U of S | Mailing List Archive | alt-photo-process-l | Another attempt to understand (long) (was Miracle size for gum nowton

Another attempt to understand (long) (was Miracle size for gum nowtonal range

Since Marek hasn't answered the questions I posed in an attempt to help me understand his argument, I've kept picking away at it in my own head (one has a lot of time to think while cutting and carrying piles of brush). I'm the kind of person who can't let something go until I understand it, so I just keep puzzling away, with or without help, until I can find a way to make it make sense to me.

Paring away and discarding the false premises that got in the way, like the somewhat mistaken characterization of gum chemistry and the assumption that a "typical" comparison of dichromate concentrations as to tonal range should involve exposing different dichromate concentrations the same amount (which would produce utterly misleading and useless results) I think I've maybe discovered the source of the difficulty, why we're not understanding each other.

My definition of the optimal exposure for a gum emulsion, explained on my page on exposure, requires that the optimal exposure produces, and retains over development, the maximum number of steps that emulsion is capable of printing. If you look at the example I show on that page, you'll see that after what I consider the optimal exposure, more exposure doesn't produce more steps, it just moves the same number of steps farther up the scale, requiring longer development to move the scale back down where it belongs and open up the shadows; the strips shown there were developed for three hours. So when I'm talking about the "correct" exposure for a gum layer, I mean the exposure beyond which more exposure will not get you anything in terms of *either* DMax or the number of steps printed. In other words, at correct exposure, you should get the maximum amount of hardening; you're not going to get *more* of anything by adding more exposure or more dichromate.

Re-reading Marek's post (quoted below) again carefully, especially this statement: "now of course case one will show short scale, because we have learned to underexpose gum with thin negatives" seems to suggest that we are talking at cross purposes about exposure. Of course if someone is underexposing their gum layer, then more exposure would give them more retained steps, but I'm not assuming that anyone is underexposing their gum layer. So we're talking past each other, but by my definition of proper exposure, which assumes that the exposure is right to start with and that you are already getting optimal hardening of the gum layer, exposing more, or adding more dichromate, isn't useful and will just add to the development time without gaining anything in terms of tonal scale.

However, having said that, and having looked closely at my dichromate demonstration which I did some 7-8 years ago before I came to my present understanding of exposure, I do believe that the 1/5x dichromate is underexposed. I exposed it 5x as long as the saturated dichromate, but it seems pretty clear, looking at this from my present understanding, that it's underexposed even at that. So I'm willing to concede that this might not be an adequate demonstration of the principle, and that increasing the exposure even more might well result in more retained steps. Whether you could get it all the way from four steps to eight by increasing the exposure I have no idea, and why anyone would choose to print with such long exposures when a longer tonal scale is available with short exposures, I couldn't hazard a guess.

Not that I'm going to abandon on the spot the well-established principle that more dichromate not only prints faster but also prints with a longer tonal scale; that's been too well established by others for me to abandon it without clear evidence to the contrary, and the early guys who noticed and established that principle also understood well the relationship between dichromate concentration and speed (better than many people seem to understand it now) so I doubt they would make the dumb mistake of exposing different dichromate concentrations the same amount when determining the effect of dichromate concentration on a third variable. But as always, I am open to be persuaded by evidence, and if someone can show that different dichromate concentrations, properly exposed, produce the same tonal range, I will be happy to concede the point.

But even though this discussion started with Marek disagreeing with the long-established principle, it doesn't look as if the comparison he's proposing actually addresses that principle. What he's asking is not "When two different dichromate concentrations are properly exposed, do they produce the same tonal range?" but a different question: "if you grossly overexpose two different dichromate concentrations, to the point that a very long development time (8-10 hours) is needed for either, do you achieve the same tonal range, and is it longer than the tonal range you would achieve with either, with normal exposure (and here I mean what I consider proper exposure) and a normal development time? That's a different question and the answer to it would not address the generally established principle one way or the other. I don't know the answer to that question, but I do know that with saturated ammonium dichromate, overexposure with extended development doesn't get you any more steps than normal exposure and normal development.


