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

Re: Gum tonal range (was Miracle size for gum)

I've just started looking at responses to my post, and while I appreciated Etienne's cogent response, the next response I glanced at seemed to be responding to something I didn't say at all, so maybe a general clarification would be helpful:

I was not arguing against the use of step wedges, for heavens' sake; I would have thought that was quite clear when I said that the number of steps printed is the useful measure for gum printing (besides, after all, I use and post step prints all the time), but the number of steps printed shouldn't be used to extrapolate any general rule about print density. Step wedges are very useful, especially for quantifying one's own practice. I was simply arguing against the translation of number of steps, or the number of "stops" to a print density range by multiplying by .15 or .30. Hope that point is now perfectly clear.


On Oct 11, 2009, at 11:01 AM, Katharine Thayer wrote:

Goodness, this thread has been confusing. It seems a perfect example of what Judy was talking about the other day; I can't find it now, but as I recall it was the post that made Paul snort a peanut butter sandwich out of his nose. The main idea, written in Judy's inimitably witty prose, was that people who come to gum from platinum (I would broaden that to include silver as well) come with a set of preconceptions that don't translate well to gum. I couldn't agree more, and this thread is a good illustration of what happens when you try to import conventions that work for metal processes into a discussion about gum, where they don't apply at all.

With metal processes there is a well-understood relationship, which can be expressed in an equation that every photographer knows by heart, between exposure, measurable amount of deposited metal reaction product, and measurable amount of light reflection from the print. One follows in a lawful way from the others, and back again (in other words, besides being able to predict the print log density of an area from the exposure, you can also start from the reflection log density and translate it back into exposure). In those systems, it makes perfect sense to say that if six steps are printed from a Stouffer 21-step wedge, that will translatein a lawful way to a tonal density range of log .9, but in gum there is no such relationship between exposure, steps printed, and tonal range, and such a statement is misleading, if not meaningless. The attributed statement "Gum prints two stops" is simply nonsense; it can't be connected to anything meaningful in terms of gum. And then on top of that, there's also a confusion in this thread between negative DR and print tonal range, which has added to the confusion.

Loris has it right. With gum, the print tonal range is more a function of pigment and pigment concentration than of negatives or exposure. The maximum and minimum absolute densities are determined by the pigment and the concentration of that pigment in the emulsion. I have an example on my site of the same pigment (PBk11 --iron oxide black) printed at six different pigment concentrations, with a table showing the measured densities of the lightest and darkest values in the print. The lightest pigment load printed from log .10 to log .60, or a tonal range of .50. The heaviest pigment load, which was deliberately overexposed and then treated with violent methods of forced development to bring out the lighter tones, had a DMax of log 1.00 and lightest tones of .25, or a tonal range of 1.00; however the extreme methods of forced development didn't produce a print with natural tonal gradations, so I don't think it really counts; like a lot of early gum prints it's just mostly dark with some light values forced out of the darkness by power-washing and vigorous scrubbing.

The three pigment concentrations that fall into the range most people would use for most gum applications (the lighter two of these you'd use for one of two or three layers, say, and the darkest for a one-coat gum print) the DMax ranges from .95 to 1.05 and the Dmin from .20 to .30, but in each case the tonal range printed (difference between absolute density for darkest and lightest values) was log .75.

This log .75 is a range of measured reflection density, but that measure of optical density has nothing, I repeat NOTHING, to do with stops of exposure. I'm repeating myself, but it bears repeating; the equation that relates exposure to density of reaction product to optical density of tonal values *does not hold for gum,* and it makes no sense whatever to talk about tonal range in gum, or even steps printed in gum, in terms of stops of exposure.

Yes, the Stouffer 21-step has incremental steps of log .15, or "half a stop" for each step. This means that when the 21-step was printed on the film, for every half stop of increased exposure the density of the silver reaction product deposited on the film increased by log .15 and the opacity of the film also increased by that much. But that relationship, that holds for how the silver was deposited on the film strip, does *not* hold for the printing of the gum through the test strip, and does *not* mean that for every step of gum printed, there is .15 more optical density in the print.

I did that demonstration only with the one pigment, and I gave back the borrowed densitometer, so I can't check to see if that .75 holds for other pigments; I suspect not, since there are few pigments (lamp black is really the only other one) that can print such a dark value) so it wouldn't be reasonable to state as a principle that "Gum prints a tonal range of log .75" (although it would be somewhat more reasonable than saying "Gum prints two stops" which makes no sense at all). The fact is, and my point here is, that it's really meaningless to try to make use of those photographic conventions when talking about gum, on the contrary it's misleading and confusing. To try to establish how many "stops" gum will print is a question that has no answer, a question that makes no sense.

To say many *steps* gum will print, without making the mistake of translating the steps into stops of exposure or into density, is more useful in a practical sense, but as experienced gum printers know, this is influenced by a great many variables and will vary widely across emulsions. So to state as a general rule how many steps gum will print is also something of a chimera; the answer, as for almost everything in gum, is "it depends."


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.


Christina Z. Anderson schrieb:

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?


Christina Z. Anderson