Pyrogallol

Every other developing agent in this encyclopaedia is judged partly by how little of its oxidation product ends up in the film. Pyrogallol is the one where the oxidation product is wanted, printed through, and counted as density. Everything strange about pyro follows from that.

In photography

Where it sits among the agents. Kodak’s 1928 primer ranks the developing agents by reduction potential — how much bromide is needed to hold them back — and puts hydroquinone lowest, then Athenon, then pyro, then Kodelon, then Elon highest. Pyro therefore behaves like the low-potential end: highlights first, shadows filling in once the highlights are established. The primer also states the trade-off directly, describing hydroquinone as somewhat less powerful a reducing agent than pyro but with less propensity to give stain.

The stain is the point. The primer’s account of the sulfite is written around pyro precisely because pyro shows it best: the oxidation product of pyrogallol is yellow, it is deposited in the film along with the silver, and a pyro developer without sulfite gives a very yellow negative whose image consists partly of silver and partly of oxidised pyrogallol. Add sulfite and the image is much less yellow, because the sulfite is oxidised instead. Add a great deal of sulfite and you get almost as blue an image as with Elon. One reagent, one dial, and the whole range from a stain image to a plain silver one.

Bergger’s current datasheet for PMK, a pyrogallol developer adapted to modern films by Gordon Hutchings in the 1980s, describes the same effect as a feature rather than a fault: “a yellow-green tint surrounds each silver grain and fills the usually empty space between them and becomes an intrinsic part of the image”, so that “the density of a pyro negative is therefore the conjunction of two densities, that of the silver and that of the coloration”. Bergger claims increased effective film speed and printing quality, more pronounced definition and reduced grain from that second density.

Everything downstream has to respect it. Bergger’s processing instructions are unusual, and each oddity protects the stain. A pre-wet of three to five minutes is called essential, and its absence gives uneven development. The stop bath is to be non-acid. The fixer must be “a non-tanning bath”, and Bergger recommends its own acid-free product. The wash runs 20 to 30 minutes because the colouring intensifies during washing, and a hypo eliminator must not be used because it weakens the colouring. Kodak’s primer gives the printing-stage consequence from the other end: reduce a pyro-developed negative with ferricyanide and the silver goes while the stain stays, so the negative looks yellower as it is reduced; reduce it with permanganate and the stain is attacked in preference to the silver.

What more or less of it does. Kodak’s D-1, the standard A.B.C. pyro, carries 60 g of pyro per litre of stock A with 9.8 g of sodium bisulfite and 1.1 g of potassium bromide, 105 g of sodium sulfite per litre in B and 75 g of sodium carbonate per litre in C. Mixed for tray development at one part of each with seven of water, the working solution holds 6 g of pyro, 10.5 g of sulfite and 7.5 g of carbonate per litre; the tank version dilutes the same three stocks with eleven parts of water instead of seven and runs 13 to 15 minutes at 18 °C (65 °F) against 7 to 9 in the tray. PMK is the modern shape of the same idea, two concentrates diluted 1+2+100 — one part of A, two of B, a hundred of water — with a capacity of 1,000 cm² of film per litre and a working range of 21 to 27 °C, above which Bergger warns of emulsion damage.

⚗Three hydroxyl groups, two of them useful

Pyrogallol is a triol: three hydroxy groups in a row on the ring, against hydroquinone’s two on opposite corners. The IUPAC dissociation-constant dataset records pKa₁ between 9.05 and 9.28 at 20 °C, pKa₂ between 11.19 and 11.34, and pKa₃ around 14.

C₆H₆O₃ → C₆H₅O₃⁻ + H⁺

First ionisation: pKa1 about 9.05 to 9.28 at 20 °C, ionic strength 0.1

A carbonate developer straddles the first constant; a caustic one passes the second; nothing a darkroom mixes goes near the third. Three ionisable groups also mean more ways to be oxidised than hydroquinone has, which is the bookkeeping behind Kodak’s observation that pyro oxidises far more readily than Elon or para-aminophenol for a given amount of preservative.

