Carbon transfer from digital negatives: an odd contrast issue

[koraks]

As part of my color carbon endeavor, I was mucking about with some tests yesterday and I hit upon something I can't quite explain.

TL; DR: I'm not seeing much difference in the dmax I get with a print made at the same time series using one tissue with a low, and one with a high concentration sensitizer.

Here's what I did:

I took two identical tissues from the same batch. Around 5x7", 0.5% India ink, 8% gelatin, 0.25% or so glycerin, 3% sugar.
One tissue I sensitized with 0.5ml 16% ammonium dichromate and a few ml. ethanol. Sensitizer applied with a foam roller, tissue dried completely by running room temperature air over it in a drying box with a powerful fan.
Second tissue sensitized and dried using the same procedure, but 0.5ml 4% dichromate. So effectively only 25% of the dichromate load compared to the first tissue.

Then I exposed both tissues using the same contact printing frame, light source and negative. The negative is a digitally printed job onto screen printing film using an Epson 3880 using its black + yellow inks, with normal ink load (so no additional ink density). Each tissue I printed as a series of test strips, with one-stop increments. The 16% dichromate tissue got exposures of 30 seconds, 1 minute, 2 minutes, 4, 8, 16 and 32 minutes. The 4% tissue got exposures of 1 minute, 2, 4, 8, 16, 32 and 64 minutes. The strips were realized by incrementally covering the tissue with a piece of rubylith and then giving additional exposure.

The negative is a simple series of 10% density increments from 0% to 100% density fashioned in GIMP. No correction curve was applied to this file. Once more: both tissues were printed from the same negative. This is the digital version of the negative; the print onto transparency film is pretty faithful (you'll have to take my word for it; my old Epson actually had a good day when I printed it):

(The rubylith seems to do an OK job overall, but with long exposures (16 minutes and longer) there seems to be some fog due to halation in the contact printing glass and perhaps the tissue itself. While this may skew the results somewhat, I don't think it explains what I think I'm seeing.)

I then processed both tissues in the same way; transferred onto Yupo, dried, and then scanned together in one go. Which looks like this:


(click for larger)
Note the annotations; the second tissue is actually the top one. Sorry about that. I noted the number of minutes (or seconds) of each of the horizontal bands on the left.

Used GIMP to take Lab measurements and recorded the L (lightness) measurement on a scale of 0 to 100 of the darkest column on the left and the lightest (white) column on the right. So effectively I measured dmax and dmin of each of the strips on both prints. I tabulated the values for the 1 minute, 2, 4, 8, 16 and 32 minutes exposures:

4% 0.5ml tissue; L value	16% 0.5ml tissue
Time (minutes)	Darkest value / dmax	Lightest value / dmin	Darkest value / dmax	Lightest value / dmin
1	31.4	94.9	35.5	95.3
2	25.9	93.9	29.3	95.3
4	22.5	93.8	24.2	95.2
8	20.5	93.8	21.4	94.5
16	18.3	92.8	17.8	91.9
32	17.1	72.2	16.5	75.6

Maybe the above is a bit abstract, but if you look at the numbers, you may already spot the source of my surprise. To make it easier, this is what it looks like in a simple line-connected scatter plot:

Number of minutes on the horizontal axis, L-value in % on vertical axis.
The orange set of lines is the 16% sensitizer tissue, the green set of lines are the 4% sensitizer tissue measurements. The top lines are low density (high values) and bottom set of lines are the high density (darkest values).

Alright, so the obvious observations are:
* Both tissues respond in a logarithmic fashion to increased exposure. No surprise.
* At around 16 minutes, the tissue starts to build density through the black parts of the negative. In other words, from there onward, the blocking power of the ink in the high density parts of the negative is no longer sufficient to make a 'paper white'. The only thing surprising about this is that it actually takes so much exposure to print through this pretty normal amount of ink. Not bad for cheap transparency film.

A much less logical but very observation is: there seems to be only a minor difference between the maximum density of the 4% and 16% tissues at the same exposure times, although there is a factor 4 difference in sensitizer strength. Huh? Yes, there's a difference. The 16% tissue makes slightly higher dmax for the same amount of exposure. It's a visible difference in real life, but you actually have to look pretty hard to spot it. In other words: it's quite minimal. That is, relative to my expectations.

