Alternative Developing Agents: p-Aminophenol, Ascorbate, Glycin, Catechol, Pyrogallol

Metol and hydroquinone answer most of the questions a developer is asked, which is why they answered them for a century. The five agents on this page exist because of the questions they do not answer: how to sell a developer as a small bottle of syrup, how to build one with no quinol in it at all, how to make a bath whose development time you can compute from its dilution, and how to make an image that is partly not silver.

Each section here asks the same question — what does this agent do that metol and hydroquinone do not? — and refuses the alternative question, which is which one is best.

The five, and where their donor groups stand

1. 1p-Aminophenol — hydroxyl and amino, para — the parent of metol and of every Rodinal-type concentrate
2. 2Glycin — p-aminophenol with a carboxymethyl group on the nitrogen; not the amino acid glycine
3. 3Catechol — two hydroxyls, ortho — hydroquinone’s isomer and nothing like it in behaviour
4. 4Pyrogallol — three hydroxyls in a row; three ionisable groups and more ways to be oxidised
5. 5Ascorbic acid — a lactone, not a benzene ring; it came from food chemistry, not from the dye works

Skeletons rather than structures: the positions of the donor groups are the claim, and bond lengths, angles and hydrogen atoms are not shown.

p-Aminophenol: a solubility problem solved backwards

A Rodinal-type developer is a small bottle of dark syrup used at one part in twenty-five or one in fifty, and the reason it can exist in that form is a defect turned into a feature. Para-aminophenol barely dissolves in water — the published figures span more than sixteenfold, from under 1 mg/mL to 16 mg/mL, but every one of them puts it one to two orders of magnitude below hydroquinone’s 70 g/L. Put enough sodium hydroxide in with it and it dissolves in its own alkali, and what leaves the factory needs nothing but water.

Wall’s 1924 formulary prints the working of it, in an Ermen formula of the rodinal type: 50 g of the hydrochloride and 150 g of potassium metabisulfite in about 625 cm³ of water, with caustic soda added under constant stirring. “At first a precipitate of the paramidophenol base is formed, but, as more caustic soda is added, this dissolves.” Then the instruction that makes the formula: add enough soda to nearly dissolve the precipitate, and “it is very important to leave some undissolved”. The bottle is held deliberately at saturation, with solid base in equilibrium above the solution. Wall gives the instruction and not the reason, and no source this course holds supplies one.

The IUPAC dataset puts a number on the acid–base half of it: 4-aminophenol has a pKa of 10.46 at 25 °C for loss of the phenolic proton. A carbonate developer at pH 10 has a minority of the molecules ionised; a caustic concentrate is well past it. Kodak’s own primer ranks para-aminophenol, which it sold as Kodelon, between pyro and metol on its bromide-tolerance scale, and notes that Kodelon can be substituted for metol but that more of it is needed for a developer of the same strength.

📷What the concentrate buys, in figures a datasheet will give you

Foma still sells one: Fomadon R09, described on its sheet as a liquid concentrate of a fine-grain, normal-working para-aminophenol negative developer, used at 1+25 or 1+50. Set its stated capacity beside the phenidone–hydroquinone concentrate in the same range and the economics of the type are visible without any chemistry at all.

Foma concentrate	Type	Dilution	Films per 250 mL package
Fomadon R09	para-aminophenol	1+25 or 1+50	25
Fomadon LQN	phenidone–hydroquinone	1+10 to 1+14	12

Twice the work from the same bottle, because the agent can be carried at a concentration the others cannot reach. That is the whole commercial argument for the type, and it is why it has survived since 1891 while more elegant developers have not.

The catch is in Wall’s other measurement. Time is not proportional to dilution. Wratten and Wainwright found photometrically that a plate needing three minutes in rodinal at 1+20 needed, at 1+200, not the thirty minutes arithmetic predicts but 42 minutes in air-free distilled water, 46 in ordinary distilled water and 52 in tap water. The water is part of the formula at that dilution, and so is the air dissolved in it. Wall’s temperature coefficient for paramidophenol is 2.4 against 1.9 for metol–quinol, so a long dilute development in a cold room is a different development again.

!Rodinal, and what this course will not tell you about it

The lineage is documented. Eder credits Momme Andresen, chemist to the aniline company in Berlin that became Agfa, with German patent 60174 of 27 January 1891 for paramidophenol — rodinal — and records that it was still in general use decades later. Wall’s 1924 list of the proprietary versions runs to eight names.

