Using a Safer Mordant for Etching on Aluminum, Copper and Zinc

Now Called Salt or Saline Etch

This paper contains very much of the one published in LEONARDO, the international refereed journal for technology in the arts (Vol 31, No. 2, pp,. 133-138, 1998). Since retirement, I have done more work on the possibilities of using modified copper sulfate to etch copper, also eliminating all toxic compounds from the spent mordant before pouring down the drain. I did the original research at home and the printmaking studios at the university. After getting the original process to work in 1992, I was interested in why a copper salt would chemically attack some metals. For clarification, I went to Dr. L. W. Bader of the chemistry department. He became intrigued with the approach because it did not require dangerous acids and agreed to help.  It even became a study for a group of students taking chemistry as a project to explain what they think was happening.  I have been told by other doctorate faculty that it is not as simple as one may think, with various reactions taking place between the materials. Dr. Bader has long since retired and I am left alone to do anything more research into this fascinating process.

Unknown to me at the time, I have been informed that Goya used plain copper sulfate on zinc for his editions sometimes in the late 18th Century. I do not know anything more about what the artist did.  Cedric Green has started to use copper sulfate alone and promoted the technique as Bordeaux etch on his website.  While my process with the addition of common sodium chloride will work on zinc just as well, it seems to have been missed by printmakers because I emphasised aluminium for the sake of students.  In my research I have found that the use of salt actually increases the mordants’ strength for metals and advised printers to add it to their bath. Even before my article was published in LEONARDO, I had increased the sodium chloride content to the point the bath turned green, indicating that the copper has been converted into copper chloride.

This updated paper describes an etching bath containing copper sulfate, table salt, a weak acid and available oxygen; for use on aluminum, zinc and copper plates. The bath in use has a pH reading of 2.5 - 4.5 and will not harm skin or clothing, as it does not contain strong acids when used for aluminium and zinc. During etching, the copper sulfate is converted into fine copper particles, leaving common chemicals that can be disposed of down the drain quite safely in many places. The bath can be regenerated very easily so that copper compounds can be reused instead of being discarded.


Etching of intaglio images on metal has been practiced since the seventeenth century, being produced mainly on copper and later on less expensive zinc. Today, with artists printing smaller editions, zinc has become quite common, as it is readily available from suppliers for use in making photo-mechanical engravings for newspapers and other relief printing processes. It is not considered as good a material as copper, but the high cost of polished copper makes zinc an acceptable alternative. While etching can be done on other metals and alloys, these are not used by the average printmaker. Since etching of metal usually calls for the use of acids or other corrosive solutions, the handling and disposal of spent mordant has become more of a problem in all environmentally concerned societies.

While research into electro-etching (Semenoff & Christos, 1991) is being carried on by some printers, the more common use of corrosive etching solutions has prevented that technique from becoming more popular. Printers and artists are not familiar with electronics, and therefore continue to use methods known to them. A simple, environmentally safe, inexpensive, and readily available mordant would be helpful for today's etchers. With the high cost of artists' materials, even the price of zinc has become too much of a financial burden for many students. Since editions at art schools are usually limited to small numbers, a search was undertaken for materials that react much like traditional metals, yet stand up to the printing of smaller editions.

Use of Aluminum in Place of Zinc

Aluminum, an inexpensive material already in use by some artists, would easily stand the printing of small editions as required by schools. If unblemished common-grade metal could be purchased from local metal suppliers, the saving could be even greater. The large sheets could be sheared to appropriate sizes, and since both sides of the metal can be used, the price of aluminum is reduced to about 1/20th that of zinc plates. To protect the back of the metal for later use, the plate is covered with Mac-Tac, a self-adhesive vinyl commonly used as kitchen shelving material. All that may be required is extra polishing of small scratches that appear on some mishandled sheets.

It is generally accepted that aluminum and magnesium are less desirable metals for the process because they are soft and also produce a coarser grained image than copper. This may be of concern for some artists because of the images they choose to create, but for many artists these softer metals are not a problem. For artists who prefer to make large prints, but in small editions, the problems with aluminum become less of a consideration. Certainly for students who have a limited material budget, aluminum will serve their purpose. At this University, the metal was purchased in both 12 and 16 gauge, but it was found that the thinner was more than adequate. Because of the reduced cost of aluminum plates, the size of intaglio prints has greatly increased since this process has been introduced.

Techniques Possible with Aluminum

While engraving and dry point are both practiced on aluminum, the softer nature of the metal limits the edition. Although the electro-facing with harder metal is possible, the authors doubt if the cost would warrant the use of cheaper aluminum in the first place. For the small editions required in this Department, the direct intaglio techniques such as dry point have not been overlooked, and are used in conjunction with etching and aquatinting.

Hard ground, soft ground and aquatinting are the most commonly practiced techniques and commercial grade aluminum has been more than adequate for these techniques. Aquatints are processed as with zinc, with the results on aluminum being indistinguishable from the harder metal except by the finest connoisseur. Large editions, although smaller than that possible in copper, could still be impressive. One senior student has pulled an edition of 25 with no noticeable deterioration of the plate, indicating that a larger edition is possible.

Mordants for Aluminum

A number of chemicals will corrode aluminum, but are either expensive or not readily available to the printer. The suggested chemicals have been stannous or ferric chlorides, both having been successfully used by printers. One of the problems found with both chemicals was local availability and a fairly high cost. Attempts to order stannous chloride from a supplier in the USA were hindered by the fact it could not be exported by common carrier without an expensive surcharge for the handling of hazardous materials. Experience with ferric chloride made it less desirable because of the red cast that forms around the etching area. Also, the need to etch the plate upside down in this chemical was a concern. While electro-etching was possible, the need for numerous power supplies to accommodate the many students simultaneously working in the etching area made the technique too costly.

