510-Pyro

Fifteen grams of solid, one liquid, and no water anywhere in the bottle. 510-Pyro is a staining developer that dissolves its three developing agents in triethanolamine instead of water, and the consequences of that one decision run through every section below: there is no sulfite in it, there is no separate alkali in it, it is one bottle rather than two, and its concentrate is claimed to keep for years rather than months.

Jay DeFehr published it on a site of his own in December 2006. It is packaged and sold under his name by others: Bostick and Sullivan in New Mexico and Little House of Pyro in Belgium are the two whose pages this course has read.

Purpose

To make a stained, tanned negative from a single bottle of concentrate that survives being left on a shelf. Every other staining developer in this formulary is at least two bottles, and for a reason that is not arbitrary: pyrogallol oxidises in an alkaline solution faster than almost any other developing agent, so the agent and the alkali have to be kept apart until the moment of use. Kodak’s 1928 primer states the rule and gives the remedy — readily oxidised developers are stored in two or three solutions — and D-1, the standard A.B.C. pyro, is three bottles because of it.

510-Pyro solves the same problem differently. It puts the agents into a liquid that is itself the alkali and that dissolves practically no atmospheric oxygen, so there is nothing for the pyrogallol to react with until water arrives. The alkali and the agent share a bottle for years and nothing happens. That is the whole design, and the formula is what falls out of it.

The name is the composition: 5 grams of ascorbic acid and 10 of pyrogallol.

!What this page can print, and the two places where the course's data model does not fit

The formula is well sourced. It comes from the formulator’s own publication of it, and two independent printings agree with every figure in it but one. Under the course’s ruling of 5 September 2026 that is primary evidence, and this entry is status: verified.

Two things about the entry are compromises, and both are the course’s, not the source’s.

First, triethanolamine has no page in the chemical encyclopaedia. The encyclopaedia’s 132 entries do not include it, and the formulary’s own validator refuses a formula carrying an ingredient the reader cannot look up — which is the right rule, and it is why the substance that makes up something like nine-tenths of this bottle is not in the ingredient list above. Everything the course can source about it is on this page instead, under Function of every ingredient and under Safety, cited to PubChem. It is not in the page’s chemicals: list either, because that list may only name entries that exist or are planned.

Second, the structured entry has one field for a make-up volume and it is called water. There is no water in 510-Pyro. The 100 mL in that field is the make-up volume of the concentrate, the note attached to it says so, and this page prints its own quantity table rather than the shared component, so that nobody is ever shown a row headed “Water” for a developer that contains none.

Both are recorded here rather than worked around silently, because a reader who cannot see the shape of the evidence cannot judge it.

Recommended uses

Roll film and sheet film, in a tank or on a rotary processor, and above all in the regimes where other developers are awkward. The published record for this developer is unusually weighted towards reduced agitation, and that is where its own formulator worked.

• Normal development at 1+100. Five to seven minutes as a starting point, eight to fourteen for most of the common films in the chart. This is the dilution the sellers name first and the one most of the published times use.
• Semi-stand and stand development at 1+300 and 1+500. DeFehr’s own Technical Pan procedure is 1+300 for twenty minutes with a single ten-second agitation at the half-way point. Digitaltruth’s note gives 1:500 with one minute of initial agitation and one inversion every ten to fifteen minutes for about an hour. Fifty minutes at 1+500 is in the chart for FP4 Plus and sixty for Fomapan 100.
• Negatives for ultraviolet-printing processes. A stain deposited in proportion to exposure adds density where the silver is densest, and it does so most effectively against printing materials sensitive to blue and ultraviolet light. Sandy King’s account of stain sets that out in general terms for all pyro developers, and Little House of Pyro claims specifically that 510-Pyro negatives suit scanning, silver gelatin and ultraviolet printing at the same development time. That last is a seller’s claim and no measurement supports it here.
• Pushed film. The chart carries Tri-X at EI 800 and 1600 and T-Max P3200 at 3200, and Jim Byers’s data on DeFehr’s blog gives Tri-X at EI 1600 in eighteen minutes at 1+100 on a rotary processor with what he describes as surprisingly small grain for a two-stop push. Byers’s report is a named photographer’s account published by the formulator, not a measurement by either of them.
• Travelling. This is a real use and DeFehr demonstrates it: two 50 mL bottles of concentrate, a Paterson tank and a packet of fixer processed a trip’s worth of film in a hotel bathroom with no thermometer and no timer.

When another formula is preferable

When you want a developer whose behaviour is documented by its maker. This is the weakest part of 510-Pyro’s record and it is worth being blunt about it. PMK has two manufacturers’ kit sheets and a datasheet with a time-and-temperature chart, a stated capacity in square centimetres per litre and a stated working-solution life. Pyrocat-HD has its formulator’s own long article with contrast-index targets for each printing process. 510-Pyro has a blog, two compilations of its formula, and a seven-page time chart whose data is credited to a community database and which contradicts itself on five common films.

