Sulfide and Sepia Toning: Bleach, Redevelop, Convert

Kodak’s 1928 primer states the chemistry of sepia toning and its consequence in the same breath. Silver sulfide is a very insoluble compound of silver, so a silver image or a silver halide treated with sulfur or with a sulfide is at once transformed into it; that transformation is by far the most popular method of toning developing-out paper prints; and where great permanency is required, prints should preferably be toned to a silver sulfide image, because experience has shown this form of silver to be one of the most stable. The last of those three is a manufacturer’s claim about permanence, and it is examined rather than accepted in the archival-evidence lesson.

The previous lesson established that a toner replaces the substance of the image rather than colouring it. This page is about how that one replacement is arranged in a tray, by two quite different routes; what each published ingredient is there to prevent; and the single rule every route to it obeys.

Two routes to one compound, and why both survived

The 1928 primer names them and the count has not changed since: direct toning, with the hypo alum bath, and bleaching and redevelopment. Wall’s 1924 handbook draws the same line in the same place — two principal methods of obtaining warm brown or sepia to purplish brown tones, the direct in which the image is converted into silver sulphide direct, the indirect in which the silver is first converted into chloride, bromide or iodide and then into sulphide.

The distinction is not a matter of taste. It decides what the toner has to attack. A direct toner works on metallic silver in place; an indirect toner works on a silver halide that the previous bath made out of that metal. Both arrive at Ag₂S, and both are properly called sepia toning, but they behave differently in the tray and they fail differently.

Kodak’s own explanation for keeping both is industrial rather than aesthetic. It is troublesome to use a bath which has to be heated, so hypo-alum toning was used on the large scale, where a hot tank running all day tones hundreds of prints, while smaller quantities were bleached and redeveloped at room temperature. That trade-off is still the one you make. The hypo-alum bath takes an hour to come up to temperature and a quarter of an hour per print; the bleach-and-redevelop sequence is over in ten minutes and needs no thermometer.

The indirect route, bath by bath

Bleach, rinse, redevelop: the sequence Kodak has printed since 1928

1. 11. A thoroughly washed printFully developed, fixed in a non-hardening fixer, washed. Nothing here is optional: residual thiosulfate turns the bleach into a reducer, and residual silver compounds turn the toner into a stain
2. 22. Rehalogenating bleachPotassium ferricyanide oxidises the image silver; potassium bromide catches it as silver bromide on the spot. The print goes pale yellow-brown. In T-7a this takes approximately 5 to 8 minutes
3. 33. Rinse, at least 2 minutes in cold running waterThis is a step of the formula, not a courtesy. It is what keeps an acid bleach out of an alkaline sulfide bath, and it is the control every safety source for this process names
4. 44. Sulfide redeveloperThe sulfide ion takes the silver from the bromide. About 30 seconds in T-7a, until no further change occurs in the tone. The image comes back brown
5. 55. Rinse immediately, then harden if requiredKodak follows with a dilute hardening bath for 2 to 5 minutes on fibre base, eliminated if a hardening fixer was used - which, for a print meant for toning, it should not have been
6. 66. Wash30 minutes at 18 to 24 degrees Celsius for fibre base, 4 minutes for resin-coated

The steps and times are Kodak's for T-7a; the reasons attached to them are argued in the sections below. The quantities themselves live on the formulary page rather than here.

Two reactions carry the whole route. The first is the rehalogenating bleach, which oxidises metallic silver and immediately traps the silver ion as an insoluble halide before it can diffuse anywhere:

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

Bleaching: the image becomes a halide again, in place

The second is the redevelopment, and it is an exchange rather than a reduction. Nothing is developed in the ordinary sense — no electrons move — because the silver is already Ag⁺ and stays Ag⁺:

2 AgBr + S²⁻ → Ag₂S + 2 Br⁻

Redevelopment: the halide becomes the toned image

⚗Why the exchange goes to completion, and goes fast

The driving force is the difference in solubility. OpenStax’s table gives silver bromide a solubility product of 5.0 × 10⁻¹³ and silver sulfide 1.6 × 10⁻⁴⁹ at 25 °C. Those are not adjacent numbers: they are thirty-six orders of magnitude apart. Any sulfide ion that reaches a grain of silver bromide takes its silver and does not give it back, and the reaction runs until either the halide or the sulfide is gone.

