Gum-Bichromate Printing

The gum-bichromate printing process has been around for more than 150 years. The process was derived from a series of origins, the first being Mongo Ponton’s discovery of the light sensitivity of dichromate salts in 1839, second, the discovery by William Henry Fox Talbot in 1852 that soluble colloids such as gum arabic lose their solubility when mixed with a dichromate, exposed to sun light and dried, and third, the ability of adding a pigment to the dichromate and gum mixture, as discovered by Alphonse Louis Poitevin.

Thus the gum printing process is a technique whereby watercolour paper is coated with a base of gum arabic in solution with dichromate salt and watercolour pigment. When the paper is exposed to sun light, the exposed areas become insoluble. The development process simply requires washing out the soluble, or unexposed areas, by floating the print face down in a bath of warm water.

Originating in the 1830’s, the gum printing process was criticized for its lack of tonal scale and image definition. It gained popularity when photographers realized that executing multiple exposures would extend the tonal scale and add depth to the shadows.

Materials

A. A gum arabic solution, a ready for use gum arabic solution is readily available at a local art supplier.

B. 10% Dichromate Solution. This solution is prepared by combining 10 grams of ammonium dichromate with 100 ml of water.

~ Ammonium Dichromate, available at a chemical supply house, is an oxidizer and is toxic, therefore care should be taken when handling this product. A dichromate solution can cause a chemical burn when in contact with skin. Chromium salts are also potential carcinogens.

C. Watercolour pigment, professional grade watercolour pigments that will not fade over time are recommended.

D. Negatives

~ Negatives can be created in the traditional means, using high contrast litho sheet film and

developed according to the manufacturers specifications or with a digital camera and a laser

printed transparency.

~ Negatives should be of three types

1.             1. single continuous tone.

2.             2. single posterization type

3.             3. different densities

 

E. Paper, BFK Rives or Carlyle Japan (a premium watercolour paper is recommended).

~ Preparing the paper.

Presoaking

Paper should be presoaked as to minimize shrinking making it difficult

to register subsequent exposures

Sizing
Sizing the paper with artist gesso (mixed 1:1 with water) will prevent
the pigments from staining the paper in the unexposed areas.

F. Wide soft applicator brushes, mixing bowls, measuring beakers, masking tape, developing trays, push pins, watercolour pencils.

Procedure for making a single print

Preparing the paper. Presoaking the paper will also help in reducing the shrinkage of the paper and assist in registering subsequent exposures. Some watercolour papers will require no further preparation where as others will require the paper to be sized prior to printing in order to maintain a whiteness in the print, avoiding to stain the paper.

Preparing the emulsion. The emulsion should be prepared just prior to use. The quantity of chemicals mixed will depend on the number of prints desired. For a manageable number of prints is six 9” x 12” sheets use the following formula:

15 ml of gum arabic

2 to 3 cm strip of pigment

5 ml of 10% ammonium dichromate solution

Slowly combine the gum arabic and the pigment in a glass or metal mixing bowl, then add the dichromate solution.

Applying the emulsion. Using a pure bristol brush, approximately 2” wide, gradually sweep the emulsion across the paper, first in one direction and then in the other, until a smooth and uniform surface is achieved. The emulsion becomes light sensitive once dry. Drying may be assisted using a fan. When the emulsion is dry to the touch it is ready for exposure. Exposure should take place within a few hours of drying.

Exposing the image. The prepared paper requires to be contact printed with the negative in direct sun light. The exposure time varies depending on the intensity of the sun. A longer exposure time is required in the early morning, late afternoon or winter sun. An exposure time of approximately 5 to 7 min. is sufficient in the summer months where as 15 to 20 min. will be required in the winter months.

Developing the print. This image is then developed using warm water (27ªC) to dissolve the unexposed solution by placing the image face up in warm water until the paper is thoroughly soaked. Once the paper is saturated with water it is placed face down allowing the pigment from the unexposed areas to settle to the bottom of the developing tray. Some of the unexposed emulsion may be brushed away with a soft brush however care should be taken not to brush away any of the soft exposed areas of the print. Once all of the unexposed areas are washed away the print may be hung up to dry.

Multiple Printing. Multiple prints are carried out in the same manner. Push pins can be used to precisely register subsequent negatives. For each additional colour the same procedure is followed as for the first negative. These are the colours that I use most commonly and are similar to the colour separation process used in printmaking. The colours can be varied depending on the subject matter and desired effect. Additional variations can be achieved by the length of the exposure time, the ratio of pigment to dichromate salt to gum-arabic, the temperature of the water during development and by brushing off the unexposed areas while the image is being developed.

Bibliography.

Crawford, William. The Keepers of Light, A history and working guide to early photographic processes. Morgan & Morgan Dobbs Ferry, New York House, Suda. Artistic Photographic Processes. Amphoto Books, New York Photographers’ Formulary, Box 5105, Missoula Montana 59806