Use of Copper Salts in Mordant

My experience with printmaking processes, and interest in chemistry began the research for a better etching bath. My observation that acidified copper sulfate would erode aluminum was the basis for the mordant. The first experiments with copper sulfate were to combine it with hydrochloric acid (which was purchased as technical grade muriatic acid), commonly used in cleaning fresh concrete, or for controlling the pH of swimming pools. The copper sulfate was purchased from a local garden supply outlet (where it is sometimes labeled bluestone) and sold for controlling algae growth in dugouts.

Since both of these chemicals need not be reagent grade, the less expensive, more readily available materials are quite adequate for this purpose. It was found that this mordant would etch both aluminum and zinc, though not as fast acting on zinc. There was no need to use separate trays, as chemical action was not affected, as is the case of nitric acid and the use of zinc and copper in the same bath.

As the etching action takes place, a deposit of very fine copper is formed at the opened surface. Very fine bubbles of hydrogen are also formed, which remove the copper created by the electro-chemical exchange with the aluminum. This keeps the process ongoing as the copper is flushed by the gas from the thin lines and fine dots. A soft brush or feather may be used to remove the copper sludge and speed the action slightly, although it was found that most students simply leave the plate for a specified time. In time, the reaction will slow down as the acid content is reduced and a clear gel-like substance of aluminum hydroxide is formed. Increasing the acid will not completely refresh the solution as the copper is depleted and the action eventually stops. By adding more copper sulfate, the action will continue, as long as the other chemicals are present is sufficient quantity.

Use of Common Salt in the Mordant

To find a safer and less expensive source of chlorine, I decided to try common table salt, which is pure sodium chloride. Most salt sold as table salt is labeled "iodized" and contains small amounts of sodium iodide. This makes no difference to its use in this technique, but non-iodized or coarse salt is much cheaper. Since it is available in bulk quantity for use in water softeners, it is the most practical way to purchase the chemical. It is available in fine granulation or as rock salt; use of the fine grade is preferable as it dissolves easier to make up a bath. Although a simple solution of copper sulfate and sodium chloride worked, it was found that an acidified solution would perform much better. This would prevent the formation of the gel-like aluminum hydroxide and consequent reduction of etching.

As each batch of mordant became exhausted, different formulations were mixed for the studio and tried by the students. In the early experiments, hydrochloric acid was used as the acidifier, but since it entered into reaction with the aluminum, it would become depleted, and the aluminum hydroxide gel would form in the etching bath. What was need is an acid with an oxygen containing radical, such as nitric (HNO3) or sulfuric (H2SO4). By choosing sulfuric acid, which prevented the formation of aluminum hydroxide, we prolonged the use of the mordant. Since sulfuric acid does not react extensively with aluminum in this technique, it is not depleted during the life of the bath and the solution remains acidified. It will be neutralized by the aluminium hydroxide produced, and slowly raises the pH reading of the bath. It was decided to find a dry compound that would be safer to handle than liquid acids and replace the sulfuric acid - this turned out to be very successful.

There are, on the market, a number of products that produce a weak form of sulfuric acids when combined with water. They all contain sodium bisulfate (sodium hydrogen sulfate) as the major ingredient and in many cases the only chemical; usually found in an impure form. The most common form is sold as a toilet bowl cleaning agent (Sani Flush ) which contains deodorizers and other materials to meet market demands. Another source is a vehicle radiator-flushing compound that is available at automotive supply houses. It may come as a two-part package, but only the bulkier powder containing sodium hydrogen sulfate is needed. The third source is from jewelry craft stores selling Sparex at goldsmithing supplies. Sold under a number of labels, sodium bisulfate is the main or only ingredient. This chemical is used as a pickling solution to remove fluxes and oxides after hard soldering silver and gold articles, as it forms a weak sulfuric acid when mixed with water. Another very good source is from stores selling products for swimming pools and hot tubs.  It should be noted that sodium bisulfate, sodium hydrogen sulfate and sodium acid sulfate are one and the same compound. It should be listed on the container under one of these names.

Formula Starting with a Dry Powder

By using only dry materials, the chemicals can be premixed and conveniently stored in its powder form. This reduces the chance of accidental spillage and uses less storage space. The dry powder may absorb humidity in the air because of the salt content, resulting in the blue copper sulfate changing to green copper chloride. When added to water, the blue color returns. Since a number of various proportions worked, the exact ratio of each chemical does not seem to be critical. It was found that the quantity of copper sulfate was the most important chemical in controlling the reaction and establishing the amount of metal displaced. The amount of aluminum that can be removed is about eight to ten percent of the copper sulfate available. This is the ratio of actual copper molecules as compared to the hydrogen, sulfur and oxygen in the compound. The other chemicals were in sufficient quantity to carry on the reaction until all the copper sulfate was depleted. It is important that excessive copper sulfate not be added directly to the solution as a violent reaction on the plate would likely be the result. As the normal etching action decreases, the addition of copper sulfate alone will bring the bath back to life. Increasing the sodium bisulfate will increase the amount of hydrogen gas produced to some extent, which calls for constant brushing of the plate surface to prevent broken lines.

The etching of the metal and the production of small amount of hydrogen gas are two independent reactions take place on the plate. The removal of metal is an electro-chemical reaction that will be explained later by Dr. Bader. When salt and sodium bisulfate are mixed in water, they form hydrochloric acid that will chemically etch both metals. In this reaction, hydrogen gas is produced as you might have learned in a high school chemistry lab by adding zinc to a test tube of dilute  hydrochloric acid, then test for the gas by use of a match. The weak acid produced as a byproduct of mixing the mordant, is useful for pushing up the small particles of pure copper being formed on the surface of the metal plate. The metal salts in the solution will react differently depending on what they are and how much of each is present. This complicates the explanation of this mordant, especially on how it gets to regenerate itself

The etching time is comparable to the use of traditional mordants, with total etching being completed within 10-30 minutes, or less in the case of light aquatints. One important factor is the fairly constant etching action over the life of the etching bath as long as there was sufficient copper sulfate in the solution. Only when most of the copper was depleted, did the action slow down. By observing the intensity of blue coloring in the bath, the strength of the solution can be estimated. With such an indicator, students were less likely to over-etch their plates.