That is why Kodak’s published redevelopment time for T-7a is approximately 30 seconds while its bleach takes five to eight minutes. The bleach is a redox reaction with a rate; the redevelopment is a precipitation that is over as fast as the sulfide can diffuse into the gelatin. It is also why there is no partial-redevelopment technique: you cannot stop this step half way in any controlled sense, so all the control in the indirect route lives in the bleach.

The word redeveloper is Kodak’s and it is worth flagging as a piece of shop language rather than chemistry. In a redevelopment intensifier, run on a negative, the bleached halide really is developed back to silver. Here it is converted to a different compound instead, and the same bleach serves both purposes. The bath that follows decides which process you are running.

What is in a published bleach, and why

A bleach can be two salts. Kodak Limited’s London formula T-52 is exactly that — equal weights of potassium ferricyanide and potassium bromide in water, and nothing else. Kodak’s American T-7a puts two more ingredients in the same bath, and the 1928 primer says in one paragraph what each is answering. This is the paragraph to read if you want to know how a formula gets written.

Potassium ferricyanide is the oxidant. Hexacyanoferrate(III) takes one electron from metallic silver and becomes hexacyanoferrate(II). It is the only ingredient that acts on the image.

Potassium bromide decides whether this is a bleach or a reducer. The silver ion the oxidation produces will go wherever the chemistry lets it. In a bath loaded with bromide it meets a bromide ion and comes down as silver bromide on the site of the grain it came from; in a bath loaded with thiosulfate it forms a soluble complex and walks out of the print, which is Farmer’s reducer. Same oxidation, different anion, opposite outcome. Moersch’s practical rule follows from this: the ratio of the two salts has little effect on the result of toning, while a higher bromide content and a higher pH make the bleach work faster.

Potassium oxalate is there because of enamelled trays. The 1928 primer: if a trace of iron is present in the bleach — for example from a defective enamelled tray — blue spots of ferric ferrocyanide are liable to form, and this tendency is reduced to a minimum by adding potassium oxalate, because the blue iron salt is soluble in the oxalate. Iron(III) meeting the hexacyanoferrate(II) the bleach has just made gives Prussian blue, which is one of the most insoluble and most visible pigments in the darkroom. Oxalate complexes the iron and keeps it in solution, so the pigment never gets the chance to precipitate on the paper. This is a formulation answer to a hardware problem, and Kodak’s other answer to the same problem is the instruction not to use metal trays or tanks at all.

Acetic acid is there to prevent blisters. The 1928 primer states the purpose and no more: acetic acid is added also, to prevent possible formation of blisters. That is the whole of the published reason, and this page does not extend it into a mechanism the source does not give.

!The trade you are actually making between T-52 and T-7a

T-7a’s two extra ingredients each prevent a real defect. But the acid one of them puts into the bleach sits one bath away from a sulfide bath, and the section below is about why that matters more than either defect does. Kodak Limited’s T-52 bleach has no acid in it at all.

So the choice is not “better formula against worse formula”. It is: accept a mildly acid bleach and the two-minute rinse that must then be treated as a step of the formula, or accept a plain bleach with no protection against blue spotting and no protection against blisters. A reader who wants the shortest hazard chain takes the London bleach and gives up the oxalate. A reader who has had blue specks takes T-7a and rinses properly. Both are Kodak’s; neither is a compromise the course invented.

How strong should a bleach be? The one systematic answer in the older literature comes from R. Bullock of the Kodak Research Laboratory, reported in Wall’s 1924 handbook. In the indirect process there is no advantage in increasing the bromide beyond one-third of the ferricyanide. At 10 per cent ferricyanide the colour is more yellowish and the bleaching very rapid; between 3 and 1 per cent the bleaching is practically no longer and the colours are normal; below 1 per cent bleaching is much slower but the colour is just as good. The most advisable strength is therefore about 3 per cent ferricyanide with 1 per cent ammonium bromide. Two of Bullock’s conclusions are warnings rather than settings: excessive bromide in the bleach leads to loss of image, and too long a wash after bleaching is to be avoided. Both are the kind of finding that only shows up in a series, which is what the lab after this page is for.

