Working Process for Cuprotypes

Cuprotype toningThe 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:

  1. 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).

  2. 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).

  3. 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.

Cuprotype with toning

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.
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.

Examples of toned Cuprotypes

Cuprotype with Simple FeCN toned
Cuprotype with Simple FeCN toning.
Cuprotype with Simple FeCN toned then Iron toned
Cuprotype with Simple FeCN toning and then Iron toning.
Cuprotype with Simple FeCN toned then Citric Acid toned
Cuprotype with Simple FeCN toning and then Citric Acid toning.
Cuprotype with Complex FeCN toned
Cuprotype with Complex FeCN toning.
Cuprotype with Complex FeCN toned then Iron toned
Cuprotype with Complex FeCN toning and then Iron toning.

Reference

Cuprotype curve – but designed for salted paper printing.

Added March 2023

Download curve Cuprotype.acv here, or see the table below for input values.

Input values
0
13
27
38
51
64
89
115
140
166
191
204
217
229
242
255
Output values
0
15
21
24
29
33
40
48
61
81
110
132
169
204
230
255
Salted paper and cuprotype curve

This curve is essentially the same as described for salt printing in Christina Z. Anderson’s book Salted Paper Printing. A Step-By-Step Manual Highlighting Contemporary Artists

Frank Gorga is an artist and retired chemistry professor working in several alternative processes. Frank Gorga’s website can be found here.

4 thoughts on “Working Process for Cuprotypes”

  1. Hello Dominic,

    I do not think that there is anything to worry about with regard to copper sulfate toxicity when making cuprotypes.

    The amount of copper sulfate used in cuprotype is pretty small… 2 mL of sensitizer (which is more than enough for an 8×10 inch print) contains about 1 g of copper sulfate. While a small amount of this does enter the wash water, the amount ending up in the wash is only a small fraction of the total.

    When making a cuprotype, the copper(II) of copper sulfate is photo-reduced to copper(I) which is then reoxidized to copper(II) when the print is treated with ferricyanide. However the reoxidized copper forms the insoluble pigment which comprises the image and therefore won’t wash out of the print. Thus only the copper(II) present after exposure to light, but before treating with ferricyanide, is going to end up in the wash. Exactly how much copper(II) enters the wash is difficult to say and will vary greatly with the image being printed.

    That said, even if the entire 1 g of copper(II) sulfate entered the waste stream and if you are on a public sewer system the power of dilution quickly brings the concentration of copper well below concentrations that are likely to cause trouble.

    If you are in a rural area where folks have private septic systems, and where the volume of waste water is much smaller than with a public sewer system, an alternative approach is probably warranted. To be on the safe side you might want to collect (and evaporate, see below) the initial washes of the cuprotype (before treating the print with ferrocyanide). These washes will have the large majority of the ‘washable’ copper in them.

    If you work this way you can collect the initial wash (or may the first two if you want to be extra cautious) in a container (a large flat tray is best) and allow the water to evaporate. The waste container ends up looking like a horrible mess (I do this with multiple processes), but I have been using the same container for years. Eventually, I will dispose of this container (with the dry solids) in the regular trash.

    In the cold months, evaporation can be slow but I’ve never had to start a second container. Also, I would be sure to keep this waste tray in a place where pets are not tempted to drink from it!

    I have never had an issue with my septic system using this approach for all of the processes I print with. My view is that if whatever I put into my septic tank is not harmful to the microorganisms there, that it is unlikely to be a problem further ‘downstream’ from there.

    I do this even with silver-based processes (my fixer from salt printing is collected and evaporated) and silver is much more toxic to microorganisms than is copper.

    Lastly, folks often ask about “ferricyanide” in terms of toxicity. After all the name contains ‘cyanide’ and we all know that cyanide is a horrible poison. The bottom line here is that the cyanide in ferricyanide is very tightly bound to iron (i.e the ‘ferri’ part) making ferricyanide much less of a problem than free cyanide. The reason cyanide is so toxic is that it binds to very tightly to the iron in proteins (especially hemoglobin and cytochrome oxidase) and thus interferes with the reversible binding of oxygen to the iron in these proteins which is essential for life. Since the cyanide is ferricyanide is already bound up with iron it is not particularly toxic.

    I hope this helps,

    — Frank

  2. hello,

    I have a question regarding the safety of the waste used in this process. I was wondering if the rinses in water are safe to go down the drain. I know copper sulfate can be bad for aquatic life if washed down the drain, But the copper itself is reduced in the process and I guess forms insoluble copper compounds in the fibers of the paper. I’m unaware of what’s actually going down the drain though and was curious as to what precautions I should be aware of when giving this process a try

  3. Hello Fausto,

    As for the color… yes, green is the expected color. The solution will become cloudy and brown over time. The time for this to occur seems variable. Sometimes it happens almost immediately other times it takes hours. Both states seem to work just fine. I don’t actually use the complex toner often so these statements are based on only a few batches and not on any systematic testing.

    As for re-usability… again, yes. I have used this solution successfully even several weeks maybe as long a a month after preparation. However, I have done no systematic testing so I don’t know exactly how long it will last nor have I studied its capacity so I can’t say how many prints can be toned.

    — Frank

  4. Hello, thanks a lot for all this info and the research. I have a couple of question about the “Complex” FeCN toner:
    1) after adding HCl the toner becomes green, is it expected?
    2) is it reusable?
    thanks a lot!

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