Not sourced here. The course cannot name the yellow oxidation product, cannot give an equation for its formation, and cannot describe the mechanism by which it hardens gelatin. Kodak establishes that the product is yellow and that it deposits with the silver; Bergger establishes that the tint surrounds the grains and adds density; Eder establishes that the tanning happens. The chemistry joining those three statements is real and is not on this page.

Properties

C₆H₆O₃, relative molecular mass 126.11, CAS 87-66-1. Kodak Limited’s 1949 handbook gives the identification in one line: “pyro is 1:2:3 trihydroxybenzene, also known as pyrogallol or pyrogallic acid”. It is a white solid in various forms that turns grey in light and air, melting at 133 to 134 °C (271 to 273 °F) and decomposing before it boils, with a specific gravity of 1.45 at 20 °C (68 °F). CAMEO carries a reactivity alert reading simply Strong Reducing Agent, and its health hazard entry notes that skin exposure can cause discolouration, irritation and sensitisation.

Solubility is high and imprecisely known. HSDB’s clearest statement is that 1 g dissolves in 1.7 mL of water, about 590 g/L; the ILO-WHO safety card gives 60 g per 100 mL at 20 °C; two further HSDB entries for 25 °C disagree with each other at 625 and 507 g/L. The sources span a fifth of their own value, which does not matter for dissolving pyro and does matter for quoting a figure.

Stability is the whole practical problem. Kodak’s primer is emphatic that a pyro solution is kept in two parts — agent and preservative in one bottle, carbonate and bromide in the other — because pyro oxidises much more readily than the other agents, and that pyro keeps better with sodium bisulfite, which is acid, than with sodium sulfite, which is slightly alkaline. Scum on a standing developer is oxidation product, and the primer says it is worst with pyro. Set against all of that, Bergger states a shelf life of up to ten years for both PMK concentrates even in half-filled bottles: a hundred years of formulation between the two claims, and the difference is in what the agent is dissolved in rather than in the agent.

Handling

The aggregated ECHA notifications give Warning with the irritant and health-hazard pictograms. Three statements are unanimous across all reporting notifiers: H312, harmful in contact with skin; H332, harmful if inhaled; and H341, suspected of causing genetic defects. H302, harmful if swallowed, stands at 93.1 per cent and H412 at 93.4 per cent. There is no NIOSH Pocket Guide entry and no EH40 workplace exposure limit, and HSE’s own introduction to that list states that the absence of a substance from it does not indicate that it is safe.

The most useful hazard text the course holds is the manufacturer’s. Bergger’s PMK datasheet says that pyrogallol is toxic to health, causing kidney, liver and circulatory disorders or even death; that it is toxic by inhalation, skin contact and ingestion; that it is a phenol and can cause burns; that brief skin contact may cause a dark, non-scalding stain and prolonged contact a chemical burn resembling a heat burn; and that gloves are to be used and all equipment cleaned with soap and water. Princeton University’s guidance adds that pyrogallic acid is among the developers that can be absorbed through the skin to cause severe poisoning.

Why Level B. Level A of the course rubric admits substances classified no further than irritant or harmful if swallowed; a unanimous mutagenicity statement is past that. The question is B against C, and the course’s own precedent is hydroquinone, which carries the same H341 at 99.9 per cent and sits at B because the recognised control is ventilation, gloves and eye protection rather than a fume cupboard. Pyrogallol has the same shape of evidence and, unlike catechol, has no acute-toxicity pictogram, no carcinogenicity statement and no skin notation; it also has something catechol lacks, which is a current manufacturer publishing a control regime for a product sold to home darkrooms. Level B’s own criteria then describe it exactly: a toxic reagent, and a fine powder that must not be inhaled.

That last clause is where this page’s caution belongs. Inhalation is notified as harmful by every notifier, and Kodak moved its own product from the flaky powder to the crystal a century ago because the fine form flew about the darkroom and settled on plates. Florida Atlantic University’s guidance for photographic chemicals does not tell darkroom workers how to weigh pyrogallol; it tells them to avoid pyrogallol and catechol and to substitute phenidone. A reader who wants a pyro negative and has no extraction should buy the liquid concentrate rather than the jar of solid.

