Kodak T-52

The classical sepia toner, printed by Kodak Limited in London as three solutions and four sentences. It is the formula the schema work was done for: a bleach, a keeping stock, a working bath drawn from that stock, and a sequence of baths that is as much the formula as the quantities are.

⚠Read this before the formula: what is Level C here, and what is not

The formula is classified at Level C, and the operation that puts it there is the weighing. Sodium sulfide sits at Level C on three separate criteria, any one of which would be enough, and the rubric makes engineered extraction the recognised control for weighing it. This entry is at Level C because it publishes the whole formula, including the making of the 200 g/L stock, and that is a Level C operation.

The working bath is not. The course’s ruling of 5 September 2026 is that the operation sets the level rather than the substance: the deliquescent solid is weighed once, outside the session, under the controls its own encyclopaedia entry states — or the weighing is avoided entirely by buying a sulfide stock ready made — and the tray of dilute Solution C is then a Level B operation. A lesson may therefore run at Level B on this formula provided it says so and keeps the stock-making out of the session.

Nothing about the substance’s classification is disputed by that, and nothing on the sodium sulfide page is softened here. What changes is where the Level C work happens: once, at a bench set up for it, and not on the evening you tone a print.

Bleaching Solution — the rehalogenating bleach

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	50 g
Potassium bromide	50 g
Water	to make 1000 mL	Five per cent of each salt. No temperature is given; neither salt needs warm water, and a ferricyanide solution should not be warmed.
This solution has its own entry, because Kodak Limited prints it again unchanged as Solution A of T-56 and the 1928 primer uses the same bath to intensify a negative. Kept in a dark or yellow bottle.

Stock Sulphide Solution — the redeveloper, as a keeping stock

Ingredient	Quantity	Form the source specifies
Sodium sulfide	200 g	pure
Water	to make 1000 mL	A 20 per cent solution. Kodak's 1928 primer and Wall's 1912 dictionary both say why the salt is kept this way rather than as a solid: it deliquesces, and a strong stock made up as soon as the chemical is bought is the only form in which the quantity stays known.
This is a keeping stock and not a bath. Nothing is toned in it.

Toning Solution — the working redeveloper

Ingredient	Quantity	Form the source specifies
Stock Sulphide Solution	50 mL
Water	to make 1000 mL	Kodak Limited prints this as a solution with its own make-up volume rather than as a dilution, which is how the handbook writes every working bath drawn from one of its own stocks.
Thrown away after use. Kodak Limited says so in as many words, and Wall's 1912 dictionary gives the reason: a dissolved sulfide decomposes, and what it decomposes into eventually dissolves the image instead of toning it.

Used in this order — toning a bromide print or lantern slide

1. Bleaching Solution — until the image is bleached — No time is published. The end point is the disappearance of the black, and the print is left holding a faint yellowish image of silver bromide.
2. Water — until the yellow stain is removed — A criterion, not a time. The yellow is bath rather than image, and it is also the acid and ferricyanide that must not reach the sulfide bath.
3. Toning Solution — until the tone is complete — Rapid. Wall's 1912 dictionary describes the action as taking place very rapidly; Kodak's American sheets, for their own formula, give about thirty seconds.
4. Water — brief washing in running water

Bleach in Solution A, wash until yellow stain is removed, and tone in Solution C. Complete with brief washing in running water. Throw away Solution C after use.

Four baths, of which one is the formula's most important and is water. The wash between the bleach and the sulfide is the step that keeps the two chemistries apart, and it is the step every safety source for this process names.

Purpose

To convert a finished bromide print into an image of silver sulfide, in two stages, and change its colour from black to brown in the process. Kodak Limited’s own header is “Sulphide toner. A two-solution sepia toner for bromide paper and lantern slides”, and the footnote adds that a toner of this type was sold ready made as Kodak Sepia Toner.

The 1928 primer explains why the trade went this way rather than any other. Silver sulfide is a very insoluble compound, so a silver image treated with a sulfide is at once transformed into it; and where great permanency is required, prints should preferably be toned to a silver sulfide image, since experience has shown that this form of silver is one of the most stable. Colour and permanence arrive in the same bath, and the permanence was the point long before the colour was fashionable.

