Origin of Copper Sulfate Mordant

Over the weekend some students from the university paid me a visit to talk about printmaking. During the afternoon we got talking about their use of the copper sulfate mordant for etching aluminium. One of them had encountered some articles about it on the Internet and the fact many there think it was developed in 1998; according to my article published in Leonardo. So I went on to explain how it came about for use at the University of Saskatchewan.

While I was teaching a first year printmaking class in 1992, in February, a student came complaining to me she could not afford the sheet of zinc needed to start the intaglio section of the class. Was there any cheaper metal she could use? I suggested aluminium and talked about changing to that metal with the head of the department, who refused to have ferric chloride in the area. While stannous chloride would work, the cost and availability put it out of the question.

Years earlier when I was printing traditional lithography from aluminium plates, I discovered a formula used by commercial printers to produce a better ink base on their plates by first plating the image area with copper. It involved using cuprous chloride in the solution, but when I tried copper sulfate instead, the plate became eroded instead of plated properly. With that background knowledge, I wondered if a safer mordant couldn’t be developed for use in a classroom.

I was able to come up with a simple formula for that class, consisting of copper sulfate and hydrochloric acid, using common aluminium sheets available locally. In the spring of 1992, the University of Saskatchewan held an international conference to promote new safer technology, including the toner technique and the new waterless litho process I had developed. At that time we only talked about the discovery of using copper sulfate as a mordant, but it never got into the official program of the conference, but became common knowledge for the printers who were interested. During teaching summer session that year, I developed the formula farther by removing the dangerous hydrochloric acid and replacing it with common table salt for the chlorine molecules. I quickly found that aluminium hydroxide formed as a gel like material, making the bath unsightly and not as effective in biting the metal. What was needed was an acid that would not interact with the metal and keep acidic levels high enough to prevent formation of the hydroxide. Of course both nitric and sulfuric acids would work, but I didn’t want to use any dangerous materials if possible. The obvious solution was to use sodium acid sulfate, which I was accustomed using in goldsmithing. This was rather expensive, but a cheaper and easier to get form was the toilet bowl cleaner SaniFlush, available in all grocery stores in North America. The stage was set for really researching the mordant.

Amongst my record keeping of my printmaking research, I found the tests I did to see how much aluminium would be eroded for a given weight of copper sulfate. These are dated November 12 and 13th, 1992. By this time I had retired from faculty, but became artist-in-residence, so could still access the resources at the university. I went to the chemistry department to see if they had a better understanding of the reactions taking place. Dr. Bader accepted the challenge, along with other professors and their students. They produced a report on their understanding of the process that was presented to me and their instructors. This was used by Dr. Bader and myself to write up an article for publication in the international refereed journal Leonardo. It was submitted and received by them on April 5, 1994.

Cu mordant

When contacting the editors of Leonardo, I pointed out that I had already been sending a similar paper to interested printmakers who were in contact with me by mail and over the Internet. The referees accepted the article with no changes, but suddenly someone at the office of Leonardo in San Francisco realized that copper sulfate was responsible for contaminating the water in the bay from its’ use in the grape growing area nearby. I protested that little or no copper compounds need to get released if proper care was taken. A second set of referees were picked and the process started again. Finally in Volume 31, No.2, the article was published in 1998.

When I started this research, I was not aware that Goya used copper sulfate to etch some of his zinc plates – but used my own experience with lithography. I also found and stated at the start, that this mordant would etch zinc with a slight unneeded modification. The use of salt has seemed to be overlooked by others publishing on the copper sulfate mordant. You will notice the received date at the end of the Leonardo article. Not really interested in printing intaglio works, I have not bothered much with this process other than trying to find a safer and cheaper method for my students while I was teaching. On the university website, this paper is requested much more than the paper dealing with waterless lithography and has become one of the worlds printmaking processes.
April 94

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