⚠Safety

Weigh the solid only with extraction or inside an enclosure, and never as loose powder in a room where film or paper is open. Gloves, eye protection and an apron for the working solution; expect the stain on skin and treat it as a marker of contact rather than as the injury. Keep it away from oxidisers, permanganate, iodine and metal salts, and keep the alkali in a separate bottle until the moment of use — the reaction that makes a developer and the reaction that destroys it are the same one. Bergger’s instruction to keep solutions tightly closed and protected from light is not about shelf life alone; a browned pyro solution is an undescribed mixture of oxidation products with no classification of its own.

Waste

Pyro is the one developing agent on these pages whose waste route is settled by an aquatic classification rather than by argument: H412, harmful to aquatic life with long lasting effects, is notified by 93.4 per cent of reports, and P273 — avoid release to the environment — sits in the precautionary list. Bottle the spent developer on its own, label it with the substance and the date, and take it to a household hazardous waste route; in England and Wales the government’s service finder lists them. Do not combine it with fixer: Kodak’s J-300 guidance puts developer’s silver content at a negligible level, so the mixture recovers nothing and spoils the silver-bearing stream. The wash water is a larger volume than usual, because Bergger’s instructions ask for 20 to 30 minutes of running water and forbid a washing aid that would cut it. Check your local regulations; they decide.

History

Pyrogallol reached photography before photography had developers. Eder records that Henry Braconnot, professor of natural history at Nancy, produced pyrogallic acid in a pure state in 1831 and found that it rapidly reduced metallic silver from silver nitrate solutions where gallic acid did so only very gradually. Twenty years later, in 1851, the physicist Regnault in Paris and the chemist Liebig at Giessen each found independently and simultaneously that it was a much more energetic and rapid developer than gallic acid; Regnault developed paper negatives in an aqueous solution of one part in a thousand and exhibited the results early that year at the Société Héliographique, where Eder says they attracted attention for their vigour and the modulation of the middle tones. Pyro went on into the Talbotype, the Niepceotype and the beginning of wet collodion.

When gelatin dry plates arrived they were developed exclusively with the pyro-ammonia developer carried over from collodion. The results were delicate, Eder writes, but the negatives carried a yellowish or brownish stain that often spoiled the whole gelatin layer, and the solutions were difficult to keep. The fix, in 1882, was H. B. Berkeley’s addition of sodium sulfite, which protected the solution from browning and the negative from the stain, and which Eder calls an enormous advance applied within a few years to every other organic developing agent; Mawson and Swan added potassium metabisulfite for the same purpose in 1886. Every preservative in this encyclopaedia descends from a repair to pyro.

🕰The tanning nobody was looking for

At the end of the 1870s Leon Warnerke, a Russian working in England, discovered the tanning action of pyrogallol in the development of silver bromide plates, and reported in 1881 that only the parts of such a film which had not been exposed remained soluble in warm water, the exposed and tanned parts being insoluble. Eder had pointed out in the same year that the swelled relief left by alkaline pyrogallol without sulfite could be made high enough to mould from, and used as a photomechanical printing plate. The property that Bergger’s datasheet now guards against — “a non-tanning bath is essential” at the fixing stage — was, for two generations, the reason to use pyro at all.

Wall’s 1924 formulary records the decline of the original form: pyro-ammonia “was once the favorite developer, but is rarely used now on account of its variability, due to the evaporation of the ammonia”. What survived is the alkali-plus-sulfite version, and what survives today is a small speciality trade. Bergger’s datasheet calls it “the most used developer in the 19th century, nicknamed the king of developers”; Bostick & Sullivan sell Pyrocat-HD as “a semi-compensating, high-definition developer formulated by Sandy King as an alternative to PMK”, claiming a third of a stop more film speed, shorter times, more consistent staining and lower toxicity — although that sheet names no developing agent, and this page attributes it no composition.