The sulfide redeveloper, and the salt it is made from

Sodium sulfide is the classical redeveloper and it is an awkward substance to keep. The 1928 primer describes white transparent crystals with a strong affinity for water, deliquescing quickly unless carefully protected from the air, and gives the storage rule that follows: it is best kept in a strong stock solution. Kodak went further and sold a fused grade of definite purity, greyish white, because so much trouble had been caused by impure sodium sulfide — and printed the conversion, one part by weight of the fused salt being equivalent to approximately three parts by weight of the crystals.

That deliquescence is why the course’s ruling of 5 September 2026 matters to this page. Weighing the solid is a Level C operation and remains one. It is done once, outside the toning session, or avoided entirely by buying a sulfide stock ready made; the tray of dilute working toner is then a Level B operation. The classification rubric and the part overview carry that argument, and the formulary entries for T-7a and T-52 are published at Level C precisely because a formula page publishes the whole formula, stock-making included.

⚗Why the bath is alkaline, and why it smells

The sulfide ion is the conjugate base of an extremely weak acid, so in water it takes a proton and leaves hydroxide behind. A sulfide toner is therefore strongly alkaline whether or not any alkali was added to it.

S²⁻ + H₂O ⇌ HS⁻ + OH⁻

Hydrolysis: the reason a sulfide bath is alkaline

Push that equilibrium a second step and you have dissolved hydrogen sulfide, which leaves the solution as a gas. The rotten-egg smell over the tray is not a by-product of toning. It is the equilibrium above, running in air, and it is present the entire time the bath exists. Moersch is explicit that it gets worse in the first rinse water, where the print carries alkaline sulfide into a dish of neutral water and dilutes it.

The working bath is dilute, and it is disposable. Moersch gives 0.5 to 2 per cent w/v sodium sulfide, toning in 30 to 60 seconds after a bleach; Bullock’s optimum is 3 per cent, with the instruction that the bath should be unsparingly used so as not to exhaust it. Kodak Limited’s T-52 working bath falls inside Moersch’s range and Kodak’s American T-7a is stronger — though the same formula number was printed at less than half that strength in 1928, a disagreement the T-7a entry sets out in full. Two things survive the spread. A working bath is a dilution of a stock, not a weighing, which is the practical form of the Level C ruling above. And the bath’s condition matters more than its exact strength: Kodak Limited’s instruction for T-52 is one line and it applies to all of them — throw away the toning solution after use.

A sulfide bath does not stop working. It starts dissolving the picture. Wall’s 1912 dictionary gives the taxonomy of failure, and it is the most useful paragraph in the older literature on this subject: as the dissolved sulfide deteriorates, hypo is formed. The first sign is a yellow-brown image; the next is an apparent failure to act at all; the last is that the bleached image gradually disappears, because the bath has become a fixing bath. CAMEO gives the chemistry underneath it — aqueous sodium sulfide solutions gradually convert to sodium hydroxide and sodium thiosulfate on exposure to air. The 1928 primer adds the same fault from the other end: old sodium sulfide often contains hypo, and if any considerable quantity is present some of the silver bromide will be dissolved and the print will lose strength in the highlights.

The rule that governs every sulfide bath

Everything above is chemistry you can choose to use. This section is not optional and it is the reason this page exists as a separate lesson from the lab that follows it.

⚠Sulfide solutions never meet an acid

Acidify a sulfide and you make hydrogen sulfide. CAMEO records it as a reactivity fact of the substance, and the Environment Agency’s waste guidance writes the equation out. It is the same acid-base chemistry as the hydrolysis above, taken two protons further, and the product leaves the solution as a gas instead of staying in it:

S²⁻ + 2 H⁺ → H₂S

What an acid does to a sulfide bath

The gas is acutely toxic and its warning property fails. HSE’s EH40 gives hydrogen sulfide — its own table spells it sulphide — a long-term workplace exposure limit of 5 ppm (7 mg/m³) as an eight-hour average and a short-term limit of 10 ppm (14 mg/m³) over fifteen minutes. The NIOSH Pocket Guide sets a ceiling of 10 ppm over ten minutes, an IDLH of 100 ppm, and carries the note that decides how this substance is handled: the sense of smell becomes rapidly fatigued and can NOT be relied upon to warn of the continuous presence of hydrogen sulfide. The capital letters are NIOSH’s. A darkroom has none of the instruments those numbers assume, and the alarm everyone trusts is the one that switches itself off.