When you want more highlight contrast on variable-contrast paper. Finch reports Barry Thornton’s comparison: the green-yellow stain of a pyrogallol developer acts as a soft filter on variable-contrast paper, where the brown stain of a pyrocatechin developer such as Pyrocat-HD gives better highlight contrast. Finch says he uses catechol-based developers for that reason and adds that a worker who scans rather than prints in the darkroom is much less affected by the difference. That is one practitioner reporting another practitioner’s finding, and this course marks it as such rather than repeating it as chemistry.

When you would rather not weigh pyrogallol at all. Florida Atlantic University’s environmental health guidance for photographic chemicals does not tell darkroom workers how to handle pyrogallol; it tells them to avoid it and substitute phenidone. D-76 and D-23 will develop film perfectly well with no staining agent in them at all, and the sellers who supply 510-Pyro ready-mixed remove the weighing step without removing the developer.

When you need a working bath you can measure the pH of and trust. No source in this course’s corpus publishes a pH for 510-Pyro at any dilution, and the page below explains why the course declines to calculate one.

Mixing

One bottle, mixed once. The quantities are DeFehr’s, in the order in which his own line-up and both independent printings give them.

Ingredient	Quantity	Form the source specifies
Ascorbic acid	5 g	The solid acid, not the sodium salt
Pyrogallol	10 g	—
Phenidone	0.25 g	0.375 g in the revised formula; see Variants
Triethanolamine	to make 100 mL	The vehicle, in place of water. Finch and Digitaltruth print it twice: 75 mL at the head of the sequence, then triethanolamine to make 100 mL at the foot

The sequence. Digitaltruth’s only mixing instruction is “add chemicals in specified sequence”, and the sequence both printings specify is: about 75 mL of triethanolamine into the vessel; the ascorbic acid; the pyrogallol; the phenidone; then triethanolamine to the 100 mL mark. DeFehr’s sidebar lists the three solids in the same order. Dissolve each completely before the next goes in, which is Kodak’s general rule from 1928 and matters here for the reason it always matters: an undissolved crystal sitting in the vessel when the next chemical arrives is a crystal oxidising at its surface.

No source in this course’s corpus states a mixing temperature, and the course will not invent one. Two sourced facts bear on it. PubChem gives triethanolamine a melting point of 21.5 to 21.6 °C, so a bottle standing in a cold room may be a solid or a slush and will need warming before it can be poured or stirred. And PubChem describes it as a highly hygroscopic, viscous liquid: it takes up water from the air, so it should be worked with reasonably quickly and re-capped, and a vessel that has just been washed and not dried is the wrong vessel.

🧮What the standard dilution actually puts in the tank

The concentrate is 100 mL. One part of it to a hundred of water makes 101 parts, so each figure below is the concentrate’s own strength divided by 101 — and per litre of working solution the concentrate contributes 1000 ÷ 101 = 9.90 mL.

10 g per 100 mL × 9.90 mL/L = 0.990 g per litre of working solution

Pyrogallol

5 g per 100 mL × 9.90 mL/L = 0.495 g per litre

Ascorbic acid

0.25 g per 100 mL × 9.90 mL/L = 0.0248 g per litre

Phenidone

In moles, because the proportions are the design:

0.990 g/L ÷ 126.11 g/mol = 7.85 mmol/L

Pyrogallol

0.495 g/L ÷ 176.12 g/mol = 2.81 mmol/L

Ascorbic acid

0.0248 g/L ÷ 162.19 g/mol = 0.153 mmol/L

Phenidone

Fifty-one molecules of pyrogallol and eighteen of ascorbic acid to every molecule of phenidone. By weight the ratio is 40 to 20 to 1; the difference between the two sets of numbers is only the difference in molar mass.

Set the pyrogallol against the other two staining developers in this formulary and the surprise is how little there is to see. PMK’s working bath carries 0.971 g of pyrogallol per litre and this one carries 0.990 — a difference of two per cent, which is smaller than the uncertainty in either printing. Pyrocat-HD carries 0.490 g/L of catechol, about half as much agent, and 0.0196 g/L of phenidone against this bath’s 0.0248. Three developers with quite different reputations put almost the same amount of staining agent in the tank. What differs is the alkali, the preservative and the vehicle.

At 1+500 every figure above is divided by very nearly five: 0.200 g/L of pyrogallol, 0.0998 of ascorbic acid and 0.00499 of phenidone — a fifth of a gram of staining agent in a litre, developing for an hour.

🧮How much triethanolamine is in the bottle, and why the page will not be more precise

The sources bracket it and do not fix it. Finch’s sequence starts at 75 mL and tops up to 100 mL, so the finished concentrate holds between 75 and 100 mL of triethanolamine, the balance being whatever volume 15.25 g of solids occupies. The course has no sourced density for solid pyrogallol or ascorbic acid in its corpus and will not compute the figure from an unsourced one.

That gives a range rather than a number for the alkali in the tank. Per litre at 1+100 the concentrate contributes 9.90 mL, of which between 75 and 100 per cent is triethanolamine: 7.4 to 9.9 mL per litre. At PubChem’s relative density of 1.124 that is 8.3 to 11.1 g/L, and at 149.19 g/mol, 56 to 75 mmol/L.

