The gum bichromate process of photography is an old process, and is only suitable for large work, and for subjects that do not need much definition. The process itself is as follows. Cut some sheets of good cartridge paper into pieces rather larger than the negative to be printed from. Prepare a 10 per cent, solution of potassium bichromate and in it immerse the cut paper for from two to three minutes, taking care that the paper is evenly wetted. The immersion may be done in ordinary daylight, as the paper does not become sensitive until it is dry. In a room free from dust pin up the paper by the corners to dry. As soon as the paper is dry it must be kept in the dark, or as carefully guarded from actinic light as silver paper would be. Make up a 40 per cent, solution of gum arabic and filter and mix with it the pigment that is to be used, which would be either ordinary powder colours as obtained from the oilshop, or the water colours sold by artists' colourmen. The latter colours are preferable, as they are usually in a finer state of division. A thin coating of the mixture is then evenly applied to the paper, smoothing out with a large badger brush; dry thoroughly.

The exposure may be timed by an actinometer, but is practically a trifle longer than would be required to make a print in albumen from a negative of similar density. Lay the print face downwards in cold water for half an hour and note the result. If correctly exposed there will probably be by this time a dim outline of the principal objects. Raise the temperature of the water and bathe very gently until the image is well out. Soak for a few minutes in alum and rinse well to remove the bichromate; this is all the fixing required. The paper should not be kept long after sensitising. Some examples of the gum process have been obtained by working up the softened gum with a brush. Carbon tissue allows of similar modifications.