There's a currently fashionable notion that the pigments in the gum emulsion act as color filters and affect the exposure of the gum layer in the same way a color filter affects the exposure of a photographic film or paper. Much of what's been said about the supposed relationship between pigment hue, exposure and curves doesn't square with my experience, and much of what's been said has been contradictory. There have been scant if any actual data offered to support these statements, and I suspect that this is yet another example of the many misconceptions about gum that are passed from one person to another without being questioned or tested, simply because the theory seems to make sense.
The theory as I understand it goes something like this: since we "know" that when choosing colors for colorized inkjet negatives, magenta ink blocks more UV than cyan ink, then it follows automatically that when printing tricolor gum, the blue layer will print faster than the red layer, and since yellow ink blocks even more UV than magenta, the yellow layer will take the longest to print of the three tricolor gum layers. What's more, an orange-red gum emulsion will take longer to print than a bluish-red gum emulsion, and so forth.
They say it's not what you don't know that can hurt you; it's what you know that ain't so, and in this case, both these assumptions are simply wrong. First, the ranking of inkjet colors by UV blocking capability varies by printer and inkset; for some yellow blocks more UV than red or green, for some it's the other way around. For some cyan blocks more UV than magenta, for some it's the other way around. Obviously, if different inksets yield different rankings of colors by UV-blocking, then it is nonsense to say that UV-blocking is a function of the color of the ink and that we can draw inferences about the UV-blocking capability of an ink by knowing the color of the ink.
Consequently it should come as no surprise to discover that the further conclusion drawn from this false premise is also false: there is no reason to suppose, and in fact I have not seen in all my years of gum printing any evidence to support, that the color of the pigment has any particular bearing on the length of the exposure (which would reflect the UV-blocking capability of the pigment) any more than the color of an inkjet ink is the determining factor in the relative amount of UV blocked by that particular ink. We don't know exactly what determines the UV blocking of an ink or pigment, but its visible color is not a determining factor.
First, let's look at the assertion that because the pigments are acting as color filter, the different hue ranges (red, yellow, blue) will require different exposures, simply because of their yellowness, blueness or redness (and the inherent ability of that hue to block UV); the exposures for blue will be shortest, because "blue blocks UV least" and the exposure for yellow will be longest because "yellow blocks UV most."
When I was printing a lot of tricolor using PR 175 (a dark orange red), pthalo or ultramarine, and PY110, all at fairly saturated concentrations on unsized Arches Aquarelle, in a year-round damp climate, my times for all three colors were almost always the same, about 3 minutes. It never occurred to me that I'd be required to defend that practice; I just used exposures that worked (see my tricolor gallery page for a few of the many many tricolor prints I made in those days). And when someone came along recently stating categorically that "blue" "red" and "yellow" require quite different exposures, I shrugged my shoulders and said well, not always.
I'm in a different climatic environment now and using different pigments and a different paper, so my current times can't be compared in any meaningful sense with my earlier times, but just the same, in a different climate, with different pigments and different paper, the times for "blue" "red" and "yellow" are still quite similar. Running tricolor tests with PV19, Prussian blue and PY97, on Arches bright white sized with gelatin and glyoxal, my recent exposure times:
Not much difference between them, but blue is the longest, not the shortest, and yellow is the shortest, not the longest. These exposure times were determined using my method for determining optimal exposure, and employed a constant development time so that the exposures can be precisely compared.
As for the assertion that an orange red requires a much longer printing time than a blue red, I compared PV 19, a blue red, and PR 209, an orange red, and found that their optimal exposures (determined as described on the exposure page) for the mixes I use for tricolor, holding development constant, were exactly the same.
It has also been asserted that different hue ranges require different curves, but my curves generated by ChartThrob were essentially the same curve for each of the three pigments as mixed for tricolor.
To summarize: recent controlled observations are consistent with what I've observed more informally in decades of printing gum: I've seen no evidence to support the currently popular notion that pigments behave in predictable ways in gum printing purely as a function of their hue (wavelength), in other words that "red" "yellow" and "blue" pigments as a group print faster or slower than each other, or require a particular kind of curve, as a function of their "redness" "blueness" or "yellowness." Pigments behave differently based on their chemical and physical structure, strength, saturation, concentration, opacity and other characteristics of pigments, in combination with each other as well as singly, but the hue of a pigment does not reliably predict its behavior with regard to exposure, or for that matter, with regard to any other variable I can think of.
Copyright Katharine Thayer, all rights reserved. December 2007