Gum bichromate is a printing process that was at the height of its popularity at the turn of the 20th century. It is a process of contact printing where the negative must be exactly the same size as the desired final image. The image is formed on a sheet of watercolor paper which is coated with an emulsion consisting of gum arabic, pigment and photosensitive material (dichromate). Placed in contact with the negative, the support is exposed to light from a UV mercury lamp (sunlight is equally effective). Where more light is absorbed (the more transparent areas on the negative), the dichromate will harden and become insoluble; the emulsion under the denser areas of the negative will remain soluble in the water in which the image is developed. While sensitizing the support, it is possible to add pigments of different kinds and colors, and we can also vary the chromatic intensity, and step in during the developing to enhance highlights. This is a process that can give results that are anything but standardized: every image, always strictly produced by hand, is a unique example which can never be reproduced identically. The image gradually acquires detail and depth of color through the progressive application of layers of pigment which become fixed on the paper thanks to the gum and sensitizing material (dichromate). As each new coating is applied, the image takes on form, density and tonal range, while any excess material is washed away by the water to reveal the highlights.

TECHNIQUE
You will need : gum arabic, watercolor paper, a wide flat brush, potassium dichromate, tempera paints, watercolors, or powder paints, large format negatives (18x24 / 24x30...), a UV lamp, a sheet of plate glass larger than the negative you want to print from, two developing trays, a graduated cylinder, disposable gloves. The first step is to prepare the definitive negative (in the format you wish to print). This can be done directly if you are using a slide, or through an intermediate process (internegative) if you are using a negative. Under a safelight enlarge the original positive onto orthochromatic film. When your negative is ready, mix the emulsion to sensitize the paper. All the steps of the process can be carried out in normal light, except for drying the emulsion coated paper where a dark room is required. First prepare the gum, by dissolving 500 g of gum arabic in 500 cc of water. Then prepare the dichromate solution to photosensitize the paper, by dissolving 20 g of potassium dichromate (see Safety Precautions) in 200 cc of water. These solutions (gum and dichromate) can be used for a large number of prints, whereas the quantities given below are just enough for sensitizing 7-8 sheets of paper as supports for a 24x30 cm negative. Take 15 cc of the prepared gum solution and further dilute it with 15 cc of water, mixing thoroughly. Blend into 30 cc of this solution 3 g of pigment (or 2 cm of a tube of tempera paint or water color), again mixing very thoroughly (if you do not add dichromate to this solution it can be kept for several days). Add 7 cc of the prepared dichromate solution, mixing well. Once the dichromate has been added, the emulsion must be used immediately: in a few hours it will deteriorate to the point of being unusable. Use a brush to spread this emulsion on a high quality watercolor paper (grammage 250-300), brushing in all directions to spread a thin layer to the size of your desired format. Peg your paper up to dry in a dark room for a couple of hours. Then place the negative on the emulsioned side of the paper, and with this side up expose the paper to a UV light source. Place the lamp at a distance of 40-50 cm for an exposure time of 5-10 minutes. Since the appropriate distance and times depend on the type of lamp used, it is advisable to use the first few sheets to test the developing process. Before beginning to expose the negative fill 2 developing trays with 3-4 cm of water and put on gloves to protect your hands from the dichromate. Place the exposed paper face up in the water for 2 minutes; then turn it face down for another 5 minutes. In this first bath most of the dichromate will dissolve in the water. Move the paper into the second tray and leave it there face down for at least 30 minutes. This is where the actual developing takes place and it can take longer than the estimated 30 minutes (variable factors include the strength of the emulsion, the exposure time and the workroom temperature. If the image does not develop spontaneously, you may have to use a brush or running water to gently remove unwanted pigment. When the image is developed hang the print up to dry.

SAFETY PRECAUTIONS
Potassium dichromate is toxic and an irritant for the skin and eyes. It can cause serious allergic reactions and ulceration. If absorbed into the body it will attack the immune system. It is also suspected to be carcinogenic. Do not inhale or swallow. Use gloves, protective goggles and mask while handling this substance. See a doctor if potassium dichromate comes into contact with cut or grazed skin.

Luciano Leoni