Paper meant to carry a gum bichromate image will need shrinking and sizing – at least if more than one print layers are planned. To accurately register the layers the paper should keep its dimensions in each consecutive printing. This can be achieved by shrinking the paper before printing – we do this by soaking the paper in hot water for an hour or so.

The more printed layers (and washes in between), the greater the probability to weaken the paper’s inherent sizing. Finally pigments will adhere to paper fibers and stain them permanently. So most papers need some sort of extra sizing when printing multiple gum.

Perhaps the majority of gum printers use gelatin for sizing, but there are other possibilities too. We thought why not use the chemicals that we have on hand anyway: We coated paper with a mixture of gum arabic and potassium dichromate (with no pigment) in 1:1 proportion and exposed 3 minutes with an Osram Ultra-Vitalux lamp, from a distance of one meter. This sensitized gum arabic hardens with exposure, and after washing the paper in water for half an hour, all soluble gum/dichromate should be washed away. The sized paper will then stand several layers of printing without pigment stain.

This sizing has discolored from using too strong, undiluted sensitizer and too long exposure.

After the exposure the dichromate tends to stain paper slightly – if so, water can be added to the size. With proper gum/dichromate/water mix, suitable exposure and development, the paper tone should keep unchanged.

We tested this “exposed sizing” with our three UV light sources: Eiri 2000W UV exposure unit, array of Sylvania 20W BL-350-UV tubes, and Osram Ultra-Vitalux 300W lamp.

Our method for sizing paper for gum printing