Carbon Transfer: anyone try Calvin's 'supercoat' tissue concept?

This is a concept Calvin Grier launched recently: you make a regular DAS-sensitized carbon tissue, but you coat a thin layer of clear gelatin on top. This is supposed to help retain delicate highlights, and combat the 'tonal threshold' problem (as Calvin calls it).

Here's the video in which he explains it all:

He's also published an eBook that contains the same material as the video above: https://thewetprint.com/product/carbon-printing-with-inkjet-negatives/
The video is free, the eBook isn't.

I've done my first set of tests, but no luck so far. See my meager results here: https://tinker.koraks.nl/photograph...et-first-test-with-supercoated-carbon-tissue/

I'm honestly also not quite clear on how the concept it supposed to work at a theoretical level. The supercoat is unsensitized, so I don't really see how it's supposed to survive the warm water development step. In my tests, indeed it didn't and most of the image simply slid off the support during development. I'm wondering if others have already tried this (or are willing to dedicate some time to it).

I do not have the correct pair of gelatins for this to work, so perhaps that's why I'm running into trouble with it. IDK; I struggle to see how the unsensitized gelatin layer is supposed to work here.

koraks said:

I'm honestly also not quite clear on how the concept it supposed to work at a theoretical level.

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Let me hazard a guess. Without the supercoat, the hardened highlight regions of the tissue absorb very little water and don't swell much. When the supercoat is present, because it is not hardened and hence can absorb more water than hardened gelatin, it perhaps helps in increasing the swell of the highlight regions and thereby increasing their contact with the final support.

That's an interesting thought @Raghu Kuvempunagar - I had not considered that option. Thinking along the lines of swell, one of the parameters in the process is how long you allow the print to swell before the transfer is made to the final or temporary support. In my experience, longer swell times correlate with adhesion problems, so that would go against what you said here (shorter swell times tend to result in ripple marks).

Other mechanisms I have considered, but not thought through diligently:

• Some of the DAS in the image-forming layer diffuses into the topcoat layer as the latter is coated on top of the (still wet) image-forming layer
• Exposed DAS in the image-forming layer somehow causes hardening in the topcoat layer, perhaps also assisted by some kind of diffusion process. This might occur either during exposure or during soaking of the exposed tissue. If this is part of it, then I'd expect it to occur during the water soak since I assume mobility of molecules in the dry tissue (during exposure) to be severely limited.

I have no proof or credible arguments for any mechanism; at this stage I'm happy to hear about any additional clues, ideas etc.

If the idea is to get a very thin layer of unpigmented but hardened gelatin on top of the image making pigmented gelatin, to protect the delicate highlights from washing out during the warm water treatment, then diffusion seems an ungainly mechanism to get precise control. Why not use a lightly sensitised supercoat?

Raghu Kuvempunagar said:

Why not use a lightly sensitised supercoat?

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My thought exactly, so that's what I intend to try next.

When I first read Calvin's doc & watched the video, I had to check multiple times whether I had not messed a sensitization step for the topcoat - but it really isn't there in his process, to my surprise. I'm sure he must have tested this (he did something similar with gum, years ago), so there's probably a reason why he doesn't do it that way. I guess I'll have to find out the hard way why that is.

I believe the unsensitized gelatin is just to provide temporary protection the extremely thin highlights during development, keeping the very small amt of pigment in the very thin layer from washing out of the exposed highlights.

I get around this by printing with very low pigments levels that allow for a thicker layer of gelatin in the highlights that resist losing that small amount of pigment. I am also using dichromate and printing from camera negatives which perhaps has less issues.

Vaughn said:

I believe the unsensitized gelatin is just to provide temporary protection the extremely thin highlights during development, keeping the very small amt of pigment in the very thin layer from washing out of the exposed highlights.

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So how's that supposed to work without the unsensitized gelatin washing away (as it did in my experiment)? Why would the pigment not wash away from the unsensitized gel (btw, in my experience, it doesn't)?

I'm aware you're using dichromate and thick tissues; both help a lot with highlight retention. The tonal threshold issue is far less problematic with dichromate than it is with DAS. Camera negatives are a mixed blessing since they can in some ways help with highlights, in other ways pose a problem. Esp. pyro-stained negatives are a liability more so than a blessing.
Btw, my work is also predominantly with in-camera negatives.