The formula that used in the Department at present is as follows:

CuSO4 (copper sulfate -- bluestone) 1 kilogram
NaCl (sodium chloride -- table salt) 1 kilogram or more to make the solution green
NaHSO4 (sodium bisulfate -- Sani Flush ) 25-50 grams
H20 (water) - depending on bath strength 10-20 liters
(Note: I have found in my latest research that by increasing the salt content, the action of the mordant becomes more effective on both aluminium and zinc)

The powder is mixed by volume after an approximate weight is established for the chemicals. This is premixed dry in a five-gallon plastic container and stored until needed. It is advised that an appropriate mask is worn while mixing the dry copper sulfate and sodium bisulfate as they come in fine powder form. When the salt in the mixture takes on humidity, there is less risk in handling the powder mordant.

Water is added to a tray to make up the working solution. Then the dry chemical mixture is added to bring it up to strength, using a rough calculation for the copper content or by judging the blue color of the mordant. You should conduct tests to find the initial strength of the solution you like.

We have also experimented with the use of saturated solution of copper sulfate, which was kept on hand to be added to the depleted solution to see what might be the total life of the bath. It was found that the bath could be extended considerably, but the eventual build-up of aluminum salts was a limiting factor. On first mixing the bath, it would test about 1.0 pH; but on using the bath for only a very short time, the solution would stabilize anywhere between 2.5 - 3.5 pH or higher. It is still an effective mordant at pH levels of 4.0 and above. This relatively weak acid solution makes it a very safe mordant to use in the classroom setting.

Eventually I would find that if enough salt and bisulfate were present it the mordant, the exposure to air would help the production of cupric chloride as the bath regenerated overnight. More on that is explained farther on in the paper.

Advantages to Copper Sulfate/Sodium Chloride Mordant

Acids are extremely dangerous chemicals to handle, especially in a classroom situation. Since most art students are not concerned with technical matters (over the creation of the image), they tend to be lax in their handling of materials in a proper manner. While health and safety has become more important in the classroom, the elimination of as many dangerous materials as possible seems a step in the right direction. Since none of the above chemicals are considered to be dangerous to handle, there is now less concern about the safety of the student. Spills on skin or clothing are of only minor importance, as the weak chemical will not stain or do harm if removed quickly.


A grid used to keep plates off the bottom of the tray so not to disturb sediment. A CU of an area of aluminium plate

An important improvement in how metal is etched occurs with open bites. To obtain a solid black area, it is customary to apply aquatinting. Due to the particular electrochemical etching action of this bath, the surface becomes corroded in a rough fashion that holds ink very well. If the metal is homogeneous and does not have a longitudinal grain as a result of production, the mid-range tints can be achieved by stopping out the open bite when the proper tint is produced. Unfortunately, the common grade of aluminum we are currently purchasing has a pronounced grain in one direction, which is revealed in lighter than solid color tints. I have seen some wonderful prints using no aquatinting techniques to produce very light tints. The grain of the metal is only seen with a loupe.


Aluminium plate etching by Yoko Imabayashi using a splash of mordant to get the effect of water

Some students have had concern in creating the deep etches needed for viscosity printing, but this needs only the addition of copper sulfate to keep the bath alive. A higher concentration of salt and bisulfate may be needed to supply the chlorine and acid molecules for the metal exchange. In viscosity printing, a very large amount of metal is removed, especially on the larger plates now being used by our students. If there is a problem of the bath being depleted too quickly from the large amount of metal removed, the only cure is to make sure that the bath starts with sufficient copper sulfate. Constant monitoring may be needed in extreme cases.  Since there is no open bite like with acids, the depressed areas have to be burnished smooth if traditional viscosity print effects are needed.

If the bath is mixed with too much copper sulfate, a very violent reaction will take place. This produces a great amount of heat, which can destroy the hard ground resist. It may also lead to some of the fine metallic copper being bonded to the now coarse grain of the aluminum. Rubbing only forces the copper deeper into the rough metal and it can then take on a burnished state. To remove this unwanted copper, use a dilute solution of nitric acid, which will attack it, but leaves the aluminium alone

Recycling the Spent Bath

Over the last decade, we have started to recycle the spent bath rather than throwing it away. It is was once placed in plastic containers and left to stand for a month or more. The other chemicals in the bath start to react on the copper sludge at the bottom and soon the blue color returns. In the place of the dark metallic copper on the bottom, a whitish powder appears. If one wanted faster exchange of the copper, I suggested periodic stirring of the solution until the reaction is completed. The clear blue solution is drawn off and put back into service. It is less corrosive in some ways and preferred by many students. Salt and acid content seem to be sufficient and only copper sulfate is added as needed to keep the bath alive. This procedure has greatly reduced the amount of copper sulfate we require in the etching studio. It has been found that the whitish sludge contains copper compounds that can be reclaimed by adding sulfuric acid solution or a fair amount of the sodium bisulfate material. If your solution is not recycling, try adding more sodium bisulfate and salt to act on the copper particles. What you will be getting is cuprous or cupric chloride from the hydrochloric acid and oxygen within the solution.


Spent mordant would regenerate even without access to air, leaving white sediment.

One of the issues raised is the contamination of sewer system from the copper salts put down the drain. This is unnecessary if care is taken. There are procedures that can eliminate all the copper and zinc compounds before the spent bath is filtered, diluted and flushed away.

When etching aluminium I would suggest the best way to remove all of the copper is to use scrap pieces of the metal to completely make the bath clear to show the removal of every molecule of copper from the solution. The solution is then filtered through a cloth to remove the copper particles, leaving only salt, sodium sulfate, sodium bisulfate and aluminium hydroxide. None of these cause problems in sewer systems, but check with your local authorities about this. If there is a problem, the aluminium hydroxide can be removed from the clear solution by adding some alkaline product such as borax, sodium carbonate or metasilicate. This will precipitate the aluminium hydroxide into a sludge as the solution loses its acidity, which can also be filter out through a cloth and  disposed as dry garbage. You should notice that I used synthetic dish cloths as filters that are easier to reclaim. A finer mesh cloth would have removed all the white material as a slight amount has been able for get through. Enough alkaline solution has to be added to get the pH very close to 7 or a bit higher to make sure all the metal has been captured.




