The Copper Way

Introduction

I

I arrived at my first copper salt print independently in April 2020, by taking advantage of the different solubilities of copper bromides.
Despite my long-standing interest—spanning over a decade at that time—in historical printing techniques, I had never heard of cuprotype until then. Only after conducting some online research did I discover Patterson’s method and, through it, the original historical processes of Burnett and Obernetter.

Of course, I am not the only one who has developed a formula by following a different path: as experience teaches us, there are many ways to achieve a particular result, and a single method can have multiple variants. In this book, everything concerning the practice of cuprotype is influenced by my personal experience. Therefore, the information is presented and discussed through that lens. It is necessary to point out that my ideas and methods may differ from the general opinions and practices of other practitioners dedicated to this specific technique.
I decided to embark on writing this book after leading a workshop on cuprotype at the Academy of Fine Arts in Bari, organized by Professor Grazia Tagliente.

This project was driven by a desire to put down in black and white—and in "red on white"—the experiences I accumulated through experimenting with this alternative photographic printing technique. Although few in number, there are already online articles about cuprotype, mainly referring to Patterson’s method—the only notable modern research devoted to this technique. Aside from his excellent work—concise in detail yet complete in its essence—there are other occasional mentions, which I will not consider here.
Only since 2020 has this technique begun to appear in manuals on historical and alternative printing, but still in a very brief manner. At the time of writing this text (February 2024), there appear to be no books entirely dedicated to cuprotype.

This book has had a dual genesis.
It was conceived as a manual: a handful of historical notes, some technical-scientific information, and a lot of practice.

Modern literature on the subject, in addition to being scarce, has proven to be inaccurate and full of gaps. In some cases, the historical sources themselves have been ambiguous: one in particular contributed to "polluting" the original processes of both Obernetter and Burnett—a contamination that has been passed down to the present day. Original documents, both old and modern, even call into question the very chemistry of the reddish-brown pigment obtained through the cuprotype process, which has long been believed to be Hatchett's brown.

In attempting to reconstruct the origin of discrepancies among sources, a new book was born—a historical-scientific research project rich in surprises, which in some cases challenges what has been accepted until now.
An archival investigation that extended beyond the bounds of photography has uncovered a substantial amount of information, scattered across a network of documents almost exclusively in English and German. These fragments have made it possible to construct a coherent historical reconstruction of the origin and development of the process, its limited dissemination, and its rapid decline—up to its modern rediscovery.

The practical sections of this text are based on my own experience, as is the experimental research section. Where necessary, my own knowledge has been supplemented by the experiences of other experimenters. Much more could have been added on technical and theoretical matters, but these were omitted because the aim from the beginning was to create a work that is easy to use in its technical and practical aspects—without limiting it solely to those. It is an open-ended research, where the technical manual alternates with historical-scientific reconstruction.



II

A cuprotype print, like any creative work in any other medium, reflects the taste and skill of its creator.
But simply using an "exotic," historical or alternative photographic process does not make a print a beautiful one, nor does it make a photographer a great artist.
It is this mistaken habit—of treating mastery of the medium as the ultimate goal of creative work—that leads many to criticize its overuse, rightly considering it a "gimmicky" expressive tool.
Fortunately, in most cases, ambitions grow, and the photographer themselves no longer settle for mere technical tricks to move forward.

This text may be useful for two kinds of people: first, for enthusiasts of historical printing techniques—photographers, both advanced and beginners, who find pleasure in experimentation and in the manual, independent creation of their work; and second, for the naturally curious, starting with students who, in their academic journey, choose or come across this "art form."

Vincenzo Caniparoli

The invention of photographic processes based on the light sensitivity of iron salts is undoubtedly owed to Sir John Herschel (1792–1871). His investigations opened a path from which many later insights would emerge.