The modern product’s composition is not disclosed and this course does not guess at it. Under the course’s provenance rule a proprietary formulation is taught as behaviour and disclosed components only. There is a second, duller reason on this occasion: the course fetched the current Adox Rodinal datasheet, and the mirrored PDF carries no extractable text. No dilution, pH, keeping claim or statement about stand development on this page comes from it, and where a modern figure was wanted the course used Foma’s sheet for its own para-aminophenol concentrate instead. That is a gap in the research and it is recorded rather than papered over.

Ascorbate: the developer with no quinol in it

Kodak’s XTOL sheet leads with two claims about the same product: “ascorbic acid-based” and “no hydroquinone”. The first says what it contains, the second what it does not, and the fact that both are printed as benefits is the whole story of the last thirty years of developer chemistry.

Ascorbic acid is the one agent in this part that is not a substituted benzene ring. ChEBI describes a five-membered lactone carrying a two-carbon side chain, and CAMEO’s datasheet carries a reactivity alert reading simply Strong Reducing Agent. It reaches a developer already ionised: the IUPAC dataset gives pKa₁ 4.04 at 25 °C, so at any developer pH the molecule has long since given up its first proton and what is present is the ascorbate anion.

That has a formulation consequence which is the reason the salts exist.

C₆H₇NaO₆ → Na⁺ + C₆H₇O₆⁻

Sodium ascorbate dissolving: the anion is the reducing species and the sodium is a spectator

A solution of the free acid is acid, and it will eat the carbonate or borate the developer needs before the developer can use it, in proportion to how much reducing agent you wanted. Sodium ascorbate does not, and CAMEO puts its aqueous pH at 5.6 to 7.0 or higher. Mole for mole the swap is a ratio of molar masses, 198.11 ÷ 176.12 = 1.125, and that factor answers the question about ascorbate ions and no part of the question about pH.

Where it works is the striking number. Kodak specifies XTOL’s working tank solution at pH 8.20 ± 0.05. Set that against ILFORD’s 10.30 to 10.50 for Bromophen stock and the ascorbate developer is running two orders of magnitude lower in hydrogen-ion concentration than the paper developer, and half a unit below ILFORD’s own ID-11. Kodak’s primer supplies one consequence immediately: alkalis soften the gelatin, so too alkaline a developer over-swells the coating and gives trouble with frilling. A bath at pH 8.2 asks less of the emulsion than one at 10.4.

⚗Keeping: two sourced accounts that do not agree, and the folklore the course leaves out

The photographic literature’s standard claim is that ascorbate developers fail suddenly rather than gradually, because there is no route back for the oxidised agent of the kind sulfite provides for quinone. This course cannot source that mechanism, and what it holds instead is a genuine tension between two Tier-1 documents.

CAMEO, on ascorbic acid: aqueous solutions are oxidised by air at a rate depending on pH and oxygen, accelerated by alkaline conditions, iron and copper — and, strikingly, degradation also occurs under anaerobic conditions, so excluding air is not a complete defence. On sodium ascorbate its entry is one sentence: “Aqueous solutions are subject to quick air oxidation at pH greater than 6.0.” A developer at pH 8.2 is on the wrong side of that line by construction.

Kodak, on XTOL: excellent keeping properties, good shelf life of six months after mixing when stored in full bottles, and high resistance to breakdown from oxidation during storage or in replenished processes. Its storage figures — six months full, at least two months partially filled, because partially filled containers allow oxidation — are the same shape and the same numbers as its figures for D-76.

Those two statements are about the same chemistry and they point in opposite directions. The course records both and resolves neither. What it will not do is import the community reports of sudden failure that are usually offered as the tie-breaker: those are Tier-3 evidence under the course’s source standard, they may never establish a chemical claim, and Kodak has not documented them. What would settle it is a keeping trial against a fogged-strip activity test, which is exactly the shape of the experiment aerial-oxidation-experiment runs on hydroquinone.

One practical note is safe to give because it is a hazard of the ingredient rather than a claim about the product: iron and copper accelerate the oxidation, so an ascorbate developer is a poor candidate for a rusty tank or a hard-water supply used unfiltered. Kodak Limited’s 1949 handbook lists Calgon, sodium hexametaphosphate, among the chemicals it sold for darkroom use, which places water treatment on the darkroom shelf; no sheet in this course’s corpus states that any commercial ascorbate developer contains a sequestrant, and this page does not say one does.

