric acid without fully extracted and filtrated glove units similar to those used in the nuclear industry. Regrettably, despite these compelling facts, many artists, workshops, and printmaking departments still maintain that their existing measures provide adequate protection. It is hoped that this resource will contribute to the dissemination of current thinking about safe practice for artists, specifically in the field of printmaking but also in a wider art-making context (see HEALTH + SAFETY).

(Above) Extract from a nitric acid material safety data sheet
(Below) Friedhard Kiekeben, Project David, seven steel etchings with plates;
etched in nitric acid, 1988, click to enlarge



Metal Salt Etching: Safer to use; Use it Safely

The Metal Salt Etching System has no toxic gas emissions that are comparable to acid etching. During etching, the chemical reaction by-products are largely contained within the etching solution without polluting the workshop atmosphere. The processes are not heat generating, and volatile reactions do not occur during normal use. The processes may, however, involve  a degree of exposure to a chemical odor which may be an irritant to some and should be controlled through adequate local ventilation and good air flow in the etching room. Both the art safety experts Monona Rossoll and Michael McCann recommend copper sulfate solutions as the safest option among all commonly used etching chemistries, chiefly due to their comparatively low airborne irritation and toxicity, but care should still be taken with any use of chemicals.

REMEMBER: Although considerably safer, the new systems still utilize harmful corrosive chemicals and their reactive properties, and it is very important to follow all safety instructions in their use. Medical science suggests that as an extra precaution women should not engage in extensive etching practice of any kind (acid or salt based) during pregnancy to safeguard against the possibility of reproductive damage.

Some Practical Safety Advice when using the Metal Salt Etching System:
  • Strong, long-sleeved gloves, goggles, and protective clothing (e.g. plastic lab apron) should be worn when handling metal salts and metal salt solutions.
  • Use a dust mask when handling copper sulfate or etching residues.
  • In professional shops there should, ideally, be an emergency shower and eye-wash fountain in the etching space. In an artist's personal studio, it is important to make sure you have easy access to clean running water or an eye-wash bottle, in case of splashes. Any splashes in the eyes must be rinsed immediately with plenty of water. Immediately wash off any splashes on the skin.
  • Have a supply of sodium carbonate in the studio for neutralizing any accidental spills and run-offs in the etching space.
  • Frequently clean the etching space with warm water to prevent any build up of salt residues.
  • Keep all solutions and etching residues in closed containers, and keep etching supplies in locked non-metal containers and cabinets.
          Eye Wash Stations like the one shown above can be purchased from suppliers such as LSS (Lab Safety Supply).
          Visit for details.
In the past two decades I have received just two reports of cases of an allergic reaction to the smell of ferric chloride, and one case of an allergy to copper sulfate. In these rare instances, alternative methods should be used by the sensitized person. Prolonged exposure to concentrated ferric chloride fumes may damage the mucous membranes, but in practice this is easily avoided by adopting the following measures: ensure good airflow, etch in a contained environment (use a tank that is fitted with a lid), and ideally use mechanical agitation such as a pump or a propeller in the tank rather than air tubes and bubbles.

In the wine growing industry copper sulfate is widely used as a fungicide. Some workers who  regularly sprayed copper sulfate aerosols onto grapes eventually developed health problems due to frequent inhalation of the corrosive mixture. For safety avoid producing mists or dusts when working with metal salts.

In general though, Metal Salt Etching is considerably safer than acid etching because, like a battery, it is based on the principle of electrolysis: metal ions are elegantly exchanged between the poles of two kinds of metal that have an energy potential between them, whereas the old acid etching approach has a much more aggressive chemistry. Common sense would suggest that care should be taken with any process, and if care is taken with the metal salt system it can provide the  printmaker with all the excitement of the chemical reaction without any serious health implications. Of course, no new printmaking process, however safe or simple, would be worth its salt if the results did not equal or exceed those produced by traditional means. Perhaps, the most exciting fact of all about the Edinburgh Etch and the Saline Sulfate Etch is that the results are startlingly good!


A Note on Air Regenerated Cupric Chloride Etching: A Safe Alternative?

