Pyrocat-HD

A staining developer built to be predictable. PMK and the pyrogallol developers before it stain beautifully and misbehave in a rotating drum; Sandy King’s answer was to change the agent rather than the technique, and almost everything else on this page — the brown stain instead of a green one, the tolerance of an acid stop bath, the forty-minute semi-stand times, the absence of the aerial-oxidation precautions that dominate a pyrogallol page — follows from that one substitution.

Stock Solution A — the two developing agents, the restrainer and their acid preservative

Ingredient	Quantity	Form the source specifies
Sodium metabisulfite	10 g	King's table names sodium metabisulfite; his own prose two sections later calls the same ingredient sodium bisulfite, and Photographers' Formulary's sheet names sodium bisulfite at the same weight. The two salts are not the same weight of sulfur; the page works the difference out under Function of every ingredient.
Catechol	50 g	Named pyrocatechin in both printings. Catechol, pyrocatechol and pyrocatechin are three names for benzene-1,2-diol.
Phenidone	2 g
Potassium bromide	1 g
Water	to make 1000 mL	King's instruction is to start with 750 mL of distilled water and to make up to 1000 mL with distilled water, and he gives no temperature for either. Photographers' Formulary's sheet, working at a fifth and a half of this scale, starts its water at 125 °F (52 °C) and makes up at 68 °F (20 °C). The starting temperature differs between the printings; the make-up volume, and therefore the strength, does not.
King's order, and each ingredient is stirred until dissolved before the next goes in. The phenidone is the exception to a plain addition: it is first ground to an even paste with about 5 mL of isopropyl alcohol and the paste is then stirred into the stock. The alcohol is a mixing aid and is not an ingredient of the formula, so it is not listed as one.

Stock Solution B — the alkali, alone in its bottle

Ingredient	Quantity	Form the source specifies
Potassium carbonate (anhydrous)	750 g
Water	to make 1000 mL	700 mL of distilled water to start and distilled water to make 1000 mL. King's warning belongs to the volume: the carbonate must be added very slowly with constant and rapid stirring, because added too fast, or stirred too little, it will be impossible to dissolve it all. The solution warms appreciably as it goes in.

Mixed in the ratio — the standard working solution, for negatives to be printed on silver gelatin papers

1 part Stock Solution A + 1 part Stock Solution B + 100 parts water

To make a standard working solution mix 1 part A with 1 part B with 100 parts water. One Part Stock Solution A + One Part Stock Solution B + 100 parts water. In most localities it is safe to mix the working solutions with tap water; if you see any unusual development artifacts such as sledging, streak marks, or uneven staining, the use of distilled water is recommended.

102 parts in all, so each stock is diluted 102 times. The bath then carries 0.490 g/L of catechol, 0.0196 g/L of phenidone, 0.098 g/L of sodium metabisulfite, 0.0098 g/L of potassium bromide and 7.35 g/L of potassium carbonate.

Mixed in the ratio — for AZO and for the alternative processes - carbon, kallitype, platinum and palladium, Van Dyke, printing-out paper, albumen and salted paper

2 parts Stock Solution A + 2 parts Stock Solution B + 100 parts water

For printing with AZO and most alternative processes, including carbon, kallitype, Pt/Pd, Vandyke, POP, albumen, and salted paper, a 2:2:100 solution is recommended. Two Parts Stock Solution A + Two Parts Stock Solution B + 100 Parts water.

104 parts in all, so each stock is diluted 52 times and every concentration is almost exactly double the standard bath's: 0.962 g/L of catechol and 14.42 g/L of potassium carbonate.

Mixed in the ratio — the energetic bath, for zonal expansion and for negatives intended for alternative processes

1 part Stock Solution A + 2 parts Stock Solution B + 100 parts water

The working solution can be made quite a bit more energetic (faster working) by doubling the amount of B solution. For example, with a 1:1:100 dilution, Ilford FP4+ develops to a CI of .52 in 8 minutes. With a 1:2:100 dilution, development time to the same CI is only 5:30. This fact makes the 1:2:100 dilution very useful for zonal expansion, especially for negatives intended for use with alternative processes.

The one dilution that changes the alkali without changing the agent: 0.485 g/L of catechol, which is the standard bath's, against 14.56 g/L of potassium carbonate, which is the alt-process bath's. The same paragraph, word for word, is on Bostick and Sullivan's sheet.

Mixed in the ratio — semi-stand development of sheet film in trays

1 part Stock Solution A + 1 part Stock Solution B + 200 parts water

Sheet film in trays, semi-stand agitation: special working solution of 1 part A with 1 part B with 200-400 parts water. Agitation is for one minute at start of development, followed by 30 seconds at the half-way point. Development time for slow and medium-speed films is 40-50 minutes, 70º F. Development time for fast films is 50-60 minutes. Dichroic fog may result from extended development of high speed films. If this is a problem in your work use a 1:1:200 dilution and reduce development to about 30 minutes.

The strong end of a range the sheets print as 200 to 400 parts of water. `parts` holds one number and the source gives two, so the range is in Dilutions and in the page body. At 1:1:200 the bath carries 0.248 g/L of catechol and 3.71 g/L of potassium carbonate.

⚠Level C, and the level is about the jar, not the tray

Catechol is classified Level C by this course: signal word Danger, an acute-toxicity pictogram, a substantial minority notification of “may cause cancer”, and a NIOSH skin notation meaning that absorption through intact skin contributes materially to exposure. The control that classification calls for is engineered extraction and managed waste, which a home darkroom does not have, and the course’s position — set out in the staining and tanning developers lesson — is that a reader may study catechol chemistry and may not weigh catechol powder at home.

Weighing fifty grams of solid catechol is the operation that sets this page’s level. It happens once per litre of Stock A and nowhere else. Both makers sell the developer ready-portioned — as a liquid concentrate, or as pre-weighed solids in a bottle you add water to — which removes that step; the reasoning, and what remains once it is gone, is under Safety below.

Purpose

To develop a film to a silver image and a brown stain image at once, and to do it the same way in a tray, a tube, a drum and a tank. The first half of that is what every staining developer does. The second half is what this one was made for, and King says so plainly: in the spring of 1998 he set out to find a staining developer that would work in Jobo processors, print drums and BTZS tubes, in trays with intermittent agitation, and in tanks with minimal or semi-stand agitation. His pyrogallol experiments failed on the first of those — rotary processing gave high fog and uneven staining — and reduced agitation failed on the last, with streaking and mottling.

The substitution that answers both is catechol for pyrogallol. King’s stated reason, quoting Anchell and Troop, is that catechol stains and tans as well as pyrogallol, is generally considered more stable and reliable, and is much less sensitive to aerial oxidation. Aerial oxidation is the mechanism behind general stain — the fog-coloured stain that lies over the whole film whether silver developed there or not — so an agent that resists it is an agent that will survive being whipped through air in a drum.

The second decision was to keep the alkali off the caustic end. King’s account of why previous catechol formulas had not succeeded is that many used sodium hydroxide and were, in Bill Troop’s words as King quotes them, “too alkaline for modern films, tending to create unnecessary fog”, and that too much alkalinity also coarsens grain. Pyrocat-HD uses potassium carbonate instead, and the carbonate page gives the reason the potassium salt rather than the sodium one: solubility. 750 g of potassium carbonate will go into a litre; 750 g of sodium carbonate will not go anywhere near it.

What the name probably says. Neither King nor either kit sheet expands the initials. What both sheets do is open by calling it “a semi-compensating, high-definition developer”, and King’s own list of the developer’s attributes puts very high acutance first and a tight grain pattern second, so high definition is what the documents point at. It is a reading, not a statement any of them makes.

