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)

Katharine wrote:

Goodness, this thread has been confusing.
Amen. Discussions of "stops" always are, because so many people fail to observe the distinction between the *exposure scale* of a printing process and the *density scale* of the resulting print. And you are right, the "more literal" processes (silver, carbon, and PT for sure, and some of the other iron processes) obey a substantially law-like behavior that tempts folks to make this lazy and incorrect gloss. But even with respect to these processes, I believe it is a dangerous conflation (see below).

The same sloppiness affects most proponents of the Zone system, who forget that zones are print densities and often talk of the "zones" of a scene that hasn't been photographed. The whole point of all of the sensitometry/densitometry is that we have significant control we can use to map the various luminance values in the scene to print values (densities) of our choosing. IMO, this is properly done by, first, deciding how much density range one wants in the final print, then working backwards toward the range of important scene luminance values. Not every print demands a reflection density range from 0.01 log to 2.5 log (though it seems that many zonies would die before showing a print that doesn't contain the full range of reflection densities available with the chosen process). Some incredibly beautiful and moving photographs have reflection density ranges of less than 1.00 log placed everywhere within the 2-or so log range of what I'm calling the "more literal" processes.

However, note that the mapping from negatives to prints is rarely 1:1. That is, rarely does one stop (0.3 log) of increased exposure produce one stop (0.3 log) of density increase in the print. Thus, I must disagree to this extent with Katharine's statement:

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.
With this niggle, I generally agree wholeheartedly with Katharine's points.

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.
* * *
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.
Not being a gum printer, I cannot say from experience, but one would hope that the more methodical gum workers would get at least an approximation of repeatable results if they used the same pigments and concentrations from one session to the next. Granted, the specific mapping will be different from the mapping of what I have called the "more literal" processes -- but then again, the "more literal" processes are not monolythic in this regard. A high-contrast silver-gelatin paper will produce a reflection DR of around 2.0 log with an exposure scale of around 1.0 log, while pure PT with no contrast agent, or salt paper, will produce a reflection DR of 1.7 log or so with an exposure scale of as much as 3.0 log. (Of course, everyone's mileage will vary.)

(Now, here's where I risk antagonizing partisans of gum. Honestly, I'm not trying to.)

The real differences, it seems to me, are the low reflection DR of gum prints and the substantially nonlinear mapping of exposure scale to reflection densities that seem to be persistent characteristics of gum. (Actually, it appears to me that it's a bit more complicated than that. I have seen gum prints that exhibit reflection density ranges that appear to me to be in excess of 0.75 log, but in general to my observation, the greater the reflection DR, the greater the contrast and the less smooth and linear are the tonal transitions. A reflection DR of 0.75 seems to be what is achievable by a careful worker trying to get smooth tonal transitions. So, it appears to me that you can have a greater reflection DR, or you can have smoother tonal transitions, but you can't have both at the same time. But again, I am not a gum printer.) I must confess that because of these characteristics, many of the gum prints I see do not impress me particularly. That said, some workers -- many of whom are on this list -- have made absolutely stunning gum prints, and my hat is off to everyone who has persevered with the medium until they can achieve the results they are after. I believe gum may be the hardest alt process to really master.

Best regards,