Is this a result of messing with inkjet digital negatives, which turn out to behave more like halftone screens than actual continuous tone negatives? In which case I'd be surprised, because the maximum density strip in the negative is not a 100% perfectly even density either, and I don't see very apparent dot gain issues in that band - they should have been there if the system did behave like a true halftone screen setup.

Or is this due to the self-masking effect of dichromate in gelatin? That the 16% tissue is trying (in a way) to make more density, but the exposed dichromate itself inhibiting this?

So here's my question: how would you explain this minimal difference between these two sets of density measurements especially in terms of the highest density measurements for each exposure time? I would have expected that the 16% tissue would create far more density at the same exposure time than the 4% tissue. Turns out it doesn't...!?

In case you're wondering: yes, there is a difference in response curve, and that's pretty significant. Here is what it looks like:

For this, I measured the density of each vertical band in both prints for the 8 minute exposure. I then normalized these values; that is, I mapped the readings onto a 0-100% scale. The orange plot is the 4% tissue, the blue one is the 16% tissue. Note that you can no longer make an absolute comparison between the tissues based on this plot, but the difference in curve shape seems quite significant.

Well, blame on me for not having done more systematic testing before - or not paying sufficient attention when I did. But this is kind of surprising to me. I kind of feel rather stupid for not having spotted this before, I must say.

Am I going crazy? (I mean, with regards to this test; I perfectly well know I am in general, you see.)

 

[Vaughn]

My first thought is how accurately can you measure 0.5 ml? And how precisely can you dupilcate the coating of the tissue to eliminate error?

 

[koraks]

More than sufficiently accurate for the inevitable marginal errors not being the explanation of this phenomenon. I've been making hundreds of test prints over the past few weeks and the printmaking process is quite consistent.

Mind you, we're comparing two tissues of which one of them has 4 times as much sensitizer as the other, and they print nearly the same in terms of dmax. I can't explain that from the typical 20% worst case measurement error when pipetting 0.5ml from a liquid, you see?

 

[nmp]

What it means is you hit the "global" Dmax at 4% dichromate. At that concentration the sensitizer is able to fully crosslink (beyond the so called "gel transition" at which point the matrix beomes insoluble under the development conditions, given an appropriate UV dose.) Using any higher percent may change the pathway to the full density but not the value of it. If you measure the density of the unprocessed film itself, it should be near the Dmaxes of the two samples or thereabouts - I am guessing.

If you want to see a difference in the end point, perhaps you can go the other way - i.e. 4%, 2%, 1%...

:Niranjan.

 

[koraks]

Thanks for chiming in, and this is exactly the kind of explanation I'm looking for. Although it does surprise me mighty, because 4% at 0.5ml is very little to begin with. I had already planned (but not yet gotten round to) going further down like you said, so that's the way forward for sure. At the least, that would help in confirming your view, which in itself is valuable.

The nasty implication of this is that I'll probably be facing rather long exposures for these tissues - or I have to do a serious job at a light source instead of the half-assed one I've done so far. Mind you, this light source is already >2 stops brighter than the big bank of UV tubes I used before...

Anyway, thanks for your insight, it's very much appreciated indeed!

 

[nmp]

You are welcome. As long as I don't have to do any of the work....

Another interesting thing to note from the data table is that 4% has consistently higher density (low value, darker) than the 16% - all the way to 16 minutes when they flip to the other way around. One would have expected 16% to give darker readings at lower exposures too. You can actualy see it pretty clearly on the two scans. The 4% looks much darker overall than the 16%, even after accounting for one stop higher exposure. I don't know if that is statistically significant or not. The Dmin relationship is also similar.

Also, per my reading, somewhere between 8 and 16 minutes, say 12, seems about the right exposure time based on best combination of Dmin and Dmax. Anything above, you would start to get stains.

:Niranjan.

 

[Vaughn]

More than sufficiently accurate for the inevitable marginal errors not being the explanation of this phenomenon. I've been making hundreds of test prints over the past few weeks and the printmaking process is quite consistent.

Mind you, we're comparing two tissues of which one of them has 4 times as much sensitizer as the other, and they print nearly the same in terms of dmax. I can't explain that from the typical 20% worst case measurement error when pipetting 0.5ml from a liquid, you see?