And the isomer. Foma’s Fomatol P is described on its own sheet as a phenidone–isoascorbate developer. Erythorbic acid and its salts are a different stereoisomer of the same formula, with a different PubChem record and a different CAS number, and the photographic literature runs the two together constantly. The course names what the sheet names and treats them as distinct substances until it has a source that says otherwise. The practical point for a formulator is the one that always applies to isomers: the same formula is not the same substance, and the previous page made that argument with three molecules of C₆H₆O₂ of which one does not develop at all.

Glycin: the developer whose time you can calculate

Dilute a glycin developer to a tenth and it takes very close to ten times as long. That sentence is the whole of glycin’s reputation, and after the Wratten and Wainwright measurements above you can see why it is worth having.

Wall states the rough rule that a developer’s energy varies with its dilution, then the exception: Mees had shown a large departure from it with metol and hydroquinone, and also when air is dissolved in the water, but “with glycin and pyro-soda time seems proportionate to dilution”, and those two “seem to be the only developers that are not affected by the water”. A developer whose time you can compute, and which does not change its mind when the tap does, is a developer you can leave alone for an hour.

That is exactly how it was used. The 1906 British Journal Photographic Almanac prints Hübl’s directions for stand development: one ounce of the concentrated glycin stock in 80 to 90 ounces of water with 80 minims of ten per cent bromide, in which a properly exposed plate “should make its appearance in 15 or 20 minutes, and obtain full density in several hours”, with caustic soda added for under-exposure and more bromide for over-exposure.

Two more numbers characterise it. Wall’s tables of the Watkins factor — total development time divided by the time the image first appears — give glycin with sodium carbonate as 8 and with potassium carbonate as 12, against 30 for metol, 14 for metol–hydroquinone and 5 for hydroquinone with minimum bromide. A small factor means the image appears late in its own development, which is what an even-working agent does; and the fifty per cent jump from swapping one alkali for another is a reminder that an agent’s behaviour is never the agent’s alone. And on fog, the 1906 almanac reports Lüppo-Cramer’s comparison under a heavy bromide addition: pyro and adurol gave appreciably more fog than the hydroquinone standard, “edinol and glycin, less; pyrocatechin, none; metol, a trace” — the experimental version of Wall’s word clean.

Its structure explains none of that, and the course says so. Glycin is para-aminophenol carrying a carboxymethyl group on its nitrogen — not a dihydroxybenzene, and not the amino acid glycine. Why that added acid group should slow the agent down, and why it should fog so much less under bromide, is outside what this course’s sources establish, and the IUPAC dataset carries no dissociation constant for the molecule at all, so even the alkali dependence is read off the formulas rather than calculated.

Catechol and pyrogallol: when the oxidation product is the point

Every other agent in this part is judged partly by how little of its oxidation product ends up in the film. These two are judged by how much.

They are the ortho and the triol of the same family: catechol with two hydroxyls adjacent, pyrogallol with three in a row. The IUPAC dataset gives catechol pKa₁ 9.23 to 9.45 near room temperature and pyrogallol pKa₁ 9.05 to 9.28 at 20 °C, so both sit close to hydroquinone’s 9.88 and both need a real alkali; pyrogallol’s pKa₂ near 11.2 and pKa₃ near 14 are the bookkeeping behind Kodak’s observation that pyro oxidises far more readily than Elon or para-aminophenol for a given amount of preservative.

One dial: what sulfite does to a pyrogallol image

1. 1No sulfite — a very yellow negative — image part silver, part oxidised pyrogallol
2. 2Sulfite present — much less yellow: the sulfite is oxidised instead of the agent
3. 3A great deal of sulfite — almost as blue an image as with Elon, whose product is not deposited coloured

The three cases are Kodak's, from the 1928 primer's account of the preservative; the drawing renders them as hatching density because the course has no photograph of the comparison and will not describe one it does not have.

Kodak’s account of the preservative is written around pyro precisely because pyro shows it best, and it is a graded experiment rather than an assertion: no sulfite gives a very yellow negative whose image consists partly of silver and partly of oxidised pyrogallol; sulfite gives a much less yellow one, because the sulfite is oxidised instead; a great deal of sulfite gives almost as blue an image as Elon, whose product is not deposited in a coloured form. Sulfite is the dial, and a staining developer is simply this dial turned down.