Recommended uses

Large-format sheet film, which is what it was tested on. King works in 4x5, 5x7, 7x17 and 12x20, and every kit sheet repeats that his experiments have centred on sheet film. The published capacities are written per sheet, and the contrast-index charts are made from BTZS-tube processing of sheets.

Rotary processing above all. This is the case the formula exists for. King used PMK for a decade, switched to rotary processing, and began to get high general stain, streaking on the emulsion side and pressure marks on the base side that were invisible in white light and appeared under UV in printing. Rollo Pyro reduced the general stain and did not cure the streaking. His statement about Pyrocat-HD is categorical: it can be used with rotary processing “with absolutely no fear of uneven development, streaking, or staining”.

Dual-purpose negatives — one negative for silver and for an alternative process. A stained negative has two effective printing density ranges, one seen by blue-sensitive silver paper and one by a UV-sensitive process, and King’s curves show Pyrocat-HD opening a wider gap between the two than PMK, Rollo Pyro or WD2D. His reason is the colour: brown is a more efficient actinic filter for UV than green. For a reader who prints both ways from the same negative, that gap is the whole argument.

Negatives for the UV processes on their own, where the required contrast index runs from about 0.68 for platinum to 1.00 for albumen and salted paper. King’s remark on PMK and WD2D is that reaching even a modest density range takes them 20 and 15 minutes, which makes them “only marginally useful for alternative processes in normal SBR conditions, and quite impractical” for expansion development. Pyrocat-HD at 2:2:100 reaches the same place in six.

35 mm and roll film, which King says his later tests support, on the grounds of high acutance and a tight grain pattern. Note that the published roll-film times on the Formulary’s sheet are not his: they are Ed Buffaloe’s, offered as what he actually used.

Minimal, semi-stand and stand development, which is the other end of the same design. King’s minimal-agitation scheme — continuous for the first minute, then 10 seconds every third minute — is claimed to buy maximum adjacency effects, a compensating effect and increased emulsion speed together, at the cost of about 50 per cent longer development.

A darkroom on a budget. King’s tenth attribute is cost: mixed from scratch, Pyrocat-HD is much more economical than the pyrogallol developers, because there is less than half a gram of developing agent in a litre of the working bath.

When another formula is preferable

• Where no fume cupboard or equivalent extraction is available and the reader wants to mix from solids. This is the first question, not the last. See Safety: the answer is to buy the liquid concentrate or to choose a developer whose agent is not catechol.
• Where a stain is not wanted at all, D-76 or D-23. A hundred grams of sulfite to the litre is the opposite end of the same dial, and D-76 is the developer every other one in this formulary is described against.
• Where a green stain is wanted rather than a brown one, PMK. The two colours behave differently on variable-contrast paper, and the difference is not a matter of taste: a green stain shoulders the highlights and holds them, a brown stain does not and gives more highlight contrast instead. King states both halves.
• Where the process must be a familiar one and the developer must be forgiving. King’s own warning is that “the use of Pyro developers is incompatible with sloppy technique” and that photographers careless in the darkroom should not consider using pyro at all.
• Where the negatives are 35 mm and the printing is by projection at a large magnification. The grain claims for this developer are the formulator’s and are not measured, and there is no silver solvent in the bath to reduce grain by dissolution. D-76 has a hundred grams of sulfite doing exactly that job.
• Where a rapid dish developer with a stain is wanted, D-167, which develops in two to three minutes.
• Where high contrast is the point rather than a long scale, D-19. Pyrocat-HD will reach a very high contrast index, but it does it by time and dilution rather than by an energetic bath, and D-19 is what an energetic bath looks like.

Mixing

Two bottles, mixed once, then a working bath mixed at the tank and thrown away. King’s order for Stock A is the order below and each chemical is stirred until it has dissolved completely before the next is added — which is Kodak’s general rule from 1928 and matters here for the reason it always matters: a crystal still sitting in the solution when the next ingredient arrives oxidises at its surface.

1. Stock Solution A, 1,000 mL. Start with 750 mL of distilled water. Add 10 g of sodium metabisulfite and stir until dissolved. Add 50 g of catechol and stir until dissolved — this is the step that needs extraction, and it is the only one that does. Mix 2.0 g of phenidone with about 5 mL of isopropyl alcohol into an even paste, add the paste to the stock and stir until dissolved. Add 1.0 g of potassium bromide and stir until dissolved. Distilled water to make 1,000 mL.
2. Stock Solution B, 1,000 mL. Start with 700 mL of distilled water. Weigh out 750 g of potassium carbonate and add it very slowly, with constant and rapid stirring. Distilled water to make 1,000 mL.
3. The working developer. 1 part A, 1 part B, 100 parts water for silver printing; 2 parts A, 2 parts B, 100 parts water for AZO and the alternative processes. Tap water is acceptable in most places, and distilled water is the remedy if you see sledging, streaks or uneven staining.

⚗Why the phenidone goes in as an alcohol paste and the others do not

This is the one mixing instruction on the page that is not about oxidation, and the phenidone entry says why the course cannot give a number for it: PubChem’s solubility heading for phenidone returns no values at all, so the encyclopaedia has no figure for how much of it a litre of water will hold. What that page holds instead is practical evidence, and it is the same knowledge as King’s instruction in a different form: it records that ILFORD’s two-part powder developers tell the user to dissolve the smaller, phenidone-bearing part first in warm water, and that a few grains left undissolved are normal. Its closing advice is “do not assume a phenidone powder will simply dissolve”.

Two grams is a small weight in 750 mL and a stubborn one. Ground to a paste with a few millilitres of isopropyl alcohol it goes in as a suspension of already-wetted particles rather than as a floating powder, and it dissolves. The alcohol is a handling aid: five millilitres in a litre is half a per cent, and it is not treated as an ingredient by either printing of the formula.

⚗Why 750 g of carbonate goes in slowly, and why the beaker gets hot

King’s warning is unusually specific: add the carbonate very slowly with constant and rapid stirring, because “if you add the solution too fast, or don’t stir enough, it will be impossible to dissolve all of it in the water”, and “as you add the chemical an exothermic reaction takes place and the solution will warm up appreciably”.

Both halves are the same fact. The potassium carbonate page gives the solubility from Chemical Safety Card 1588 as 112 g per 100 mL of water at 20 °C, so 750 g in a made-up litre is well inside the limit at equilibrium — but a heap of carbonate dumped into water sets locally, and once it has set there is very little water left to attack it with. Dissolution also releases heat, and a solution that is warming while you stir is a solution whose local concentration is running ahead of your mixing.

The second warning is the deliquescence one, and it is the more expensive mistake. Potassium carbonate takes up water from the air. King: weigh out 100 g of carbonate that has absorbed a lot of water and “the actual chemical weight might be only 60-80g, which of course would produce a much weaker B solution than the formula requires”. A damp tub does not change the developer’s ingredients; it changes its development time, silently, in the direction of underdevelopment.

🧮What the two standard dilutions actually put in the tank

One millilitre of A, one of B and a hundred of water make 102 parts, so each stock is diluted 102 times:

50 g/L in A × (1 ÷ 102) = 0.490 g per litre of working solution

Catechol

2 g/L in A × (1 ÷ 102) = 0.0196 g per litre

Phenidone

10 g/L in A × (1 ÷ 102) = 0.098 g per litre

Sodium metabisulfite

1 g/L in A × (1 ÷ 102) = 0.0098 g per litre

Potassium bromide

750 g/L in B × (1 ÷ 102) = 7.35 g per litre

Potassium carbonate

At 2:2:100 the total is 104 parts and each stock is diluted 52 times, so every figure above is very nearly doubled: 0.962 g/L of catechol and 14.42 g/L of carbonate.