Some print studios have recently adopted cupric chloride etching systems in favor of ferric based ones. The cupric chloride etching process originates in the electronic industry where expensive safety and containment equipment is utilized to conduct this process safely. But the benefits of this method are in its very economical use of chemicals (some of these are recycled during etching), not in enhanced safety. The method involves the use of large amounts of hydrochloric acid, chlorine, and chlorine gases in its operation, so it should be classed as acid etching not metal salt etching. Even the electronics industry is currently debating whether the significant hazards of chlorine gas exposure inherent in the process are worth the risk. 
Chemistry of the Cupric Chloride Etch: "...However, the practical difficulties of using Chlorine for replenishment, not just the actual safety issues, but more importantly, the perceived problems, in the heavily legislated, risk-averse, environment of today, make the use of Chlorine not very common, even in the largest facilities. It should be noted here, that much of the perceived safety of alternate replenishers is just that, perceived. Any replenishing system in use today, if the controls do not function correctly, can generate significant quantities of Chlorine gas. Chlorine gas can, of course, be very dangerous to any form of life, even to chemical process engineers." 

"A safer substitute for etching copper plates is ferric chloride (iron perchloride). This forms acidic solutions so should be handled accordingly, but does not have the dangers of handling concentrated acids.  Ferric chloride solution might cause minor skin irritation from prolonged contact.
(c) Copyright Michael McCann, Center for Safety in the Arts 1992"


Lithography, Intaglio and Relief Printing: Acids

Selected Excerpts by Angela Babin, M.S., Michael McCann, Ph.D., C.I.H., and Devora Neumark
The University of Illinois at Chicago, Online Health & Safety in the Arts Library (UIC) is a comprehensive resource for artists, art schools, print studios etc. The mission of the Health in the Arts Program is to diagnose, treat and prevent arts-related disorders among people working in all aspects of the arts

Acid Etching Hazards By Michael McCann, Ph.D., C.I.H.

Etching of copper and zinc plates using diluted acids is a common intaglio technique.  Acids can also be used for etching metals for jewelry and metalworking.  Acids used include nitric acid and Dutch Mordant (prepared by mixing hydrochloric acid and potassium chlorate).


Skin Contact

Concentrated acids are highly corrosive and can cause severe burns if splashed on the skin or in the eyes.  The dilution of concentrated acids with water can be very hazardous because of the risk of splashes.  When mixing, always add the acid to the water, never the reverse.  If you add water to concentrated acid, large amounts of heat are produced which can result in boiling concentrated acid spewing all over. 

Gloves, goggles, and protective clothing (e.g. plastic laboratory apron) should be worn when mixing concentrated acids.  There should be an emergency shower and eyewash fountain in the area where concentrated acids are mixed.  It is important to ensure that there are no electrical outlets or switches within splash range of the shower or fountain.

Dilute acids are less hazardous than concentrated acids. However, prolonged or repeated contact can cause skin irritation (irritant dermatitis).  Even dilute acid solutions can cause eye irritation if splashed in the eyes.  Gloves and goggles should be worn at all times even when working with dilute acids.



Nitric acid etching produces brownish nitrogen dioxide gas. Unfortunately, nitrogen dioxide gas has poor odor warning properties and you cannot tell when you are being overexposed. Large acute overexposures (e.g. from using too concentrated a nitric acid solution) can result in chemical pneumonia.  Low concentrations of nitrogen dioxide over a long period of time can cause emphysema.  There have been a number of printmakers who have developed emphysema as a result of doing nitric acid etching without proper ventilation.  I have inspected many printmaking studios and found that all the light fixtures and other metal surfaces in the acid room were corroded, a clear indication of inadequate ventilation of the nitrogen dioxide.

Dutch Mordant has a different problem.  When the potassium chlorate and hydrochloric acid are mixed, highly toxic chlorine gas is released, which can cause chemical pneumonia.  Over 15 years ago, five students and teachers in Alberta, Canada got chlorine poisoning from mixing Dutch Mordant without adequate ventilation.