Bergger’s current datasheet for PMK describes the same effect as the product feature: a yellow-green tint surrounds each silver grain and fills the space between the grains, so that the density of a pyro negative is the conjunction of two densities, that of the silver and that of the coloration. Everything downstream on that sheet exists to protect the second density — a non-acid stop bath, a non-tanning fixer, a twenty to thirty minute wash because the colouring intensifies during washing, and no hypo eliminator, which weakens it.

Tanning is the other half of the same oxidation. Eder records Warnerke’s report of 1881 that a gelatine film developed with pyro was soluble in warm water where it had not been exposed and insoluble where it had. The course can name neither the yellow oxidation product nor the mechanism by which it cross-links gelatin, and asserts neither; what is established is that the product is coloured, that it deposits with the silver, and that the gelatin hardens where development happened. staining-and-tanning-developers takes the practice, the published formulae and the hazard assessment.

Comparative hazards, and the two that are not treated alike

The five differ far more in their hazard records than in their chemistry, and the differences are not where photographic folklore puts them.

Agent	Signal word	The statements that decide the handling	Encyclopaedia level
Ascorbic acid	none	97.3% of 588 reports say it meets no GHS criterion	A
Sodium ascorbate	none	98.9% of 361 reports say it meets no GHS criterion	A
Glycin	Warning	H315, H319 at 100%, H335 at 97.7% — on only 44 reports	B
para-Aminophenol	Warning	H302, H341, H410 unanimous; H332 harmful if inhaled at 99.5%; H317 and H373 at 34.3% each	B
Pyrogallol	Warning	H312, H332 and H341 all at 100%; H302 and H412 above 93%	B
Catechol	Danger	H315 above 99.9%, H319 at 94.6%; H301 and H311 (toxic by mouth and skin) at 16.4%; H350, may cause cancer, at 11.4%	C

Three readings follow, and each one contradicts something people say about these substances.

Pyrogallol is not the most heavily classified agent on the page — catechol is. Catechol carries Danger, a minority classification as toxic rather than merely harmful by two routes, and a carcinogenicity statement from 11.4 per cent of 2,962 reports, which PubChem reinforces with the California Office of Environmental Health Hazard Assessment’s statement citing IARC. That is why the encyclopaedia places catechol at Level C, where a fume cupboard or a licensed waste route is the recognised control, and pyrogallol at Level B. Remember what a level means, though: the letter is a hazard assessment of a procedure, so a page’s rating comes from what that page asks you to do, and staining-and-tanning-developers makes its own assessment rather than inheriting these.

Pyrogallol’s danger is not the one the datasheets emphasise. Its three unanimous statements are harmful in contact with skin, harmful if inhaled, and suspected of causing genetic defects, while the two irritation statements are notified by only 12.3 per cent — the reverse of the usual pattern. Bergger’s own datasheet is blunter than the codes: pyrogallol is toxic by inhalation, skin contact and ingestion, causes kidney, liver and circulatory disorders, is a phenol and can cause burns, and brief skin contact may leave a dark stain while prolonged contact gives a burn resembling a heat burn. There is no NIOSH entry and no EH40 exposure limit, so there is no airborne figure to work to.

The mildest record on the page belongs to the newest agent, and thinness is not the reason. Ascorbic acid’s 588 reports and sodium ascorbate’s 361 are respectable numbers, and 97 to 99 per cent of them report no GHS criterion met. That is a real finding rather than an absence of one — unlike glycin, whose 44 reports across four notifications are among the smallest evidence bases in the encyclopaedia, and for which the course could locate no CAMEO datasheet, no safety card, no NIOSH entry and no exposure limit. Glycin’s mild classification and ascorbate’s mild classification are not the same kind of statement, and reading them as if they were is the most common mistake made with hazard data.

⚠What sets the controls when the agent is mild

On an ascorbate developing session the protective equipment is not set by the ascorbate. It is set by the alkali, the sulfite and the bromide beside it, and by the general rule that gloves and eye protection go on whenever a solution is handled. Two specifics are worth naming. Every powder here is still a powder: HSE’s guidance for manual film development and Princeton’s for mixing powdered developers both apply to a substance that is also a food additive, and para-aminophenol’s H332 makes that particular one a weighing hazard first and a solution hazard second. And sodium ascorbate is a base that neutralises acids with heat, so it does not share a shelf with a stop-bath concentrate.

