There's a currently fashionable notion that the pigments in the gum emulsion act as color filters and affect the printing of the gum layer in the same way a color filter might affect 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 is never any evidence offered to support these statements, that I've been able to discover, 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.

Some of the statements that have been made, without supporting evidence, are that an orange-red gum emulsion takes longer to print than a bluish-red gum emulsion, and that red takes longer to print than blue. Neither of these statements is consistent with my observations and experience.

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 hue, and that exposures for blue should be shortest, because blue blocks UV least. When I was printing a lot of tricolor some years ago, 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 about the same, about 3 minutes as I recall. It never occurred to me that I'd be required to defend that practice; I just used exposures that worked. But when someone came along who stated categorically that "blue" "red" and "yellow" each require quite different exposures, I shrugged my shoulders and said well, not always. I thought maybe it was that the red and yellow I was using were deep-valued as red and yellow go, and so maybe similar to the value of cyan, and that might explain why my exposures were so similar for different colors. And in fact when I ran some tests later on different pigment combinations for tricolor, I did find that a very light-valued yellow, such as PY151, did require a different exposure than the darker-valued PY 110, for example.

I'm in a different climatic environment now and using different pigments and a different paper, so my current times can't be compared exactly with my earlier times, except to say that 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: Prussian blue, 3:00 minutes; PV19 (quinacridone rose, 2:45; and PY97 (hansa yellow), 2:15 minutes. Not much difference between them, but blue is the longest, not the shortest, and the exposure for red isn't way longer than the others as asserted.

As to 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, for the saturated mixes I use, 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. In other words, these recent observations are consistent with what I've observed more informally in years of printing gum: I haven't seen much evidence to support the currently popular notion that pigments behave in predictable ways in gum printing as a function of their hue (wavelength). What I see is that pigment mixes behave differently based on their strength, saturation, concentration, opacity and other characteristics of pigments, probably in combination with each other, but that the hue range of a pigment doesn't provide predictive power as to how it will behave, particularly with regard to exposure.