Earlier, I’ve signaled the challenge of achieving sufficient consistency for a feasible color carbon process. Part of that would be aided by halftone negatives, as discussed in the previous installment. The other part I’ll highlight here is about the tissue itself, and how to sensitize it. Applying sensitizer to a pre-made tissue doesn’t seem to be ideal, and it would work better to incorporate the sensitizer into the glop. But what are the possibilities and challenges of doing so?
First of all, it’s technically possible to incorporate dichromate into a carbon tissue by adding it to the glop. However, the so-called ‘dark reaction’ makes the gelatin harden over time even without exposure to UV light, which essentially renders the tissue unusable – especially if good consistency is required. Theoretically, one could freeze the tissue as soon as it dries, more or less stopping the dark reaction, but I consider this unpractical. So dichromate is out. What other options do we have?
Ferric Ammounium Citrate
The first candidate is in fact very appealing, as it’s easily available, quite cheap, and relatively non-toxic. It’s the same ferric ammonium citrate (FAC) that’s used for e.g. cyanotypes or Van Dyke Brown prints. Research was done by Halvor Bjørngård at Chiba University, resulting in the so-called Chiba System for gelatin and gum-based pigment prints.
The Chiba process itself is somewhat cumbersome; because the carbon tissue must not be exposed to oxygen during processing (otherwise the hardening action fails), it requires a thin film to be overlaid on top of the gelatin tissue. Bjørngård used agar agar for this, but the flimsy agar film limits prints to very small sizes. Well, that’s a very crude and brief summary, but as far as I can tell, the Chiba approach as published originally isn’t very practical. But it’s promising.
Fortunately, Sandy King has done extensive work on the FAC front, trying to make it work (better) for carbon printing. The best approach he seems to have come up with, is to pour the carbon tissue on a thin, transparent film (instead of e.g. Yupo or paper), and then expose the tissue through the base. Development of the print is then also on the same substrate, making this a direct carbon process and not so much a carbon transfer.
The fact that the thin film also becomes the final support frankly doesn’t appeal to me very much. One of the main reasons for me for doing carbon transfer is the relative freedom in choosing a paper substrate (or even glass or metal sheets). There is also the problem of degradation of fine detail if the substrate isn’t thin enough and if the exposure light source isn’t sufficiently collimated. So there are additional process and equipment considerations to take into account.
I did do a small test with FAC-incorporated tissue, just to get a feeling for what it does. And it’s tantalizing, for sure! Although the experiment wasn’t exactly a success, it does show potential. I made some tissues and processed them as I’d normally would – so no special transparent base or anything, and no film overlaid on top of the tissue. Just exposed a step wedge and tried to transfer it to Yupo.
Well, the transfer didn’t work. The gelatin relief just floated away freely from the Yupo it was supposed to adhere to. It was very nice and firm though, and I could in fact ‘capture’ it in the warm water development bath on a transparent sheet. A bit like a Polaroid transfer or emulsion lift, in a way. Here it is – I repeated the experiment, so the step wedge is shown twice:
The problem is that the transfer evidently doesn’t work well this way. But otherwise, it does demonstrate that the FAC successfully hardens the gelatin. I should mention that it takes a little hydrogen peroxide to do this after exposure, but only a tiny bit proves to be sufficient.
The other option, one that’s actually tried and tested, is a compound usually referred to as DAS (more correctly, “4,4′-Diazidostilbene-2,2′-disulfonic Acid Disodium Salt”, CAS # 2718-90-3). It used to be used, as I understand, in the manufacturing of cathode ray tubes – old-fashioned TV’s and computer monitors. Since that’s basically a dead industry, the material is barely being produced anymore and prices are (in my opinion) annoyingly high. But it’s obtainable from a handful of sources worldwide, and kept cool and dark, it apparently keeps fairly well.
I purchased a little and have been doing some experiments with it. Going by what King, Nelson & Lockhart write in their carbon book, I had expected that the stuff would be pretty straightforward to use. Sadly, this doesn’t appear to be the case.
Firstly, upon soaking the exposed tissue, it creates a massive number of microbubbles. Apparently, this is normal, and I’ve been handling this by brushing them off in the cold water bath with a soft brush. More problematically, I just can’t get this tissue to transfer reliably – or, in fact, almost not at all. The problem is always the same: the transfer seems to be going OK, but as warm water development proceeds, the highlights first lift off of the final support, followed by the mid tones and ultimately most of the shadows as well. This happens with the supports that give me a 100% reliable transfer rate with dichromate, so the problem really does have to do with DAS. Since I duplicated one of the recipes from the book mentioned and King mentions that the developed relief should be quite ‘tenacious’, I’m puzzled.
This should be the way forward, but there are evidently some issues to iron out. So far I’ve also not been able to find any mention of the exact issues I’m encountering, so it’s kind of an odd problem. The main possibility I can think of so far is that the DAS content of the glop is on the low side, even though I took the DAS to gelatin ratio directly from the carbon transfer book. I have yet to try a new batch of tissue with a higher DAS content to see if that makes any difference.
All this means I haven’t even gotten to the point where I have to handle the rather strong yellow stain that DAS leaves in the developed relief. It’s going to take potassium permanganate to clear it (which softens the gelatin, so it’s a bit tricky) and then sulfite to clear the permanganate stain. This should work OK, but as said, testing this will have to wait for the point where I can actually get this stuff to work in the first place.
In conclusion, as with the story about the negatives, there are promising options, but they come with their own challenges. I’m definitely going to go ahead with the DAS. For now, I’ve been having fun with plain old B&W carbon transfer using dichromate lately, and using in-camera negatives. I have to admit that I doubt if I’ll enjoy a hybrid digital/’analog’ color process as much. I still am planning to get the color thing to (sort of work) just to see how it fits me, but I doubt it’s something I’m going to stick with. Well, to be continued!