Gelatin

OtherCAS: 9000-70-8Shelf life: 24 mo

Physical Properties

Also known as: Photographic Gelatin, Bloom Gelatin

Gelatin (hydrolyzed collagen protein; CAS 9000-70-8) is the universal colloid binder of silver-halide photography — the transparent, hydrophilic polymer that holds silver halide crystals in controlled suspension to form a photographic emulsion.[1] Every silver gelatin film and paper ever manufactured depends on photographic-grade gelatin. Beyond emulsions, gelatin is the binder in carbon-transfer tissue, the sizing layer in platinum/palladium paper preparation, and a secondary colloid in several alt-process variants.

Photographic uses

  • Silver halide emulsion making (DIY and commercial): Photographic-grade gelatin (typically 200–250 Bloom strength) is mixed with silver nitrate and halide salts to produce the light-sensitive emulsion. The gelatin's specific amino-acid composition controls grain formation kinetics, which determines film speed and grain structure.
  • Carbon transfer tissue: Pigmented gelatin sheet is sensitized with potassium dichromate, exposed, and transferred to a final support — producing the tonally-rich carbon-transfer print.
  • Pt/Pd paper sizing: Gelatin pre-sizing of watercolour paper stabilizes the sensitizer and improves image sharpness.
  • Alt-process colloid substitute: Gelatin can substitute for gum arabic in bichromate-family processes for slightly different tonal rendering.

Practical notes

Supplied in several forms: powder (fine granular, easiest to handle), sheets (traditional form, used in photographic emulsion making), and "Bloom" grades (the photographic-emulsion quality indicator, typically 150–300 Bloom for photographic use).[2]

Dissolution: gelatin powder is soaked in cold water for 20–30 minutes (it swells dramatically), then gently warmed to 40–50 °C to dissolve. Do not boil — high temperature degrades the polymer and reduces emulsion quality. Solutions gel on cooling; keep emulsions warm during coating.

Solutions are microbial food. Add a trace preservative (thymol, sodium benzoate) for extended storage; fresh working solutions last a day at room temperature, weeks refrigerated.

Related compounds

Gum arabic is the other common alt-process colloid, used in gum bichromate. Fish glue and albumen are historical alternatives used in specialty processes.

Safety Notes

Acute hazards

Gelatin has no meaningful acute toxicity — it is food and a supplement ingredient. Darkroom quantities pose no hazard.

  • Very rare allergic reactions in sensitive individuals.
  • Microbial spoilage of aged wet gelatin can cause mild illness if ingested (not a concern for darkroom contact).

Storage & handling

  • Personal protective equipment: None required.
  • Storage: Dry granules or sheets in closed containers. Keep cool and dry.

First aid

Not typically required.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. BOOK Crawford, William. The Keepers of Light: A History and Working Guide to Early Photographic Processes 1st ed. Morgan & Morgan, 1979. ISBN 0-87100-158-6. ↩

Reference databases

Last reviewed 2026-04-22