Gum bichromate prints a photograph in any color, fusing photosensitive chemistry with painting media. Where black-and-white photography is tinted after the fact, gum printing builds the image in pigment from the start: gum arabic, sensitized with a dichromate and mixed with ordinary watercolor, hardens under ultraviolet light in proportion to exposure; the unhardened gum washes away. Its parentage is contested, like much nineteenth-century invention — Poitevin theorized adding colored dye in his 1855 collotype patent, but John Pouncy patented pigment prints in 1858. Talbot had already imagined light “arrested on a material surface… the work of some Lilliputian artist”; gum printing puts the pigment at the end of that artist’s brush.
Gum bichromate — two formulas
Recipe I
Gum:
350 g powdered gum arabic
hot water to make 1 liter
powdered watercolor pigment, to the color and density wanted
Sensitizer (store in a dark bottle):
50 g potassium dichromate crystals
500 mL warm water
Just before coating, mix 2 parts gum to 1 part sensitizer.
Recipe II
Part A:
20 mL prepared gum arabic solution (14° Baumé)
2–3 g tube watercolor, high pigment density (Winsor & Newton, MaimeriBlu)
Part B:
13 g potassium dichromate — or 25 g ammonium or sodium dichromate
80 mL distilled water at 125 °F
~20 mL room-temperature distilled water, to make 100 mL
Dichromates are toxic and sensitizing — gloves, ventilation, careful disposal.
To print
Coat the paper with the mixed emulsion, dry, and expose under a negative to ultraviolet. Develop by floating the print face-down in water — the unhardened gum lifts away, leaving pigment where the light struck. Build full color in successive coats, one pigment at a time.