It takes time for the small amount of pigment to wash out (diffuse) of that very thin layer of highlight gelatin. A coating of unsensitized gelatin might delay the pigment from washing out in the highlights before the rest of the print can be developed.

We have twoo separate actions happening. Melting the unexposed pigmented gelatin -- and the migration of pigment out of the sensitized, exposed pigmented gelatin. Keep or slow that diffusion from happening and you'll keep your highlight detail.

The other factor is just the highlights being so thin, one can get a mechanical removal of the gelatin (over agitation, etc). A layer of unsensitized gelatin might lend a little physical protection from this happening. This is a lesser issue than the diffusion, I believe.

A third possibility -- a layer of unsensitized gelatin over a layer of sensitized tissue will get partly sensitized from being in contact with the sensitizer in the gelatin below it. This might provide a very thin protective layer over the highlights,.

Pyro negs have been a zero liability for me so far (I use both Pyrocat HD and a non-staining developers)

Vaughn said:

A coating of unsensitized gelatin might delay the pigment from washing out in the highlights before the rest of the print can be developed.

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I understand what you're saying, but I think there are several problems with the theory.

A sensitized, exposed gelatin with pigment in it (i.e. the image-forming layer) is more tightly contracted and will release pigment only with severe difficulty. Again, in my work I see very little to no evidence for this happening at any significant scale. An unsensitized gelatin is a much more spacious, flabby matrix and much less capable of trapping something. So the idea of using an unsensitized gelatin to trap a pigment that supposedly diffuses out of a hardened gelatin matrix sounds counterintuitive.

That's not the main problem I have with the theory, though. The main issue is that the implicit assumption is that the tonal threshold problem is created by pigment diffusing out of a gelatin matrix. If you observe affected prints closely (literally closely, microscopically), you'll see that this is not the mechanism at work. The mechanism is very simply the very thin layer of gelatin washing away entirely, taking the pigment along with it. It takes a certain thickness of the matrix to survive the process. This thickness is dependent on several factors, but overall it's significantly larger/thicker for DAS than for gelatin, apparently because the mode of hardening differs between them.

So the solution of trapping diffusing pigment sounds to me like one that belongs to a different kind of problem. Note that what I describe above is consistent with how Calvin explains it in several of his writings and videos, including the ones I linked to in #1. He does not mention pigment diffusion in this context AFAIK.

So that leaves us with the other one, that I do follow:

Vaughn said:

The other factor is just the highlights being so thin, one can get a mechanical removal of the gelatin (over agitation, etc). A layer of unsensitized gelatin might lend a little physical protection from this happening. This is a lesser issue than the diffusion, I believe.

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As pointed out above, we disagree on which one is the lesser issue, but let's step aside that for a moment. With this second theory, which IMO holds much more water, the pressing question I asked in #1 is how an unsensitized layer of gelatin is supposed to help with adhesion. It's unsensitized, therefore it doesn't harden with exposure, and since it sits between the image and the support, as it dissolves, it'll accomplish the opposite from adhesion. This is exactly what my initial experiment demonstrates. So my assumption is that there's something about Calvin's process parameters that makes a fundamental difference. Either it has to do with layer thickness (but I used the same thickness and same gel for the topcoat as he does), or it has something to do with the topcoat not really dissolving away at all. Insofar as Calvin says anything about the mechanisms he believes are at work, it all points towards this unsensitized layer surviving warm water development. It seems to me that for this to happen, it needs to harden. The question is how and when that happens.

Vaughn said:

A third possibility -- a layer of unsensitized gelatin over a layer of sensitized tissue will get partly sensitized from being in contact with the sensitizer in the gelatin below it. This might provide a very thin protective layer over the highlights,.

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Yes, as I said in #3. For me it's reassuring that you arrive at the same conclusion that I did.

Vaughn said:

Pyro negs have been a zero liability for me so far (I use both Pyrocat HD and a non-staining developers)

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Dichromate and DAS are different animals, which is probably why you've not experienced this as an issue. A non-stained negative, especially if it has considerable grain, helps in rendering delicate highlights. It's a bit like printing with a halftone negative, to an extent. That's what I found and documented on my blog some time ago, and interestingly Calvin has included an illustration along similar lines (but without specifically mentioning staining developers, since he talks about inkjet negs primarily) in the e-Book I linked to above. Anyway, I consider the question of staining developers and of the comparison between dichromate and DAS as out of scope of the questions I asked above; they're interesting topics, but I've explored those in depth already and for now don't really have a pressing need to dig deeper into those bits.