1/ Copper particles filtered from the solution through a coffee filter. These paper filters are dense enough to prevent any fine particles getting through, making the clear solution free of copper. This method works for both aluminium and zinc.

  2/ Aluminium hydroxide is being filtered out through double layer of synthetic dishcloth material after an alkali is added to the solution..

  3/ Aluminium hydroxide that can be dried and disposed of in dry garbage container. Note the small amount of material that has been able to get through the synthetic fabric. A finer material would prevent this. This small quantity of aluminium compound would not harm the sewer system as aluminium is present in alum - used to make pickles.


If you are etching zinc, then the zinc chloride in solution is harder to precipitate but it has to be removed before disposal down the drains. I have found that the same procedure to remove solids work well. Filter out the solid copper particles after dissolving scrap zinc to remove the copper from the spent bath. As long as the solution is clear, there are no copper compounds in solution. I have found that adding sodium metasilicate is the best way to produced a white insoluble compound (zinc silicate) that can be filtered in the same way.


Regenerating the bath for farther etching



The effect of peroxide to make the cupric chloride bath reusable. Page out of my DVD using PDF files for information

 Disposal of Spent Etching Bath

The properly prepared spent bath can be safely put down the drain with sufficient amounts of water, as all the chemicals can be accommodated by the drainage system. Table salt and sodium bisulfate are chemicals commonly put into waste disposal, and even traces of copper is already present within plumbing systems. As all of the copper sulfate will be reduced to a fine metallic copper at the bottom of the tray, it should be separated for disposal as a solid waste. For any remaining copper sulfate left in the spent etching bath, the solution is poured into a large plastic container and a discarded aluminium plates are used to reduce the remaining copper. Aluminium, the only remaining metal, is present in common household alum, and disposed down the drain. If need be, the aluminium hydroxide could be precipitated from the solution by making it alkaline with the addition of household lye, borax, or washing soda. The sludge that is formed may be disposed as a solid in traditional ways.

While alkaline solutions are needed to precipitate out the metal compounds, I now believe the most effective is the use of sodium silicate instead of the metasilicate material. Since the straight silicate is clear and only needs to be diluted by a great deal, it doesn’t confuse test results like the impure version of dry sodium metasilicate that I have used. Even if there is no zinc chloride in solution, use of metasilicate still showed a whitish liquid, making it hard to judge the test. Whilst metasilicate is used in commercial cleaning materials, it can be combined with cheaper alkaline chemicals like sodium carbonate and phosphates - as I have found in some products. I know that the purer liquid sodium silicate (waterglass) is clear, so it will give you accurate results. Sodium metasilicate is sold as a phosphate substitute. It is always soluble in water. On the other hand, sodium silicate dries into a insoluble material, so consider spills in your work area. A bottle containing the viscous or diluted material that will keep for years.

A step by step process may be easier to follow.


1. Remove all the remaining copper compound by adding more of the scrap metal that you are using for plates, until the liquid is completely clear and some metal still not dissolved. There will likely be some weak hydrochloric acid still in solution, depending on how the mordant was made up; this can be seen by small bubbles rising from the scraps. Since zinc reacts more with the acid than does aluminium, it is a matter of choice on how much you wish to reduce the acid content before the alkaline solution is added. When using zinc, it is important not to add any alkali other than sodium silicate as those zinc compounds will still stay in solution.


2. Filter out the fine copper particles through a fine cloth or filter paper and dispose into dry garbage; or find someone who wants 100% pure copper in their work.


3. If only aluminium plates are used, adding any base to will precipitate our white aluminium hydroxide. As long as the solution is acidic, aluminium hydroxide is not formed. When the pH goes above 7, the hydroxide is precipitated out. This is one of the chemicals that has been used to clear water in municipal water systems, so should not be a problem for the drains - but probably best to remove the solid material.


4. Filter out the white substance if your municipal system is upset of you putting it down the drain. After we finish a jar of pickles, the liquid containing aluminum potassium sulfate (Alum) is usually poured down the drain.


5. If you are concerned about any aluminium is getting into the sewer system, then use a diluted solution of pure sodium silicate to detect even the smallest amount of the metal.


6. For zinc plates, there will by zinc chloride/sulfate in solution that will not precipitate out with common alkaline materials. This is why very diluted sodium silicate is used, which is added to the filtered liquid before disposal down drains.


7. Dilute your filtered liquid even more to prevent a solid sludge being formed when the silicate is added.


8. Dilute the viscous sodium silicate so it doesn’t produce a harden sludge on entering the filtered liquid. A very diluted solution works for this and final testing as well.


9. Filter out the zinc silicate with a cloth or paper filter, but don’t dispose of the liquid until you test it again with the silicate test solution.


10. Make sure what you are pouring down the drain is clear, indicating there is no zinc compounds.

The filtered out solids can be put into dry garbage containers to be picked up for collection, unless the local government has concerns with metals being disposed this way. Check with your city council.  


Concerns with the Use of Copper Compounds

It has come to my attention that some environmentally concerned groups are rejecting this process as it uses copper compounds. In areas where grapes are grown, the abuse of copper sulfate over the centuries has contaminated the ground water, lakes and streams. Over the years, tons of copper sulfate has been put into the environment, leading to stricter use of this chemical. If one takes the trouble to reduce all the copper sulfate with scrap aluminium, zero amount of copper will be put down the drain. This process is no more toxic than the other copper mordants now used if care is taken in handling the materials. When dealing with health boards and concerned groups, make sure they understand the alternative methods being used and that all the copper can be removed from the solution. If there is any doubt, the spent bath can be sent to special disposal facilities, like one would with used copper etched in nitric acid baths. It should be noted that in the use of copper plates and nitric acid, there would be copper nitrate produced that is now escaping into the environment. If etchers are allowed to use traditional methods with copper, then it should be recognized that the use of this copper sulfate mordant is of no greater danger to the environment. It certainly is safer for the printer to handle.  

