A New Cuprotype?

[Jan de Jong]

That's an interesting idea though I'm unable to fathom what ferricyanide and ferrocyanide do to reduce stain. However, tartaric acid could potentially help as it is copper chelate. I didn't add dichromate as restrainer in the sensitiser and on hindsight I should have.

For intensifying, in a different test, I have tried developing in copper sulphate + tartrate + thiocyanate and it seems to work (sample size = 1). But I didn't do more tests because developing in a thiocyanate broth defeats the purpose of adding thiocyanate to the sensitiser. Your idea of toning in ferricyanide + ferrocyanide + tartaric acid could potentially help if the plain ferricyanide developer isn't converting all copper(i) thiocyanate to copper(i) ferrocyanide. John Mercer used ammonium chloride with ferricyanide in his experiments to convert copper(i) thiocyanate to copper(i) ferrocyanide. That could be another avenue if I were to redo this experiment again.

I thought it was an interesting observation and surprised me to see such whites. I could quote what CoPilot (now has renamed itself to Ferra in my conversations) tells me. It says Kferro could act as a mild complexing agent and binding stray Cu2+ ions, where the tartaric acid lowers the pH which keeps the copper ions moure soluble and may prevent precipitation of hydroxides and oxides.
I would have to try again to repeat to see if the observation is repeatable. I was hoping you may have some scraps left to confirm.

 

[Raghu Kuvempunagar]

I thought it was an interesting observation and surprised me to see such whites.
I would have to try again to repeat to see if the observation is repeatable. I was hoping you may have some scraps left to confirm.

It is indeed an interesting observation. Unfortunately, I don't have ferrocyanide at the moment and it won't be before October I can get new chemicals. I'll remember and give the ferri+ferro+tartaric mixture a try when I get the chance.

 

[Jan de Jong]

It is indeed an interesting observation. Unfortunately, I don't have ferrocyanide at the moment and it won't be before October I can get new chemicals. I'll remember and give the ferri+ferro+tartaric mixture a try when I get the chance.

you can reduce some KFerri to KFerro in a simple way though

 

[Raghu Kuvempunagar]

Jan,

I'm reluctant to use ascorbic acid (or any acid) with ferricyanide. So I tried the following:

Solution A
Water: 10 ml
Potassium ferricyanide: 0.2 g
Sodium sulphite: 0.1 g
Potassium ferricyanide: 0.2 g

Solution B
Water: 10 ml
Tartaric acid: 0.2 g
Sodium carbonate: just enough to get pH ~6

Add solution B to A and stir well.

I brushed this mix over a cuprotype print, left the print untouched for 10 minutes, washed in dilute citric acid followed by normal water wash.

I didn't notice any intensification of the darks or lightening of the lighter tones unfortunately.

Sulphite reduces ferricyanide to ferrocyanide and hence solution A is a mix of both ferricyanide and ferrocyanide. There is also some sulphate in it due to the oxidation of sulphite. And pH is higher than your mix. No idea if these differences played any role.

I will get some ferrocyanide in the near future and give it a try as I remain intrigued by the possibility of contrast increase.

 

[AntonKL]

It seems that the exposure time of this process can be shortened down to classic cyanotype times by briefly heating the exposed print to >100°C before washing it, e.g. with a clothes iron with a sheet of paper in between.
I'm not using this process productively but rather was just trying to see if it works with my UV enlarger (no, the contrast is too low), and noticed that it gave more density with warm or even boiling water, then tried the clothes iron on a whim.
I made a comparison chart, using a printer test page on a transparency as step wedge, they came out a bit overexposed but the main point gets across nonetheless. For reference I also made a cyanotype print. The exposure was made with a 90W 395nm LED from ~10cm distance, hence the low times.
The unevenness in the prints comes from my poor coating job...

I also tried complex copper toning:

I used the recipes from Frank Gorgas Article and 125gsm "Canson 'C' a grain" drawing paper (it's nice for experiments since it dries really quick and washes easily, but I wouldn't use it for anything productive)

 

[Raghu Kuvempunagar]

Nice finding! The application of heat to the exposed print is making more Sulphide available (by breaking down Thiosulphate) to the formation of Copper Sulphide, I guess. A more acidic sensitiser/developer may help further, if you want to try, as Thiosulphate breaks down more easily in acid.

BTW @nmp @Jan de Jong and others: delighted to see Hypo-Cuprotype now being taught in wokshops.

Anne Eder

Showing the range of tones available with the cuprotype process. I’m a bit in love with this palette right now. Want to learn this? I have a workshop coming up in November with @lux.et.libera . DM me...

www.facebook.com

 

[AntonKL]

The application of heat to the exposed print is making more Sulphide available (by breaking down Thiosulphate) to the formation of Copper Sulphide, I guess.

Wouldn't this lead to uniform fogging? From what I saw I think the mechanism is that exposure first forms some intermediate compound that is both heat and light sensitive, and also water soluble but a bit slow to dissolve. When doing the hot-water-only tests I spilled cold water on some prints before pouring the hot water, this only lead to a slight lightening (see the top print in the second image in #155). When a print was in cold water for longer (10-20s) and then put into hot water, there was still a slight darkening of the print.

A more acidic sensitiser/developer may help further, if you want to try, as Thiosulphate breaks down more easily in acid.

Acid does not seem to help. I just tried this with citric acid and also malic acid, if anything it slightly worsened the results, especially when the paper was completely dry.

 

[Raghu Kuvempunagar]

exposure first forms some intermediate compound that is both heat and light sensitive, and also water soluble but a bit slow to dissolve.

Interesting! How do you know this intermediate compound is light senstive? Does the print darken if exposed to UV light after washing away the residual sensitiser?

Acid does not seem to help. I just tried this with citric acid and also malic acid, if anything it slightly worsened the results, especially when the paper was completely dry.

Thiosulphate breaks down producing Sulphor Dioxide and nascent Sulphur in acidic medium turning the solution milky white, but this process is slow in room temperature and when weak acids are used. This phenomenon has been known to printmakers for more than 100 years now and has been used in the past as a Sepia toner for Silver prints. In the case of Cuprotype, Cu(+) formed after reduction of Cu(++) would form a Sulphide of Copper, when nascent Sulphur is readily available or at least that was my expectation.