Recommended uses

Bromide prints and lantern slides, which is what Kodak Limited names. The process is indifferent to the image’s size and works on a negative as readily as on a print — the 1928 primer’s redevelopment intensifier is this same pair of baths used to add density rather than colour.

Where a print is to be kept rather than looked at once. See Purpose: the permanence claim is the older of the two reasons for this formula.

Where the whole image is to change, rather than part of it. A complete bleach hands the whole picture to the sulfide. Partial toning is possible and is discussed under Behaviour, but it is a different intention and it needs a different bleach.

When another formula is preferable

• Where the sulfide reagent is the objection, the hypo-alum bath T-1a, which reaches a silver sulfide image without a sulfide ever being handled. It is the same destination by a different road, and its road is at Level B.
• Where blue spots or blisters have been a problem, Kodak’s American T-7a, whose bleach carries potassium oxalate against the first and acetic acid against the second. The cost of the acetic acid is that it puts an acid one bath away from a sulfide.
• Where a warmer, redder result is wanted, the sulfide-selenium toner T-56, which is this formula with selenium dissolved into the sulfide stock — and which is at Level D because of it.
• Where the print is to be protected without changing colour, a gold or selenium toner working directly on the silver, with no bleach at all.
• Where a modern packaged toner is available, use it and follow its own sheet. Kodak’s footnote to this formula said as much in 1949.

Mixing

Three bottles, in this order, and never in one vessel.

1. Solution A, the bleach. 50 g of potassium ferricyanide in about 700 mL of cold water, then 50 g of potassium bromide, then water to 1000 mL. Dark or yellow bottle — Wall’s 1912 dictionary gives that instruction for this exact bath. This is the same solution the formulary carries on its own page as the rehalogenating ferricyanide-bromide bleach, and everything there applies here.
2. Solution B, the sulfide stock. 200 g of pure sodium sulfide, water to 1000 mL. This is a 20 per cent solution and it is the form in which the chemical is stored, not a bath. Both Kodak’s 1928 primer and Wall’s 1912 dictionary insist on it for the same reason: the salt deliquesces, so a jar that has been open is a jar whose contents are of unknown strength, and the stock should be made up as soon as the chemical is bought.
3. Solution C, the working toner. 50 mL of Solution B, water to 1000 mL. Made when it is wanted and thrown away afterwards.

🧮What the fortieth actually gives, and what it does not

Solution B is 200 g/L. Solution C takes 50 mL of it to a litre, which is one part in twenty:

200 g/L × 50/1000 = 10 g per litre

Sodium sulfide in the working bath

Ten grams to the litre is the number to carry away from this page, because it is the figure the other published sulfide toners can be compared against. Kodak’s American T-7a in its 1928 printing works out at 10.4 g/L — the same bath by a different route — and the same formula in its 2006 printing at 22.5 g/L, more than twice as strong. Twenty-one years apart Kodak Limited and Kodak in Rochester agreed within four per cent; seventy-eight years apart Rochester disagreed with itself by a factor of two.

The arithmetic that is not done here is a conversion into moles. Sodium sulfide of commerce is a hydrate of variable water content — the 1928 primer records that one part by weight of Kodak’s fused grade was equivalent to about three parts of the crystals — so 10 g of “sodium sulfide” is a weight of a substance whose formula weight depends on the jar. Kodak Limited specifies pure, which narrows it; it does not close it. A molarity computed from 78.05 g/mol would be a number this page cannot support.

Never let Solution A and Solution C meet, in the tray, the sink or the waste bottle. The wash between them is a step of the formula, not a courtesy.

Behaviour

The bleach is judged by eye and the toner by the eye’s failure to see any further change. No time is published for either. What Kodak Limited publishes instead is a criterion for the wash: until the yellow stain is removed.

The redevelopment is fast. Wall’s 1912 dictionary describes the sulfide action as taking place very rapidly, and follows it with ten to fifteen minutes’ washing. A bath that works in seconds is a bath in which nothing can be corrected once it has begun.

The print loses density. Kodak’s own advice for its packaged sepia toners is to develop prints fully and make them slightly darker than normal, because sepia toners reduce print densities. That advice belongs to the process rather than to any one formula, and it is the single most useful piece of planning on this page: the compensation happens at the enlarger, before any of this.