Compare Pyrocat-HD’s 53.2 mmol/L of potassium carbonate. The alkali molarities are of the same order. What is not of the same order is the base strength, and that is the next section’s subject.

Behaviour

It is dilute, it is one-shot, and it tolerates being left alone. Between five and thirty minutes at 1+100 covers nearly everything in the published record, and thirty to sixty minutes at 1+500 covers the stand and semi-stand work. Nothing in any source suggests replenishment and every published procedure mixes at the tank and discards.

Agitation moves the time more than anything else does. DeFehr’s own Technical Pan instruction is the clean demonstration, because it is one film at one dilution with two regimes: twenty minutes semi-stand against sixteen minutes at ten seconds a minute. He states the rule in the same paragraph — increased agitation frequency requires a corresponding reduction in development time — and four minutes, a fifth of the longer time, from agitation alone.

✕The development chart contradicts itself, and the reason matters

Bostick and Sullivan’s seven-page chart is the only development table published for this developer, and its header reads “Data Courtesy of digitaltruth.com”. It is a community compilation, and it shows.

Film	Dilution	Meter setting	Temperature	Times published
Ilford HP5 Plus	1+100	400	20 °C	8.25 min and 13.75 min
Ilford FP4 Plus	1+100	125	20 °C	10 min and 17.25 min
Kodak Tri-X 400	1+100	400	20 °C	8 min and 16.5 min
Ilford Delta 400 Professional	1+100	400	20 °C	12.25 min and 22 min
Kodak T-Max 400	1+100	400	20 °C	9 min and 30.75 min

Four variables held constant and the answer differs by up to a factor of 3.4. The chart records no agitation regime, and that is very likely most of the explanation, because DeFehr’s own figures show a quarter of the time moving on agitation alone and his contributors’ stand times show far more than that. The phenidone revision may account for a little of it as well.

Three of these rows can be traced. The chart’s Tri-X 400 entries at EI 400, 800 and 1600 — 8, 11 and 18 minutes at 20 °C — are the times Frederic Harster and Jim Byers published on DeFehr’s own blog, all three on rotary processors. The identification is the course’s own, from matching the numbers, and it suggests that the chart is at least in part a compilation of the blog. That is worth knowing in both directions: it tells you where some of the data came from, and it tells you that the faster of each pair is the rotary one.

Use the chart the way it deserves to be used: as a range to test within, at the agitation you actually intend to use, on your own film, with a clip test. Do not read a number off it and expect it.

The concentrate keeps and the working solution does not. Every source that says anything about keeping says the same thing about the bottle — Little House of Pyro claims six years or more, Finch says years — and no source at all offers a figure for the diluted bath. That asymmetry is not an oversight; it is the design. The concentrate is protected because there is no water in it and therefore almost no dissolved oxygen. Add water and the protection is gone, along with the sulfite that a conventional developer would have had to fall back on, because this one has none.

Downstream of development, treat it as PMK is treated. No source publishes a post-development sequence for 510-Pyro. BERGGER’s PMK sheet does, and the reasons it gives are properties of the pyro stain rather than of PMK: the stop bath must not be acid, the fixer must not be a tanning one, 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. The course’s position is that those instructions follow from the stain and so apply here, and that this is an inference and not a published instruction for this developer. Jim Byers, on DeFehr’s blog, records a three-minute water pre-soak before development, which is BERGGER’s instruction for PMK as well.

Image characteristics

A stained negative carries two densities in one layer, and that is true of this developer exactly as it is of every other pyrogallol developer. The silver image is the one you can see by transmitted white light. On top of it, in proportion to how much development happened at each point, sits a coloured oxidation product of the pyrogallol, which is dense to blue and ultraviolet light and much less dense to green and red. Kodak’s 1928 primer demonstrates the same substance from the other end: with no sulfite at all the negative is very yellow and the image is “partly silver and partly oxidised pyrogallol”, and with a great deal of sulfite it is almost as blue as a metol negative.

Colour. Yellow-green. Finch reports Barry Thornton’s finding that this colour behaves as a soft filter on variable-contrast paper — it holds the highlights back and prints them at lower contrast — where the brown stain of a catechol developer gives more highlight contrast. Named practitioners reporting practice, marked as such.

Grain. The sellers make the strongest claims on the page. Little House of Pyro says most ISO 100 films “go grainless”; Finch lists very fine grain among the claims 510-Pyro “arrived on the scene with” and notes, in his own voice, that people who tested it found good results. No measurement of grain against a control exists in any source this course has read, and the working bath contains no silver solvent — no sulfite, no thiocyanate, nothing that could dissolve silver halide and reduce grain the way D-76’s hundred grams of sulfite per litre does. Whatever fineness of grain this developer gives, it does not give it by dissolving anything.

Acutance. DeFehr’s own explanation is tanning, and he states it as a contrast with other high-acutance developers: 510-Pyro “enhances sharpness through emulsion tanning and not increased graininess like other acutance developers do”. The mechanism section below sets out what tanning is and what the course can and cannot say about it.

Speed. Nothing. No source publishes an effective speed against a control developer, and the chart’s meter settings are what its contributors chose rather than what the developer delivers. The one thing the chart does establish is that people push it: Tri-X at 800 and 1600 and T-Max P3200 at 3200 all have rows.

