Gum Arabic

SensitizerCAS: 9000-01-5Shelf life: 24 mo

Physical Properties

Also known as: Acacia Gum, Acacia Senegal, Kordofan Gum

Gum arabic (acacia gum, a complex polysaccharide; CAS 9000-01-5) is the colloid base for gum bichromate printing — one of the oldest and most painterly alternative photographic processes.[1] Chemically it is a water-soluble polymer of galactose, arabinose, rhamnose, and glucuronic-acid units, harvested from acacia trees and used in photography, printing, food, pharmaceuticals, and adhesives for centuries. When mixed with watercolour pigment and a dichromate sensitizer, gum arabic becomes the UV-hardenable binder whose selective crosslinking produces the gum-bichromate image.

Photographic uses

  • Gum bichromate sensitizer base: Mixed in roughly equal parts with pigment + ammonium dichromate or potassium dichromate solution, brushed or coated on watercolour paper, exposed under UV through a negative, and washed out in water.[2] The unexposed gum dissolves; the exposed (hardened) gum retains pigment in proportion to the exposure received.
  • Multiple-coat gum printing: Serious gum-bichromate printers apply 4–8 coatings in different colors + pigments to build rich painterly images with extended tonal range and colour control unavailable in any silver process.
  • Sizing for Pt/Pd paper: A dilute gum-arabic pre-coat stabilizes fibrous watercolour paper for platinum printing, preventing sensitizer migration during coating.

Practical notes

Supplied as amber-yellow "tears" (the raw tree exudate), as fine ground powder, or as a pre-mixed 40–50% aqueous solution. Food-grade gum arabic (from culinary suppliers) is indistinguishable from chemistry-supplier grade for photographic purposes.

Dissolution of raw gum is slow; dried gum in water takes hours to solubilize. Most workers use a pre-mixed solution, or soak the dry gum overnight in water with occasional stirring. Solutions keep for months in closed refrigerated containers; room-temperature solutions may ferment after weeks and develop an unpleasant smell (sign to discard).

Related compounds

Gelatin is the colloid used in silver-halide emulsions and carbon-transfer tissues — different chemistry, different UV response. Gelatin can substitute for gum in some bichromate-family processes (gum-gelatin hybrid).

Safety Notes

Acute hazards

Gum arabic has no meaningful acute toxicity. It is a food additive (E414) used in confectionery, baking, and as an emulsifier in beverages. Darkroom quantities pose no hazard.

  • Very rare allergic reactions in sensitive individuals.

Storage & handling

  • Personal protective equipment: None required.
  • Storage: Dry powder or tears in a closed container. Solutions refrigerated or with a trace preservative (propylene glycol, thymol) to slow fermentation.

First aid

Not typically required. Rinse with water for any contact.

References

  1. BOOK Farber, Richard. Historic Photographic Processes: A Guide to Creating Handmade Photographic Images 1st ed. Allworth Press, 1998. ISBN 1-58115-024-4. ↩
  2. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩

Reference databases

Last reviewed 2026-04-22