I use 3M rubylith tape -- one layer will not always block all UV on long/heavy exposures...I double-up.

Have other practitioners used such a high concentration of dichromate? I am not familiar with color printing. The highest I have worked with in monochrome is 12% (5ml/100sq inches).

I have always felt that the significant contrast change due to increasing sensitizer strength is found mostly in the higher values. Some small change is expected in the minimum exposure required to achieve a black (slightly greater printing speed), but increasing the concentration makes more dichromate available to the little UV light that makes it through the negative in the highlights. Percentage-wise, this change (more gelatin hardened) is greater in the highlights than in the darker areas, and contrast is lowered.

One other question. Are you planning on standardizing on one percentage? People working with digital negs seem to do so.

 

[koraks]

Another interesting thing to note from the data table is that 4% has consistently higher density

Yes, it's puzzling, but I don't attribute too much to this because it might at least in part due to inaccuracies.

Also, per my reading, somewhere between 8 and 16 minutes, say 12, seems about the right exposure time based on best combination of Dmin and Dmax

Yes, you can determine optimal contrast range by subtracting some values - but its key to navigate between good dmax and good dmin. 12 minutes is close to fogged whites and I found that transposing the same parameters to, say, a magenta tissue or a cyan one, especially shifts doen the point where highlights fog. I'm on the fence which parameters I'll make specific to the various color layers and which to standardize. It's a balancing act, or a set of compromises if you will.

I use 3M rubylith tape -- one layer will not always block all UV on long/heavy exposures...I double-up

Well, I doubled up for most of the area and particularly for the parts that received short exposure so they wouldn't fog while being blocked off, but I didn't note any signs of rubylith-caused fogging I must say.

I have always felt that the significant contrast change due to increasing sensitizer strength is found mostly in the higher values.

My data so far very much support this. An increase in highlight contrast, but effectively a decrease in the shadows. It also seems to match my more subjective experiences with normal negatives and varying dichromate concentrations.

Have other practitioners used such a high concentration of dichromate?

Keep in mind it's only 0.5ml across roughly 35 square inches. So the concentration may seem high, it really isn't if you think about it.

Are you planning on standardizing on one percentage?

Certainly, this is part of the preparations for it. There are several working points I could choose, and I already did it in one iteration, but decided to see where the possibilities for optimization are. It's a long story; part of it is already on my blog, part of it is work in progress.

Keep your thoughts coming gents, they make sense!

 

[Vaughn]

Agreed

The math has me sensitizing at a much greater amount, with me using 5ml of sensitizer for 100sq inches and you a tad less than 1.5 ml per 100 sq inches. Unless my thinking is wrong, the difference between using 4% and 16% should be the same amount of difference we see between 4% and 1% (just in the other direction). But considering the small amount of sensitizer volume you use and going down to 1% (or lower), you might be skating on the edge and I do not know if you will run into some limitations.

How much carrier (acetone/alcohol) do you use with the 0.5ml of sensitizer?

I use an unusual high amount relative to most people, it seems. 1 part to 3 parts, so a 9x11 tissue would get 5ml of sensitizer mixed with 15ml of acetone. Part of the reason is that I pour thick tissues and it seems to help get the dichromate evenly and deep into the tissue before all the acetone evaporates. Another is that I usually sensitize 12.5"x19" tissues and the volume is easier for me to spread (w/ roller). The other part is just my normal crazy wishful thinking...and the liking of the smell of acetone.

 

[koraks]

That's right Vaughn, going lower than 4% @ 0.5ml would take me in the very low regions. My experience with this (years ago I went there a lot; it's a long story) resulted in numerous issues. In principle, it worked, but it just isn't optimal. Smooth tonality comes only at moderate and higher concentrations.

How much carrier (acetone/alcohol) do you use with the 0.5ml of sensitizer?

A few ml, probably 5 or 6 or so. Enough to get an even coating on the tissues. It doesn't appear to be critical, as long as it's enough for evenness and not too much to make drying annoyingly slow. The latter is my main reason for using only ethanol for the mix. Acetone evaporates a bit (too) quickly to my taste. It does smell really nice though, I agree! But plain 96% ethanol is also cheaper...

