Try everything once, or so they say. As I was struggling with a new iron-copper printing process proposed by Photrio user @Raghu Kuvempunagar, cuprotypes were brought up. And I had never made one. So let’s correct that and have a brief look at this process.
The concept of the cuprotype is a bit like a cyanotype, but instead of resulting in an iron-based image (i.e. the Prussian blue of a cyanotype), the idea is to have a copper-based end result. The light-sensitive agent is essentially the same – ferric ammonium citrate. The chemistry is somewhat different, as is the process. For my test prints, I relied on Frank Gorga’s (as always) well-documented work here: https://www.alternativephotography.com/cuprotype-process/
The reason I ventured here was really to figure out whether the papers I had been trying for Raghu’s new process actually do work. I’ve run into rather big problems trying to replicate Raghu’s appealing results and the paper is one of the main factors that will always be brought up (not in the least by myself). I know that the papers I tried work for classic cyanotype, as I’ve used them for that purpose before. But it seemed useful from a methodological viewpoint to also establish suitability for a copper-based process. Just to make sure.
So I mixed up a (very) small amount of cuprotype sensitizer, as well as a decent volume of the ‘complex’ copper-citrate-ferricyanide first toner, and a haphazard (but apparently functional) spin on the iron toner as well. I tested exposures of 3000 (arbitrary) units with a 365nm and a 395nm LED source. These correspond to exposure times of around 6 minutes at 365nm and 20 minutes at 395nm. Cuprotype is a slooooooow process, so these exposures were still on the short side.
Above are some of the prints I made. The papers you see are all produced by Dutch small-scale paper mill Schut. From left to right, these are “Laurier”, “Salland” and “Simili Japon”. Only the latter is mentioned on their website and I actually have no way of knowing whether the former two are even being manufactured anymore. Most of what I know about these papers, I have gleaned from the Polymetaal website, that also sells these products.
Note that exposure and processing for the three prints above was identical: all 3000 (arbitrary) units at 365nm, followed by a thorough wash as per Frank Gorga’s instructions, and then the ‘complex’ first toner. Btw, I mixed up that toner from dry chemicals instead of two separate solutions. The resulting mix was opaque mustard in color, a bit like the hot coco mentioned on the process description website. I don’t know if it’s supposed to be like that, but it does seem to work.
Here’s an exposure vs. density plot of the Laurier (the darkest) and Simili (the lightest):
The Simili plot shows every so slightly lower dmin, which corresponds to a lower level of fog on this process, even though the paper base itself is slightly more tan than Laurier’s pure white. Interestingly, the Simili plot sags at around step 11 of the Stoufer T2115 (logE 1.5) while the Laurier plot soldiers on towards a higher dmax. Neither reaches a particularly satisfying density, however. Exposures must have been too short for this.
As mentioned, I’ve also tried 395nm exposures. These took a whole lot longer, which is mostly due the spectral sensitivity of my DIY integrator (which I’ve briefly discussed earlier). Interestingly, despite the much longer exposure, the dmax isn’t all that higher – implying that maybe my DIY UV integrator and ferric ammonium citrate agree on how much 395nm and 365nm photos are worth, energetically speaking.
Here are prints on Laurier, 300 units at 365nm to the left and 3000 units at 395nm to the right. The difference seems very significant, but note that the darkest tones on both prints really don’t differ all that much in terms of absolute density. The tonal scale is different, though. Have a look at the plots:
Note how the dmax on the 365nm print (which looks lighter overall) is actually slightly darker. There’s also a conspicuous kink to the 395nm curve. I really don’t know what happens here, but interestingly, it’s around the same density step where the Simili and Laurier plots started to deviate earlier. Coincidentally, testing with Raghu’s new process, I also noticed that step 11 is where tonalities change. Interesting. Questions, but so far no answers.
As mentioned, I also whipped up some iron toner. I mentioned it was ‘haphazard’, which refers to the fact that I didn’t really bother weighing the ingredients very carefully. Also, the ferric chloride I had on hand was really the etchant I use for PCB manufacturing (which I think comes as a 40% w/v solution or thereabouts). I could (should) have dug up the jug of fresh solution, but being lazy, I just took the bottle of working stock etchant (used many times before, so loaded with copper) and took a few milliliters from that. Interestingly, it seems to work OK.
Shown above is the 395nm Laurier exposure and an identical exposure on Salland, with the latter being toned in iron toner after the first ‘complex’ toner. The hues are certainly more neutral; the degree/duration of toning can be used to control the tone from warm/pink towards cyan. I stopped somewhere around neutral here.
Initially I made the mistake of fairly thoroughly washing the print afterwards – which removed the toning for the most part. The tiny amount of alkalinity of our tap water is sufficient to effectively bleach the cyanotype toning. I re-toned it; the print may have lost some density in the process. It’s hard to tell.
There is some staining of the paper due to toning; I found it’s important to wash the print well after the first toner, otherwise Prussian blue forms indiscriminately throughout the paper base.
The curves show only marginal differences density-wise – the color difference is of course very apparent. Interestingly, the main difference in density seems to be a shouldering off of the toned curve, although this may be a measurement fluke due to imperfect processing. I do have a feeling that this exposure is pretty much dmax for this paper/process combination. The papers I tried seem to not be perfectly suitable for this process. I did not try to soak them in acid first, which may have helped in producing higher dmax and less fog.
Overall, an interesting experiment, but this is not a process I see myself exploring much further. As mentioned, the main reason I tested it was as a step in a problem-solving process for my troubles with Ragu’s ‘FerroBlend’ process. I was aware of the cuprotype process and had read the accounts of several people working with it in 2022-2023 on Photrio. While I find the dedication to developing a workable process admirable and some of the images shown quite enjoyable, the aesthetic is not one that appeals to me. The long exposure times and inherent tendency towards fogging don’t help much, either. Not my cup of tea. But, it has to be said – Frank Gorga’s process description is once again excellent, as I had been able to reproduce the process to a somewhat presentable result without much effort.