Now in moles, because the ratios are the design:

0.490 g/L ÷ 110.11 g/mol = 4.45 mmol/L

Catechol

0.0196 g/L ÷ 162.19 g/mol = 0.121 mmol/L

Phenidone

7.35 g/L ÷ 138.21 g/mol = 53.2 mmol/L

Potassium carbonate

0.098 g/L ÷ 190.11 g/mol = 0.516 mmol/L, which releases 1.03 mmol/L of hydrogen sulfite

Sodium metabisulfite

0.0098 g/L ÷ 119.00 g/mol = 0.082 mmol/L

Potassium bromide

Three ratios worth carrying away. There are about 37 molecules of catechol to every one of phenidone — 25 to 1 by weight, and the difference between those two numbers is only the difference in molar mass. There are about 12 carbonate ions to every molecule of catechol, which is what a developer looks like when the alkali is doing the driving. And the preservative is one hydrogen sulfite ion to every four molecules of catechol.

Set that preservative against a normal developer and the design becomes obvious. D-76 carries 100 g/L of sodium sulfite, which is 0.79 mol/L. Pyrocat-HD’s standard bath carries 1.03 mmol/L of sulfur as hydrogen sulfite — about one seven-hundred-and-seventieth of it. PMK’s working bath carries 1.86 mmol/L, so Pyrocat-HD at its standard dilution has a little over half of PMK’s already deliberately inadequate preservative; at 2:2:100 it carries 2.02 mmol/L and slightly overtakes it.

Behaviour

It is dilute, it is one-shot, and it is unusually indifferent to how it is agitated. Eight to fifteen minutes at 21 °C for the published sheet-film and roll-film times, from a bath holding under half a gram of developing agent per litre — and forty to sixty minutes when the same developer is diluted a further two to four times for semi-stand and stand work.

The alkali is the throttle and the sheets say so directly. Doubling the B solution at constant A — the 1:2:100 dilution — takes Ilford FP4 Plus to a contrast index of 0.52 in 5 minutes 30 instead of 8 minutes. The agent concentration is unchanged; only the pH has moved. That is why the alt-process dilution is 2:2:100 rather than 1:2:100: King wants both more agent and more alkali for a bath that has to reach a contrast index of 0.7 to 1.0.

It resists aerial oxidation, and that is the behavioural claim everything else rests on. King: “Because of its high resistance to aerial fog Pyrocat-HD is an excellent developer for tray development. Negatives developed in trays, even with long development times, will have very little general stain.” The reason he gives is the agent, not the formula — catechol does not oxidise as rapidly in alkaline solution as pyrogallol does.

The one exception he names is fast rotation. “With very active agitation, as in Jobo at maximum speed, Pyrocat-HD does indeed oxidize,” so he recommends the slowest rotation available. Where that is not possible he offers two remedies and is careful to say they are alternatives — “or (but not also)”: add 30 per cent more of stock A when making the working solution, or add about 0.3 g/L of sodium sulfite to each litre of working solution.

🧮The two anti-oxidation remedies are not the same size

King offers them as equivalents. They are not, and a reader adjusting a bath should know by how much.

0.3 g/L ÷ 126.05 g/mol = 2.38 mmol/L, on top of the bath’s own 1.03 mmol/L — a total of 3.41 mmol/L, more than three times the sulfur

Adding 0.3 g/L of sodium sulfite

1.03 mmol/L × 1.3 = 1.34 mmol/L — about a third more sulfur

Adding 30 per cent more of stock A

The sulfite route multiplies the preservative by 3.3 and changes nothing else. The extra-A route multiplies it by 1.3 and also raises the catechol, the phenidone and the bromide by 30 per cent, which changes the development time as well as the fog.

This is the course’s arithmetic on King’s published figures, not a correction of his advice. What follows from it is practical: the two remedies will not land in the same place, and the one that leaves the developer otherwise untouched is the sulfite.

The stocks keep for months to years and the working bath is not kept at all. Stock A for up to a year in partially full bottles by King’s account and about six months by the Formulary’s; Stock B indefinitely, which is unsurprising for a bottle containing nothing but carbonate. No source publishes any keeping time for the diluted bath, and every published procedure discards it after one use.

A pre-soak is specified, and how strongly depends on the method. King makes it optional for tray and tank work, essential for rotary processing, and strongly recommended for minimal and stand development — five minutes in each case, and distilled water for rotary. Both kit sheets shorten it to two minutes and drop the conditions. His reason for stand development is specific and worth having: an air bubble that forms during a forty-minute stand will spread an irregular circular pattern a quarter to three-eighths of an inch across, and ruin the frame.

Two capacity figures, and they differ by half as much again. King asks for a minimum of 50 mL of the standard dilution per 4x5 sheet; both kit sheets ask for 75 mL for the same operation. The course prints both.

🧮King's capacity rule is one number, printed seven ways

His list looks like seven separate recommendations and is one:

50 mL ÷ 20 in² = 2.5 mL per square inch

4x5, 20 square inches

80 in² × 2.5 = 200 mL, which is what he prints

8x10, 80 square inches

154 in² × 2.5 = 385 mL, which is what he prints

11x14, 154 square inches

240 in² × 2.5 = 600 mL, which is what he prints

12x20, 240 square inches

Every entry in the list is 2.5 mL per square inch to within a rounding — 5x7 comes out at 87.5 and he writes 90, 7x17 at 297.5 and he writes 300. So the rule for any format is 2.5 mL per square inch, or 1 litre per 2,580 cm². The kit sheets’ 75 mL per 4x5 is 3.75 mL per square inch, or 1 litre per 1,720 cm² — and that is the same figure per square inch as Photographers’ Formulary’s minimum for PMK, which is 300 mL per 80 square inches. Whether that is a considered figure for this developer or a house rule carried across from another one, the documents do not say.

Image characteristics

A brown stain, and it is stronger than it looks. King is emphatic on this because it catches people: photographers coming from PMK or Rollo Pyro “nearly always underestimate the intensity of the Pyrocat-HD stain because by comparison with the green or greenish/yellow stain of these developers the brown Pyrocat-HD stain looks almost neutral in color”. In printing it is as great or greater in effect.

On graded silver paper, slightly more contrast than a green stain gives. Graded papers have some green sensitivity, and brown blocks green better than green does.

On variable-contrast paper, more highlight contrast and less compensation. This is the sharpest practical difference between the two developers in the course, and King sets out both sides. A green stain acts as a continuously variable contrast filter that holds highlights extremely well — at the cost of shouldering the curve, which flattens the upper mid-tones and highlights, and which King notes is the standard objection to PMK. A brown stain blocks green light more effectively, so there is much less shouldering: more contrast in the highlights, less compensating effect. Which of those two you want is a picture decision, not a technical one.

On AZO and under UV, a large gain. AZO has plenty of blue sensitivity and much less green than ordinary papers, and the alternative processes are UV processes. For a given visual density a Pyrocat-HD negative prints AZO with more contrast than PMK or Rollo Pyro, and King’s general figure for what a pyro stain does under UV is that it can increase exposure by as much as a full stop and add about log 0.30 of density or more to the top of the curve.

Stain is proportional to silver, not to the film. King’s finding contradicts a widespread belief and he says so: “There appears to be a universal acceptance in some quarters that certain films work better with Pyro developers than others because they stain better… My own testing, however, does not support this conclusion.” What he found is that image stain follows the negative’s overall density rather than the film type, and that the films which appear to stain better are simply the ones with thicker gelatin — which gives more general stain, the undesirable kind.

A tanned emulsion. Catechol hardens the gelatin it develops. King attributes part of the sharpness of a pyro negative to this: the tanning reduces irradiation and infectious development, and there is little migration of silver halide during development. The catechol page records that the course can describe neither the oxidation product nor the mechanism by which it hardens gelatin, and nothing here fills that in.

⁇Do pyro negatives really print differently? Two named experts, opposite answers

King puts the disagreement on the page himself and the course keeps it there.