It is a process with a characteristic failure and Wall names its three stages. As dissolved sodium sulfide decomposes it forms hypo; first the image comes up yellow-brown, then it barely comes up at all, and finally the bath acts as a fixing bath and the bleached image disappears. There is no remedy at that last stage, which is why Solution C is mixed fresh and thrown away.

The bleached print between the baths is light-sensitive, because it is silver bromide in gelatin. Kodak’s 2006 sheet, for the packaged version of this process, records that the effect is extremely small and may not be noticeable.

Image characteristics

Warm brown to sepia, and the range is not the toner’s to choose. The 1928 primer states the governing rule: silver sulfide runs from light brown to black according to its state of subdivision, and the subdivision of the toned image follows that of the image it replaced. The paper, the exposure and the developer chose the colour; the toner keeps it. The primer’s practical version of the same rule is that to get good colours in sulfide toning a print should have been fully developed but not over-exposed.

Highlights are where a bad result shows. A print that was under-fixed, over-fixed or under-washed stains in the highlights and borders, and the stain appears only after toning. Kodak’s guidance on print processing is unusually blunt about this: improper fixing is probably the major cause of stains in toned prints.

The gradation survives, the density does not entirely. Nothing in the bleach removes silver, so the distribution of the image is preserved; the loss comes from conversion that is not quite complete and from a substance with a different covering power.

A sulfide-toned print is not identifiable by analysis in the way a selenium-toned one is. The Getty Conservation Institute’s atlas explains why on the silver sulfide page: the baryta layer’s barium sulfate supplies most of the sulfur signal, and the image’s contribution is very hard to separate from it.

The mechanism

Two conversions with a wash between them, and the wash is there because the two chemistries must not meet.

⚗Silver, to silver bromide, to silver sulfide

The bleach oxidises metallic silver and the bromide catches it where it stands.

Ag + [Fe(CN)₆]³⁻ + Br⁻ → AgBr + [Fe(CN)₆]⁴⁻

Solution A: the image becomes a halide again

The sulfide bath then exchanges the halide for a sulfide. It is a straightforward metathesis driven by an enormous difference in solubility: silver bromide’s solubility product is about 5 × 10⁻¹³ and silver sulfide’s about 1.6 × 10⁻⁴⁹, so the reaction has nowhere else to go.

2 AgBr + S²⁻ → Ag₂S + 2 Br⁻

Solution C: the halide becomes the toned image

The 1928 primer states the same thing without the numbers: silver sulfide is a very insoluble compound of silver, so a silver halide treated with a sulfide is at once transformed into it.

Why the intermediate has to be a halide rather than a hydroxide or an oxide. Because silver bromide is insoluble and stays exactly where the silver grain was. Any bleach that puts silver into solution — Farmer’s reducer, for instance — hands the sulfide bath nothing to work on and takes the picture away instead.

Why Solution C is a fortieth of Solution B rather than the stock itself. Concentration is not the control here; the reaction goes to completion at almost any sulfide concentration. What a strong bath buys is speed, and what it costs is the ability to stop. A ten-gram bath finishes in seconds; a two-hundred-gram bath would finish before the print was flat in the tray, and it would carry forty times as much sulfide into the wash water and the waste bottle.

Why the wash between the baths is chemistry and not tidiness. A print carries its previous bath with it. Ferricyanide arriving in a sulfide bath is an oxidiser meeting a reducing agent; acid arriving in a sulfide bath, from a stop bath or an acid fixer earlier in the sequence, is the reaction that makes hydrogen sulfide. The sodium sulfide page carries that rule with its sources, and Princeton’s guidance for photographic studios is explicit that a print must be rinsed thoroughly after an acid bleach before entering the toner. This formula’s bleach contains no acid, which is a real and under-noticed advantage of the 1949 London formula over the American one; the print may still be carrying acid from earlier in its processing.