Etching with both nitric acid and Dutch Mordant requires local exhaust ventilation.  The acid trays and sink where acids are mixed should have either a slot exhaust hood or laboratory-type hood.  Do not use an overhead canopy hood.



Although unlikely to be a problem with adults, accidental ingestion of small amounts of concentrated acids by young children could be very serious and possibly fatal.  For this reason, it is essential to keep acids and other adult art materials locked away from young children.



A possible substitute for acids in metal etching is ferric chloride (iron perchloride).  Ferric chloride, when dissolved in water, produces hydrochloric acid in small amounts, and repeated contact could cause skin irritation.  However, you do not have the hazards of mixing concentrated acids.  During the etching process, the ferric chloride gets converted to iron that can accumulate in the etched area. Inverting the plate or using a stirrer can solve this problem.


Storage and Disposal

Concentrated acids should be stored separately from alkalis such as ammonia and other materials such as solvents because of the risk of a chemical reaction if there is a spill.  Concentrated nitric acid can even react with other acids and should be stored separately from all other chemicals.

Acids should be neutralized with baking soda (sodium bicarbonate) before being poured down the sink with lots of water.  Schools and other large scale users of acids should have neutralization tanks and polyvinyl chloride plumbing.

Art Hazard News, Volume 12, No. 8, 1989

This article was originally printed for Art Hazard News, © copyright Center for Safety in the Arts 1989

(c) Copyright Center for Safety in the Arts 1992

Obtain material safety data sheets for all materials used (MSDS)
Acids are used in intaglio (acid etching) and in lithography. 
Strong acids commonly used include nitric acid, hydrochloric acid, and phosphoric acid, and less commonly carbolic acid (phenol), chromic acid, hydrofluoric and sulfuric acids.

1. Concentrated acids are corrosive to the skin, eyes, respiratory system and gastrointestinal system.  Dilute acids can cause skin irritation on repeated or prolonged contact.  
2. Chromic acid is a skin sensitizer, suspect carcinogen, and oxidizer. 
3. Phenol is highly toxic by skin absorption and ingestion.  It may cause severe kidney damage, central nervous system effects and even death if absorbed in large amounts.
4. Hydrofluoric acid is highly toxic and can cause severe, deep burns which require medical attention.  There is no immediate pain warning from contact with hydrofluoric acid.
5. Concentrated nitric acid is a strong oxidizing agent and can react explosively with other concentrated acids, solvents, etc. Nitric acid gives off various nitrogen oxide gases, including nitrogen dioxide which is a strong lung irritant and can cause emphysema.

1. Acids used include phosphoric, nitric, acetic, hydrochloric,hydrofluoric and tannic acids.  The concentrated acids are corrosive and even dilute acid solutions can cause skin irritation from prolonged or repeated contact. Hydrofluoric acid and phenol are the most dangerous to use. 
2.  In particular nitric acid etching releases the respiratory irritant nitrogen dioxide which has poor odor warning properties.  Large acute overexposures may cause pulmonary edema (chemical pneumonia), and chronic exposure may cause emphysema.  During the etching process, flammable hydrogen gas is also produced.
3. Concentrated nitric acid is a strong oxidizing agent and can react with many other chemicals, especially solvents or other organic compounds, to cause a fire.
Dutch Mordant Hazards
Mixing hydrochloric acid with potassium chlorate to make Dutch mordant produces highly toxic chlorine gas. Several years ago, five art students and teachers had chlorine poisoning in Canada from mixing Dutch Mordant without proper ventilation. Potassium chlorate is a key ingredient in many pyrotechnics, and is a potent oxidizing agent. It can react explosively with organic compounds, sulfur compounds, sulfuric acid or even dirt or clothing. On heating it can violently decompose to oxygen and potassium chloride. Storage and use are very dangerous, special precautions are required especially when mixing.
A safer substitute for etching copper plates is ferric chloride (iron perchloride).  This forms acidic solutions so should be handled accordingly, but does not have the dangers of handling concentrated
acids. Ferric chloride solution might cause minor skin irritation from prolonged contact.

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