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An Introduction to the Gum Bichromate Process

By Association Co-Founder Christopher John Ball

About the process:

The gum bichromate process draws on techniques first developed by Alphonse Louis Poitevin in 1855. It was in this year that he patented a process he called 'helio-plastic'. He had found that when bichromated gelatine was exposed to a light, strong in UV, under a negative and then soaked in water; parts of the gelatine stood out in relief to form an image. This was because the bichromated colloid had undergone a hardening process, making it more or less insoluble, in proportion to the action of light.

In the same year he discovered that the bichromated gelatine which had been exposed to light would allow greasy ink to adhere to it, although it repelled water. He used these properties as the basis for a photo-lithographical process. Poitevin is looked upon as the founder of photo-lithography, the carbon process and collotype printing. In 1867 he was awarded the greater part of a prize of 10,000 Francs for the discovery of a permanent photographic printing process.

The gum-bichromate process takes these techniques and adds the ability of the bichromated gelatine to trap a pigment. When exposed to light, in contact with a negative, and ' developed ' in water the resulting raised image was rendered visible because of the trapped pigment. The gum bichromate process creates is a perfect 'alternative' photographic technique for the creation of erotic imagery.

The materials required to make a gum-bichromate print are as follows:
  • Pencils
  • Water colour paper 300g+
  • Gelatin *
  • Formaldehyde ( 37% solution )
  • Pure Gum Arabic ( 12 to 14 degrees baume gum )
  • Standard darkroom trays
  • Contact frame
  • Mapping pins
  • Registration bar
  • Large format negative
  • Ultra Violet light source
  • Tube water colour pigments. N.B. Get the best available and not student grades.
  • Ammonium Dichromate
  • Distilled Hot water
  • Distilled Cold water
  • Glass containers
  • Dark Brown glass bottles
  • Soft brushes containing no metals. Japanese/Chinese style calligraphy brushes, that have been stitched, are best.
  • Thermometer
  • Stirring rods
  • Lots of Patience
* Vegetarian's may want to try alternative vegetable gelatin substitutes.

As stated use a pure liquid gum arabic and the highest quality water colour. You will find that some pigments have greater staining strengths than others, so you will need to experiment between manufacturers. It has been my experience that certain colours may react with the dichromate, in particular chrome colours, cobalt, zinc and viridian.

The pigment and gum are mixed according to the desired intensity of colour.

The following mixtures may be used as a starting point:

Alizarin Crimson = 1 gram of pigment + 18ml of gum arabic
Windsor Blue = 1 gram + 20ml
Cadmium Yellow Pale = 1 gram + 26 ml
Lamp Black = 1 gram + 40 ml
Windsor Green = 1 gram + 20 ml
Payne's Grey = 1 gram + 20 ml
Neutral Tint = 1 gram + 36 ml
Cerulean Blue = 1 gram + 20 ml
Indian Red = 1 gram + 20 ml
Burnt Umber = 1 gram + 20 ml
Cadmium Red = 1 gram + 20 ml
Charcoal Grey = 1 gram + 20 ml

To make the Sensitiser you need:
Ammonium Dichromate: 30 gramme's
Distilled Hot Water: 75mls
Distilled Cold Water: 100mls.

Mix equal parts of the gum pigment mixture and the dichromate solution, remember to stir well.

Preparation of the paper support :


To allow for proper registration the paper needs to be pre-shrunk. Indicate the back, and top, of the paper with a pencil. Then place the paper to soak in tray of extremely hot water for between 30 and 45 minutes. Hang the paper to dry.
Sizing:

Place 30 grams of gelatin, or vegetable variant, in 1 litre of water. Let the gelatin soak for about a half hour and then gently heat the container until the gelatin has dissolved.

Once dissolved place the solution in a tray. Next place a sheet of pre-shrunk paper into the tray and gently rock it. Allow the paper to soak for about 20 minutes.

Hang the paper to dry.

N.B.: If you are going to apply a number of coats of emulsion it is advisable to size the paper twice.


Hardening:
Take the 37% formaldehyde solution and dilute 25ml with 1 litre water.

N.B: DO THIS IN A WELL VENTILATED AREA AS THE FUMES ARE TOXIC.

Soak the paper in a tray of this solution for 15 minutes, then hang to dry.
How to apply:

The gum bichromate solution must be applied with a soft brush that contains no metal.

Using the mapping pins, fix the paper to a drawing board.

Apply the mixture keeping the brush strokes long and moving left and then right to avoid streaking. It is best to apply the solution with one brush then blend it with a dry brush.

The emulsion is not sensitive until almost dry

Dry the paper in the dark but do not be tempted to use heat as this will cause the gum to harden.

 

coating paper 1
coating paper 2
coating paper3
Exposure:

You should expose the materials as soon as ready and develop as soon as exposed.

The negative that will be found to print best is one that is thinner than that usually used for printing via an enlarger, i.e. without strong contrasts.

The negatives are printed in contact using a printing frame and registration pin. Remember to punch holes in the paper.

You can expose the negatives in sunlight but an artificial light source allows for greater control and repeatability.

Unlike the cyanotype process, there is only a slight visible image formed during the exposure stage.

Home made printing frame

 

Development of the exposed image:

To develop the image gently place the paper, emulsion side down, into a tray of cold water. This will allow the soluble gum and pigment, i.e. those parts of the image that have been proportionally less affected by the action of light, to dissolve out of the paper in their own time. This may take between 20 minutes and as long as 6 hours, depending upon the image depth.

Another method is to use a spray gun, such as that used to spray house plant leaves, to gently wash away the soluble parts of the emulsion.

This method works best if you first place the paper, face down, in a tray of water for about 20 minutes.

This method allows for creative development of the image. The highlight tones can be controlled by varying the force of the spray. Apply a stronger spray to remove more of the image.

When you have an image that is as you require, hang the paper to dry. Please take great care as, when wet, the image surface is very delicate. Do not allow other prints to touch each other whilst drying.

Double Coating

The contrasts, and image depth, of the print can be controlled by double coating. After the print has been made and finished, the surface is given another coating and exposed etc again.

For this to work you must use some method of registration

This can be repeated until a satisfactory image has been obtained.

 

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