Function of every ingredient

Potassium ferricyanide, 50 g to the litre of Solution A. The oxidiser. Each hexacyanoferrate(III) ion takes one electron from one silver atom, so the quantity present sets how much silver the bath can bleach before it is spent: 0.152 mol/L here. More bleaches faster and less evenly; less bleaches slowly, which is the controllable direction. It is also the ingredient that carries this bath’s hazard, and the hazard is what it does with an acid rather than what it does with silver — see Safety, and read its own page before weighing it.

Potassium bromide, 50 g to the litre of Solution A. The halide, and the reason this is a toner rather than a reducer: it precipitates the oxidised silver as silver bromide in place. At 0.420 mol/L it is in nearly threefold molar excess over the ferricyanide, which keeps the precipitation ahead of the oxidation. Less and some of the silver comes down as the ferrocyanide instead, which redevelops badly; more and the bromide begins to dissolve silver bromide in its own right, which Wall’s 1924 handbook warns costs image intensity. The full argument is on the bleach’s own page.

Sodium sulfide, 200 g to the litre of Solution B. The redeveloper, and the whole reason this formula sits at Level C. It supplies the sulfide ion that converts silver bromide to silver sulfide, which is the toned image. Kodak Limited specifies pure, and both Kodak’s 1928 primer and Wall’s 1912 dictionary explain what impure means in practice: iron, which separates as a black sludge from a hot solution and can be decanted off, and hypo, which is what a sulfide turns into as it decomposes and which dissolves the bleached image rather than toning it. Wall’s visible test is the crystals themselves — the ordinary greenish-brown sample is to be avoided and white crystals bought instead. More sulfide in the working bath tones faster and buys nothing; less tones slowly and, past a point, incompletely. Its own page carries the hazard, the classification and the incompatibilities, and none of them is restated here.

Solution B, 50 mL of it in every litre of Solution C. Kodak Limited’s own ingredient line for the working toner is a volume of its own stock rather than a weight of a salt, and it is recorded that way here for the same reason the handbook wrote it that way: the reader measures out of a bottle they have already made, and the strength of the bath is a property of that bottle. Writing “10 g of sodium sulfide” in its place would be this course’s arithmetic standing where Kodak’s instruction was, and it would quietly discard the fact that the sulfide is never weighed twice.

Water, three times, and once as a bath in its own right. Each solution is made to its own litre. The fourth appearance of water — the wash between the bleach and the toner — is not a diluent at all but the step that keeps two incompatible baths apart, and Kodak Limited gives it the only criterion in the whole formula: until the yellow stain is removed.

Interactions

With acid, in any form, at any point in the sequence. This is the interaction that governs the process. Ferricyanide and acid give hydrogen cyanide; sulfide and acid give hydrogen sulfide. Both rules are on the chemical pages with their sources, and the practical consequence is the same: no stop bath, no acid fixer and no acid cleaning agent anywhere in the run, and a generous wash between the baths.

With iron. The 1928 primer records that a trace of iron in the ferricyanide-bromide bleach, from a defective enamelled tray, forms blue spots of ferric ferrocyanide. This bleach carries no potassium oxalate, so it has no protection against it. Plastic or glass trays; no steel tongs, clips or hangers.

With hypo, in both directions and for two different reasons. Hypo carried into the bleach turns it into a reducer. Hypo forming inside an old sulfide stock turns the toner into a fixer, which is Wall’s terminal failure. And a print that was fixed badly brings its own hypo to both.

With unexposed photographic materials, which is not a chemical incompatibility but ruins them all the same. The 1928 primer states it as a rule of the darkroom: a very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, so no photographic material should be stored in a room where sulfides are kept or where sulfide toning is done.

With the previous processing, more than with any other formula in this section. Kodak’s guidance is that improper fixing is probably the major cause of stains in toned prints, that an exhausted fixer leaves insoluble silver compounds that form a dark yellow stain on meeting a toner, and that prolonged fixing traps fixer in the paper base and turns sulfide-toned prints yellow. Two-bath fixing and a full wash are part of this formula even though they appear nowhere in it.

With drying, before toning. Wall’s 1912 dictionary makes a claim no other source here repeats: a print should be dried after fixing and washing before it is toned, and toning a print still wet from the wash invites uneven results and muddy tones. It is presented here as his, because nothing corroborates it.

