Tonal Inversion: a Special Case of Pigment Stain

Let's start with an example: a somewhat overpigmented mix of burnt umber produced this step print, with visible tonal steps of hardened gum (steps 1-5) then white (or more accurately, slightly toned) paper (steps 6-7) then stain (steps 8 and up). Some gum printers call this "tonal inversion" because the steps which should be lightest are darker than some of the steps below them. But it's important to realize that the "tone" on the higher steps is just stain, not tone in the usual sense.

What seems to be happening is that steps up to 11 or so received enough exposure to harden some of the gum underneath them; you can see that the paper isn't white in steps 5-8 but is a slight tone, the lightest highlights of the tonal scale of the gum. There is enough hardened gum there to serve as a gum resist to repel the stain, so the excess pigment washes off those areas rather than being deposited as stain. As the hardened gum becomes less dense in steps 9-11, it resists less of the stain with each ascending step, and so the stain is darker on each of those steps. Then starting with step 12, there is no longer any hardened gum to resist the stain, and from there on, the stain is unchanged.

Here's how the same pigment mix looks, printed in an image:

On the left print, the gum emulsion dissolved off the paper due to underexposure; all the "tone" seen on the print is pigment stain. This same effect can result from a coating that is so overpigmented that it flakes off, regardless of the exposure. The lightest areas are where the gum was exposed and partially hardened, creating a gum resist that repelled the pigment stain; the pinkish areas are stain, where there was little or no exposure. The print on right shows the same condition as in the step tablet above, where there is both some hardened gum in a positive tonal image, and the stain on areas of little or no exposure. If one wanted to play with this effect, one could probablylearn to control how much reversal of tone one would achieve by manipulating exposure and development, as long as the coating mix is overpigmented to the right degree to start with.

The remedy? The remedy for tonal inversion is the same as the remedy for any pigment stain that results from overpigmentation: use less pigment. With a less pigmented mix, the inversion simply doesn't appear. For example, you can see the same building depicted above (slightly different image) printed in burnt umber with a less pigmented mix here. Notice that the print that is properly pigmented isn't less definite, it's just less pigmented. This is a common misconception, that a properly pigmented mix is paler than an overpigmented mix; it's not necessarily so. You can print as saturated or deep a color as a pigment will print, without overpigmenting the mix; adding more pigment after that point simply creates problems without giving you any useful increase in density or color saturation.

The example below, I would call a borderline example of tonal inversion. To me, this is just stain, not inversion, since while the print is heavily stained in the light areas, the dark areas haven't flaked or washed off to create a true inversion, so to my mind it's marginal whether it should be included in this section. But the fact is that some gum printers would call this an inversion because what should be the lightest steps on the step tablet are darker than the steps just below. So, in deference to them, I will include this as an example of an inversion. But this unclear borderline between stain and inversion shows how closely they are related.

The left print above is PV 19 mixed with Daniel Smith premium gum. The Daniel Smith premium gum has an odd characteristic not present in other gums I've used, in that the gum seems to dry out or evaporate over time. In this case, the evaporation of the gum increased the pigment/gum ratio to the extent that stain occurred; see for example the red stain in the areas which should be white, such as the top several steps of the step print, her teeth, the top rows of her bracelet, and so forth. The remedy was to add about 30% more gum to the mix, thereby decreasing the pigment/gum ratio. The print on the right shows the corrected mix printed in a tricolor print. It's still a bit too strong, as you can see by the slight magenta tint in the top two or three steps of the step tablet, but obviously much improved over the overpigmented mix on the left.

Here's another example: Both prints are made with M. Graham "neutral tint" paint (a mixture of PG 7 and PV 19) plus a drop of Prussian blue, printed on yupo (plastic sheet). The print on the right is made with twice the pigment in the same amount of gum as the one on the left. In this print you can clearly see the excess pigment that has been deposited as stain, in the top three steps of the step tablet and also in the face and hands. You can also see where areas of the image which should be darker (lips, shadow along edge of jaw) have sloughed off the plastic, leaving white areas there which, against the darker stain, manifest as tonal inversion. The remedy, of course, the way to eliminate the tonal inversion, is simply to reduce the pigment/gum ratio. This set of prints also shows clearly that the idea that pigment stain can't happen on hard surfaces but is only associated with printing on paper, is just that-- a myth.

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Copyright Katharine Thayer, all rights reserved.