Gum bichromate

Gum is the process that lets a photographer choose the colour. The image is whatever pigment goes into the gum, so a print can be green, red, blue grey or all three at once. That freedom is the point: gum was made to prove a photograph could be handled like a painting, and the results look handled.

It is also soft by construction. The hardened gum holds no fine structure, and the printer washes part of the image away with water and a brush. Detail dies in that wash, so the picture keeps its shapes and loses its edges. A gum print reads as a drawing at ten paces and as a photograph only up close.

Mix and coat. Gum arabic, a dichromate sensitiser and a pigment go on the paper with a brush. Light hardens the gum wherever it strikes, and the unhardened gum washes off in plain water. Ask for the brushed border, because the coat never reaches the paper edge and the brush marks stay visible.

Stack the coats. One coat gives a thin, low contrast image, so a printer coats, prints, washes and dries again, three or four times, one pigment per coat. Registration is done by hand, so the coats sit slightly out of line and the colour goes muddy or strange. Name the pigments you want, and say the coats are stacked.

Work the print by hand. During the wash the printer pushes water at the surface, or uses a brush, and lifts pigment out of the areas that should go pale. That is why a gum print has scrubbed highlights and drifting edges. Say low contrast, no true black, and visible grain in the pigment.

Use it for a landscape, a nude or a figure study that should look made rather than taken. It also suits a picture that needs a colour a camera cannot produce, such as an olive sky or a red brown shadow.

Leave it alone when detail carries the picture. Text, architecture and product work all come apart in gum, because the process removes exactly the fine structure they depend on.

A platinum print is the other pictorialist print and the exact opposite in handling. Platinum is precise, neutral and matte, with a long smooth scale. Gum is pigment, brushed, coloured and scrubbed.

Design publishes Cyanotype at /techniques/styles/, and that is the nearest relation of all. Both are contact processes printed in daylight on hand coated paper. Cyanotype gives one fixed Prussian blue with no pigment choice and no stacked coats, so it lives with the print processes there. Gum gives any colour you mix.

Cinema publishes Film Grain at /techniques/cinema/ as a texture over a moving image. Gum grain is pigment sitting in dried gum, coarse and uneven, and it changes from one coat to the next.

John Pouncy showed gum prints at the Paris exhibition of 1855 and published the method in 1858, but the process only took hold when the pictorialists revived it from about 1894 in London and Paris. It became the signature print of the salon movement, because a hand worked gum print could not be mistaken for a commercial studio product. Photo-Secession exhibitions in New York from 1902 hung gum prints beside platinum ones, often gum over platinum on the same sheet. Every print is a contact print, so the negative is as large as the picture, and no two prints from one negative match.

The model returns a sharp photograph with a watercolour filter over it. Say the image itself is pigment in hardened gum, the detail is soft, and parts of it are scrubbed away with a brush.

The second failure is a clean rectangle. Say the coat is brushed on, so the edge is uneven, the brush marks show, and bare paper sits outside the coated area.