Phil Davis, asked by Steve Simmons whether prints from a PMK negative and a non-PMK negative of identical curve would look the same: “if it were possible to produce identical film curves with PMK and some other developer on the same film, and if the prints were matched at two or more density levels, and if the tests were conducted with a panel of unbiased viewers under totally ‘blind’ conditions, I suspect that the prints would be judged to be indistinguishable.”

Gordon Hutchings, on the same question: “Would two prints from identical gamma pyro and non-pyro negatives look the same in the prints? Absolutely not. If there was no difference in developers we would never have needed but one.”

King’s own position is between them and is offered as experience rather than measurement: he believes most pyro users would say the prints look different, that the reason may be edge effect, highlight separation, internal acutance “or simply the ‘mystery of Pyro’”, and that in his own work he has found pyro developers an extremely useful tool.

What the course holds: the sensitometric claims on this page that carry measurements — the speed figure, the contrast-index charts, the UV-versus-blue gap — are King’s own and are cited as his. The claim that a stained negative prints differently from an identically curved unstained one is not settled by any document the course has read, and this page does not settle it.

The mechanism

Two developing agents reduce exposed silver halide to metallic silver, and one of them leaves its oxidation product in the gelatin on purpose.

AgBr + e⁻ → Ag + Br⁻

What both agents are ultimately doing

The course does not write catechol’s oxidation. It cannot name the coloured product, cannot give the equation, and cannot describe how the same chemistry hardens gelatin. That gap is stated on the catechol page and argued out in the staining lesson, and nothing on this page quietly fills it in. What can be explained is the architecture: why two bottles, why an acid preservative, why so little of it, why potassium carbonate, and what happens the moment the stocks meet.

⚗Why the alkali has to be above pH 9, and what the carbonate is doing there

Catechol is a weak acid twice over. The IUPAC dissociation-constant dataset gives its first ionisation at pKa₁ 9.23 to 9.45 near room temperature and its second, less certainly, between 12.6 and 13.7.

C₆H₆O₂ → C₆H₅O₂⁻ + H⁺

First ionisation: pKa1 about 9.2 to 9.5

Below pH 9 the molecule is essentially undissociated, and an undissociated phenol is not a developing agent. That single constant explains the whole history of catechol formulas: Wall’s 1924 carbonate formulas are slow because a carbonate bath sits close to the first constant, and the caustic ones are fast because they run well past it.

The potassium carbonate page supplies the other half: carbonic acid is weak, so a dissolved carbonate is alkaline, and the carbonate–bicarbonate pair buffers near the pKa of 10.33 that follows from the second ionisation constant. A bath held near 10.3 is comfortably above catechol’s 9.2 to 9.45 and comfortably below the caustic region that King says fogs modern films.

No source the course has read publishes a pH for the Pyrocat-HD working bath, and none is given here. What can be said is where the alkali sits relative to the agent’s first constant, and that the buffer — a base with a conjugate acid — is what keeps it there while the film releases acid into it. Sodium hydroxide has no conjugate acid in the bottle and therefore no buffer, which is the structural difference behind King’s warning about the caustic variation.

⚗Why the preservative is acid, and why it stops being a preservative on contact

Kodak’s 1928 primer states the rule that governs every two-bottle staining developer: “bisulphite is preferable as a preservative in the case of a two-solution developer, since oxidation progresses less readily in acid than in alkaline solution.”

That is why Stock A keeps. It is not the quantity of preservative — ten grams per litre is not much against fifty grams of a readily oxidised phenol — it is the pH. The metabisulfite page gives what happens when the salt meets water:

Na₂S₂O₅ + H₂O → 2 NaHSO₃

Metabisulfite becoming bisulfite in the beaker

and hydrogen sulfite in solution is acidic. In that bottle the catechol is nearly stable.

The moment 10 mL of B goes into the tank, the protection is spent, and King describes exactly this. His account of why the developer needs so little sulfite is worth reading twice: the sodium bisulfite in the A solution “serves two roles: 1) the sodium bisulfite acts as a preservative in the stock solution, and 2) when it mixes with the potassium carbonate alkali it forms sulfite through reaction”. The bisulfite page carries the equation, which Kodak writes for sodium carbonate:

NaHSO₃ + Na₂CO₃ → Na₂SO₃ + NaHCO₃

Kodak's equation, written for sodium carbonate rather than for the potassium salt in this formula

The course prints Kodak’s equation rather than rewriting it with potassium, exactly as the PMK page declines to rewrite it for metaborate. What needs no rewriting is the stoichiometry, because that follows from the published weights alone: the working bath holds 1.03 mmol/L of hydrogen sulfite against 53.2 mmol/L of carbonate, so at most about 1.9 per cent of the alkali is consumed. Kodak notes that the bisulfite “exerts an apparent restraining action” by destroying an equivalent of the alkali; in a pyro-soda formula that term matters, and here it does not.

⚗Superadditivity, and how little phenidone it takes

King’s claim for the second agent is one sentence: Pyrocat-HD “is based on Pyrocatechin but also contains a small amount of Phenidone that is strongly super additive with Pyrocatechin”. He offers no data for it, and the course records it as the formulator’s statement.

What the arithmetic adds is the scale. There are 37 molecules of catechol in the tank for every molecule of phenidone, and 0.121 millimoles of phenidone in a litre — about a fiftieth of a gram. A pair of agents that together develop faster than the sum of their separate rates is the definition of superadditivity, set out in the superadditivity lesson, and the signature of the effect is exactly this: a trace of one agent transforming a bath made almost entirely of the other.

The comparison with PMK is instructive and is arithmetic rather than opinion. PMK’s second agent is metol at 0.28 mmol/L of the salt, which releases two molecules of the active amine per formula unit; Pyrocat-HD’s is phenidone at 0.121 mmol/L. So Pyrocat-HD runs on roughly a fifth of the second-agent molarity — and King’s own substitution rule points the same way from the other end, since replacing the phenidone takes ten to twelve times its weight of metol.

🧮Where Pyrocat-HD sits against PMK, in moles

Both are two-bottle staining developers used at about 1 per cent of stock. The PMK page works out its own bath; here they are side by side, per litre of working solution.

Pyrocat-HD 1:1:100, catechol 4.45 mmol/L — PMK 1+2+100, pyrogallol 7.70 mmol/L

Developing agent

Pyrocat-HD, potassium carbonate 53.2 mmol/L — PMK, sodium metaborate 42.3 mmol/L

Alkali

Pyrocat-HD, hydrogen sulfite 1.03 mmol/L — PMK, 1.86 mmol/L

Preservative

Pyrocat-HD, phenidone 0.121 mmol/L — PMK, metol salt 0.28 mmol/L

Second agent

Pyrocat-HD’s standard bath carries about 58 per cent of PMK’s developing agent and about 126 per cent of its alkali, and King’s published claim is that it reaches an equivalent contrast in about 70 per cent of PMK’s time for silver printing and about 50 per cent for a UV process. Less agent, more alkali, shorter times: that is a developer whose speed comes from pH rather than from concentration, which is the same lesson the 1:2:100 dilution teaches inside the formula itself.

Function of every ingredient

Catechol, 50 g in Stock A — 50 g per litre of stock, 0.490 g/L in the standard tank. The developing agent the formula exists for, printed as pyrocatechin in both sources; catechol, pyrocatechol and pyrocatechin are three names for benzene-1,2-diol, the ortho isomer of hydroquinone. It reduces exposed silver halide like any other agent, and like pyrogallol and unlike every non-staining agent in this formulary its own oxidation product is deliberately retained: coloured brown, laid down in the gelatin in proportion to the development that happened there, adding a second density to the negative. It also tans the gelatin it develops, which is part of King’s explanation for the sharpness and is why a hardening fixer is the wrong fixer here. Its two properties that decide the formula’s architecture are its first dissociation constant, 9.23 to 9.45, which sets the alkali needed to make it work at all, and its comparative resistance to aerial oxidation in alkaline solution, which is the entire reason for choosing it over pyrogallol. More catechol gives more silver density and more stain together and needs the times re-established; less gives less of both, and the 2:2:100 dilution is exactly this knob turned up for processes that need a contrast index near 1.0. Three facts belong to its own page and matter at the bench: it browns in air and light, it has a high enough vapour pressure that you can smell an opened bottle of the solid, and it is the ingredient that sets this page’s safety level.