Compensation. Strong at the dilute end, and the published record is almost entirely at the dilute end. A bath at 1+500 holds a fifth of a gram of pyrogallol per litre; in a highlight, which develops fast, the local supply is used up long before the hour is out and development slows to a stop, while the shadows are still working. Byers, on DeFehr’s blog, describes exactly that behaviour on camellias — dark leaves and blown highlights in the same frame — and it is the classic argument for stand development rather than anything peculiar to this formula.

The mechanism

Three developing agents, one of which is the picture, one of which is the engine, and one of which nobody in the corpus explains.

What pyrogallol does

Pyrogallol is benzene-1,2,3-triol: a benzene ring with three hydroxyl groups in a row. It gives up electrons to silver ions at an exposed grain and becomes an oxidation product, and the property that makes it a staining developer is that its oxidation product is coloured and stays where it was made. Metol’s is not deposited in a coloured form; pyrogallol’s is. Kodak’s graded experiment, in which the same developer is run at increasing sulfite concentrations from a very yellow negative to an almost blue one, is the demonstration that the stain and the oxidation product are the same thing.

It also tans. The oxidised agent hardens the gelatin where development happened, in proportion to the amount of development. DeFehr’s own account is that the hardening produces a relief image and inhibits the migration of the developer both into the depth of the emulsion and between adjacent areas of different density, so that shadows develop deeper into the layer than highlights do. That is the formulator’s explanation of his own developer, and it is a good one; it is not a Tier 1 statement and it is not treated as one here. What Eder’s history establishes independently is that the tanning action of pyrogallol was known in the nineteenth century and was used to make relief images.

What the course cannot tell you is what the coloured substance is. No source in this corpus names it, gives it a structure or writes a balanced equation for its formation, and this page will not supply one. An equation with an unnamed species in it is a diagram with an equals sign.

What the vehicle does

Triethanolamine is 2-[bis(2-hydroxyethyl)amino]ethanol: a nitrogen atom carrying three hydroxyethyl arms. It is a tertiary amine, which makes it a base, and PubChem gives the pKa of its conjugate acid as 7.76 at 25 °C and the pH of a 0.1 N aqueous solution as 10.5.

Those two figures do the work of the accelerator in a conventional developer. Finch states the design directly: the triethanolamine “provides the alkalinity to activate the developing agents so no accelerator required”, and being a non-aqueous solvent it “removes the need for an anti-oxidant / preservative, so there’s no sulphite required”.

⚗Why the course prints no pH for the working bath

It would be easy to produce a number here and it would not be worth anything.

What is sourced. Triethanolamine’s conjugate acid has a pKa of 7.76, so a triethanolamine buffer holds a solution in the region of pH 8. Ascorbic acid is an acid — the ascorbic acid page carries its dissociation constants — and there are 2.81 mmol/L of it in the tank against 56 to 75 mmol/L of the amine, so most of the amine is left unprotonated after the acid has been neutralised. For comparison, Kodak publishes the working tank solution of XTOL, an ascorbate developer, at pH 8.20, and ILFORD publishes Bromophen stock at 10.30 to 10.50.

What is not sourced. The actual pH of a 510-Pyro working bath. Nobody has published one, the course has not measured one, and a pH computed from a buffer equation would be a calculation about a three-component mixture in which two of the components are themselves weak acids of unstated concentration once dissolved in an amine. The targetPH field of this entry is therefore empty, and the honest summary is: this is a low-alkalinity developer by the standards of the carbonate and hydroxide baths, working somewhere in the region a triethanolamine buffer commands, and the figure is not established.

The practical consequence of low alkalinity is the one to carry away, and it is visible in the times: a bath that develops FP4 Plus in ten to seventeen minutes at 1+100 is not a fast one.

And the oxygen. Finch’s claim is that triethanolamine, unlike water, does not dissolve oxygen, so the agents in the concentrate are not in contact with any. The course has verified the melting point, density, hygroscopicity and base strength of triethanolamine against PubChem; it has not found a sourced figure for the solubility of oxygen in triethanolamine, so Finch’s reason is reported as his and the six-year keeping claim is reported as the packer’s. What is not in dispute is the outcome the sellers and the formulator agree on: this concentrate does not go off the way a pyro solution in water does, and Kodak’s 1928 chapter on the keeping of pyro is the measure of how remarkable that is.

What the three agents do together, and what nobody says

This is the honest gap on the page. A pyrogallol-ascorbic-acid-phenidone combination is unusual — Finch calls it “an unusual combination” in exactly those words — and no source the course has read explains the division of labour among the three.

