Government Building
By Peter Sundstrom

By Barry Tingey

There are many old photographic processes that have largely gone out of fashion due to a variety of factors.

• The availability of the necessary materials.
• The time you have available to experiment with the processes.
• The lack of a darkroom or the space needed.

It is proposed to present a small series of articles on the older processes that can be reasonably undertaken.

The majority of photographers these days have little idea of the range of processes that are available. The first of the older processes we will look at is the Gum Bichromate Process, which relies on the property of a solution of a dichromate to render insoluble a gum bichromate emulsion when exposed through a negative to sunlight or ultra-violet light.

The Gum Bichromate Process

You have to have a negative of the same size as the print you want to make. With most of us using 35mm it leaves the prints rather small, however there a re a couple of ways that you can get around this.

Have your negative scanned in a photo scanner and then printed out onto a sheet of clear plastic of the required size.

Alternatively, make a contact, or camera copy of your negative and have this scanned out onto a clear plastic sheet as above.

To make a test strip from your enlarged negative, place the negative face down onto a sheet of face up gum bichromate paper, preferably in a printing frame or under a sheet of heavy glass. Make a test strip starting with about 30 minutes exposure to sunlight or UV light, then masking the negative give a normal series of test exposures in proportion to the total time, say adding about ten minutes at a time. Develop the test strip and then see which is the best exposure.

To make your final print, use the exposure you have arrived at. It is a good idea to measure the light levels when you do your test exposure to give you a guide for the actual printing, as light levels can change during a long exposure.

Preparing the Materials

• Use tubes of good-quality water colour paints (colours as required). Use a four inch (100 millimetres) length for strong colours. As your experience grows you will be able to estimate the amount of colour needed. Mix the paint with 25 millilitres of gum solution.
  
• Gum Solution: use 14 degree Baume gum solution from a printing supply house or make up a concentrated Gum Arabic solution (nice & syrupy and really thick).
  
• Sensitiser: dissolve 29grams of Ammonium Dichromate in 25 millilitres of water. When dissolved make up to 100 millilitres with water.
  
• Mix 25 millilitres of the sensitiser with 25 millilitres of Gum Solution which has been mixed with the pigment.
  
• Sizing solution: make a strong warm solution of gelatine. Sufficient gelatine must be dissolve so that the solution sets hard on cooling.

Use Cold Press (rag paper) with a smooth surface and adhere to the following procedure.

• Preshrink the paper in a hot water bath.
  
• Size the preshrunk paper with a good even coating of the warm gelatine solution. Cool and dry. I personally think that it is a good idea to let the paper dry for a day, as this allows more water to dry out of the gelatine layer.
  
• Coat the paper with the emulsion: Brush the Gum, pigment, sensitiser onto the sized paper surface. All coatings should be thin and even. Dry the coated paper in the dark or in safelight conditions. Many experienced workers recommend two or more coats but I think one coat is enough provided that you have used enough pigment.
  
• Expose in contact with the negative in a printing frame. Using sunlight for the time determined from your test strip.
  
• Contact must be close between the negative and the emulsion.
  
• Development: place the print face down in water at room temperature. Change the water as soon as it becomes pigmented and cloudy (with Pigment). Handle the prints with care and make any water movements gently. Sometimes it is necessary to use a soft brush to clean the highlights. The water temperature may be raised slightly if the print is not clearing.
  
• Place the print on a screen or blotter to dry.
  
• A 5% solution of Potassium Metabisulphite may be used to clear prints.