Phenidone, 2.0 g in Stock A — 2 g per litre of stock, 0.0196 g/L in the standard tank. The second developing agent and the smallest ingredient by weight and by moles: a fortieth of the catechol by weight and a thirty-seventh of it by moles. It is the fast, low-alkali agent that starts the image in the shadows, and King’s stated reason for it is that it is strongly superadditive with catechol. It is also the reason the developer holds its film speed: pyrogallol alone loses speed badly, which King names as one of the historic objections to pyro, and a superadditive pair does not. Its practical peculiarity is solubility, which is why it goes in as an alcohol paste and not as a powder. More phenidone would give a faster, flatter negative with proportionally less of the development done by the staining agent, and therefore less stain for the same silver; less would slow the start of development and lose the speed. King’s own alternative is 10 to 12 times its weight of metol, which he says does not affect stain intensity or acutance but may lose a little effective film speed — the three published figures for that substitution disagree, and Variants sets them out.

Sodium metabisulfite, 10 g in Stock A — 10 g per litre of stock, 0.098 g/L in the standard tank. The preservative, and the most carefully judged number in the formula after the phenidone. It does two different jobs in two different vessels and King says so himself: in the bottle it is a preservative, holding Stock A acid so that the catechol is nearly stable, on Kodak’s 1928 rule that oxidation progresses less readily in acid than in alkaline solution; in the tank, the moment it meets the carbonate, it is converted to sulfite and becomes the working bath’s entire sulfite content. That content is one seven-hundred-and-seventieth of D-76’s. The reason it must be that small is the reason every staining developer is under-preserved: sulfite destroys the coloured oxidation product that the stain is made of, so a normal preservative would remove the image the developer exists to make. More would keep the stock a little better and visibly reduce the stain; less would give a stock that browns on the shelf. The one place a reader is invited to add some is fast rotary processing, where King offers 0.3 g/L of sodium sulfite added to the working bath — which is a tripling, as the arithmetic under Behaviour shows.

🧮Metabisulfite or bisulfite? The two printings name different salts at the same weight

King’s own table says sodium metabisulfite, 10 g. His prose four sections later says “the small amount of sodium bisulfite in the A solution”. Photographers’ Formulary’s sheet says sodium bisulfite at the same weight. Three statements, two salts.

They are not interchangeable by weight, though they are by function. Metabisulfite hydrolyses to two bisulfites, so:

10 g ÷ 190.11 g/mol = 52.6 mmol, releasing 105.2 mmol of hydrogen sulfite

King's metabisulfite

10 g ÷ 104.06 g/mol = 96.1 mmol of hydrogen sulfite

The Formulary's bisulfite

The Formulary’s stock therefore carries about 9 per cent less sulfur than King’s, at the same printed weight. To match King’s figure with the bisulfite you would need 10.95 g.

Nine per cent of a preservative that is already at one seven-hundredth of a normal developer’s is not a difference a reader can see on a negative, and the course does not present it as one. It is here because the page prints a weight against a named substance and the two sources name different substances, and Rule 1 says what the sources say rather than what is convenient. The formula object on this page carries King’s salt, because King devised the formula.

Potassium bromide, 1.0 g in Stock A — 1 g per litre of stock, 0.0098 g/L in the standard tank. The restrainer, and by any ordinary standard there is almost none of it. D-19 carries 6.3 g/L in its working bath and DK-50 carries 0.5 g/L; this bath carries under a hundredth of a gram, about a fiftieth of DK-50’s dose. What it is for is the long development: a bath that will sit on a film for forty minutes with almost no agitation has forty minutes in which to develop unexposed grains, and a trace of bromide raises the barrier to spontaneous development without measurably slowing the exposed grains. More bromide would slow the developer and could shift the tone; less would invite fog in the long-development schemes, which is where King’s warning about dichroic fog on extended development of high-speed films belongs. This is the ingredient the two printings disagree about: King gives 1 g per litre of stock and Photographers’ Formulary gives the equivalent of 2 g. Doubling a restrainer is not nothing, and the course prints King’s figure with the Formulary’s beside it rather than choosing.

Potassium carbonate, 750 g in Stock B — 750 g per litre of stock, 7.35 g/L in the standard tank and 14.4 g/L at 2:2:100. The accelerator, the whole of the second bottle, and the developer’s main control. Its job is to deprotonate the catechol past its first dissociation constant into the anion that actually reduces silver halide, and to hold the bath there while development releases acid into it. Its concentration in the tank is what sets the development time, which is why the sheets’ one published time comparison is a change in the alkali alone: FP4 Plus to the same contrast index in 5 minutes 30 at 1:2:100 against 8 minutes at 1:1:100. Potassium rather than sodium for one reason, solubility: 112 g per 100 mL of water at 20 °C against about 30 g for sodium carbonate, which is what makes a 750 g/L stock possible at all. More carbonate means shorter times and more contrast in a given time, and eventually the fog that King says spoiled the earlier caustic catechol formulas; less means longer times and a flatter negative. Two handling facts belong to it: it is deliquescent, so a damp tub weighs partly water and gives a weak stock, and it dissolves exothermically, so it must go into the water slowly and with vigorous stirring.

Water, 1,000 mL as a make-up volume in each bottle, and a hundred parts of it in the tank. Distilled in both stocks, in both printings — which for Stock B is about avoiding a precipitate in a very concentrated solution and for Stock A is about not introducing anything that will oxidise a phenol. In the working bath, King allows tap water in most localities and names the symptoms that say otherwise: sledging, streak marks, uneven staining. The hundred parts are not a diluent so much as an ingredient: dilution is what buys the long times, and the long times are what the acutance and the adjacency effects of this class of developer are credited to.

Not an ingredient: isopropyl alcohol, about 5 mL. It appears in the mixing instructions and in neither ingredient table, and the page keeps it out of the formula object for that reason. It is there to wet the phenidone into a paste so that two grams of a poorly soluble powder will go into solution, and half a per cent of alcohol in a stock that is diluted a further 102 times is not doing anything in the tank.

Interactions

Between the two bottles, and this is the formula’s defining interaction. Stock A is acid and keeps; Stock B is strongly alkaline and keeps; together they are a developer that oxidises. Mix at the tank, use once, discard. The bisulfite is converted to sulfite at that moment, which is the only sulfite the bath ever has.

Between the bisulfite and the carbonate, negligibly. The neutralisation consumes at most about 1.9 per cent of the alkali, so unlike a pyro-soda formula the preservative does not measurably restrain this developer.

With the stop bath: a dilute acid stop is permitted, and this is a real difference from PMK. King recommends “a dilute acetic acid bath of about 1/4 to 1/2 normal strength” and warns that “if the stop bath is too strong it will reduce image stain”. PMK, by contrast, is given a plain water stop by both its makers, and this course’s PMK page records their reason: an acid stop strips the pyrogallol stain outright. Both kit sheets for Pyrocat-HD nevertheless prescribe a plain water stop for one minute, so the developer’s originator and its two suppliers do not agree, and the conservative reading — plain water, or a genuinely weak acid bath — is the one that cannot cost you the stain. Neither SB-1 nor the citric acid stop bath is at a quarter to a half of normal strength as published, and a reader following King would be diluting one of them further.