Variants

Kodak T-7a, the American formula for the same process. A stronger bleach carrying potassium oxalate and acetic acid, a 45 g in 500 mL sulfide stock, and a working toner at 22.5 g/L in the 2006 printing or 10.4 g/L in the 1928 one. Its quantities, and the disagreement between its own two printings, are on the T-7a page. It is not a variant of this formula and this is not a variant of it: they are two makers’ formulas for one process, and the course keeps them apart.

Kodak T-56, the sulfide-selenium toner. This formula with 5.7 g of selenium powder dissolved into the sulfide stock, giving warmer, redder browns. See its own page; the selenium takes it to Level D.

Wall’s 1912 mercury-bleach series, which produced tones from warm brown through cool brown to a black richer than the original print by mixing a ferricyanide-bromide bleach with a mercuric chloride bleach in stated proportions, then clearing the mercury out with three changes of dilute hydrochloric acid before toning. It is recorded here as history and nothing more: the course does not use mercury(II) chloride at any level, and a procedure whose middle step is three acid baths on a print destined for a sulfide toner is a procedure this page will not print.

A course variant is not offered, and the reason is that there is nothing to reformulate. The Level C operation in this formula is weighing the sulfide, and a version carrying less of it would be the same deliquescent solid on the same balance for a longer toning time. What the course does instead is what the ruling of 5 September asks for — weigh once outside the session, or buy the stock made up — and where a route with no sulfide at all is wanted, that is a different formula: T-1a, the hypo-alum bath, which reaches the same silver sulfide image and is a different formula rather than a variant of this one.

Safety

Level C. The classification comes from sodium sulfide, which its own page places there on three separate criteria, and from the rubric, which makes engineered extraction the recognised control at that level. Read that page before reading further here; this one does not restate its classification, its exposure limits or its first aid.

Where the Level C work is, and how a lesson routes around it. The course’s ruling of 5 September 2026 is that the operation sets the level rather than the substance. Making Solution B — opening a jar of a deliquescent Level C solid and putting 200 g of it on a balance — is a Level C operation and is done once, outside the session, under the controls the sulfide page states, or avoided altogether by buying a sulfide stock ready made. Diluting 50 mL of that stock to a litre and toning a print in it is a Level B operation. A page may run at Level B on this formula provided it says so and does not put the stock-making inside the session; this entry is at Level C because it publishes both halves.

⚠What makes the sequence more dangerous than either bath

The hazard of this process is not the sulfide alone. It is that a print carries the bath before it into the bath after it, and that both of this formula’s active baths react badly with acid — the ferricyanide to give hydrogen cyanide, the sulfide to give hydrogen sulfide.

One thing this 1949 formula gets right that its American cousin does not: its bleach contains no acid at all. T-7a’s bleach carries 28 per cent acetic acid, and Princeton’s guidance names exactly that combination — rinse thoroughly after an acid bleach before the toner — as the control. Here the only acid in the sequence is whatever the print brought from its stop bath and fixer, and the wash Kodak Limited already demands is the control for it.

The rest of the controls are the sulfide page’s and are not repeated: they include extraction rather than an open window, separate tongs and dishes for the two baths, and no unexposed paper or film in the room.

Storage

Solution A in a dark or yellow bottle, per Wall’s 1912 instruction for this bath, because a ferricyanide solution photolyses to Prussian blue.

Solution B sealed, labelled, dated, and nowhere near an acid. Kodak Limited publishes no keeping figure for it. What the other sources say — and it is theirs, not Kodak Limited’s — is that a dissolved sodium sulfide has very uncertain keeping properties, that it decomposes to hypo, and that a stock which has reached that state will dissolve a bleached image rather than tone it. Wall’s three stages of failure under Behaviour are the test.

Solution C is not stored at all. Kodak Limited’s direction is to throw it away after use, and it is the one storage instruction this formula gives.

Not in the same room as photographic materials. The 1928 primer’s rule, and it applies to the stored bottle as much as to the tray.

Label every bottle with the formula, the solution letter, the strength and the date. A 20 per cent sulfide stock and a 5 per cent bleach look nothing alike but their labels will, and the consequence of confusing them is not a spoiled print.

Incompatibilities

Acids of every kind — stop baths, acid fixers, acetic and citric acid, and any acid used to clean a tray. Two different toxic gases, one from each bath.