Three acids are already in a printing darkroom, and each has a route into a sulfide tray: the stop bath, an acid fixer, and — in T-7a — the bleach itself. The controls follow from that list rather than from a general principle.

• Rinse between the bleach and the toner, at least two minutes in cold running water. Princeton’s guidance states the rule in exactly those terms: sulfide toners must not be contaminated with acid, and a print must be rinsed thoroughly after an acid bleach before it enters the toner.
• Treat the pairing as a storage problem too, not only a tray problem. A darkroom holding an acid stop bath and a sulfide stock holds an incompatible pair, and the separation rules of chemical storage apply to the shelf as much as to the bench.
• Never combine the waste. Kodak’s instruction in G-23 is unambiguous: do not discard sulfide-type toners with stop baths or fixing baths, because the combination of these solutions will generate hydrogen sulfide gas. Bottles are separate, labelled and kept apart. What may then lawfully be done with each bottle depends on where you are — the disposal ruling and the general chemical waste procedure govern the containers, and local regulations govern the route. Check your local regulations.
• Ventilation is a control here, not a comfort. Princeton requires local exhaust ventilation for toning solutions. Moersch’s version is blunter and is the one a home darkroom can act on: in a darkroom with no ventilation and little supply of fresh air you should not use sulfur solutions, and if you do not want to give up the advantages of straight sulfide toners you can tone outdoors and water the print twice before returning indoors. Run the ventilation check before the session, not during it.

The second contamination problem, which is not about you

Hydrogen sulfide is also a fogging agent, and an extraordinarily efficient one. The 1928 primer states the consequence as an inventory rule rather than as a warning: a very small quantity of hydrogen sulfide will convert enough of the silver bromide or chloride of a photographic material into sulfide to produce a severe fog, so no photographic materials should be stored in a room where sulfides are kept or where sulfide toning is done. Kodak’s 2006 sheet says the same about the gas released from mixed waste — it can fog unexposed paper and film, and will oxidise unprotected silver images in negatives and prints.

Read those two sentences together and the practical shape of a sulfide session appears. The gas does not distinguish between the print you meant to tone, the box of paper on the shelf, and the negatives in the file. It reaches all of them, and it reaches them by the same chemistry that is toning your print: sulfide finding silver.

Why a sulfide session is separated in time as well as in space

Printing

Enlarger, paper boxes open, negatives outPaper and negatives sealed and removedNo sensitive material in the room

Toning

No sulfide openVentilation checkBleach, rinse, sulfide, rinse, washAiring out

session startchangeoversession end

Two activities that cannot share a room at the same time. Printing happens first, with sensitive material out; the paper and negatives are sealed and taken out of the room before any sulfide is opened, and the room is aired before either returns. Drawn as a sequence, not to a scale of minutes.

This is also why the toned print is washed thoroughly rather than briefly. A print carrying residual sulfide into a drying rack is a source of the same gas in a different room.

Thiourea, the odourless alternative

Sepia toners sold as odourless are thiourea toners; Moersch states it flatly, and the substitution is straightforward at the tray. Thiourea — CH₄N₂S, urea with its oxygen replaced by sulfur — delivers sulfur to a bleached image without a sulfide bath’s smell and without its gas.

The alkali, not the thiourea, steers the colour. This is the interesting part of the chemistry and it has no equivalent in a sodium sulfide bath. Moersch prints the two Agfa formulas as an alkaline thiourea and potassium bromide solution: 520 at thiourea 5 g, potassium bromide 40 g and sodium hydroxide 3 g per litre, and 525 identical but with 15 g of sodium hydroxide. His rule is that the higher the pH, the darker the brown tone: 520 gives a delicate yellowish hue, 525 a stronger and darker one. The baths are long-lived and are regenerated with more alkali rather than more thiourea, which is behaviour consistent with the thiourea acting as a slow reservoir that the alkali unlocks.