With the fixer: it must not harden, and King asks for it to be alkaline. He names Kodak and Ilford rapid fixers, Formulary TF-4, and the TF-3 formula from Anchell and Troop, and says he uses TF-3 himself; the kit sheets say “an alkaline fixer (rapid fix without hardener) for 5 minutes”. The formulary’s own non-hardening bath, F-52, is non-hardening but acid; Reilly’s alkaline plain-hypo bath is alkaline but published for printing-out papers rather than film; and the course’s TF-2 entry is planned and not yet written. The course therefore has no written alkaline film fixer of its own to recommend here, and says so rather than improvising one.

With a hypo clearing agent, favourably — and this is the second real difference from PMK. King prescribes one minute in a hypo-clearing agent and says he uses a 1 per cent solution of sodium sulfite, which is the formulary’s sodium sulfite washing aid. The PMK page records the opposite instruction from BERGGER’s datasheet for that developer, which forbids a hypo eliminator on the grounds that it weakens the colouring. Two staining developers, two opposite instructions about the same tray — and the course has read King’s sheet at first hand and BERGGER’s through its own PMK entry.

⁇The alkaline after-bath: Hutchings prescribes it, King refuses it

Gordon Hutchings recommends an alkaline after-bath for PMK. King names him doing so, and this course’s PMK page prints the procedure from Photographers’ Formulary’s own sheet for that developer: the fixed negative goes back into the used developer for two minutes, because “the alkali after bath induces the formation of stain in the developed negative”.

King’s instruction for Pyrocat-HD is the exact opposite and gives a reason: “Do not use an alkaline after-bath after fixing as recommended by Hutchings for PMK. My tests show that the stain added by this after-bath is primarily b+f, or general stain. General stain increases printing times and does nothing to enhance the printing qualities of your negatives.”

These are two named specialists disagreeing about a step, each writing about his own developer. It is possible that both are right — the stain chemistries are different, and King’s whole case for catechol is that it is less prone to the aerial oxidation that produces general stain — and it is possible that one of them is describing a general property of alkaline after-baths. No document the course has read tests the after-bath on both developers, and the course does not adjudicate. What it records is that the originator of this formula tested it on this formula and says not to.

With agitation, and unusually tolerantly. This is the formula’s design goal and the claim to test first. King: vigorous agitation is not needed as it is with pyrogallol developers; minimal agitation works with most films; stand agitation works with some. The one agitation warning that runs the other way is fast rotation, which oxidises the bath.

With the wash, for 30 minutes by King and 10 to 15 by both kit sheets. The disagreement is a factor of two to three and neither document gives a reason, so a reader who cannot test should take the longer figure: an under-washed negative is a permanence problem that appears years later.

With water quality, mildly. Distilled for both stocks. Tap water for the working bath in most localities, with sledging, streaking and uneven staining as the symptoms that say to switch.

Variants

The sodium carbonate substitution, King’s own. Where potassium carbonate is not available, he gives Stock B as 200 g of sodium carbonate added to 1,000 mL of water, used at 1 part A + 5 parts B + 94 parts water to match 1:1:100, or 2:10:92 to match 2:2:100, and adds that for practical purposes 1:5:100 and 2:10:100 will do with minor adjustments to the times. His reason for the weaker stock is solubility: sodium carbonate is much less soluble than potassium carbonate.

🧮The course's arithmetic does not reproduce the stated equivalence

King says the two carbonates “give virtually identical results when used at equivalent chemical weight in the working solution”, and offers 1:5:94 as exactly equivalent to 1:1:100. The equivalence the course can check is the molar one, and the potassium carbonate page already carries the conversion from Wall’s 1924 formulary: 75 g of dry sodium carbonate is replaced by 98 g of potassium carbonate, a ratio of 1.31 against a molar ratio of 1.30.

750 g/L × (1 ÷ 102) = 7.35 g/L ÷ 138.21 g/mol = 53.2 mmol/L of carbonate

The potassium bath

200 g/L × (5 ÷ 100) = 10.0 g/L ÷ 105.99 g/mol = 94.3 mmol/L of carbonate

The sodium bath, anhydrous

That is 1.8 times as much carbonate, not an equivalence. To match 7.35 g/L of the potassium salt you would want about 5.6 g/L of the anhydrous sodium salt, which at 5 parts in 100 means a stock of about 113 g/L rather than 200.

The obvious reconciliation is the hydrate, and it does not work either. King writes “sodium carbonate” without qualification. The monohydrate at 200 g/L gives 80.6 mmol/L in the bath, still 1.5 times the potassium figure; the decahydrate, the washing soda of the older formularies, gives 34.9 mmol/L, which is 0.66 times it. Neither lands on 53.2. Note also that his sodium stock is 200 g added to a litre of water rather than made up to a litre, which pushes the concentration down a little further and still does not close the gap.

This is the course’s arithmetic against a formulator’s claim, and the course does not know which is wrong — the ratio, the stock strength, the unstated hydrate, or the assumption that “equivalent” means molar. What follows for a reader is practical rather than theoretical: treat the sodium version as a different developer and re-establish your times, which is what King’s own fallback sentence about 1:5:100 and “minor adjustments to development times” quietly concedes.

The sodium hydroxide substitution, King’s own and hedged by him. He reports that some people use a 10 per cent sodium hydroxide solution as Stock B, mixed 1 part A + 1.5 parts B + 100 parts water, and says it gives similar results to the regular 2:2:100 dilution with potassium carbonate. He hedges it twice: the increased alkalinity may give slightly increased grain with some films, so test carefully; and he recommends it only for negatives meant for contact printing. Two things are worth noticing. The agent concentration in that bath is the 1:1:100 one, not the 2:2:100 one — 0.488 g/L of catechol — so if the results really do match 2:2:100 it is the alkali alone doing it. And sodium hydroxide is a strong base with no conjugate acid in the bottle, so unlike the carbonate it does not buffer: the pH of that bath will fall as development proceeds in a way the carbonate bath’s does not. Neither point is King’s; both are the course’s reading of published properties, and neither is a reason to prefer one over the other without testing.

The metol substitution, and its three incompatible numbers. King’s formula table carries an asterisk against the phenidone: “Or substitute 25 grams of metol (with a slight loss in film speed).” His Variations section says metol can be substituted “at the rate of about 10 parts Metol to one part Phenidone”, which against 2 g of phenidone is 20 g. Photographers’ Formulary’s sheet records that “an earlier version of the formula published on the rec.photo newsgroup called for .25 grams of metol in place of the phenidone” — at its 100 mL scale, 2.5 g per litre. Three figures spanning a factor of ten, two of them King’s own and in the same article.

The course cannot resolve this and does not choose. What it can say is what each would put in the tank: 25 g/L of metol is 0.073 mol/L of the salt in stock and about 1.42 mmol/L of the active amine in the working bath, roughly twelve times the phenidone it replaces on a molar basis; 20 g/L is about nine and a half times; 2.5 g/L is about 1.2 times. King notes separately that his published times are based on the phenidone version and may need adjustment for metol, and the Formulary adds that the metol formulation may be more stable on the shelf. A reader who wants the metol version is testing an untested formula, and should say so in the notebook.

The Post-Factory Photography original, which the course has not read. Photographers’ Formulary’s sheet states that its printing differs from the version published in issue 4 of The World Journal of Post-Factory Photography, and names the difference: King “has modified solution B to use a 100% solution of potassium carbonate instead of a 10% solution of sodium carbonate”. That is a second party confirming that the carbonate stock printed above is the later form. The course has not seen the Post-Factory printing and prints nothing from it.

Pyrocat-HD is offered by its own formulator as an alternative to PMK, and that is how both kit sheets introduce it. It is not a variant of PMK in this schema’s sense — it shares no ingredient with it except water — and no basedOn link is made. The comparison is under Image characteristics and in the arithmetic above.

