Here´s an evolution of a multilayered gum print. The final print actually became an exercise of a single negative technique, where the final tone range is achieved by printing several layers on top of each other, using the same or different pigments each time.

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A correction curve for the image
Actually this is more or less “lossy” way of curving an image – some data is lost both in shadows and highlights, but that doesn’t seem to matter too much as regards the final print.

Once again I’m using my long-term “standard test image” of the old tree. There’s been a Photoshop correction curve applied to the positive grayscale image. This time the curve isn’t based on any particular measurings, but is more or less guesswork – its meaning is to flatten contrast, because gum emulsion cannot produce very long density range in a single layer.

The paper was Guarro Casas 250g, sized with gum arabic and potassium dichromate 1+1 exposed for 3 minutes. The light source used in exposing was Osram Ultra-Vitalux, from one meter distance. Pigments were powder pigments by Maimeri. The sensitizer was 13% potassium dichromate solution.

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The negative
This file was printed with an Epson 1800 onto a sheet of Agfa CopyJet transparency material.

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First layer
Sensitizer: Ivory Black 1,6 g + gum 10 ml + dichromate 10 ml, Exposure: 3 min., Development: 3 x 10 min.

The proportions of pigment/gum/dichromate and the time for the first exposure were mainly guesswork. All that came out were the darkest shadow tones. The goal was to get a full scale image (more or less), but the pigment concentration and the exposure time probably were not suitable for this. On the other hand, gum printing is such a “progressive” process that often the print is not ruined after making an unsuccessful layer, but corrections can be made to the next layers.

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Second layer
Sensitizer: Ivory Black 0,5 g + gum 5 ml + dichromate 5 ml, Exposure: 5 min., Development: 3 x 10 min. (The black lines outside the image area were used for registering the negative after each consecutive sensitizing)

Not much better – deeper shadow tones, but middle tones and highlights lacking.

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Third layer
Sensitizer: Ivory Black 0,5 g + gum 20 ml + dichromate 20 ml, Exposure: 14 min., Development: 3 x 10 min.

By using substantially less pigment and exposing much longer the tone range gradually extended.

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Fourth layer
Sensitizer: Alizarin Carmine 0,3 g + Cadmium Yellow Medium 0,2 g + gum 10 ml + dichromate 10 ml, Exposure: 20 min., Development: 45 min.

Warming the general tone with a red layer (mix of carmine & yellow).

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Fifth layer
Sensitizer: Ivory Black 0,5 g + gum 20 ml + dichromate 20 ml, Exposure: 23 min., Development: 45 min.

A little more depth with another layer of black.

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Jalo Porkkala: In Times Past, gum bichromate print.
Sixth (and final) layer
Sensitizer: Prussian Blue 0,5 g + gum 14 ml + dichromate 14 ml, Exposure: 7 min., Development: 70 min.

Finally the shadow tones were deepened and cooled down slightly with prussian blue and a shortish exposure. As the print was considered finished, the dichromate stain was cleared in a bath of 1% potassium disulfite.

Normally you wouldn’t need a stack of six gum/pigment layers to make a print like this. I bet a more experienced gum printer would do the same with three layers. Anyway, the fun lies in interactivity – after each layer you can see what you got, and plan the following layers accordingly.