Working Process for Cuprotypes

The cuprotype is a process that uses similar chemistry to a cyanotype and a relatively forgiving, inexpensive and flexible process. Frank Gorga shares his working method for cuprotype.

Writer and photography / Frank Gorga

Background of cuprotypes

Copper-based images (cuprotypes) were first described by Burnett in the 1850s. The process was modified/refined by Obernetter in the 1860s. However, the process was never particularly popular and sat more-or-less dormant for roughly a century and a half. In 2000, Jim Patterson (the “Darkroom Doc”) updated the process once again (see: Darkroomdoc.com/post/cuprotype).

In mid-2022, stimulated by a question on the Photrio forum, several folks (including Niranjan Patel, Peter Friedrichsen, Alberto Novo, and myself) revisited and further updated the cuprotype process (see AlternativePhotography.com/cuprotype-history/, for the details).

The chemistry of cuprotype is related to that of cyanotype. The photosensitive agent in both processes is ferric ammonium citrate. In cuprotype, the photoreduction of iron is coupled to the reduction of copper(II) to copper(I). In the case of the variant described here, the initial image is formed from copper(I) thiosulfate and is of low density and contrast. The initial image is toned with ferricyanide, giving an image with increased density and contrast. The toned image is formed from copper(II) ferrocyanide, a pigment known to painters as Hatchett’s Brown. While prints composed of Hatchett’s Brown are perfectly acceptable, the tone of these prints can be further altered by a second toning process. The chemical changes involved in these second toning processes have not been characterized.

“I made my first cuprotypes in mid-August of 2022. Since that time, I have spent many enjoyable hours in the dim room exploring this process.”

This document is my attempt to summarize, in one place, a detailed description of my working method for cuprotype as of February 2023. As with many things in the alternative process ‘arena’, cuprotype is subject to evolution/improvement and further development. Thus, things may change! But, for now, what follows is how I currently make cuprotypes.

Cuprotype is a relatively forgiving, inexpensive and flexible process. I urge readers to give it a try.

Negatives suitable for the cuprotype process

I use digital negatives made with a curve designed for salted paper printing. See the reference section below for specifics of the curve. These have worked well enough that I have not attempted to make a cuprotype specific curve.

I have not tried analog (film) negatives.

Substrates that can be used for cuprotype printing

Cuprotype does not seem to be particularly sensitive to the paper one uses. Certainly, any paper that works for cyanotype should work here. Other papers are also likely to work as well. Some papers exhibit mild staining of the highlights, especially when toned with iron.

Cuprotype does not seem to require acid pretreatment of papers to remove the carbonate buffer present in many papers.

One of my favorite papers for cuprotype is Fabriano Unica, an inexpensive, 50% cotton paper.

Additionally, cuprotype works well on plain cotton cloth such as one would use for cyanotype. I suspect that it will also work on silk, but I have not tried this.

Sensitizer and Coating

Stock Solutions:

50% (w/v – weight in volume) ferric ammonium citrate
– Dissolve 50 g of ferric ammonium citrate in about 75 mL of distilled water.
– Bring volume to 100 mL with distilled water.
– Mix well and store in a brown bottle away from light.
10% (w/v – weight in volume) copper (II) sulfate
– Dissolve 30 g of copper (II) sulfate pentahydrate in about 200 mL of distilled water.
– Bring the volume to 300 mL with distilled water.
10% (w/v – weight in volume) sodium thiosulphate (‘hypo’)
– Dissolve 20 g of sodium thiosulfate (“hypo”) in about 150 mL of distilled water.
– Bring the volume to 200 mL with distilled water.

These stocks are quite stable and should keep for months.

The ferric ammonium citrate will tend to grow mold. This can be prevented by adding a few drops of 10% (w/v) thymol in isopropanol (rubbing alcohol) as for cyanotype. Alternatively, one can simply filter out the mold just before use.

Working Solution:

Mix the stock solutions in a 1:3:2 (A:B:C) ratio by volume.

Coating Paper:

I brush the working solution onto paper using about .03-.05 mL of working solution per square inch (.05-.08 mL per 10 square cm). The exact amount depends on the paper and is determined by experience.

I dry paper in a dark environment for a minimum of 1 hour before exposure.

Coated paper seems to be quite stable. I have made nice prints using paper coated up to ten days prior to exposure. However, I have not tested stability in a systematic way.

Exposure

Expose coated paper under a negative to UV light. I have used both black light (495 nm) LED strips and BL fluorescent bulbs. Sunlight should work as well, but I have not tried this. Exposures are long; 20 – 45 min.

Cuprotype is a printing out process, so one can monitor the exposure by “peaking” at the paper. The native print is mustard yellow color, somewhat faint and low in contrast. It is not something one would hang on the wall! However, with some experience it is possible to judge a good exposure by inspection.