At around the 10 minute mark, Calvin explicitly says that the supercoat is a stop faster than the main layer. So it's definitely sensitised.

revdoc said:

At around the 10 minute mark, Calvin explicitly says that the supercoat is a stop faster than the main layer. So it's definitely sensitised.

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I initially thought so as well the first time I watched the video. However, I don't think so. He's referring to the gelatin itself when he talks about the speed difference, which is why he discusses the different gelatin types in the first place. If you look at both the video and the eBook, there's no step where DAS goes into the mix for the topcoat. It's not listed in the ingredients for that layer, he's not shown adding it in the video, and he doesn't mention adding it in the book. I could have imagined him forgetting to include it in one place, but it's systematically absent, which just cannot be a mistake.

revdoc said:

At around the 10 minute mark, Calvin explicitly says that the supercoat is a stop faster than the main layer. So it's definitely sensitised.

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koraks said:

it's systematically absent, which just cannot be a mistake.

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At time 16:20 in his video demonstration, Calvin says "the top layer doesn't have DAS."

At 16:20 he stresses that the top layer is not sensiitized (and no sugar, interesting enough.) -- whoops someone beat me to it.

Makes sense. The top layer of unsensitized gelatin ends up against the final support. Since it is a very thin layer (.9% gelatin), it probably becomes the 'glue' once the print is developed, and with the pigmented gelatin above it it is well protected and does not need to be hardened.

So...it is not a thin coating over the highlights, but a thin coating under the highlights (and the rest of the print, of course). So perhaps the action is to provide more support for the highlights to keep them from lifting off?

Vaughn said:

it probably becomes the 'glue' once the print is developed, and with the pigmented gelatin above it it is well protected and does not need to be hardened.

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yes, supercoat could be just that - a thin glue/goo that protects the delicate pigmented highlights from abrasion when mating with the final support.

Vaughn said:

So perhaps the action is to provide more support for the highlights to keep them from lifting off?

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That's definitely the idea. What puzzles me is that this 'glue' layer is unhardened, and consequently, it's liable to dissolve in a warm water bath, i.e. during development.

It also brings the question why this thin glue layer would be any different, mechanically, from a sizing layer on the support that the image can latch onto. I've done loads of sizing/subbing and substrate tests, as has Calvin, and yet he ends up with the solution being a topcoat on the tissue. So something special is surely happening in/on/with that layer.

koraks said:

It also brings the question why this thin glue layer would be any different, mechanically, from a sizing layer on the support that the image can latch onto.

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The sizing layer won't prevent the detrimental effects (if any) of abrasion at the time of mating as the delicate highlights on the primary support are still unprotected. OTOH supercoat can potentially provide the required protection by taking the brunt of abrasion.

That's an interesting line of thought. Apart from mating, there's the pull-apart phase which may even be more risky in mechanical terms. Mating seems relatively benign; there's a little shear force, but not much, as the water of the mating bath lubricates everything quite effectively. During squeegeeing the tissue and the support are simply pressed together and I expect that this rather mild compression is not a major risk factor.

Things are quite different during the pull-apart phase of development, with significant pull and shear forces potentially exerted on the print. I say 'potentially' because it depends greatly on the amount of water the tissue has soaked up initially, which affects how readily the tissue support will let go of the transferred print.

Interestingly, I've never seen a clear pattern that suggested a relationship between these mechanical forces and highlight retention. Initial soak time does seem to have a bit of an effect in that too long a soak time results in poor adhesion, but this tends to affect the shadows just as well as the highlights (causing frilling etc.)

Thinking about it a little more, I think it's inescapable that the transfer is inherently an imperfect and fairly weak bond - at least considerably weaker than the bonds within the gelatin matrix of the image itself. By making the topcoat an inherent part of the emulsion, the transfer interface should no longer interact with the tonal threshold, solving the problem. The exact mechanism and mechanics involved are still not entirely clear to me, though.

If you've purchashed Calvin's eBook, you should ask him directly what is the role played the supercoat. I assumed that the eBook would have an explanation for the use of supercoat as his webpage for the book says that the book covers theory behind variable contrast and supercoated emulsions.

Raghu Kuvempunagar said:

If you've purchashed Calvin's eBook, you should ask him directly what is the role played the supercoat.