What can be said:

• Phenidone is the most active agent by weight in the bottle and the least abundant by moles. One molecule of it to fifty-one of pyrogallol. That proportion is the signature of a superadditive pair, in which a small amount of a vigorous agent transforms the behaviour of a much larger amount of a slower one.
• The course does not assert the regeneration mechanism, here or anywhere. The superadditivity page sets out why: the regeneration account is the consensus of the photographic literature and no Tier 1 source held by this course states it. The page teaches the phenomenon and marks the mechanism. This one does the same.
• Ascorbic acid is doing at least one job that is not development. It is a strong reducing agent — strong enough that CAMEO’s datasheet leads with a reactivity alert saying so — and it is the only thing in the bottle that could take the role sulfite takes in an ordinary developer.
• And there the course’s confidence stops. An antioxidant that intercepted the oxidised pyrogallol would destroy the stain, and this developer stains. At 2.81 mmol/L against 7.85 mmol/L of pyrogallol there is not enough of it to intercept everything even if it wanted to, but “not enough to intercept everything” is not a mechanism. Whatever balance DeFehr found among the three agents, he described it as a balance and not as a mechanism, and the one time he is quoted on the subject he says he changed the phenidone “to optimise the three way relationship between pyrogallol, ascorbic acid and phenidone”. That is a formulator describing an empirical optimum. The course reports it as one.

Function of every ingredient

Pyrogallol, 10 g — the staining and tanning agent, and the point of the formula. A hundred grams per litre of concentrate, which becomes 0.990 g per litre of working solution at 1+100 and 0.200 g at 1+500. It develops silver like any other agent and, unlike almost any other, leaves a coloured, gelatin-hardening oxidation product behind in proportion to the development that happened. More of it and you get more of both densities, and a negative whose printing behaviour on variable-contrast paper moves further from what its silver density alone would predict. Less of it and the developer converges on being a phenidone-ascorbate developer with a tint. It is also the ingredient that sets the safety level of the page, and the one that a reader who does not want to handle it can avoid by buying the concentrate ready mixed. Its solubility in water is high — the encyclopaedia gives 500 to 600 g/L — but its solubility in triethanolamine is not published anywhere the course can find, and the only evidence for it is that DeFehr’s formula dissolves 10 g in less than 100 mL of it.

Ascorbic acid, 5 g — the second reducing agent, and the nearest thing this developer has to a preservative. Fifty grams per litre of concentrate, 0.495 g/L in the standard bath. It is the one agent here that is not a substituted benzene ring: a lactone from food chemistry rather than a product of the dye works. In a bath with no sulfite it is the only sacrificial reducing agent present, and it is also half of a well-established developing pair with phenidone — Kodak’s XTOL and Foma’s Fomatol P are both phenidone-ascorbate developers. More of it would push the bath further towards being an ascorbate developer and would, on the face of it, be expected to suppress stain; less of it removes the only protection the agents have once water has been added. Neither prediction is tested in any source, and the course marks both as reasoning rather than as evidence. Its own hazard record is the mildest in the bottle: 97 per cent of the notifications to the European inventory report that ascorbic acid does not meet the criteria for classification at all.

Phenidone, 0.25 g — the primary developing agent by activity and a fortieth of the pyrogallol by weight. Two and a half grams per litre of concentrate, 0.0248 g/L in the tank — which is close to Pyrocat-HD’s 0.0196 g/L and is the usual order of magnitude for phenidone in a staining developer. It is present in the quantity it is present in because a pyrazolidone does a developing agent’s work at a small fraction of a quinol’s concentration; the Kendall patent that introduced it states a substitution ratio against metol and offers no mechanism, and neither does this page. More of it, and DeFehr’s own revision is the experiment: he raised it by half, to 0.375 g, for consistency, which is the change described under Variants. Less of it and the bath loses most of its activity, because the other two agents are working at a pH where neither is fast.

Triethanolamine, to make 100 mL — the solvent, the alkali, and the reason the bottle keeps. This is the ingredient the schema cannot hold and the encyclopaedia has no page for, so it gets the longest paragraph on the page.

It is a tertiary amine and a triol, CAS 102-71-6, C6H15NO3, 149.19 g/mol, a viscous, hygroscopic liquid with a faint ammoniacal smell that melts at about 21.6 °C and browns on exposure to air and light. It does three separate jobs here and no other ingredient does any of them:

1. It dissolves the solids. Fifteen and a quarter grams of them, in less than 100 mL.
2. It is the alkali. With a conjugate acid pKa of 7.76 it holds the diluted bath in the alkaline region without any carbonate, metaborate or hydroxide being present. There is no separate accelerator in this developer because the solvent is one. More of it, relative to the agents, is what dilution does, and dilution moves development time by an order of magnitude across the published range.
3. It excludes oxygen from the concentrate, which is the property Finch names and the reason offered for a shelf life measured in years. This is the one of the three that the course reports on a Tier 2 author’s word rather than a Tier 1 measurement.

Its own hazard record is discussed under Safety, and it is mild by the standards of everything else on this page. Because the encyclopaedia has no entry for it, a reader who wants its safety data has to go to PubChem or to the supplier’s safety data sheet directly, and this page says so rather than pretending the link exists.

Interactions

Agent and alkali, in the same bottle, for years. This is the interaction the formula is built to prevent, and it is prevented by removing the water rather than by separating the bottles. Every other staining developer in this formulary solves it by separation: PMK keeps its pyrogallol and its metaborate apart, Pyrocat-HD keeps its catechol and its carbonate apart, D-1 uses three solutions. The consequence of 510-Pyro’s approach is a single-bottle developer; the cost is that the alkali cannot be adjusted independently of the agents, which is exactly the control Pyrocat-HD’s two bottles give you when you double the B solution to raise contrast without touching the agent concentration.