Equally significant was the influence of another great scientist and pioneer of photography: Robert Hunt (1807–1887). To understand the context in which cuprotype arose, it is fitting to begin with him. Hunt had studied in depth the effects of sunlight on a wide variety of chemical compounds, especially in relation to their photographic applications.
In 1844, he published his Researches on Light, in which he presented detailed studies on the photochemical properties of many materials. In particular, he examined the photochemical behavior of potassium dichromate, with which he had already developed several photographic processes. In these, it was typically mixed with other chemicals—as in the process known as the chromatype.

This was a positive direct process, meaning it produced an image or print without the need for a negative. The reproduction occurred in a single step, an extremely simple method, and the resulting images had such a pleasant character that, although they were not sensitive enough to be used in a camera, they were extremely valuable for copying botanical specimens, engravings, or similar materials. In practice, the process involved immersing paper in a copper sulfate solution, drying it, then immersing it in a potassium dichromate bath. After drying again, the sheet was exposed to sunlight. The image was then developed in a silver nitrate solution, yielding—as Hunt himself wrote—“a very pleasing (positive) image,” red in tone on a yellow background.

The process was presented publicly at the Meeting of the British Association in August 1843.
It made use of the light sensitivity of copper chromate, but under the action of the silver nitrate developer, the image was converted into silver chromate; thus, the resulting print cannot yet be considered a true cuprotype.


The Birth of the Term and the Development of the Technique

The term cuprotype—meaning “copper print”—was coined between 1856 and 1857 by Charles John Burnett (1820–1907), an innovative photochemical experimenter and founding member of the Photographic Society of Scotland (PSS). The neologism initially referred to a development of the chromatype process invented by Robert Hunt nearly fifteen years earlier.
Burnett’s new process started from the same foundation, using copper chromate as the photosensitive compound. However, unlike Hunt, Burnett developed the print in a potassium ferrocyanide solution.
As a result, the final image consisted of copper ferrocyanide, a pigment known at the time as Hatchett’s Brown.

To better date the coinage of the term and the development of the process, it is worth quoting from a letter dated July 10, 1857, published by Burnett in Photographic Notes (P.N. vol. II, 1857):

“I have experimented extensively and in many ways with chromates and dichromates, and have succeeded in various ways in obtaining excellent results. A very important process, useful for many purposes, is to float or immerse the paper in a mixed solution of potassium dichromate and copper sulfate, as is done in Mr. Hunt’s chromatype, [...] but instead of developing with a silver [nitrate] solution, wash away the unexposed salts, and develop by floating the sheet on a solution of potassium ferrocyanide.”

The print was then toned with iron salts to obtain neutral tones—at the time, a quality nearly exclusive to silver-based photographic processes. As Hunt had done before him, Burnett noted that using copper chromate directly as a sensitizer produced better results than the two-step sulfate–dichromate method:

“In my letter I referred to potassium dichromate and copper sulfate as the most readily available materials for my copper printing process. I now send you a sample [...] of copper chromate. Dissolve it in ammonia, sulfuric acid, or—better yet—chromic acid. I believe you will find it more satisfactory than potassium dichromate and copper sulfate [...]. I am also sending you uranium sesquioxide [...] which you will find to be an excellent toning bath for copper printing. Use it instead of the iron bath, and you will obtain browner, richer prints than with copper alone, with purer highlights than those from the uranium process described in my earlier article.”

Of particular interest is the response from Thomas Sutton, editor of the journal:

“Your letter is indeed (in its chemical section) a mine of wealth. You have flooded us with good and useful suggestions. The process you call ‘cuprotype,’ and of which you have kindly enclosed a beautiful specimen, may well prove to be an improvement [...]. I encourage you to continue experimenting and to reduce it to something close to a defined formula. It will then become your ‘otype,’ and may very well turn out to be quite valuable.”

All possibilities, therefore, remained open.

Obernetter’s Iron-Copper Process

The period spanning from the 1830s to the end of the 19th century represents what can be described as the infancy of photography. Scientific and technological progress had a profound impact on 19th-century society, and in the field of photography, it provided the fertile ground for the proliferation of an impressive number of new printing processes. The invention of new image reproduction methods, aside from their scientific importance, also held considerable commercial value.

