Pyrogallol

Developing AgentC6H3(OH)3CAS: 87-66-1Shelf life: 24 mo
Pyrogallol
Image: NEUROtiker — Public domain

Physical Properties

  • Molecular Weight: 126.11 g/mol
  • Solubility (20°C): 400 g/L

Also known as: Pyrogallic Acid, Pyro, 1,2,3-Trihydroxybenzene

Pyrogallol (1,2,3-trihydroxybenzene, C₆H₃(OH)₃; CAS 87-66-1), commonly called pyro, is the classical staining / tanning developer of photography — the oldest developing agent still in serious use, first applied to photographic work in the 1850s.[1] Pyrogallol develops a normal silver image but simultaneously deposits a yellow-brown stain proportional to image density, produced by oxidation polymerization of the developer around each developed silver grain. The stain acts as a density-boosting mask on the negative, extending effective printing contrast — particularly valuable for contact printing on silver chloride papers and alt-process materials (platinum/palladium, cyanotype) that respond to UV transmission rather than visible density.

Photographic uses

  • ABC Pyro / Wimberley's formula: The classical three-stock pyro developer. Solution A = pyrogallol + sodium sulfite; B = alkali activator; C = potassium bromide restrainer. Mixed in ratios just before use; working solution has ~4 minute life.[2]
  • PMK (Pyro-Metol-Kodalk): Gordon Hutchings's 1991 reformulation — the most widely-used modern pyro developer. Balances staining with cleaner highlights than ABC pyro.
  • Pyrogallol-HD: High-dilution stand-development variant using pyrogallol at 1:100 or greater dilution with minimal agitation.
  • Tanning developer for specialty carbon processes: Hardens gelatin in proportion to image density, allowing etching-style selective dissolution for certain art printing techniques.

Practical notes

Supplied as white to pale tan crystalline powder. Darkens rapidly on air exposure — a freshly-opened container looks almost white; the surface yellows to brown within hours. Darkened crystals are still usable but weigh slightly more than their active content (the surface is partially oxidized to melanin-like polymer). Store in amber glass with desiccant for extended shelf life.

Solutions oxidize vigorously in open air — this is why pyro developers are mixed as separate stock solutions and combined immediately before use. Working-strength pyro in an open tray turns dark brown within a few minutes, and images must be developed promptly.

Related compounds

Pyrocatechol is the 1,2-dihydroxybenzene sibling used in the lower-toxicity Pyrocat-HD formula. Hydroquinone is the 1,4-dihydroxybenzene cousin — different chemistry, no staining.

Safety Notes

Acute hazards

Pyrogallol is the most toxic developing agent in common darkroom use:

  • Toxic if ingested or absorbed through skin: pyrogallol crosses intact skin readily. Systemic absorption causes hemolysis (red blood cell destruction), kidney damage, and liver toxicity.
  • Skin sensitizer: repeated exposure builds contact dermatitis in a majority of workers over time.
  • Severe eye irritant: concentrated solution causes painful irritation; extended flushing needed.
  • Suspected mutagen (H341) and possible carcinogen: limited but cautionary animal evidence.
  • Dust inhalation: fine powder is a strong upper-respiratory irritant with systemic absorption potential.

Chronic hazards

  • Long-term skin contact is associated with cumulative liver and kidney stress. Pyrogallol is the developer where "wear gloves every time" is not optional.
  • Suspected genotoxic effects; minimize cumulative exposure.

Storage & handling

  • Personal protective equipment: Nitrile gloves (strict compliance), safety goggles, long-sleeved clothing. A particulate respirator (N95 minimum) is appropriate when weighing dry powder. Work under local exhaust ventilation when possible.
  • Storage: Tightly sealed amber container with desiccant in a cool dark cabinet.
  • Segregation: Keep separate from strong oxidizers and bases.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[3] In brief:

  • Skin contact: Flush with copious running water for at least 20 minutes. Remove contaminated clothing. Seek medical attention — pyrogallol skin absorption is medically significant.
  • Eye contact: Flush with running water for at least 20 minutes, lifting the lids. Seek medical attention immediately.
  • Inhalation: Move to fresh air. Seek medical attention if breathing difficulty or systemic symptoms develop.
  • Ingestion: Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately; pyrogallol ingestion is a medical emergency.

References

  1. 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. ↩
  2. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩
  3. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Reference databases

Last reviewed 2026-04-22