Agent and preservative — the interaction that is deliberately missing. The defining fact about a staining developer is that it cannot carry a proper preservative, because sulfite intercepts the oxidised agent and the oxidised agent is the stain. The corpus lets that be quantified across four developers:

Developer	Sulfur-based preservative in the working bath
D-76	100 g/L of sodium sulfite — 0.79 mol/L
D-1, tray dilution	10.5 g/L of sodium sulfite
PMK	0.194 g/L of sodium bisulfite — 1.86 mmol/L
Pyrocat-HD	0.098 g/L of sodium metabisulfite — 1.03 mmol/L of hydrogen sulfite
510-Pyro	None

510-Pyro is the end of that series. PMK and Pyrocat-HD carry a deliberately inadequate preservative; this one carries none at all and replaces the function, in the bottle, with the absence of water. In the tank it has no protection beyond the ascorbic acid, which is one reason every published procedure is one-shot.

Agent and agent. One molecule of phenidone to fifty-one of pyrogallol and eighteen of ascorbic acid. Superadditive pairs in that proportion are the ordinary architecture of modern developers; a superadditive triple is not, and no source explains this one. See the mechanism section.

Developer and everything downstream. A tanning developer hardens the gelatin, which changes how the film behaves in the stop bath, the fixer and the wash, and a staining developer leaves behind a dye that those baths can attack. BERGGER’s four instructions for PMK — non-acid stop, non-tanning fixer, long wash, no hypo eliminator — are the standard practice for the class. Applying them here is the course’s inference, stated as one. A plain water rinse or a citric acid stop used sparingly, an alkaline fixer, and a long wash are the conservative choices.

Variants

DeFehr’s own revision: phenidone from 0.25 g to 0.375 g. This is the only variant of 510-Pyro the course has evidence for, and the evidence is unusual in shape: three independent sources report the change and attribute it to DeFehr, and the course has found no printing of it by DeFehr himself.

• John Finch prints the revised formula as the formula, with the footnote: “Updated from the original 0.25g of phenidone by Jay DeFehr ‘To optimise the three way relationship between pyrogallol, ascorbic acid and phenidone’”. The words inside the inner quotation marks are Finch’s attribution to DeFehr.
• Digitaltruth prints the revised formula and notes that the “original formula called for 0.25g phenidone, but was updated by Jay DeFehr for greater consistency”.
• Bostick and Sullivan sell the concentrate as “Formulated by Jay DeFehr. Updated formula with extra phenidone added”. They publish no composition at all, so this is a report of the change and not a printing of the result.

What the course does with that. The entry above carries 0.25 g, because that is the figure its originator published under his own name and because the course’s rule is that the originator’s own publication is the primary evidence. The revision is not a different developer and it is not treated as a separate formulary entry: it is the same formula with one ingredient raised by half, by the same person, and it is what you are buying if you buy a bottle today. A reader mixing from the table above is mixing the original; a reader opening a bottle from any of the three sellers is very probably using the revision. Both facts are on this page because neither is safe to leave out.

What changes. The phenidone goes from 0.153 to 0.229 mmol/L in the standard bath, and the ratio of pyrogallol to phenidone from 51:1 to 34:1. The stated reasons — “consistency”, and optimising the three-way relationship — are the formulator’s own and are not accompanied by data in any source the course has read. Nothing in the corpus establishes what the change does to development times, which is one candidate explanation for some of the disagreement in the chart and is only a candidate.

No safer variant is offered here. The hazard on this page is pyrogallol, and there is no version of a pyrogallol staining developer without pyrogallol in it. The route that removes the hazardous step is buying the concentrate rather than weighing the powder, and it is described under Safety.

Safety

Level B, and pyrogallol alone decides it. The other three components are milder than the developing agents in most of the developers in this formulary.

Pyrogallol. Its aggregated GHS classification is signal word Warning with the health-hazard and exclamation-mark pictograms: harmful if swallowed, in contact with skin and if inhaled; skin irritation; serious eye irritation; suspected of causing genetic defects; and harmful to aquatic life with long-lasting effects. Kodak’s 1928 primer records a practical hazard as well — the flaky commercial form “flies about the darkroom” and settles as spots on plates and paper, which is why Kodak supplied the crystal. Weighing ten grams of pyrogallol powder is the operation that sets the level of this page, and it happens once per bottle. The controls are gloves, eye protection and extraction or good ventilation for the powder: things a well-equipped home laboratory can provide, which is what separates Level B from Level C. Florida Atlantic University’s guidance for photographic chemicals does not explain how to weigh it; it tells darkroom workers to avoid pyrogallol and catechol and substitute phenidone.