No course variant is offered. The obvious safening move would be to reduce the catechol, and reducing the catechol produces a different developer rather than a safer version of this one. The available safening move is the one both makers already sell: buy the ready-portioned kit — the liquid concentrate where it is offered — rather than weighing raw catechol.

Safety

Level C, and catechol alone decides it.

Read the catechol page before the jar is opened. Its aggregated GHS classification is signal word Danger with three pictograms including the acute-toxicity skull. Skin irritation is notified by essentially every notifier and serious eye irritation by 95 per cent of them; the acute-toxicity statements come in two grades from the same set, with about 84 per cent notifying “harmful” by mouth and skin and about 16 per cent notifying the higher “toxic” by both routes; “suspected of causing genetic defects” stands at 16 per cent and “may cause cancer” at 11 per cent, and PubChem separately carries the California Office of Environmental Health Hazard Assessment’s statement that catechol can cause cancer according to IARC. NIOSH carries a skin notation, meaning absorption through intact skin contributes materially to the exposure — which is why gloves are a control and not the control.

Photographers’ Formulary’s own sheet is unusually specific about the two properties that matter at the bench. Catechol “has a high vapor pressure and it is a phenol. The high vapor pressure means that solid catechol evaporates readily. When you open a bottle containing solid catechol, you can smell it. Always store solid catechol in a tightly capped glass container. When mixing a solution containing catechol, work in a ventilated area.” And: “The fact that catechol is a phenol means that it is corrosive and can cause skin burns.” The sheet adds that once catechol is in solution, its high vapour pressure is not a problem.

Weighing fifty grams of solid catechol is the operation that sets the level, and it happens once per litre of stock. It is a volatile, absorbable, corrosive phenol with a carcinogenicity notification, and the control its encyclopaedia entry asks for is engineered extraction with managed waste — a fume cupboard, not an open window. Florida Atlantic University’s environmental health guidance for photographic chemicals does not explain how to weigh it; it tells darkroom workers to avoid pyrogallol and catechol and to substitute phenidone.

⚠Why this page is a level above PMK when its formulator's own claim is lower toxicity

Both kit sheets quote King’s advantages over PMK and one of them is “lower toxicity”. King’s own article says that catechol has no history of the long-term health problems associated with pyrogallol, and then immediately says it “should nevertheless be treated with the same level of care since it is also a toxic chemical”.

The course’s classification does not follow the comparison between the two developers. It follows the hazard record of each agent and the control that record calls for, which is the rule the classification page states. Pyrogallol is Level B: its statements are serious and unanimous, but its controls — gloves, eye protection, extraction for the powder — are ones a well-equipped home laboratory can provide. Catechol is Level C: an acute-toxicity pictogram pyrogallol does not carry, a substantial minority carcinogenicity notification, and a skin notation, which together put the recognised control at a fume cupboard with managed waste.

Those two positions are not in conflict. King is comparing chronic outcomes reported in practice; the classification is reading what the notifiers and NIOSH say about the substance. Where a formulator’s comparative claim and a hazard classification point different ways, this course follows the classification and says which it followed.

⚠The route that removes the Level C step

Both makers sell Pyrocat-HD as a liquid concentrate — Photographers’ Formulary as catalogue numbers 01-5081 and 01-5083, Bostick and Sullivan with solution B “sent in solution and ready to use” and solution A supplied dry in its own bottle, to be made up by adding distilled water to the neck, shaking and standing for ten minutes. In neither case does the reader weigh catechol.

Read the two kits carefully, because they are not the same offer. Photographers’ Formulary’s liquid catalogue numbers arrive as solutions and never present the solid at all. Bostick and Sullivan’s kit arrives with solution A already weighed out as a dry chemical in the bottle, so the reader opens a container of solid catechol once, for as long as it takes to pour water in — which is exactly the operation the Formulary’s own sheet describes when it says that you can smell an opened bottle of the solid. Do that outdoors or in moving air, do not put your face over it, and cap it as soon as the water is in.

What remains after that is handling and diluting a concentrate that is about 5 per cent catechol, and a working bath that is 0.05 per cent. The Formulary’s own statement is that once catechol is in solution its vapour pressure is not a problem. That is a Level B operation on this course’s rubric — splash goggles, nitrile gloves, no skin contact, tongs rather than fingers in a tray, and eyewash within reach — and it is what the staining lesson means when it says that a reader who wants a stained negative and has no extraction should buy the liquid rather than the jar.

This is not a way of downgrading the page. The page carries the higher level because it publishes the formula, and a published formula is a set of weighings. It is a statement of which step the level is about, in the terms the rubric uses.

Phenidone is Level B in its own right and there is very little of it: two grams per litre of stock. Handle the powder with the same care as any developing agent — gloves, no dust, no eating in the room.

Potassium carbonate is the alkali burn on this page. A 750 g/L solution is strongly caustic, it is made in a beaker that warms as you stir it, and it is the one step where a splash reaches your eyes rather than your hands. Splash goggles are not optional while Stock B is being made. Keep it off skin, and keep it away from acids.

Sodium metabisulfite is the mildest ingredient here and is the one that gives sulfur dioxide if a strong acid reaches it. Stock A is already acid; keep acids away from it and from its waste.

The working bath is far more dilute than the stocks and is not the dangerous part of this process. Half a gram of catechol per litre, alkaline, in an open tray: gloves and no skin contact, tongs rather than fingers, and a tray that stains is a tray to keep tongs in. King’s own instruction for tray work is “always wear protective gloves with tray development to avoid skin contact”. The stock bottles are where the hazard lives.

Storage

Two bottles, both stoppered, both labelled with the word catechol. The Formulary’s specific instruction for the solid applies to the stock as well as to the jar: a tightly capped glass container, because the vapour is the problem with catechol and a loose cap is a slow leak into the room.

Stock A: up to a year in partially full bottles by King, about six months by the Formulary. The course prints both figures and takes neither as the other’s correction. What is not in dispute is that it is the acid pH of that bottle rather than the quantity of preservative that gives it any shelf life at all, and that a stock which has gone dark has oxidised.

Stock B keeps indefinitely. It is a carbonate solution with nothing in it to oxidise. Its enemy is carbon dioxide from the air, which slowly converts carbonate to bicarbonate and weakens the alkali, so keep it stoppered for the same reason you keep the dry tub sealed.

Keep the dry potassium carbonate sealed above everything else. It is deliquescent and a damp tub weighs partly water; King’s example is that 100 g of a wet tub may be 60 to 80 g of chemical. That error does not announce itself — it produces a weak Stock B and consistently thin negatives.

The mixed working solution is not stored. No source publishes a keeping time for it, and every published procedure discards it after one use.

Label both bottles with the formula, the letter, the strength and the date, per the labelling SOP, and record the batch on the formula version record. Write “catechol — toxic, do not inhale, absorbed through skin” on Stock A in words someone who has never read this page would understand.

Incompatibilities

Strong oxidisers of every kind, which the catechol page lists from NIOSH and CAMEO. A developing agent is a reducing agent, and this one oxidises more readily than most.

Nitric acid, specifically and separately. NIOSH lists it apart from the general oxidisers, and CAMEO records that catechol reacts violently on contact with concentrated nitric acid.

Acids with Stock A, which will drive sulfur dioxide off the bisulfite, and acids with Stock B, which will neutralise the alkali and fizz carbon dioxide vigorously out of a very concentrated carbonate solution. Carry-back from an acid stop bath or fixer is the darkroom’s ordinary version of the second, and it is one reason the stop bath here is dilute.

