Color potential – Remaining dyes in processed RA4 paper

As I was setting up for a color printing session, I did a quick experiment to prove something to myself that I already knew. It’s just that I had never before empirically witnessed it. It’s about the dye couplers that remain in the emulsion of chromogenic color materials, such as RA4 paper. Yes, they just stay there, even if they’re not part of the image. Pretty neat – or disconcerting?

Let me start with that awkward word: “chromogenic.” It’s pretty essential in this context, because ‘chromo’ refers to color, and ‘genic’ is of course creation. Color creation. That’s exactly what happens in materials like RA4 paper, but also C41, ECN2 and E6 film. Inside the emulsion, there’s silver halide (in RA4 paper it’s virtually all silver chloride), which is light sensitive.

There are also so-called dye couplers, which are basically incomplete molecules of color dyes. These couplers themselves are colorless. They need another molecule added to them in order to form a complete and colorful dye. This molecule comes from the developer the paper or film is processed in. As this developing agent oxidizes on contact with exposed silver, it breaks down into the exact molecule the dye coupler needs to become a true dye. This, in a nutshell, is how color processing creates an image.

From this, you can infer that the silver image is really juts a side-show. It’s instrumental in oxidizing the developer, but after that, it serves no purpose anymore. That’s why the silver image is bleached and fixed out after development. Nothing remains of it, anymore. It’s just the dyes.

And the remaining dye couplers.

Because there’s nothing in a color process that actually removes the unreacted, colorless couplers. They just sit and wait patiently inside the emulsion until their counterpart molecule shows up to marry with them. They wait until…well, forever! So every chromogenic print (C-print, RA4 print) is actually loaded with molecules that are just anxious to become colored dyes. If they would all reach their full potential, every print would be pitch black!

This thought fascinates me. Not the pitch black prints, but the fact that even a white piece of blixed out RA4 paper is really a bunch of saturated colors just waiting to happen! I wanted to experience this, first-hand, just to see if it really works.

So I set up a tiny, informal experiment. As I mentioned initially, I was setting up for a color printing session, so I had the RCP20 running, heating up. I always have little scraps of discarded paper lying around in the darkroom. I cut off a small snippet and ran it through just the blix bath of the RCP20. No development, otherwise the paper would have turned pitch black. This is one of the purposes of these scraps of paper – I generally run some through the machine at the start of a session to see if there are not transport issues and the paper comes out fully developed. Anyway, I skipped development for this little snippet, fed it straight into the blix roller rack and obtained a bright white, entirely colorless piece of paper (after washing). Colorless – but with the potential for color still in it!

Since I had been making salt prints recently, I had little bottles of sodium chloride and silver nitrate solutions sitting around anyway. So I placed a few drops of sodium chloride onto the paper, followed by a few drops of silver nitrate. This forms an insoluble white mess, which is really silver chloride. I left this to sit on the paper and then took a pipette, and dropped some RA4 developer onto the paper with the white silver chloride sitting on top of it. Boom – the white silver chloride developed immediately into a black mess. So far, so good.

What was I trying to do, there, anyway? As I explained earlier, the molecule that the dye couplers are waiting for is oxidized developer. I figured I could make that oxidized developer in situ by adding some developer to some exposed silver halide (silver chloride in this case). My bet was that I would create enough of the oxidized developer that at least some of it would diffuse far enough into the emulsion to mate with the dormant color couplers. That’s the theory.

And this is how it turned out, after washing:

Nope, it’s not some nebula far away in outer space captured by a radio telescope. These are the colorful dyes that formed as a result of the experiment outlined above actually working, and surprisingly well, at that!

Alright, so given that the theory was pretty basic and it was only to be expected that the experiment would work – why am I a little surprised, still? It’s mostly because I did not expect the oxidized developer to be sufficiently mobile and penetrate into the emulsion layers. But I guess there was such an overdose of oxidized developer that the odds were favorable.