Triethanolamine. The largest component by volume and the mildest by classification. PubChem’s aggregation of the European notifications is unusually lopsided: 4,848 of 6,052 reports — 80.1 per cent — state that it does not meet the criteria for GHS classification at all. The classified minority notify serious eye irritation at 18.5 per cent, and the fuller minority set adds skin irritation, skin sensitisation and respiratory irritation. PubChem’s own hazard text says undiluted triethanolamine is slightly to moderately irritating to skin, that a burn may result from prolonged and repeated contact, and that it has been identified as causing allergic contact dermatitis in occupationally exposed workers. IARC evaluated it in Monographs volume 77 (2000) and placed it in Group 3, not classifiable as to its carcinogenicity to humans, on inadequate evidence in both humans and animals. Its vapour pressure is negligible at room temperature, so it is not an inhalation hazard in a darkroom, though it gives off irritating or toxic fumes in a fire.

⚠The one thing about triethanolamine that is worth knowing before you buy it

Do not let it meet a nitrosating agent. Triethanolamine can be nitrosated to N-nitrosodiethanolamine, which is a known carcinogen and mutagen; the reaction and its conditions are in the literature PubChem carries, and the Cosmetic Ingredient Review panel’s published conclusion on triethanolamine ends with the instruction that these ingredients “should not be used in cosmetic products in which N-nitroso compounds can be formed”.

In a darkroom this is a storage and contamination rule, not a processing one. Nothing in this formula and nothing in a normal black-and-white sequence is a nitrosating agent. Nitrites and nitrous acid are, and so is a nitric acid spill. Keep the bottle away from them, do not decant it into a vessel that held anything of the kind, and buy a grade intended for laboratory or cosmetic use rather than an unlabelled industrial one, since the panel’s conclusion is also conditioned on the level of free diethanolamine in the material.

The course states this because the substance has no encyclopaedia page to state it on. It is a compatibility rule, not a reason to avoid the developer.

Ascorbic acid carries no classification: 97 per cent of the notifications report that it does not meet GHS criteria. Phenidone is Warning, harmful if swallowed and toxic to aquatic life with long-lasting effects, and the aquatic statement is the one that matters for the waste section below.

What is not a hazard here, and why. There is no strong alkali on this page: no hydroxide, no carbonate at 750 g/L, nothing that will burn skin on contact, because the alkali is a weak amine base. There is no sulfur dioxide risk, because there is no sulfite or bisulfite in the formula to be acidified. There is no exothermic dissolution to manage. And the working solution is extremely dilute — a litre of it holds about a gram of pyrogallol, a fifth of that at the stand dilutions — so the concentrated-powder step and the tank step are two quite different risk problems, and only the first of them sets the level.

The route that removes the Level B step entirely is to buy the concentrate. Bostick and Sullivan and Little House of Pyro both sell it made up; the reader who does that never opens a jar of pyrogallol, and handles instead a dilute solution of it in a viscous liquid that does not fly about.

Storage

The concentrate, in a full, tightly closed bottle, in the dark. Little House of Pyro claims six years or more; Finch says years. Two properties of the vehicle drive the storage rules and both are sourced. Triethanolamine is highly hygroscopic, so an open or loosely closed bottle takes up atmospheric water, and water is the one thing this concentrate is designed not to contain. And triethanolamine browns on exposure to air and light, so a clear bottle on a windowsill is the wrong place for it even before the pyrogallol is considered.

A cold store will solidify it. The melting point is 21.5 to 21.6 °C, which is ordinary room temperature. A bottle kept in an unheated darkroom may be a solid or a slush; it is not spoiled, and it needs warming to be poured or measured. That is a consequence of the sourced melting point, not a published instruction.

The working solution is not stored. No source publishes a life for it, every published procedure discards it, and the reasoning is in the Behaviour section: dilution removes the only thing protecting the agents.

Label the bottle with the phenidone figure you used. This is the one storage instruction that is specific to this developer rather than to staining developers in general, and the reason is the section on Variants: two formulas circulate under one name, differing by half a gram of phenidone per litre of concentrate, and nothing on the outside of a bottle will tell you which one you made.

Incompatibilities

• Water, before use. Not a hazard, a design constraint: water in the concentrate is what the formula exists to exclude. Dry the vessel and the measure.
• Nitrites, nitrous acid and nitric acid, with the triethanolamine, for the nitrosation reason set out under Safety.
• Oxidising agents, with the pyrogallol and the ascorbic acid, both of which are strong reducing agents. In a darkroom the practical instances are the bleaches: ferricyanide, dichromate, permanganate and persulfate solutions and their measures.
• Acid stop baths and acid hardening fixers, after development. This is the standard staining-developer caution and it is inference here rather than instruction, but the direction of the risk is clear: an acid bath and a tanning hardener both act on the two things the developer has just put into the film.
• Iron. Kodak’s 1928 primer records that iron gives a dark colour in a pyro solution and that bisulfite is difficult to prepare free from it. There is no bisulfite here, but a rusty spatula or an unlined steel measure is still the wrong tool.
• Aluminium and its alloys are the usual caution for amine solutions in general; the course has no sourced statement for triethanolamine specifically and marks this as unverified. Glass, stainless steel and the common darkroom plastics are the ordinary choices and nothing in the corpus argues against them.