Etching of Zinc

If the bath is to be used for both aluminium and zinc, then the addition of sodium bisulfate is necessary. If only zinc is to be etched, then the weak form of sulfuric acid is not essential, as the zinc chloride produced in the reaction is soluble in water and does not form a gel. Since all of our students now use only the cheaper aluminum, our experience with zinc is somewhat limited, but a great number of European etchers seem to have taken some interest in this process. In my early research I did many test with zinc to produce more than enough data to have tested the process.  It has become known as Bordeaux etch because of the use of copper sulfate and lime on grape vines.

If you plan to regenerate your mordant, then you will have to use the sodium bisulfate too, for the acid solution that will dissolve the fine copper particles.

Etching Copper Plates with Cupric Chloride  

I was surprised to discover that several amateur electronic workers were etching their copper circuit boards with cupric chloride instead of ferric chloride like in the past. It turns out that this chemical can be regenerated easily with the addition of oxygen from hydrogen peroxide or other oxygen containing chemicals. The electronic industry had taken to using this technology as it is cheaper than ferric chloride that had to be purchased, then discarded through costly disposal fees. While electronic amateurs had found the process, I was amazed that intaglio printmakers had not. Since it seemed to be a continuation of the copper sulfate mordant I discovered in 1992, research into this technology seemed the natural thing for me to do.  

Cupric chloride is expensive and not readily available locally or even from some chemical suppliers. The electronic workers have found that a bath can be produced from a starter solution of hydrochloric (cheaper Muriatic acid) and hydrogen peroxide into which a piece of copper was added for the chemical reaction. As they etched their circuit boards, the cupric chloride would increase and make it more active until a critical point was reached and etching eventual stopped . By adding more hydrogen peroxide, the bath would regenerate immediately so etching could continue. There was no need to discard the mordant as only oxygen, and a small amount of acid might be required from time to time.

There is on the Internet a very good article by Adam Seychell,, in which he describes many aspects about making and using cupric chloride. Since his explanation is excellent, there is no need for me to go over that information.


Some of the earliest tests on scrap copper in my workshop. Toner wash on scrap copper produced the proof above

Because of my search for safer materials in the classroom and studio, I decided to find a substitute for the dangerous hydrochloric acid or Muriatic that was suggested. Hydrochloric acid was discovered by medieval Islam when salt was added to sulfuric acid.  Since I was already using a weaker form of sulfuric acid in with my copper sulfate mordant, along with common salt, I already had the basic ingredients for farther research. By adding 3% hydrogen peroxide from the drug store, I found the solution corroded a test piece of copper very well. I had found a purer form of sodium bisulfate at a store supply chemicals for swimming pools which was very inexpensive - so materials seemed easy enough to find locally. While 3% peroxide worked, it diluted the bath each time I regenerated it, so a stronger solution had to be found if possible. Health food stores sell a 35% solution, luckily I was able to get a liter. I had potassium chlorate and found this worked very well, but I don’t know how much of the residual potassium compounds would affect a well used bath.

As the copper was dissolved and went into solution, the color would change from a light green to dark brown, when etching would basically stop. By adding oxygen, the test could continue to see how many cycles the 100 ml bath would take before contamination took place. After ten cycles and continued good corrosion of copper, I decided it was time to try actual test images on plates.    

Because I had earlier developed a technique using toner washes that is basically a “sugar lift” like process, I felt this would be a tougher test for both mordant and metal than an aquatint. This technique requires the use of Ferstman ground (Graphic Chemical waterbased relief ink) as first described in my university website (more farther down on the page). Toner can be applied directly to a plate or transferred from Mylar as it would be for lithography. The ground is rolled on with a small brayer, making sure that the first application is pressed well into the areas between the toner particles that had been already bonded to the plate. After heating the ground until it starts to smoke a bit, it becomes very tough and can take much abuse during washing out of the toner with turpentine and a soft brush. This leave bare metal in place of the toner, so that mordant can attack it easily. The results were as good as expected and much better than on aluminium.


Toner washes on zinc were produced by using Ferstman ground as described farther on in the article

While I may not have created a completely new process, by removing liquid acid from the studio, it may be of some help to make it safer. It seems to be a continuation of my basic etching process developed in 1992 and made available to printmakers shortly after that. Why it took sixteen years to get this far puzzles me.

  Making a Special Etching Tank

Because of the sludge produced by this process, it is advisable to have some method of keeping it at the bottom and not become stirred up and obscuring the plate. I have proposed and have seen one of these tanks built for use with copper sulfate mordant. It consist of a plywood box with an angled bottom to encourage the sludge go into one side of the tank. Fiberglass and resin is used to line the tank and make it water proof and long lasting. To keep the plate out of the sludge, a plastic grill is suspended about an inch below the surface of the mordant, allowing the reduced copper to fall to the bottom. A practical grill is available from lighting stores as there is one model consisting of 1/2 inch squares. This is kept in place by making a ledge at the proper height below the place you want the surface of the mordant to be. Since the plastic grill will float, I suggest attaching some lead weights to keep it down - use nylon cord or other plastic monofilament materials that should last. The plastic grill should be reinforced by some means to prevent it from breaking while in use.


This tank should hold a larger amount of mordant and so give more consistent times in etching plates. As etching solutions are exhausted, etching time increases, making it hard to tell if the etching is completed. If the volume of solution is such that it will last a long time, then it is easier to judge the depth of the line. For adding more copper sulfate to bring the solution up to strength, in this case it is important to use saturated copper sulfate solutions as there is no way you can stir the batch to help dissolve the crystals. I would suggest a commercial plastic valve that is meant to handle coarse materials that is connected to a drain hose. The sludge and corrosive action of the bath would not make any kind of metal tap practical in this case.