How about the colors? Well, these are pretty much as expected as well. The color palette is dominated by cyans and blues, with virtually no yellows. That makes perfect sense, as the cyan layer is the top layer in RA4 paper, so the muck I deposited onto the paper surface would first encounter that layer. In fact, not much developer made it to the layer below it, which is the magenta layer. There’s still some, as you can tell from the fact that the image isn’t all cyan, but also a bit blue (which is cyan + magenta). Since the yellow layer is all the way at the bottom of the emulsion stack, very little oxidized developer made it through to these depths.

This experiment might raise a few questions about how RA4 paper is supposed to work – let’s agree that what I did here was pretty off-label. It has definite creative potential, but let’s focus on doing it by the book, for a minute.

The first question is: in a normal color process, why doesn’t oxidized developer diffuse all over the place and touch layers it shouldn’t be touching? E.g. if you expose the magenta layer (with green light), how come there’s no oxidized developer going astray in the neighboring yellow and cyan layers?

Well, the answer is that this does actually happen, a little, but the manufacturer takes precautions to limit this. I wrote earlier about the importance of interlayers, and that’s really what this is about. Between the color forming layers, there are barrier layers that contain compounds (especially hydroquinone or a derivative thereof, apparently) that ‘capture’ the oxidized developer and prevent it from reaching the other color layers.

If these interlayers are less effective (for instance, because they are rather thin), there will actually be color contamination, or literally physical crossover. This is one of the key differentiators between higher- and lower-grade papers. If you optically enlarger negatives and process the exact same print on DPII vs. entry-level Crystal Archive, you’ll experience this first-hand. The colors on the DPII really ‘pop’, while the plain Crystal Archive print will look much more muted.

I can also imagine that something similar happens upon over-developing paper, especially in heavily used developer. At some point, I imagine the protection mechanisms of the interlayers will give way and some overall staining and color crossover will result. In my experience, RA4 paper takes a massive amount of abuse for this to happen, however. I do believe (but have never tested) that if you overdevelop prints substantially (100% or even more), contrast will increase somewhat, but saturation will decrease, exactly because of this layer crossover of oxidized developer. But I’m theorizing here, mostly.

Then there’s another question, one that I’ve actually seen pop up online from time to time. This issue sometimes occurs in the form of someone cautiously suggesting that processing steps like blix or stabilizer washing out the unreacted dye couplers. This, to my understanding, is really false. Firstly, the little experiment above was done on well-blixed paper. Secondly, contemporary stabilizer baths for RA4 paper are mostly intended (to the best of my knowledge) to prevent retained silver to ‘print out’ and cause yellowing, and there is probably a biocide in it to prevent organic deterioration of the emulsion.

What is extremely unlikely, or even impossible, is to actually wash out the color couplers. The reason for this is that they’re inherently resistant to being washed with water. This is because the dye couplers are actually water-repellent, oily molecules – which is one of the reasons why a gelatin emulsion (and it really is an emulsion in the chemical sense!) works so well for color materials: it allows these oily molecules to be separated from each other and distributed over the water-based gelatin matrix during emulsion making, and the whole mess is just stable enough to be coated onto the paper or film and then be dried. The gelatin really helps in trapping the tiny oily globules and immobilizing them.

And the color couplers stay that way: immobile, oily, and certainly not washable. They’re in there, forever. The only way to get them out, is by destroying the gelatin matrix they’re embedded in. And that means destroying the actual image as well. So for all intents and purposes, it’s not possible to ‘wash out’ unreacted color couplers.

From an archival viewpoint, this raises at least a theoretical concern. If the color couplers are there, in all our prints, in albums, on walls in museums, art collections etc., aren’t they in a way an accident waiting to happen? What if these prints come into contact with chemicals (potentially airborne) that react with the color couplers and form colored dyes? Or, more likely: what if molecules already present in the emulsion find ways to migrate to color couplers, creating stain and colored fog?