Recap

• Every one of these agents exists because of something metol and hydroquinone cannot do, and the four benzene-ring agents all satisfy the same para-or-ortho rule as the classical pair. Ascorbic acid satisfies none of it.
• Para-aminophenol dissolves in its own alkali, which is why a Rodinal-type developer can be a bottle of syrup used at 1+50 and why Foma’s para-aminophenol concentrate develops 25 films per package against 12 for its phenidone–hydroquinone one. Its pKa is 10.46; its temperature coefficient is 2.4; and at high dilution the time is not proportional to the dilution, as Wratten and Wainwright showed with 42, 46 and 52 minutes for three different waters where arithmetic predicted 30.
• Ascorbate arrives already ionised — pKa₁ 4.04 — which is why the acid form eats the alkali and why the salt exists, at a substitution factor of 1.125 by mass. XTOL runs at pH 8.20, two orders of magnitude below a paper developer.
• On ascorbate keeping the course has two Tier-1 sources that disagree: CAMEO’s rapid air oxidation above pH 6, accelerated by iron and copper and continuing anaerobically, against Kodak’s advertised resistance to oxidation and six-month shelf life. The community’s sudden-failure reports are Tier-3 and are excluded rather than used as a tie-breaker.
• Glycin’s development time is proportional to its dilution and indifferent to the water, which is the property that made it the stand developer of its period. Its Watkins factor is 8 with sodium carbonate and 12 with potassium, and Lüppo-Cramer found it among the cleanest agents under heavy bromide. Why, is not something this course can source.
• For catechol and pyrogallol the oxidation product is the image. Sulfite is the single dial between a stain image and a plain silver one, and the tanning of the gelatin is recorded from 1881 without the course being able to name the chemistry that does it.
• Catechol, not pyrogallol, is the most heavily classified agent here — Danger, a minority toxic classification, a carcinogenicity statement, and Level C in this encyclopaedia. And a mild classification means one thing for ascorbate, with 588 reports behind it, and something quite different for glycin, with 44.

Check your understanding

Question 1. Why can a para-aminophenol developer be sold as a concentrate used at 1+50 when a metol-hydroquinone one is sold at 1+10 or as a powder?

• Because para-aminophenol is far more soluble in water than metol
• Because it is barely soluble in water but dissolves in strong alkali, so the concentrate carries the agent as its alkali salt at a concentration the other agents cannot reach
• Because it needs no preservative
• Because it is a liquid at room temperature

Show the answer and why

Answer: Because it is barely soluble in water but dissolves in strong alkali, so the concentrate carries the agent as its alkali salt at a concentration the other agents cannot reach

The published water solubilities for para-aminophenol disagree by more than sixteenfold but all put it one to two orders of magnitude below hydroquinone. The formula turns that into an advantage: Wall prints an Ermen rodinal-type preparation in which caustic soda is added until the precipitated free base very nearly dissolves, with the explicit instruction to leave some undissolved. Foma’s current sheet shows what it buys — 25 films per 250 mL package against 12 for the phenidone-hydroquinone concentrate in the same range. The price is that at high dilution the development time stops being proportional to the dilution and starts depending on the water.

Question 2. A formulator replaces 10.0 g of ascorbic acid in a developer with the equivalent quantity of sodium ascorbate. How much should be weighed, and what else has to change?

• 10.0 g, and nothing else
• 11.3 g, and the alkali must be reconsidered, because removing the acid frees the alkali it was consuming and the pH will rise
• 8.9 g, and the sulfite must be doubled
• 11.3 g, and nothing else

Show the answer and why

Answer: 11.3 g, and the alkali must be reconsidered, because removing the acid frees the alkali it was consuming and the pH will rise

The ratio of molar masses is 198.11 ÷ 176.12 = 1.125, so 10.0 g of the acid is 11.3 g of the salt for the same number of ascorbate ions. That factor answers the question about ascorbate and no part of the question about pH. A solution of the free acid consumes carbonate or borate in proportion to how much agent you wanted; the salt does not, and CAMEO puts its aqueous pH at 5.6 to 7.0 or higher. How far the pH moves depends on the buffer in the bath, and this course cannot compute it, which is why the substitution is a formulation change and not a weighing change.

Question 3. What does the course say about the claim that ascorbate developers "die suddenly" rather than fading gradually?