On Oct 11, 2009, at 10:42 AM, Marek Matusz wrote:

I think I have to disagree with this statement. Here is my rationale. Let's think about the chemistry of the gum printing. A photon of light interacts with chromium (VI) reducing it to Cr (III), which binds to gum hardening it. So the more light you use the more Cr(III) you produce and more hardening of gum. The same goes with dichromate concentration, the more dichromate you have the more cr(III) is produced at the same amount of light.
In a typical experimnet that would be done to judge the effects of dichromate concentration on gum printing one would compare two test prints exposed for the same amount of time one with base dichromate concentration, second with 2x concentration. But wait, the print with 2x concentration will produce 2x the Cr(III) amount and twice as much hardening. SO the proper comparative example is to expose the 1x chromium concentration for 2x the exposure. Now the development will be long or very long..
SO actually 3 test strips should be compared.
case one 1x dichromate 1x exposure (your usual working scenario), it will require shorter development
case two: 1x dichromate 2x exposure
case 3: 2x dichromate 1x exposure

I have done these and can not find the dfifference between cases 2 and 3, now of course case one will show shot scale, because we have learned to underexpose gum with thin negatives
WOuld somebody show me test prints of cases 2 and 3 showing any difference?
I actually ruined my set by leaving them a bit longer for development and then forgetting about them for a day, so I need to make another set for show and tell.

> Date: Sun, 11 Oct 2009 09:33:57 -0700
> From: kthayer@pacifier.com
> Subject: Re: was Miracle size for gum now tonal range
> To: alt-photo-process-l@usask.ca
> Yes, phritz, this observation corresponds with mine and with the
> observations of observant gum printers through more than a hundred
> years; more dichromate= a longer tonal range (more steps), less
> dichromate= fewer steps and a more contrasty print. And I think
> you've got the right idea, at least part of it, for why that's so.
> See, you aren't at complete odds with gum at all, even though it
> feels that way at the moment; you do have a sense of how it works.
> Katharine
> On Oct 10, 2009, at 1:02 PM, phritz phantom wrote:
> > hi chris,
> > i did tests for tonal range about a year ago and again two months
> > ago. i printed step wedges with saturated and 5% ammonium
> > dichromate. since i don't have a proper step wedge (yet), i can't
> > say how many steps, but for me i get about twice the tonal range
> > with saturated than with 5%.
> > with the (uncurved) chart throb scales and 5% am-di there appr,2.5
> > lines between max. densitiy and white. while with the saturated
> > solution there are 5 or 6 lines between white and black
> > another thing i noticed is that the 5% solution does significantly
> > clear better than the saturated (on the cheap paper i use). i
> > wouldn't really call it stain, but the whites of the 5% sheet are
> > noticably more brilliant also i think the tonal range of the 5%
> > does break off more abrupt in the highlights. maybe it's just the
> > fine highlights that wash off quicker in the development, because
> > of the lesser light sensitivity of the 5% solution.
> >
> > phritz
> >
> >
> > Christina Z. Anderson schrieb:
> >
> >> Loris,
> >> Someone said in the literature recently that gum prints 2 stops
> >> (log .6). I was surprised at how low this was, as I always hedge
> >> my bets and say 4-6 stops, with 6 a stretch. Partly if you don't
> >> clear with pot metabi you can get a false read of maximum black
> >> because the brown stain of the dichromate can read a darker step
> >> when it is not really hardened anymore "goo" on top. So I always
> >> figured that the 2 stop person cleared and found that to be true.
> >> Or maybe was guessing it from the seat of his/her pants.
> >>
> >> But it's all kind of a moot point I suppose once you fit your
> >> negative curve to the exposure/dichromate you use.
> >>
> >> I use a 15% solution of am di (2 tsp to 100ml quick n' easy
> >> approximate).
> >>
> >> So Marek is figuring a 3 stop range, you a 4-5 stop range....
> >>
> >> You say that with weaker di and greater exposure you are getting a
> >> longer tonal range--do you have a picture on the web of that
> >> comparison somewhere?
> >> Chris
> >>
> >> __________________
> >>
> >> Christina Z. Anderson
> >> http://christinaZanderson.com/
> >> __________________
> >>
> >>
> >
> >

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