I was talking to a conservator recently, who told me about 1970s Kodak color materials where the magenta coupler has a tendency to react out into a magenta dye. Apparently, this problem is very real. But allegedly, the risk is quite limited with today’s color RA4 paper (and, I imagine, C41 film). In any case, for me, all this does emphasize the need to ensure proper storage conditions for RA4 prints. They should probably not be brought into contact with contaminated air (think industrial districts, heavy traffic), or moisture (making elements in the emulsion more mobile).

And I think it also underlines a challenge that has not been sufficiently met: to create a better understanding of the potential failure mechanisms involving remaining color couplers in chromogenic materials, such as RA4/C-prints. We make these prints by the millions (remember that digitally-exposed RA4 prints are chemically no different from optically enlarged ones!) and in some cases, there are archival requirements on them (family albums, fine art prints). However, it seems that curators, let alone the general public, are virtually uninformed about the chemical nature of these prints. I think that’s something we need to fix, now that there is at least some remaining knowledge of these materials alive in the manufacturing industry. It’ll likely be difficult or even impossible to recreate that knowledge once it stops being used in an industrial context.

But let’s end on a positive note. There’s creative potential in this, too! I took that little snippet from above and with that thought of the space nebula, inverted it and messed about with the colors a little. I think it turned out pretty neat:

4 thoughts on “Color potential – Remaining dyes in processed RA4 paper”

  1. Never thought about it, but yes, it’s like that… Huh, that’s quite creepy – tons of chromogens all over the world, waiting for the chance to blacken all the archived color films and photographs : D
    The classical color process will once become another historical photographic technique, it’s like this. The public, even the expert one, will be satisfied with simplified explanation of color film’s inner workings and the details will become a part of hidden history. Who will care, if there will be no need to produce color films and papers one day?

    To the topic of layers separation – from what I know, the problem was that untreated chromogens tended to travel between layers during development. The effect had fatal consequences for the color purity. Agfa researchers came with idea to make chromogens more “oily” – they added long CH2 chains to them, so they couldn’t easily penetrate to adjacent layer (and let themselves get developed there). But obviously there was still some part of chromogens travelling, so IMO interlayers are there to further isolate them.
    Kodak researchers invented another workaround for this problem – the color layers developed separately and not at once – which was the basis for Kodachrome.
    There is also another interlayer effect – the area, where development takes place, tends to “suck out” the strength from the developer. Where only one layer is exposed, it gets developed stronger than in the place, where all the layers have been exposed.

    1. Thanks for reaching out and adding your views on the separation. Yes, the use of oily instead of watery couplers was made fairly early on. It’s universal technology now. The couplers in Fuji papers are just as well oily instead of watery, and they’re in fact the only true ’emulsion’ involved in the emulsion-making process! (The rest being solutions or dispersions).

      > Who will care, if there will be no need to produce color films and papers one day?
      Well, that’s the million dollar question…it’s one of the reasons I’ve been asking questions and posting about this, although fortunately, there’s a lot of pretty good information on this in the thousands of research papers out there. I guess one of the challenges is really to combine all that information and make it more accessible.

      I’m not sure about the ‘sucking out’ w.r.t. the layers; I never heard anything about it and I would have to look into it. Do you happen to know how that would work from a chemical viewpoint? What I do know is that there are scavenger compounds in the interlayers that prevent oxidized developer from migrating through the layers. I think hydroquinone or a similar compound is used for this in paper, but I never really looked into it deeply.

      1. Yes, the effect is exactly called an “inter-image effect” or “vertical Eberhardt’s effect”. It has nothing to do with interlayers, sorry for wrong naming in my previous comment.
        It consists of two causes – the oxidized developer penetrates into adjacent layers, like you mentioned in your article. It happened in early color stock and probably solved by interlayers you mentioned.
        Other cause is given by partial depletion of developer during diffusion through the layers. When upper layers happen to be exposed, the bottom layer gets less developed, because the developer is partly depleted. When upper layers get unexposed, the bottom layer, if exposed, gets overdeveloped. But maybe the solution has been already found, too.

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