For and ordinary opened tray, I have introduced a simpler method of preventing the sludge being stirred up and masking the image.  By attaching sturdy styrene legs to the plastic grill with acetone, I then tied strips of lead to the edges of the grill with heavy nylon cord, to prevent them floating. This keeps the plate well out of the sludge, which falls to the bottom, giving the student a clear view of what is happening on the plate surface.

Understanding the Chemical Reaction (Dr. Bader)

In chemical terms, the etching/dissolution of the metal from the plate is an oxidation reaction... requiring the concurrent reduction of some other species. Which species is oxidized and which gets reduced (a loss and gain of electrons respectively) can be determined qualitatively from the electrochemical series, reproduced in part below, and oxidation/reduction can be quantified by calculations involving reduction potential data.

Reduction Reaction Potential (volts)

Cu2+ + 2e- = Cu (metal) +0.34
2H+ + 2e- = H2 (gas) 0.00
Zn2+ + 2e- = Zn (metal) -0.76
Al3+ + 3e- = Al (metal) -1.66

The negative sign for zinc and aluminum means that they resist undergoing the reduction reaction written, and rather prefer the reverse, oxidation reaction when placed together with hydrogen ions H+ (the active ingredient of acid solutions). The hydrogen, having a greater reduction potential, undergoes the necessary concurrent reduction. In short, the metals dissolve in acid! As the process takes place, bubbles of the hydrogen produced can interfere with further acid-metal contact, thereby stopping the reaction.

Conversely, copper which appears above hydrogen in the table does not dissolve in (is not oxidized by) just any acid... rather, nitric acid is required to provide the extra oxidizing ability of the nitrate species. This shows why this mordant does not work with copper plates.

In the same sense that the H+ dissolves metals below it in the table, the Cu2+ ion in the solution is able (even "more able") to dissolve metals below it in the table. The result is the dissolution of the aluminium or the zinc, and the production of copper (as the dark, finely divided metal) in place of the bubbles of hydrogen. This metallic copper produced could also interfere with further reaction, were it not for the fact that the solution can still work through the wet sludge, and further, that dissolution by acid is still occurring (to a lesser extent) at the same time, producing small hydrogen bubbles on the finely divided copper - which carry it away, avoiding further interference with the dissolution of the aluminium. The result is the noted clearing of the etching site, very little loss of acid content, and considerable loss of copper ions from solution -- appearing as copper metal.

The aluminium removed goes into solution as the aluminum ion, which requires fairly high acid strength to prevent the formation of gel-like aluminum hydroxide. The relatively slow loss of acid maintains this desirable condition - assisted by the fact that chloride ions (from the sodium chloride) form fairly stable, soluble complexes with the aluminum ions. Eventually, of course, the solution becomes "over-loaded", and its high ionic strength works against further reaction.

The sulfate which came from the copper sulfate and the sodium bisulfate is a necessary artifact of the use of these inexpensive and available substances. At best, it is non-participating, at worst it contributes to the overall ionic strength of the solution and its eventual exhaustion.

If it is necessary to revive the solution, or to maintain a constant copper content over a period of time, it would be useful to use a simple color comparison technique. Addition of copper sulfate to re-establish the same color in a side-by-side comparison will ensure similar copper content.

Reference has been made throughout this paper to "sulfuric acid". The acid present is, in fact, the hydrogen sulfate ion (HS04-) which can be considered to be the result of "partial neutralization" of (strong, dangerous) sulfuric acid. The noted pH of around 2.5 is consistent with the presence of this bisulfate ion, often described as a "fairly strong" weak acid. While strongly acidic conditions are thus avoided, the solution is, nonetheless, quite corrosive (witness the fact that it dissolves metals!) so that drips, splashes and spills should not be ignored. The solution is less dangerous than the more usual etching acids with respect to skin contact, but eye contact is still very dangerous. The wearing of safety glasses is strongly advised. The potential for the staining and corrosion of studio facilities is not an inconsequential factor. Clean-up is probably best accomplished with plain water, which dilutes the solution without allowing large amounts of aluminium hydroxide to form, which would produce white smears.


Creating large etchings has been a costly endeavor because of the high price of copper and zinc. With the bold abstract designs being created by many artist/printers today, the undesirable characteristics of aluminum might not be a concern for producing that kind of image. Since commercial aluminium can be purchased in sheets up to 4'x12', the size of the image is only limited by the limitation of the printing press itself. These, along with the following attributes of the method, make it a viable option:

1/ The cost and availability of the chemicals used to make the mordant makes this process less expensive and much less troublesome in acquiring the materials.

2/ The non-hazardous nature of the chemicals involved makes it safer for the studio, specifically in the classroom situation.

3/ Disposal of spent etching baths do not require expensive environmental control as all the copper compounds are removed.

4/ The color intensity of the bath is a good indicator of the strength of the solution, so that timing of the etch can be more accurately estimated.

5/ The more consistent action of the mordant over the life of the etching bath gives the artist more control over the plate.

6/ Open bite areas produce deep blacks without the need of aquatinting.

It is hoped that teaching institutions adopt this process for not only budgetary considerations for their respective departments in the cost of etching chemicals and disposal expenses, but for the cost of metal plates to their students.

References and Notes

Semenoff, N. and Christos, C., Using dry copier toners and electro-etching on intaglio plates, Leonardo, Vol. 24, Number 4, pp. 389-394, 1991.

Permission to photocopy this paper is given by the authors. Publication without permission is prohibited. Send inquires and comments to:

Nik Semenoff, Artist-in-Residence,
Department of Art and Art History,
University of Saskatchewan,
Saskatoon, Saskatchewan, Canada. S7N 5A4


Note: This updated paper in a different version that has been accepted sometime ago and published by LEONARDO, the international refereed journal of technology in the arts. LEONARDO, Vol. 31, No. 2, pp. 1333-138, 1998.


