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)

Loris wrote:

I don't think gum has a nonlinear transfer function; AFAIK, on contrary, dichromate processes have a pretty linear transfer function (as long as the dichromate strength isn't at the extremes = not excessively weak/strong), hence the non-linear tones in the resulting print.
It's not a matter of the linearity of dichromate; rather, I believe, it's the physical means by which the exposed, pigmented gum is (or is not) retained by the substrate. This is the same phenomenon that makes non-transfer carbon processes problematic, unless you expose through the substrate. The highlight region of gum prints tends to become much contrastier than the rest of the range (and often with a "granier" appearance, as well), I believe for this reason. Once the density gets to a certain point, gum behaves more linearly for the rest of the scale. Or so it appears to me.

Our negatives are not linear in any sense, therefore, since dichromate processes are linear (or much linear when compared to the "literal" processes - if you like), the translation of non-linear negative densities to print densities are non-linear too.
Well, step tablets are linear, and that is what this discussion has been about -- the transfer characteristic of the *printing* process. So let's not add to the confusion by conflating the negative transfer characteristic with the printing process transfer characteristic. In any case, most modern films are quite linear -- look at the sensitometric data for the T-Max films, for example, which exhibit near ruler flatness for most of their range, unless one manipulates them otherwise.

Said that, it should be noted that dichromate processes are pretty high contrast in the highlights. So when thinking the transfer function as a single entity, it's like a having a stepped / double character; almost having one inclination in the shadows and midtones, another in the highlights.
I do not believe this is true for dichromate generally, although it is true of the traditional gum process (and is the basis for my comment regarding nonlinearity). I have done extensive carbon printing, and extensive sensitometric/densitometric analysis of the carbon process, and I have never found this to be the case. Rather, as I stated above, I believe it is a result of the particular physical process by which a hardened colloid (including gum here), exposed from the non-substrate side, is or is not retained by the substrate.

Best regards,