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I did, and he kindly responded.
He confirms that the topcoat is unsensitized, but that DAS is supposed to diffuse into it. I assume this happens during tissue manufacture. I wonder when I made my test batch, if I let the image layer sit for too long before applying the topcoat. I can see how a thin film of dried gelatin could form on the bottom layer as it dries, resulting in reduced diffusion of DAS into the topcoat. Although thinking about it a little more, that's not really how gelatin dries...it doesn't really skin over. Ah well, at least some confirmation on ideas we could already surmise.

Raghu Kuvempunagar said:

his webpage for the book says that the book covers theory behind variable contrast and supercoated emulsions.

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Well, this is not to be critical of Calvin's work, but I think that bit about 'theory' should be put into perspective a little. What Calvin covers in very understandable terms, is the reason why a topcoat might work, at a general level. He discusses the tonal threshold and how it can be reduced/overcome by adding layers that sort of absorb this effect (you could see the supercoat as a kind of sacrificial layer). So in that sense, there's some theory. What he does not do, is discuss the supposed mechanisms at work - and that's likely at least in part because he's still figuring that bit out himself. Here's a brief quote from his eBook:

For the supercoat, the gelatin used in each layer is
very important. I still need to confirm exactly what is
happening. I mentioned earlier that the problem with
making inverted variable contrast emulsions is due
to the lower layers holding onto the upper layers. I
think this is the main factor contributing to the massive
improvement in the smoothness of the tonal threshold.

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I also realized that Calvin and I have rather different approaches towards publishing (and we have different interests, of course). Calvin prioritizes well-formatted, 'mature' workflow descriptions that are practice-oriented and can serve as a cookbook for those looking to replicate his process. He generally does a lot (really, a heck of a lot) of research, both theoretical/literal as experimental, before publishing anything. The resulting publication reports on workflows that he has diligently verified to work well, and I suppose he doesn't find it necessary (or feasible) to extensively discuss the theoretical background. Mind you, the bits he does touch upon are offered in clear and very understandable terms, and I've never noticed any technical incorrectness in any of it so far. I work differently - I publish much more informally, for an audience of fellow experimenters, and much more in a stream-of-consciousness manner. So much of what I write involves failed experiments and lengthy attempts at making sense of what's happening.

There's pros & cons to either approach, but this did make me realize that it would be super interesting to see Calvin publish about his failed experiments and the thick of the research process once in a while. It would help me (and probably many others) to interpret the failures and problems we inevitably run into when we try as diligently as possible to replicate Calvin's work.

koraks said:

He confirms that the topcoat is unsensitized, but that DAS is supposed to diffuse into it. I assume this happens during tissue manufacture. I wonder when I made my test batch, if I let the image layer sit for too long before applying the topcoat. I can see how a thin film of dried gelatin could form on the bottom layer as it dries, resulting in reduced diffusion of DAS into the topcoat.

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Well.. sensitising the supercoat should solve your problem then. Does Calvin give any reasons why that's not advisable?

Raghu Kuvempunagar said:

Well.. sensitising the supercoat should solve your problem then. Does Calvin give any reasons why that's not advisable?

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Yep, he just did - simplicity of the process. He says sensitizing the topcoat works as well. He also says that the diffusion shouldn't be hindered by a semi-dry bottom layer, so that doesn't explain my initial failures.

I'm going to do some more tests to see if I can find a pattern to it.

PS: he also suggests that the gel combination is the probable reason for the failure I've seen, so I need to find a combination that works for this approach.

I wish in his video he had also shown the result he gets without the supercoat and compared the highlights of the two visually.

Indeed; this is one of the tests I'm planning to do.

One interesting question. How thick is the gelatin of a typical highlight relative to the thickness of the supercoat...is one doubling the thickness of the gelatin in the highlights with the supercoat? This would a significant increase.

Calvin quickly showed a handful some examples of supercoating with different gelatins (soft over hard, hard over soft) and mentioned that the tonal transitions change significantly. The differences in the migration of sensitizer to the top coat might be part the reason.

All very interesting -- but so glad I am not going in that direction. I appreciate the sharing of the video -- always good to pick up working tips. I have a pouring station along the same lines -- smaller and not as nice by a country mile. (sheets are 12.5"x19"). I built it over where the bathtub use to be.

I size with matte acrylic polymer, so no. I'll have to see if there is a difference if applied to my way of sizing...