Gum Bichromate

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Changing Pigments

Posted by on 04 Jan 2009 | Tagged as: Gum Bichromate

Many watercolourists don’t have black paint on their palette – they mix the blacks and neutral grays from other colors as needed. Thus the “muddy effect” of black pigment can be avoided, and black is built by using watercolors’ mixing and transparency properties.

In gum printing the colors are formed and mixed much like in watercolor painting. So I decided to try printing my test negative without any black layers. This time the pigments were Linel brand, made by Lefranc & Bourgeois. Stock pigments were made as follows:

Hortensia Blue 1 g + gum arabic 4 ml
Ruby Red 1 g + gum arabic 3 ml
Helios Yellow 1 g + gum aracic 3 ml

linel_gum-1.jpg Below are all six layers with the Linel pigments. The negative was the same as in the previous test print.
1: Hortensia Blue Stock 5 ml + dichr. 5 ml, exp. 12 min., dev. 50 min.
 
linel_gum-2.jpg
linel_gum-3.jpg
+2: Ruby Red Stock 4 ml + gum 1 ml + dichr. 5 ml, exp. 8 min., dev. 60 min.
+3: Helios Yellow Stock 3 ml + gum 1 ml + dichr. 4 ml, exp. 14 min., dev. 30 min.
linel_gum-4.jpg
linel_gum-5.jpg
+4: Ruby Red Stock 2,5 ml + gum 2,5 ml + dichr. 5 ml, exp. 22 min., dev. 40 min.
+5: Hortensia Blue Stock 2 ml + gum 4 ml + dichr. 6 ml, exp. 11 min., dev. 30 min.


linel_gum-6-sml.jpg
Jalo Porkkala: In Times Past, gum bichromate print.

+6: Helios Yellow Stock 2 ml + gum 9 ml + dichr. 9 ml, exp. 33 min., dev. 30 min.
This is the finished print, with all six layers printed.

A Scale Test

Posted by on 07 Dec 2008 | Tagged as: Gum Bichromate

To get the hang of gum printing it may be a good idea to print tone scales with the intended pigments. This print will be a useful reference when choosing colors and determining exposures and tone rendering.

gum_scales.jpg
A digital step wedge was printed on transparency film and plain paper which was then oiled for translucency. These “negatives” were gum printed (with different exposures) on a sheet of Somerset paper, sensitized with gum emulsion containing ivory black pigment.

Another Gum Try

Posted by on 01 Dec 2008 | Tagged as: Gum Bichromate

Here is another try to achieve results similar to the previous gum test, using only three print layers.

gum-2_curve.jpg
Thought I should make another version of the negative. Here the correction curve makes it somewhat lower in contrast, compared to the previous one.

020908_gum-1.jpg020908_gum-2.jpg
The first layer (left) and the second layer on top of it. Funny how warm ivory black looks on Somerset paper…

020908_gum-3.jpg
All three overlapping layers printed.

The Process

Three stock pigments were made and used diluted for the sensitizer:
1. Maimeri Ivory Black 1 g + gum arabic (14 Baume) 20 ml
2. Maimeri Prussian Blue 1 g + gum arabic 20 ml
3. Maimeri Alizarin Carmine 1 g + gum arabic 20 ml

Coating was made with a foam brush, then evened out with a Hake brush.
Paper was Somerset 200g, preshrinked.
Sizing: Gum + dichromate + water, 1+1+1 parts, exposed 4 min. with Osram Ultra-Vitalux from one meter, washed for 60 min.

Layer 1
Pigment: 1 part Ivory Black Stock + 1 part gum + 2 parts dichromate
Exposure: 18 min.
Development: 30 min.

Layer 2
Pigment: 1 part Prussian Blue Stock + 1 part gum + 2 parts dichromate
Exposure: 6 min.
Development: 70 min.

Layer 3
Pigment: 1,5 parts Alizarin Carmine Stock + 0,5 parts gum + 2 parts dichromate
Exposure: 10 min.
Development: 24 min.

Clearing: Potassium disulfite 5% 2 min.

Multilayered Gum

Posted by on 24 Nov 2008 | Tagged as: Gum Bichromate

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.

gum-1_curve.jpg
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.

gum-1_neg.jpg
The negative
This file was printed with an Epson 1800 onto a sheet of Agfa CopyJet transparency material.

050808_gum-1.jpg
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.

050808_gum-2.jpg
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.

060808_gum-1.jpg
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.

060808_gum-2.jpg
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).

060808_gum-3.jpg
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.

070808_gum-sml.jpg
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.

Sizing Exposed

Posted by on 19 Nov 2008 | Tagged as: Gum Bichromate

Paper meant to carry a gum bichromate image will need shrinking and sizing – at least if more than one print layers are planned. To accurately register the layers the paper should keep its dimensions in each consecutive printing. This can be achieved by shrinking the paper before printing – we do this by soaking the paper in hot water for an hour or so.

The more printed layers (and washes in between), the greater the probability to weaken the paper’s inherent sizing. Finally pigments will adhere to paper fibers and stain them permanently. So most papers need some sort of extra sizing when printing multiple gum.

Perhaps the majority of gum printers use gelatin for sizing, but there are other possibilities too. We thought why not use the chemicals that we have on hand anyway: We coated paper with a mixture of gum arabic and potassium dichromate (with no pigment) in 1:1 proportion and exposed 3 minutes with an Osram Ultra-Vitalux lamp, from a distance of one meter. This sensitized gum arabic hardens with exposure, and after washing the paper in water for half an hour, all soluble gum/dichromate should be washed away. The sized paper will then stand several layers of printing without pigment stain.

This sizing has discolored from using too strong, undiluted sensitizer and too long exposure.

After the exposure the dichromate tends to stain paper slightly – if so, water can be added to the size. With proper gum/dichromate/water mix, suitable exposure and development, the paper tone should keep unchanged.

We tested this “exposed sizing” with our three UV light sources: Eiri 2000W UV exposure unit, array of Sylvania 20W BL-350-UV tubes, and Osram Ultra-Vitalux 300W lamp.

Our method for sizing paper for gum printing

sizing_gum.jpg

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