 

[koraks]

I've extended the test with 1% 0.5ml sensitizer. This is what the plot looks like now:

The dark navy & black pair is 16% 0.5ml, the orange pair is 4% 0.5ml and the blue pair is 1% 0.5ml.

This is still consistent with what @nmp suggested: that beyond 4% nothing much happens because I've already hit the limit of how much of the gelatin will harden for any given exposure, regardless of how much additional sensitizer is dumped into the tissue.

Looking at the 1% 0.5ml pair we also see a much more logical pattern with lower maximum densities and a clear optimum in printable contrast at around 8 or 16 minutes, beyond which additional exposure simply doesn't add much anymore. This in itself is also interesting, BTW, because it implies that low sensitizer concentrations struggle with reaching high printable densities. This also matches my earlier experience with silver negatives.

I might add concentrations in-between 1% and 4% as well to get a feeling for how things develop, but from this my preliminary conclusion is that while sensitizer concentration does indeed influence contrast, the useful bandwidth in which this can be employed is rather limited. Very low levels aren't very practical and give all sorts of density and tonal distribution problems, while high concentrations perform similarly in terms of dmax, but with an increasing problem with non-linearity as sensitizer strength increases. Overall, the most effective ways of manipulating contrast are still tailoring the negative and varying the pigment concentration in the tissue.

All this is very interesting for me, although I bet I'm merely reinventing the wheel here. I should really get Sandy King's book, but I've been putting it off for some reason. Silly me.

 

[Vaughn]

A few ml, probably 5 or 6 or so. Enough to get an even coating on the tissues. It doesn't appear to be critical, as long as it's enough for evenness and not too much to make drying annoyingly slow. The latter is my main reason for using only ethanol for the mix. Acetone evaporates a bit (too) quickly to my taste. It does smell really nice though, I agree! But plain 96% ethanol is also cheaper...

That is about what I use then. 15ml for 100 sq in is pretty close to 6 ml for 35 sq in.

For fun I did the quick math -- Using a 5ml of an 8% solution, I use 0.4 grams of Ammonium dichromate per 100 square inches of tissue.

I usually recommend to new printers to standardize their process around a 4% sensitizer. That way they have room to move up or down in concentration without bumping up against the extremes.

I determine my base exposure time on achieving the first black through the rebate of the film (or base of inkjet material)...the most common way users of digital negatives use to determine their exposures for their testing -- pigment concentration, pigment type, tissue thickness, sensitizer type/strength, and UV source will all affect the amount of exposure needed to get a black. I have no idea on how one determines that with color pigments that don't go black!

 

[koraks]

I have no idea on how one determines that with color pigments that don't go black!

Uhm. Yes. That's one issue I'm still trying to figure out

There's a couple of approaches I can imagine:
* Determine what the density is required to give decent (subjectively determined) saturation on the primaries and secondaries.
* Determine at what point a convincing black (again, subjectively determined) is achieved when mixing C, M and Y.
* Trying to find some kind of 'objective' or industry standard for density and aim for that.

I already determined that in principle, it's also a matter of taste or goals. For instance, I've done quite some testing with tissues and exposure parameters that gave rather subtle/muted colors because the dmax for the C, M and Y tissues was effectively rather low. This actually gives rather nicely subtle hues, which for some images might work very well indeed. Yet, I don't think that's a proper route as you can always mix yourself downward in saturation (although the more saturated the tissues are, the more difficult it becomes to accurately mix subtle intermediate hues), but given low color density across the color layers, you can never mix upwards in terms of chroma.

Btw, for B&W it's also not necessarily so straightforward - as you've also found out, probably many moons ago. You seem to be getting very nice prints with very low pigment loads and very long exposures (and as a consequence, high relief). From a density viewpoint, it's just as 'correct' to pick a higher pigment load and adjust the other variables accordingly. This demonstrates that to a large extent, selecting a working point is really a balancing act that's personal in the sense that you pick some things you want to optimize, and that choice will likely vary from person to person. In the end I think that color isn't necessarily different.