Using Copier Toners for Both Negative
and Positive
Intaglio Images

By Nik Semenoff

In 1985, I found that dry copier toner could be used for producing tusche like washes in lithography, without the fear of overworking the plate or stone. On farther work with that material, I found it useful for imaging on Mylar and exposure to photo emulsions to be processed in all media. Being a plastic, toner had some interesting characteristics that made it useful for various techniques within printmaking. I incorporated its use into the student work at our university, seeing that freer images would be the result. While positive photo plates and screen emulsions allowed us to use the toner images, our concern about the use of KPR materials had prevented its use in intaglio. The masters’ student at the time, had access to commercial photoengraving plates at the undergraduate school she had attended, so Christine Christos proceeded to use toner on Mylar for her intaglio work. I tried a more direct approach to the use of toners in intaglio.

Presensitized photoengraving plates are expensive and out of the reach of many students. For this reason, I experimented on a more direct use of toners on metal plates. There are two methods of working directly on plates, one producing a positive image, the other a negative one. To produce a positive image, it requires the application of a resist coating over the toner.

Toner characteristics (see my paper on toner use in printmaking)

Toners are manufactured mainly from plastics, the types used varies with the maker of the machine and the model number. You will find that there are two basic types, which I have classified simply as type A or B. This important information will give you greater freedom in manipulating the image. To find out the nature of your toner, place a small amount in a saucer and apply any common hydrocarbon solvent such as paint thinner. If the powder forms a soft ball, it can be classified as type B toner. Type A toners are not affected by simple hydrocarbon solvents and retain their individual particles. It is also possible that some toners will fall somewhere in between the two major types. You should also test your toner with other solvents that will likely be used in making the plate. I would suggest common alcohols, plus some of the stronger solvents. Alcohol can be used with many as a drawing fluid, while stronger solvents will be used to remove the toner.

Because water is the commonest drawing medium, the toner will have to dispersed with a wetting agent. Use a few drops of any dispersant in about 30 ml of water. Add toner to twice the volume of the water and shake in a closed container. The toner can then be diluted in a saucer for use as a tusche-like wash. If for any reason you do not wish to use water, type A toner in a hydrocarbon solvent or a type B in alcohol can be used for your washes. The reticulation will vary with the type and amount of wetting agent in water washes; also, alcohol and hydrocarbon washes will be different from water.

After the wash has dried on the plate, it has to be bonded to the surface for farther processing. For intaglio, I would recommend the use of heat to make sure the plastic has melted sufficiently to produce a good bond. If the hot plate doesn't reach a high enough temperature, use a small propane camp stove and a hand vise to hold the plate. Do not overheat the plate as the plastic will flow at too high a temperature and close up in the dark areas.

Toner as a resist to produce a negative image

The simplest approach in the use of toner for etching is to apply the toner wash in a manner that it will become the resist in the etching process. This is a normal procedure in etching and should not be too difficult to handle for most artists. The toner image is set by heat for best results. Block-out is applied to the plate where needed and the metal is etched in your favorite mordant. Negative toner images tend to appear much darker in print than what they indicated in the drawing. Because of the very fine characteristics of toner, the fine particles may be etched away as they are under-cut by the mordants. Electro-etching is probably one of the best methods of doing away with this problem. Large areas covered with toner come up too white at times, so some experience with this technique may be needed to get the results you want.


Electro-etched plate by Donna Redl, using toner as a resist in the background

Positive Images (Early but still useable method)

After the toner image has been set on the plate with heat, diluted orange shellac is flowed over the surface to produce a very thin coating. The best material is flake orange shellac that is made up into solution by suspending some within a nylon stocking in wood alcohol. After it has dissolved, remove the stocking, letting the solution stand until there is a definite separation between the wax-containing portion at the bottom, and the purer shellac on top. Remove the pure shellac carefully with a poultry baster, leaving the unwanted wax portion. To let you see how well the shellac mask has been applied; an alcohol soluble dye should be added to the solution.

Dilute the shellac with wood alcohol so that not too thick a coating is produced. You will have to experiment with this, as it is hard to explain the right viscosity; actually a very thin film produces the best results. Use a soft brush to flow the shellac over the surface of the entire plate, starting at one edge. Try not to go over any covered areas as this produces more problems than it solves. Let the alcohol evaporate.

When shellac is heated above 150 degrees centigrade, it changes from a thermoplastic to a hard, horn like material that can resist mordants very well. Heat the plate until the shellac takes on a golden brown color, indicating all the water in its molecules has been driven off. The next step is to wash out the toner image without damaging the shellac mask. Remove the toner image with common turpentine, using a short bristle brush like those used for stenciling. If there is some damage to the thin shellac layer, then use less pressure and let the turpentine soften the toner first; another solution is to use a softer brush. There are a number on the market that will do the job.  When the shellac was dyed, you can immediately see the mask after the removal of the toner. The plate is now ready for etching with the shellac mask acting as the resist. Use block out for any areas you want perfectly white.

New Method using Ferstman Ground

Since I have started to use Gerald Ferstman hard/soft ground in my classes, it dawned on me that this material is made up of emulsified shellac as the binding agent.  While the original formula used by Gerald is impossible to make these days since the Ink Dezyne company has been bought out by Nazdar, other compounds can take the place of Hydra Plus HY-170, the plasticizing agent.  I have found that a mixture of Future Acrylic Floor Polish with about 15% of a retarder added works quite well. The retarder I have use is polyalkylene glycol that makes up nearly 100% of DOT 3 brake fluid. It is not toxic. A bit of this makes the ground less likely to chip as shellac becomes brittle on curing.