Waste

The phenidone is the reason to take the waste seriously, and it is the smallest quantity in the bottle. Its classification includes H411, toxic to aquatic life with long-lasting effects, which the phenidone entry records as harmonised at law under the European classification regulation rather than merely notified. Pyrogallol carries H412, harmful to aquatic life with long-lasting effects. Ascorbic acid carries nothing, and triethanolamine is not classified for aquatic hazard by the large majority of its notifiers.

Used developer also carries dissolved silver, as every used film developer does, and silver is the component with the strongest environmental case for collection.

The general practice is to collect used developer rather than pour it away, keep it separate from fixer so that the silver-bearing stream stays concentrated and can be recovered, and hand it to a licensed waste contractor or a municipal household-hazardous-waste facility. Local regulation governs, without exception, and it differs between countries and often between water authorities within one country. Nothing on this page is a jurisdictional instruction; check what applies where you are before the first bottle needs disposing of.

Quantities are small in this case and it is worth saying why: a 100 mL bottle at 1+100 makes ten litres of working solution containing about ten grams of pyrogallol, five of ascorbic acid and a quarter-gram of phenidone in total.

Troubleshooting

The negative is stained overall, including the rebate. Image stain sits where development happened; general stain sits everywhere, and the clear rebate is the test. King’s account of stain makes the distinction the diagnostic one. General stain means the developer oxidised in the tank rather than in the film — too long a bath in a half-empty tank, too much agitation, or a concentrate that has taken up water in store.

The negative is thin and flat and the time was right. Check the dilution arithmetic first. DeFehr writes his dilutions “1:100” and means one part in a hundred of water; a reader who reads that as one part in a hundred total has made a bath 1 per cent weak, which is negligible — but a reader who mixes 1 mL into 500 mL when the chart’s time was for 1+100 has made one five times weak, and at these concentrations that is the difference between a negative and a nothing.

The times from the chart do not work. They are not expected to. The chart holds two published times for HP5 Plus, FP4 Plus, Tri-X, Delta 400 and T-Max 400 at the same dilution, meter setting and temperature, differing by up to a factor of 3.4, and it records no agitation regime for any of them. Fix the agitation, pick the shorter time if you are on a rotary processor and the longer if you are agitating intermittently, and test.

The concentrate has gone dark or turned to syrup. Triethanolamine browns on exposure to air and light, and it is hygroscopic. Neither observation proves the developer is dead and the course has no sourced test for it; the practical answer is a clip test against a known negative before committing film to it.

The bottle has set solid in a cold darkroom. Expected: the melting point is about 21.6 °C. Warm it gently to room temperature and it will pour.

Highlights are blocked on variable-contrast paper. This may be the developer working as intended rather than a fault. A yellow-green stain acts as a soft filter on variable-contrast paper, which is Thornton’s finding as Finch reports it, and it is why some printers prefer a catechol developer such as Pyrocat-HD for darkroom printing and reserve pyrogallol developers for scanning and for ultraviolet processes.

The stain faded in the wash or after fixing. BERGGER’s PMK sheet warns that a hypo eliminator weakens the colouring and that the wash should run 20 to 30 minutes because the colouring intensifies during it. Those are instructions for PMK; the course applies them here as inference, and a reader who has lost stain should look first at what the film went into after the developer.

Experiments

1. The agitation series, which is the experiment the sources most obviously leave undone. One film, one dilution, one temperature, four agitation regimes: continuous rotary, ten seconds a minute, one agitation at the half-way point, and none after the first minute. Develop each to the same time and read the density range, or develop each to the same density range and record the time. DeFehr’s own Technical Pan figures predict about a 25 per cent difference between the second and third of those; the chart’s contradictory pairs suggest the spread across all four is much larger. This is the experiment that would let a reader use the published chart.

2. The phenidone question. Mix two 100 mL bottles differing only in phenidone — 0.25 g and 0.375 g — and develop identically exposed strips of one film side by side at 1+100. Read the density range and the time to a fixed contrast index. The formulator’s stated reason for the change was consistency rather than speed, so the interesting result may be the variation between repeats rather than the difference between the means. Record it as a formula version, 510-EB-001 against the base, in the course’s own notation.

3. Stain against silver. Read the same negative twice on a densitometer, through a blue channel and through a green one, and plot the difference against exposure. That difference is the stain, and it is the only way to see the second density on its own. Run it on a 1+100 negative and a 1+500 one from the same roll: if the stain scales with development rather than with agent concentration, the two should differ less than the times suggest.

4. The keeping claim, honestly tested. Nobody can test a six-year claim in an afternoon, but the mechanism can be tested in a week. Make two small bottles from the same batch; add 5 per cent water to one and leave the other neat. Cap both, keep them together in the dark, and clip-test both against the same film at intervals. If the vehicle’s exclusion of water and oxygen is what preserves the agents, the wetted bottle should fail first and visibly.

5. The stop bath and fixer question. The post-development sequence on this page is inference from PMK’s sheet. Test it: four identically developed strips, into a water rinse, a dilute citric acid stop, an alkaline fixer and an acid hardening fixer in the four combinations that matter, then read the blue-minus-green density of each. If the inference is right, the acid hardening fixer costs the most stain. If it is not, this page should say so.