I'm currently considering going up in pigment load in my color tissues to create a steeper response curve, thereby sacrificing relief. I'm not to concerned about less relief; in fact, relief is also a bit of a liability, as more relief tends to create more problems with multiple transfers.
On related note, Vaughn, I was wondering why you actually pour your tissues so high? It seems to me they take a lot of time drying this way. In the end, what determines contrast and relief in a tissue once it's dried is the ratio between gelatin and pigment. So wouldn't you get the same thick tissues you're using by making a glop that's extremely heavy in gelatin (I don't know what the practical upper limit is; never tried it) and low in pigment? Possibly using a higher glop temperature, it would still make for a reasonably low viscosity glop that would set rather fast and also dry quicker than your tissue does now. As I recall, your gelatin concentration is also around 10% or so. Have you investigated rising that considerably?
I came to think of it because I intend to do the opposite for my color tissues. Since I don't need the relief, but the contrast is nice to have, I was thinking to actually make a very thin glop with maybe 4% of gelatin and only slightly less pigment than I'm using now. This would dry out to a dry tissue that's roughly half the thickness of my present tissue (8% gelatin) while making for comparable mechanics in pouring them. Pouring a thinner tissue simply by using less glop per tissue wouldn't work out as the 20ml I'm using for a tissue is the lower limit of what will spread out evenly. Hence, to make a thinner (but higher contrast) tissue, I'd need to drop the gelatin content. I'm curious what you think of this, even (especially!) if you would prefer to approach it differently.

 

[Vaughn]

Interesting way to approach relief. I have been using 12% gelatin for awhile.

My guess it is the total amount of gelatin used per sq inch, rather than the total volume of glop (gelatin, water, sugar, pigment) per sq inch that will determine the amount of relief. Then it just a matter of how much water one includes in with that amount of gelatin. less water -- less drying time is the only major difference. Relief should be about the same.

Like you, I find the volume I use to pour a 12.5x19 tissue (290ml) to be a very easy one to pour and get nice tissues (I pour free-hand). When I was using fixed out 14x17 x-ray film as the tissue support, it was instructive to hold the dry tissue up to the light and check for even pours and well-mixed glop. Thinner tissues and using threaded rods, etc does make the smaller volumes easier to pour.

It would be interesting to see how the relief differs as well as drying and handling. The low pigment levels seems to help keep detail in the brightest highlights, since the same amount of carbon is enbedded in a thicker layer of gelatin, and less likely to wash out (my theory). But when one enjoys working at the extreme ends of a process, one finds that what usually happens doesn't, and the other way round. Increasing gelatin concentration and reducing the pour amount might work as expected, but may not.

 

[koraks]

My guess it is the total amount of gelatin used per sq inch, rather than the total volume of glop (gelatin, water, sugar, pigment) per sq inch that will determine the amount of relief. Then it just a matter of how much water one includes in with that amount of gelatin. less water -- less drying time is the only major difference. Relief should be about the same.

Precisely!

I also pour freehand, which means that glop viscosity and surface tension are the parameters that determine the thickness of the wet tissue. I never noted all that much difference in viscosity depending on glop temperature (within the reasonable bounds of let's say 37C and 50C), and I don't think I'd want to try and change its surface tension (too many problems might pop up elsewhere), which effectively leaves modifying total solids content (especially gelatin) as the means to vary dry tissue height while accepting that wet tissue height is going to end up roughly the same.

The low pigment levels seems to help keep detail in the brightest highlights, since the same amount of carbon is enbedded in a thicker layer of gelatin, and less likely to wash out (my theory).

And a widely accepted theory at that, as far as I know. Some formulate it in terms of adhesion (e.g. Calvin Grier), but it all boils down to the notion that very flimsy bits of tissue just won't survive processing. Indeed, this puts an upper limit to acceptable pigment concentrations. I acknowledge I've been stepping over this issue of highlight integrity mostly - deliberately, too. In the end, it's a fundamental problem; the only choice we have is to what extent we're willing to accept loss of delicate highlights in the knowledge there is always going to be some loss of them. All this has been the main reason for me to lean towards lower pigment loads, btw, but I also recognize (and have experienced) the problems associated with too low concentrations. This still leaves a pretty large stretch of middle ground that I'm trying to navigate. Hence the tests; I sometimes dive into a particular issue and try to develop a feeling for how it works, and periodically evaluate all of it and see what kind of choices there are for setting the basic parameters for a printing process in terms of pigment concentration, sensitizer strength, negative density scale, exposure, etc.