The development of cuprotype as a true photographic reproduction process is attributed to the German chemist Johann Baptist Obernetter (1840–1887). The young Johann studied chemistry in Leipzig, Heidelberg, and Munich under renowned professors such as Erdmann, Bunsen, and Liebig. It was precisely on Liebig’s recommendation that he turned his attention to photography.
In 1860, Obernetter began working as a chemist in the studio of Josef Albert, the official photographer of the Bavarian royal family. It was during this time, while under Albert’s supervision, that Obernetter made his first inventions.

According to Obernetter himself, his iron-copper process was developed in Munich, with the aim of creating printing proofs that were more economical than those produced with silver salts. While working in Josef Albert’s studio, Obernetter had the opportunity to study the main drawbacks of photographic printing processes: namely, the stability of proofs and the high consumption of silver—over 150 kilograms per year were used. This enormous amount, consumed by a single workshop, pushed Obernetter to search for a method that could achieve the same photographic results without involving that precious metal in the printing process.

Encouraged by Albert and supported by the outstanding resources of his laboratory, he developed a sensitizing solution made from an aqueous mixture of ferric chloride and cupric chloride, with an excess of hydrochloric acid. After exposure in a contact printing frame, the paper was fixed in a solution of potassium thiocyanate containing a very small amount of sulfuric acid (1 part per 1000) and a small quantity of the original sensitizing solution. This treatment transformed the image into cuprous thiocyanate, a water-insoluble salt that could then undergo further chemical transformations to obtain the desired color tone.
After a brief rinse, the proofs were treated in a potassium ferrocyanide solution with a concentration ranging from 6% to 12%. In this bath, the image acquired a reddish tone, increasing its visual intensity.
The proofs were then thoroughly washed and subjected to an additional chemical toning treatment, designed to obtain neutral tones comparable to those of silver-based processes.
The toning bath consisted of an acidic iron solution made from a mixture of ferric and ferrous salts. After toning, washing, and drying, the prints were given a glossy finish by floating them on an albumen solution, which was then coagulated through heating. The final results were so refined that Obernetter himself wrote:

“In this way, I have produced very large-size proofs which even the most skilled photographers mistook for beautiful silver-salt positives.”

Despite its promising foundations, there is no clear evidence that this process ever gained widespread use. While its economic advantages were undeniable, the method remained relatively complex and less immediate compared to the more established silver-salt processes—less affordable perhaps, but simpler and far more versatile.

All known sources, including the Encyclopedia of Nineteenth-Century Photography (2008), attribute the process to Obernetter and date it to 1864. However, as previously noted, Obernetter himself stated that the process had been developed while he was still working in Josef Albert’s studio—a period spanning from 1860 to 1863, the year in which he left Albert’s employment to start his own business. Obernetter’s entrepreneurial venture was a remarkable success, and many of his inventions were widely recognized and celebrated.

In 1868, he became the first to produce, on a commercial scale, collodion silver-chloride printing paper, known as Aristotype. He then moved on to collotype printing in 1869, and developed the version that became known as the Albertype. He further refined this technique by introducing a graphite dusting process, which earned him a gold medal at the 1873 Vienna World's Fair. By that year, his company employed twenty-two people, operating ten printing presses and producing high-quality collodion papers.

Between 1880 and 1882, he developed—and quickly began producing—a gelatin silver-chloride paper emulsion, which earned a strong reputation and wide acclaim. Together with H.W. Vogel, Obernetter worked on what is likely his most important contribution: an orthochromatic photographic plate designed for landscape photography, capable of rendering objects in accurate tonal values according to their actual colors. In addition to producing photographic materials, Obernetter also pursued an active role as a photographic publisher.

He died of cardiac syncope on April 12, 1887, at the age of 47.
The news had a wide international echo in the photographic community and generated deep mourning. His work was carried on by his son Emil, also a chemist and inventor. However, the iron-copper process remained frozen at the experimental stage developed during his early years in Albert’s laboratory.

(will follow)