Alkali and catechol, which is the developer itself. The catechol page lists bases among its incompatibilities, which looks odd on a developing agent until you read the reason: the IUPAC dataset records that catechol’s dianion oxidises readily in air. The alkaline developer is that reaction run deliberately and against a clock, which is exactly why the formula is two bottles and why the mixed bath is used at once.

Fixer, in either direction, and more so than usual with a developer whose product is partly a colour. One set of tongs per tray.

Anything the incompatibilities page lists against a reducing agent, a phenol or a strong alkali, which is the page to read rather than this paragraph.

Waste

Developer waste, alkaline, carrying catechol, phenidone and their oxidation products, plus dissolved silver from the film. It is not the fixer’s silver stream — Kodak’s J-300 guidance records that developer solutions carry negligible silver, so combining the two spoils a recoverable stream and recovers nothing — but it is not clean water either.

Catechol is the reason this stream is collected rather than poured away. Its encyclopaedia entry’s waste ruling is explicit: spent developer in its own labelled bottle, treated as household hazardous waste rather than as drain load, because acute toxicity by mouth and skin sits in the aggregated notifications along with the precautionary codes for avoiding release to the environment. And the whole quantity of catechol you mixed is still in the bottle when the session ends, because a developing agent is consumed only in proportion to the silver it reduces, and this bath reduces very little silver per litre.

pH is the other question a disposal route has to answer. The working bath is a carbonate solution at 7.35 g/L, and Kodak’s own guidance gives 5.6 to 9.4 as the pH window sewer codes most often set. A carbonate bath does not sit inside that window.

Keep it out of the acid stream, both because developer and fixer wastes are collected separately and because Stock A’s bisulfite gives sulfur dioxide with acid.

Collect it, label it, and follow the general chemical waste SOP and the disposal ruling. Photographers’ Formulary’s own sheet says the same thing in one line: “Please consult with local sewer and water authorities regarding proper disposal of darkroom chemicals in your area.” Local regulation governs, and this course cannot tell you what it says where you are.

Troubleshooting

Consistently thin negatives from a new batch of Stock B. Suspect the carbonate before you suspect anything else. It is deliquescent, and a tub that has taken up water weighs partly water: King’s own figure is that 100 g of a wet tub may be 60 to 80 g of chemical, which makes a Stock B a fifth to two fifths weak and lengthens every development time on the page.

Undissolved carbonate that will not go in. It was added too fast or stirred too slowly, and King says so: added that way “it will be impossible to dissolve all of it in the water”. There is no fix in the beaker. Start again, add slowly, stir hard.

Phenidone that floats and will not dissolve. It was added as a powder. Grind it to a paste with about 5 mL of isopropyl alcohol first.

High general stain, and a lot of it in the clear rebate. The diagnostic is King’s: look at the clear area of the film, because image stain is only ever where silver is. If the rebate is heavily stained you have general stain, which increases printing times and does nothing for the picture. The commonest cause in this developer is fast rotation — a Jobo at maximum speed — and the remedies are the slowest available rotation, or one of the two sulfite additions under Behaviour, but not both.

Streaking, mottling or pressure marks that appear only under UV in printing. These are the faults Pyrocat-HD was designed to remove and King reports having eliminated them in his own work. If they appear, look at the mechanical arrangement first — loading, contact with the tube ribs, whether the film was loaded wet where the tube requires it — rather than at the chemistry.

Dichroic fog after a long semi-stand development of a high-speed film. Both kit sheets name this specifically. Their remedy is to use the 1:1:200 dilution and cut the development to about 30 minutes.

Uneven development around one part of the frame after a stand development. An air bubble. King’s description is exact — an irregular circular pattern a quarter to three-eighths of an inch across, spread around the spot where the bubble sat and the developer could not reach. The prevention is the five-minute pre-soak, and there is no cure.

Bromide drag after semi-stand on a high-contrast scene. Buffaloe reports exactly this in the Formulary’s sheet: the bromide released by intense development in the heavily exposed areas diffused out and ruined the roll. Semi-stand is not a general-purpose contrast reduction.

Sledging, streak marks or uneven staining that appear when nothing else has changed. King attributes these to the water and the remedy is distilled water for the working solution.

A weak stain after an otherwise normal process. Look at the stop bath. King’s warning is that a stop bath which is too strong will reduce image stain, and his prescription is a quarter to a half of normal strength; the kit sheets prescribe plain water. A hardening fixer is the other candidate.

Negatives that look flat on the light box but print with more contrast than expected. That is not a fault, it is the developer. A visual reading of a stained negative sees silver only, and King’s warning is that a Pyrocat-HD negative in particular looks more neutral than it is, because a brown stain reads as less colour than a green one to the eye and as more density to the paper.

Hard-to-judge negatives when developing by inspection. Use an amber safelight rather than a green one for transmitted light, because the brown stain blocks green and makes the negative hard to read. King reports this as a conclusion that emerged from discussion among users rather than as his own test, and the course records it that way. By reflected light he says either colour works.

Experiments

Read the stain in three channels and plot all three. King’s Figure 1 is a single negative read visually, in blue and in UV, and it is the clearest demonstration in this course’s sources that a pyro negative carries two densities. Develop a step wedge, read it visual, blue and UV if you can reach a UV densitometer, and measure the gap. The gap is the stain, and whether it widens with exposure tells you whether you have image stain or general stain.

Test the two capacity figures against each other. King says 50 mL of the standard bath per 4x5 sheet; both kit sheets say 75 mL. Develop identical sheets at each and compare density, stain and evenness. One of these is being half as generous again with developer as the other, and the documents cannot say which is right.

Turn the alkali knob at constant agent and confirm the published pair. The sheets state that FP4 Plus reaches a contrast index of 0.52 in 8 minutes at 1:1:100 and in 5 minutes 30 at 1:2:100. That is a single published data point for the most useful control on the page. Reproduce it for your own film, then extend it: 1:1:100, 1:1.5:100, 1:2:100, same time, and read the contrast.

Test the speed claim rather than borrowing it. King measures FP4 Plus at EI 200 at 1:1:100 and ten minutes, against a maximum of 160 for every other developer and dilution he compared. Expose one film at 100, 125, 160 and 200, develop identically, and read the shadow densities. This is the experiment that resolves the claim for your film and your meter and for nobody else’s.

Compare the two stains on variable-contrast paper. Half a subject on Pyrocat-HD, half on PMK, developed to the same visual contrast index, printed to the same shadow value on the same VC paper. King predicts more highlight contrast and less compensation from the brown stain. This is the one comparative claim on the page that a reader with two developers and one paper can settle for themselves.

Establish whether the acid stop really costs you stain. King permits a quarter-to-half-strength acetic stop; both suppliers prescribe plain water. Develop identical sheets, stop half in water and half in a weak acid bath, fix and wash both together, and read the stain density. Then try a full-strength stop on one sheet you do not care about, which is the version of the experiment that shows you the effect at its largest.

Test the after-bath disagreement. Hutchings prescribes an alkaline after-bath for PMK and King refuses it for Pyrocat-HD on the grounds that it adds general stain. Two minutes in the used developer after fixing, against a control that skips it, and read the rebate as well as the image. The rebate is where the disagreement is decided.

Reproduce the sodium carbonate substitution and measure the discrepancy. The arithmetic under Variants says King’s 1:5:94 sodium bath carries about 1.8 times the carbonate of the 1:1:100 potassium bath it is offered as an equivalent of. Develop the same film in both to the same time and read the contrast index. If the two land in the same place, the course’s assumption about which equivalence was meant is wrong, and that is worth knowing.

Push the minimal-agitation claim until it breaks. Normal agitation, minimal agitation at 50 per cent longer, and semi-stand at 1:1:200, on the same film and the same subject. Look specifically for the faults that this developer was made to avoid — streaking, mottling, uneven staining — rather than for a general impression of sharpness. The design claim is that they do not appear, and a design claim is a thing you can falsify.