Carbonized – Notes on carbon transfer highlights and tissue thickness

The fickle issue of highlights in carbon transfer…what Calvin Grier calls the ‘tonal threshold’. The tendency of very thin layers of gelatin not surviving the transfer process, creating a jagged transition from visible tone to paper white. I’ve spent quite some time optimizing it, and now revisit the topic with two factors worth looking into: the wavelength of the UV light used for the exposure and the thickness of the tissue.

I’ve written about the ‘contrast ceiling’ with DAS-sensitized carbon transfer before. A little over a year ago I published that article, and so far the insights I gained in that episode have held up for me.

Until, of course, I bumped my head on that ceiling again. Pretty hard, too.

Remember the portrait of my (then) fiancée, and my borrowing her credit card to get some Stouffer step tablets?

OK, so update: the fiancée is now my wife, and the step tablets have arrived. The Stouffer T2115 is on the right. The one at the bottom is a Danes Picta I already had, but it’s limited to base + 1.3logD (it’s excellent though). Yippi-ki-yay.

More importantly, look at the print itself. Yes, it’s horrible. There’s lots of flagging going on, which explains the wavy lines. Ignore that for a minute; this is a print made on glass as part of testing; I’ll come back to this later. The real problem is the ugly white area on the left side of her face. Note also how step 14 transitions to pure white. Not good!

What’s odd is that I made this print in such a way that really, the tonal threshold problem shouldn’t have been there. But there it was.

So I did a lot of testing. A lot.

Part of the testing revolved around subbing / sizing of the temporary support. This is where the carbon-on-glass experiment shown above came in. I used glass as a benchmark because it’s sometimes said to be kind of a gold standard transfer medium due to the high affinity of gelatin to glass. In my hands, glass performs no better than a properly subbed plastic sheet – and in fact much worse when it comes to flagging and overall ease of use.

The diversion into subbing-land didn’t bring much additional insight compared to my earlier findings. My preferred way of subbing temporary support sheets at this moment is with a 0.2-0.5% gelatin solution, hardened with DAS (added directly to the subbing bath). The only change I did make is a more thorough cleaning/scrubbing of the reused or new sheets using a watery paste of calcium carbonate (‘whiting’) and soap and/or alcohol to remove any grease or gelatin remnants on the sheets.

Anyway, for the highlight issue, the subbing thing didn’t make a difference. It jut wasn’t the cause.

For the life of me, I couldn’t get a grip on this problem. I tested with pretty much every type of tissue I still had, but kept drawing blank highlights. Until at some point, I seemingly randomly hit upon one tissue formulation that worked very well. Success! I thought. So I recreated that tissue, using the same formulation. Fortunately, I’m fairly well organizized these days, so I could easily find the exact ingredients and ratios I used for the successful tissue. And guess what? A newly made tissue of the same formulation did. Not. Work. At. All.

Drat.

So apparently I had overlooked at least one important parameter – because my new tissue did behave dramatically differently from the old tissue I had dug up from about a year ago. Until I realized something very crucial.

Recently, when making tissues in small test batches, I’ve been making small batches of 100ml of glop at a time. I can get juuuust eek out about 7 tissues of 12x15cm from that amount of glop. Not bad! Or is it? Because I then remembered that earlier (as in, last year), I would get only 5 tissues of the same size from the same amount of glop. My tissues had been losing weight!

As you can read in the ‘contrast ceiling‘ blog, I had thought about tissue thickness. Just not well enough. I understood thickness as essentially being a limit to the maximum density, in combination with pigment load. If you expose long enough, you should be able to get the same density from a thick tissue with a lower pigment to gelatin ratio as a shorter exposure from a thinner, higher pigment concentration tissue. Fair enough, but something else is going on just as well, at least with DAS.

As it turns out, a thicker tissue shifts up the tonal threshold. For some reason, delicate highlights are just a little less delicate (physically speaking) if the original tissue was thicker.

The example above contains the evidence, although it’s kind of subtle in this particular test. The detail to look for is step #20 of the Stouffer along the bottom of both images. In the print on the left, step #20 is pure white. On the right-hand print, step #20 shows distinct tone. The transition to white is thus more smooth on this print.