Oxidisers, with the sulfide, per its own page.

Iron and steel in the bleach — trays, tongs, clips. Blue spots.

Hypo, into the bleach and inside the sulfide stock. See Interactions.

Metals generally, with the sulfide stock, which its page records as mildly corrosive to most of them.

Unexposed film and paper, in the room, in a drawer, anywhere the smell reaches.

Developer, in either direction, as for every bath in the darkroom.

Waste

Three streams, and the rule is that they never meet.

The spent bleach carries ferricyanide, ferrocyanide and bromide. Its own container, never acidified.

The spent sulfide toner is strongly alkaline and carries sulfide with the bromide and a little silver taken from the print. Its own container. Kodak’s own instruction is explicit and it is the one to remember: sulfide-type toners are never discarded with stop baths or fixing baths, because the combination generates hydrogen sulfide gas — which, the same sheet adds, fogs unexposed paper and film and oxidises unprotected silver images. The usual darkroom habit of neutralising an alkaline waste with acid is precisely the forbidden operation here.

The wash water between the baths carries a little of both and goes to the same place as the rest of the photographic chemistry.

Wall’s 1912 clean-up advice still stands as the first step: flood the measures, dishes and sink with plenty of water afterwards. Then follow the silver-bearing waste SOP, the general chemical waste SOP and the disposal ruling — and note the standing caveat, that local regulation decides and this course cannot tell you what it says where you are.

Troubleshooting

The bleached print will not tone, or tones a weak yellow-brown. Wall’s first and second stages: the sulfide stock has decomposed. Mix a fresh stock, not a fresh working bath from the same bottle.

The bleached image disappears in the toner. Wall’s third stage, and there is no remedy. The sulfide stock has turned into enough hypo to act as a fixing bath. Throw it away and start again with new chemical.

Blue spots. Iron in the bleach, from a tray, a clip or tongs. This formula’s bleach carries no oxalate to protect against it; T-7a’s does.

Yellow stain in the highlights and borders. Almost always the fixing rather than the toning: an exhausted fixer, prints stuck together, or over-fixing that drove fixer into the paper base. Kodak’s guidance on this is under Interactions.

Muddy, uneven tone across the sheet. Insufficient agitation in the bleach, or — per Wall — a print toned straight from the wash rather than dried first.

Blisters or a lifted emulsion. Not this formula’s usual fault, because it has no acid in the bleach; look at the temperature of the baths and at the wash.

The tone is right but the print is too light. Expected, and the compensation is at the enlarger. Kodak’s own advice for the packaged version is to develop fully and print slightly darker than normal.

A smell of rotten eggs. Leave the room and ventilate it, then find the acid, because there is one.

Experiments

Measure the density loss. A step wedge printed on bromide paper, read, toned to completion and read again. Kodak asserts that sepia toning reduces print density; this is how much, on your paper, in your bath. Every subsequent print you plan to tone is exposed against that number.

Put the three published sulfide strengths side by side. This formula’s 10 g/L, T-7a’s 1928 strength at 10.4 g/L and T-7a’s 2006 strength at 22.5 g/L, on identical bleached prints, timed to the point where no further change occurs. Kodak disagreed with itself by a factor of two across seventy-eight years, and nothing in the corpus says what the second number bought.

Watch a stock die. Make a sulfide stock, use part of it at once, and try the remainder at a week, a month and three months on identical bleached strips, recording the colour of the solution alongside the result. Wall gives three stages of failure and no timescale; make one for your own conditions.

Bleach half way. Strips pulled at a quarter, a half and three-quarters of the visible bleaching end point, all toned together in one bath. The result is a split-toned print, and it makes visible the fact that the sulfide can only reach where the bleach went.

Test the drying claim. Wall says a print dried after washing tones more evenly than one toned wet, and no other source in this corpus repeats it. Two identical prints, one dried and one not, through the same two baths in the same minute. This is a hundred-and-thirteen-year-old assertion that has never been checked here.

Read a toned print against an untoned one under a microscope. The 1928 primer’s claim is that the colour comes from the state of subdivision inherited from the original image, not from the toner. The grain of the two prints should be the same shape and a different colour.