Determining Pigment Concentration

The most difficult part of the gum bichromate process for beginners, in my observation, is determining how much pigment to use. Unfortunately, this is not something that can be set out in instructions, or standardized quantitatively, because it varies from pigment to pigment and even from one manufacturer to another of the same pigment. The number one mistake beginners make, in my observation, is to fail to understand the variability of pigment strength and to unthinkingly mix all pigments in the same proportion to gum. If you do that, you'll not have consistency of color saturation across pigments.

I premix gum and watercolor paint in film canisters just to the point where the color is as saturated as it can get. I standardize visually by mixing by eye to color saturation, matching the color to previous swatches of the same pigment mixed to printing strength, rather than to "standardize" arbitrarily by using the same pigment/gum proportion with no regard to the inherent variability among pigments. The first approach standardizes the results; the second approach "standardizes" the process but leads to inconsistent results. It should be obvious which approach makes sense to me.

I then use that stock mix in different ways depending on the kind of print I'm making and what my aesthetic goals are. If I want to make a one-coat gum that gives the greatest tonal range possible, I may dilute the mix somewhat (how much depends on the pigment). For tricolors, I dilute the stock solution considerably because tricolors with each layer printed fairly dark would result in a tricolor that's much too dark, the layers being additive. For the highlight layer of a multiple print, or for work where my goal is a very high-key print with no dark values, I may dilute the stock solution by as much as 10 times. But in every case, I'm starting with a fully saturated color, so I can estimate the effect of various dilutions.

For each pigment you choose, if you spend some time working with it, you will find a range of pigment concentrations from which you can choose a pigment concentration from dark to light for each particular effect you're interested in achieving from that printing (within the value range the particular pigment is capable of, of course).

An understanding of pigment concentration and its relationship to print tonality is key to mastering the gum process. I cover this in more depth in the page on tonality in gum.

Copyright Katharine Thayer; all rights reserved.

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