 Start with Graphic Chemical water soluble Peacock Blue #1669C ink. To this add a bit of the above retarder and spread out the ink on a slab as if for printing.  Roll this over the toner image adhered to the plate until you have a smooth even film. Press hard with the brayer to make sure of good contact around the particles.  Heat the plate on a good hot plate or over a flame until the shellac within the ink becomes horn-like. Set aside to cool.  Use turpentine and a soft brush to washout the toner image, then remove excess turpentine with a soft cloth.  You can now etch the plate by your favorite method. The blue hard ground can be removed with sodium metasilicate or anther strong alkali.

Etching the plate

Because of the very thin shellac mask and maybe some dust particles in the air, go over the areas you want perfectly white. Within the toner image, slight breaks in the resist will not make any difference and should not be a concern. Etch the plate very carefully and not too long as the shellac mask may be lifted in too strong a mordant. I have found that the lighter etched plates printed the complete range of tints better than a deeper etched one.


-Semenoff, N., Using dry copier toners in place of traditional lithographic grease tusche,
LEONARDO, Vol. 20, #1, pp. 71-77, 1987

-Semenoff, N. and Christos, C., Using dry copier toners and electro-etching on intaglio plates, LEONARDO, Vol. 24, Number 4, pp. 389-394, 1991.

-Semenoff, Nik, Safe Etching - latest research, PRINTMAKING TODAY, Vol. 18, No 2 Summer 2009

-Bytautas, Alfons, An ingenious mix, PRINTMAKING TODAY, Vol. 18, No 3 Autumn 2009

Printing Intaglio Images with Water-based Ink.
By Nik Semenoff and Alan Flint

This has been retained in the site because there is interesting information that can be of used by research minded printmakers. 

See the paper on my recent research into waterbased ink.

With today's concern about the dangerous materials in printmaking studios, any decrease in the use of toxic products should be welcomed by printmakers. Over the last two decade, I have been successful in reducing the use of toxic materials in lithography, screen printing, and intaglio. While not as active in intaglio as the other media, as a teacher in an institution where many eager students can quickly make the studio hazardous, I feel it is my duty to find safer products. After discovering a safer mordant for zinc and aluminum that uses copper sulfate, common salt and a weak acidifier, I had been hoping to find an ink that could be liquefied and removed with water, yet become water resistant over time. I feel that I have found a method that allows the printer to easily grind their own inks, giving them more control over the characteristics of the product than commercial merchandise allow.

While visiting Alan Flint in his Hamilton studio in 1993, we discussed the advantages of water based inks for cleanup and less toxic fumes in the work place. We carefully thought about the characteristics needed for the ink, then looked at the possible binding media and pigments in the studio. One that we tried was Lascaux clear screen printing base, colored with Createx liquid pigments. The results were very encouraging and we both intended to continue with experiments. Alan's commitment to his printing establishment left him little time for research, and since I do not actually print intaglio editions, I only periodically thought about the results we achieved. With my somewhat limited research, I found that dry pigments would produce a richer print, yet it was easy to grind up a batch of ink with a spatula and a piece of plate glass. I discovered that the ink would not dry if kept in a closed plastic container, unlike commercial oil based ink that skinned over quickly. To prevent too fast drying of the ink while wiping the plate, I tried some acrylic paint retarder in a very small amount. It started to look very promising from the results of these tests.

To see if others could get the same results, I had one of our masters student look into the materials by giving her the screen base, pigments and various retarders to try. Since she was inclined to produce larger prints, I was interested to see if my concerns of premature drying would be a problem. To my pleasant surprise, she had no trouble in wiping even larger plates with the minimum amount of retarder. Not being much interested intaglio, she only did a few plates and went on to lithography and screen-printing.

I took up the challenge from time to time as I felt the materials pointed to a safer work environment. Since I was concerned that commercial products can change with the next order because the manufacturer is concerned with other matters, I was hoping to find a basic substance that would be available to printers. As I showed printers the results of using this ink, I was encouraged to forget about finding a substitute for the Lascaux screen base, and make the method known as a stepping-stone for others to carry on. Here is the results of my research in this area.

Grinding waterbased inks

Use Lascaux serigraphic medium straight from the container and add dry artists pigments from a reputable source. Grinding can take place with an ordinary spatula, but a proper muller would be of help in getting smoother inks faster. Make the ink quite stiff, much like the commercial oil inks. While no retarder or other additives may be needed, I have found that a small amount of acrylic emulsion retarder can be useful for larger plates. It also tends to effect plate tone, depending on how much is used. With little or no retarder, the ink tends to dry as it gets to be a very thin film, allowing the printer to completely remove any plate tone with a piece of wiping tissue. The ink in the lines is much slower in drying and will print normally. After wiping, I suggest you place the plate face down on your inking slab to minimize the evaporation of the liquids in the ink.

Unlike oil inks, one does not have to soak the paper very long. For rag papers with much sizing, just a few minutes are needed, for non-rag proofing papers, immersion for 2-5 seconds is enough. After blotting the wet surface, the plate is printed on an etching press. If the paper is soaked too long, it becomes water logged, allowing the water-based ink to spread amongst the fibers. The minimal soaking of the paper lets the prints dry faster, with less problem of curling.

Any unused ink can be stored in a tightly covered plastic container for later use. I have found the inks I mixed years ago to be fresh and usable, with no skinning or other defects. I also discovered that the finished prints become water-resistant after drying for some time, just like household acrylic paints. Since the Lascaux screen medium is made from a concentrated gel of acrylic resin emulsion with glycol added as thinner and retarder, it can be considered permanent for artistic purposes.

I would hope that this disclosure will encourage other printmakers to do more research into the materials and develop ink that will become standard for intaglio printers. This would make the studio less hazardous as cleaning the plate and ink slab requires only a bit of water. As I mentioned before, intaglio is not my chosen printmaking media and I only do some research in the hope of finding better and safer materials for use in the studio. I must thank Alan Flint for his encouragement for my continued research and his initial input into the use of the Lascaux product. His printmaking knowledge and enthusiasm became an inspiration for experimentation in this area.

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A copy of the license is included in the section entitled "GNU Free Documentation License".




Updated November 2011