U of S | Mailing List Archive | alt-photo-process-l | Re: was Miracle size for gum now tonal range

Re: was Miracle size for gum now tonal range

Marek, I'm having trouble following this argument, but I'll start by saying that I don't agree with your initial premise. No, a "typical experiment," or a meaningful comparison between dichromate concentrations would *not*compare them using the same exposure; that would be a pointless comparison. Although I've seen such comparisons made and wrong conclusions drawn from them, that's a case of people not understanding what it means to compare two things in a meaningful way.

So no, I would never start from the assumption that "a typical experiment" would compare the two dichromates by using the same exposure, and in fact I assumed that phritz had done the comparison correctly, by exposing each of the concentrations properly to control for their different sensitivities,in order to compare their tonal range in a meaningful way.

I have a demonstration on my website showing what happened when I compared ammonium dichromate at saturated concentration vs ammonium dichromate cut to 1/5 of saturated concentration, and exposed accordingly (it took 5x the exposure to properly expose the dichromate at 1/5 the concentration). The actual experiment involved five different concentrations showing a linear relationship between concentration and exposure, but I only showed the two outside conditions to save space on the website. The 5x diluted dichromate printed only four steps, vs eight steps for the saturated concentration.

The second misconception is that if you keep adding dichromate, you'll get more hardening, resulting in what, a greater DMax? More steps? Or what? (I'm trying to understand how, in your mind, this argument bears on the question of how dichromate concentration affects tonal range). But the fact is that it's been shown in research studies that beyond the where the colloid has achieved maximum insolubility (I forget how long the exposure was at that point, but not a long exposure) you can keep adding to the reduced chromium by continuing exposure, but it doesn't add anything to the hardening of the colloid. Besides, there's much more dichromate in most of our mixes than is necessary to completely harden the gum, as can be demonstrated by the fact that you get bright yellow (unreacted) dichromate coming off in the wash water after a gum layer is completely hardened.

As I said, I'm not understanding your argument leading to your proposed three conditions, so let me go through it and see where I'm failing to understand what you're saying. You're comparing 1x dichromate with 2x dichromate, yes? If 1x dichromate is usually properly exposed with 1x exposure, which is how I read condition (1) then 2x dichromate should be exposed with 1/2x exposure, not 1x as in condition(3) and I don't see why you would include condition (2) of exposing the 2x dichromate for the same time you would expose the 1x dichromate; that's the meaningless (pointless) comparison I mentioned above. So maybe you could clarify what you're suggesting here, because it's not making sense to me.

BTW, the characterization of gum chemistry isn't quite correct, given what we understand now, but that's not really an issue for this thread.

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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