Gum printing

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Gum printing is a way of making photographic reproductions without the use of silver halides. The process used salts of dichromate in common with a number of other related procceses such as sun printing.

When mixtures of mucilaginous, protein-containing materials together with soluble salts of dichromate are exposed to ultraviolet light, the protein content becomes tanned and resistant to solution in water. The untanned material can be washed away in warm water leaving a hardened, tanned protein negative.

Gum printing using a solution of gum arabic mixed with either potassium or ammonium dichromate. The higher the proportion of dichromate, the more sensitive the mixture. However, increasing the concentration of dichromate also reduces the contrast which is very low at best. The right concentration of dichromate is always a compromise between speed and contrast.

Using ammonium dichromate allows concentrations up to 15% of the active ingredient whereas potassium dichromate is limited to about 10%. Exceeding these concentrations results in deposits of chromic acid in the dried film which ruins any attempts at printing. The greatest sensitivity expressed as an ASA rating is estimated to be about ASA 0.003.

In gum printing, the gum-dichromate mixture is supplemented by high quality powered pigment. The resulting mucilaginous mixture is spread on a suitable base and allowed to dry in the dark. A contact negative the same size of the finished print is then placed on top of the dried coating and exposed to an ultraviolet light source, typically bright sunshine.

Often more than one negative is used to provide detail in all tonal ranges. Using multiple exposures requires very careful registration.

In exposing the paper, the thinnest parts of the negatives will allow the most exposure and cause the areas to be darker. The densest parts of the negative, which require more exposure, will have less colour.

The exposed print is then developed gradually in a succession of trays of still water (approximately ten minute intervals) at room temperature until the bath water is clear. The gum is soft and easily removed at this stage. Gentle agitation with a soft bristled brush can be used to remove pigment in highlights, give a painterly brushed appearance, and to release extra pigment from over-exposed areas.