• That it is established, because ascorbate has no sulfonate route
• That it cannot be sourced: CAMEO describes rapid oxidation above pH 6 accelerated by iron and copper, Kodak advertises high resistance to oxidation and a six-month shelf life, the two disagree, and the community reports usually offered as a tie-breaker are Tier-3 evidence
• That it is false, because Kodak says so
• That it applies only to the sodium salt

Show the answer and why

Answer: That it cannot be sourced: CAMEO describes rapid oxidation above pH 6 accelerated by iron and copper, Kodak advertises high resistance to oxidation and a six-month shelf life, the two disagree, and the community reports usually offered as a tie-breaker are Tier-3 evidence

Two Tier-1 documents point in opposite directions about the same chemistry, and the course records the tension rather than resolving it. The mechanism usually given — that there is no route back for oxidised ascorbate of the kind sulfite provides for quinone — appears in no document the course holds, so it is not asserted. What would settle the question is evidence rather than argument: a keeping trial with a fogged-strip activity test at intervals, which is the shape of the experiment this part already runs on hydroquinone. One practical consequence is safe to act on now, because it is a property of the ingredient: iron and copper accelerate the oxidation.

Question 4. Wall records that a plate needing three minutes in a rodinal-type developer at 1+20 needed 42 minutes at 1+200 in air-free distilled water, 46 in ordinary distilled water and 52 in tap water. What are the two lessons?

• That dilution has no effect on time
• That development time is not proportional to dilution, and that at high dilution the water — including the air dissolved in it — is part of the formula
• That distilled water should never be used
• That the developer had gone off

Show the answer and why

Answer: That development time is not proportional to dilution, and that at high dilution the water — including the air dissolved in it — is part of the formula

Arithmetic predicts thirty minutes and the shortest measured time is 42, so the relationship is not proportional; and the three waters differ by ten minutes among themselves, which is a quarter of the difference from the dilution itself. Dissolved oxygen consumes developing agent that would otherwise reduce silver, and at 1+200 there is very little agent to spare. Set against that, Wall singles out glycin and pyro-soda as the developers whose time does track dilution and which are not affected by the water — which is precisely why glycin became the stand developer of its period.

Question 5. A pyrogallol developer is mixed with no sulfite in it at all. What does the negative look like, and why?

• Neutral black, because sulfite affects only keeping
• Very yellow, because the oxidation product of pyrogallol is coloured and is deposited in the film alongside the silver, so the image is part silver and part oxidised agent
• Fogged, because sulfite is the restrainer
• Blue, like a metol image

Show the answer and why

Answer: Very yellow, because the oxidation product of pyrogallol is coloured and is deposited in the film alongside the silver, so the image is part silver and part oxidised agent

Kodak’s 1928 primer sets out the graded experiment: no sulfite gives a very yellow negative whose image consists partly of silver and partly of oxidised pyrogallol; sulfite gives a much less yellow one because the sulfite is oxidised instead; a great deal of sulfite gives almost as blue an image as Elon, whose oxidation product is not deposited in a coloured form. Sulfite is a single dial running from a stain image to a plain silver one. Bergger’s modern PMK sheet treats the same effect as a feature, describing the density of a pyro negative as the conjunction of the silver density and the coloration.

Question 6. Which of these two agents does this encyclopaedia classify more severely, and what is the practical consequence?

• Pyrogallol, because it is toxic by three routes; it is Level C and catechol is Level B
• Catechol: it carries the signal word Danger, a minority classification as toxic rather than harmful by mouth and skin, and a carcinogenicity statement, so the encyclopaedia places it at Level C where pyrogallol is Level B
• Neither; they are classified identically
• Glycin, because its evidence base is smallest

Show the answer and why

Answer: Catechol: it carries the signal word Danger, a minority classification as toxic rather than harmful by mouth and skin, and a carcinogenicity statement, so the encyclopaedia places it at Level C where pyrogallol is Level B

This inverts the usual folklore, in which pyro is the substance to be frightened of. Catechol’s aggregated notifications give Danger, with H301 and H311 — toxic by mouth and by skin — from 16.4 per cent of 2,962 reports and H350, may cause cancer, from 11.4 per cent, reinforced by the California OEHHA statement citing IARC. Pyrogallol carries Warning, with three unanimous statements and no carcinogenicity statement at all. One caution about how to use that: a safety level in this course is an assessment of a procedure, not a property of a bottle, so the staining developers page makes its own rating for the specific formulae it teaches.

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