Processing

Processing consists of three stages:

Wash the print extensively to remove the iron using four changes of water (2 min. for the first wash; then 4 min. for each subsequent wash).
Tone the print with ferricyanide to convert the image to the brick red brown copper (II) ferrocyanide (see below), rinse briefly in water, then wash three times in water (4 min. each).
Optionally, one can tone a second time to shift the tone of the print again (see below). After the second toning, rinse the print briefly in water, then wash twice in water (4 min. each time) These washes are unnecessary if the print is not toned a second time.

Dry and flatten (if necessary) the print.

Ferricyanide Toners

The “simple” toner requires a longer exposure and gives a somewhat flatter print, but generally acceptable print compared to the “complex” toner.

Soak prints in either of these toners for 5-10 minutes.

I have not systematically investigated the capacity of either of these toners and so use them only for a one or two printing sessions (maybe a eight or dozen prints on 8×10 paper at most). I do know that the color of these solutions is not related to their effectiveness, so one can not visually tell when they are spent. Thus, to eliminate one source of variability I am very conservative in the use of these toners preferring to ‘under-use’ them rather than over-use them and obtain inconsistent results. The same holds true for the ‘second toners’ described in the next section.

“Simple” FeCN toner

2 % (w/v – weight in volume) potassium ferricyanide in water:

Dissolve 10 g potassium ferricyanide in about 300 mL water.
Bring volume to 500 mL with water.

“Complex” Cu/Cit/FeCN toner (Thanks to Niranjan Patel for this ‘invention’)

Solution A: 20 g copper sulfate pentahydrate, 50 g sodium citrate in 500 mL using distilled water.
– Dissolve 20 g copper sulfate pentahydrate and 50 g sodium citrate in about 400 mL distilled water; bring volume to 500 mL using distilled water.
Solution B: 20 g potassium ferricyanide in 100 mL using distilled water.
– Dissolve 20 g potassium ferricyanide in about 75 mL of distilled water; bring volume to 100 mL using distilled water.

Working solution: Slowly add solution B to solution A with constant stirring. Then add 5 mL concentrated HCl. (muriatic acid).

This solution seems to develop a fine brown precipitate and takes on the appearance of cocoa upon standing. The precipitate does not seem to interfere with the toning action. However, one could make the working solution immediately before use to minimize the formation of the precipitate.

Comparison of the “simple” and “complex” ferricyanide toners and the difference in citric acid toning.

Second Toners (optional)

Using a second toner after one of the FeCN toners allows one to significantly alter the brick red-brown of the native cuprotype.

Solution [1% (w/v – weight in volume) citric acid]: Dissolve 5 g citric acid in 500 mL of water.
Soak the print in the above solution for about 5 min.

Citric Acid Toning

Soak the print in 1% (w/v – weight in volume) citric acid for about 5 min.

Gives a chocolate brown tone and a boost in contrast mainly by deepening the shadows. This toner only works in conjunction with the “simple” FeCN toner.

Iron Toning

This toner is adapted from Jan Arnow’s 1982 “Handbook of Alternative Photographic Processes“, pp 113-114.

Used in conjunction with the “simple” FeCN toner, the iron toner gives a blue-black tone. Some papers show staining of the highlights and thus a lessening of contrast.

Used in conjunction with the “complex” FeCN toner, the iron toner gives a warm gray-black tone, most of the time! As with many things in alt process printing, there are uncontrolled, unknown variables that occasionally lead to unexpected results… all part of the adventure!

Iron Toning solution:

150 mL distilled water
6.5 g ferric chloride (I used anhydrous)
12 mL concentrated hydrochloric acid (muriatic acid from the hardware store)
16.2 g ferrous sulfate (I used the heptahydrate)

If one does not like to use concentrated mineral acids, use 170 mL of water and substitute 14 g of sulfamic acid for the concentrated HCl in the above recipe. Place the water in a beaker and add the other three ingredients in the order shown making sure each is dissolved before adding the next. This solution develops a small amount of very fine precipitate. Sometimes, I filter out the precipitate with a coffee filter, but this is not necessary. This solution is quite stable, I have not done a rigorous study but it seems to keep for weeks, at least.

Working solution:

Dilute the above stock about 1:50 with water. I typically make 500 mL of working solution by mixing 10 mL of stock with 490 mL water. This solution is used once and discarded. I have not investigated the capacity of this solution. One can use it for multiple prints at the risk of seeing varying results in the final tone as the solution loses potency.

Soak the print in this working solution for 2-10 minutes. It seems to act more quickly on prints toned with the “complex” FeCN toner than it does on prints toned with the “simple” toner.