The difference between both prints is tissue thickness, with the right-hand print being made from the thicker tissue. The thinner tissue was a 7% gelatin glop poured to about 0.83mm wet height. The thicker tissue was a 12% gelatin glop poured to ca. 1.1mm wet height. Note that this is a difference on both aspects, so there is really a big difference in tissue thickness. I have no way to reliably measure dry tissue thickness, but I expect the thicker thickness is roughly twice as thick as the thinner variant. I did not specifically test for a ‘critical thickness threshold’.

Here’s one more example, that’s a little more pronounced, using the same tissue formulations for the thin and the thick tissue:

The thicker tissue is on the right again. The tonal scale here extends down to step 14; on the thinner tissue, the print poops out on step 12. This is with the same exposure!

You might argue this proves only that tissue thickness dictates density, and it is not specifically a difference in the highlights. This, however, is not what I see if I plot the curves for both sets of examples above. Look at this:

The pink set of lines is the first example pair, the blue set is the second pair. The round markers are the thick tissues, while the diamond markers are the thin ones.

Note how both blue curves are virtually identical until we get to the highlights. Here, there’s no absolute difference in image density in relation to tissue thickness, except in the highlights. The curve of the thin tissue basically takes a steep dive and then crashes abruptly into paper white. The thick tissue curve slopes down gently and eases its way into white.

The pink curve set of the first example shows a similar pattern in the highlights, although slightly less pronounced. Yet, the ‘missing’ step #20 is clearly visible and if you look at that curve, it also basically bumps into white, while the curve for the thick tissue is more gradual.

Even more puzzling is the fact that in the ‘pink example’, the thick tissue is actually the one showing the lower densities, yielding an overall lighter print (you can confirm this by looking at the image itself). The thin tissue, in contrast, produces noticeably denser midtones and shadows. No, I did not swap both prints! What’s going on here? I’ll come back to that in a further blog.

For now, the conclusion I draw is that a thicker tissue ceteris paribus helps with highlight retention.

By means of verification, here’s that portrait from last year again, printed with a thick tissue formulation and a low pigment load. The pigment load was actually maybe a little too low, but I made this tissue to prove a point, so to speak. It may not be very apparent from the scan, but there’s ever so slight tone down to step 15. Note also the absence of any harsh transitions to paper white in the image area.

Does this mean the problem is ‘solved’? No, and I don’t believe it can be solved in an absolute, binary sense. The tonal threshold will always be there. The trick is to shift it to a point where it’s no longer visible. And tissue thickness helps with that.

The question, though, is why a thick tissue renders better (more delicate) highlights. That’s a bit puzzling – after all, the density is dictated by the thickness of the gelatin layer. And given the same pigment concentration, the same optical density in the final print would imply the same gelatin layer thickness, regardless of how much tissue thickness there once was in the first place, right?

Simplified model of optical density (bottom) vs. gelatin layer thickness (top). Maybe a little too simplified.

Apparently it’s not that simple. I suspect that penetrating depth of the exposing light plays a role. Instead of a neat, contiguous layer, I imagine the ‘layer’ of gelatin more as a carpet of strands, a bit like seaweed covering a shallow lagoon floor, or even a tangled mat of tall grass that has partly fallen over. Maybe a bit like this:

As UV light penetrates the tissue, it may be blocked very early on by a DAS molecule, causing its hardening action there. Or it may travel a little deeper if just happens longer for the photon to hit a molecule and connect with it. The hardened gelatin layer will thus consist of a mesh with lots of voids, more so than a solid mat.

The mesh will be thicker on the side of the tissue that faced the light source. The mesh will overall be heavier in areas where more exposure was received. But the absolute height of the mesh may very well vary with tissue thickness. And as such, it may behave a bit like paper. If the mesh is a little higher, there may be more opportunity for strands of hardened gelatin to interlock, retaining integrity of the mesh as the unhardened gelatin dissolves away during hot water development.

Do I have any proof of this? Nope, not a sliver. I’m really making it up as I go along. But it seems to match my observations while developing carbon prints with highlight problems. The highlights actually retain their integrity for some time during warm water development, but at some point, they disintegrate. They look like a more or less solid layer at the start, but at a certain stage it just falls apart. The thought of a mesh with more height having more strength, despite the voids created by unhardened gelatin washing away, makes sense to me. It’s a bit like buttresses on a gothic cathedral holding the structure together.

Regardless of the mechanism, the overall effect seems to be convincing enough. Size does matter after all. I’ll have to keep pouring somewhat thicker tissues than I’ve been doing these past few months. Keep gelatin concentration on the high side, and pour to the highest wet height easily achievable.

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