⁇The direction of the alkali effect, in one document

Moersch’s text and Moersch’s own product table point opposite ways, and the course reports the disagreement rather than picking a side.

The text says the higher the pH the darker the brown, and its example is Agfa 520 against 525: three grams of sodium hydroxide per litre give a delicate yellowish hue, fifteen grams a stronger, darker one.

The matching table for his own MT3 Vario toner runs the other way. With the thiourea part held at 50 mL, 5 to 30 mL of alkaline activator gives brown-black, 50 to 60 mL medium brown, 90 mL yellowish brown, 130 mL dark yellow and 180 mL bright yellow. More alkali, yellower.

The two are not obviously reconcilable and no source in this course’s corpus resolves them. The possibilities include a different alkali, a different concentration scale, or a maximum in the middle of the range that both accounts sample from different sides. What is not in doubt is that the alkali is the control, that the range of hues available runs from yellow through brown to brown-black, and that the direction has to be established by test on the paper in front of you rather than read off a page. The purplish end of the sepia range is reachable by an alkali step too, but by a different one: Bullock’s finding, below, is a dip in sodium carbonate before the sulfide bath.

And the reason this course teaches the chemistry and gives no procedure for it. Thiourea’s harmonised classification under Regulation (EC) No 1272/2008 is H302 harmful if swallowed, H351 suspected of causing cancer, H361d suspected of damaging the unborn child, and H411 toxic to aquatic life with long lasting effects. Princeton’s guidance calls it a probable human carcinogen because it causes cancer in animals, and advises avoiding it wherever possible. Those codes are the whole of the argument, and reading them off a label is a skill this part assumes rather than teaches. The chemical page sets it at Level C on the rubric’s criterion for carcinogens and reproductive toxins, and the objection there is not the weighing but the substance in an open tray at pH 13 — which is a condition the “weigh it once, outside the session” ruling cannot reach.

That is a genuinely uncomfortable result and it is worth stating plainly rather than burying: the odourless option is not the safer option. It removes a gas with a workplace exposure limit and substitutes a suspected carcinogen and reproductive toxin, in a bath the reader stands over. The course’s answer is to teach thiourea as chemistry, to name the commercial odourless toners for what they contain, and to put its own practical work on the sulfide route with the sulfide route’s controls. One further status note, under the course’s ruling on formulations known only at second hand: the Agfa 520 and 525 numbers above are Agfa’s formulas as reported by Moersch, not Moersch’s own, and the originals are not in this course’s corpus. They are printed here as chemistry and as evidence for the alkali effect, and they are not published as a formula the course endorses mixing.

The direct route: hypo alum, and sulfur the bath makes itself

T-1a reaches a silver sulfide image without a sulfide reagent existing anywhere in the darkroom. The 1928 primer’s explanation is three sentences long and every property of the bath follows from it: if an acid is added to a solution of hypo it tends to precipitate sulfur; a solution of alum in water is weakly acid; so alum added to plain hypo without any sulfite present becomes turbid in time and precipitates sulfur.

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

What acid does to thiosulfate

The primer’s description of the useful state is careful and worth copying exactly: the solution of alum and hypo, at the point where it is ready to precipitate the sulfur, may be considered as having free sulfur in solution. A print in that bath at about 49 °C has its silver converted directly.

2 Ag + S → Ag₂S

Direct toning, with no halide in between

Why it must be hot and must not be hotter. The primer gives 49 to 52 °C as the working range and warns that above 54 °C there is danger of blistering and bleaching of the image; the 2006 sheet forbids anything above 49 °C and more than twenty minutes there, because prints will blister or stain. Wall’s 1924 handbook adds the rule the Kodak sheets omit: with hot toning baths the prints should be allowed to cool before washing, otherwise blisters are very likely to form.

And why it is not simply run cooler. Heat is a rate control here rather than a threshold. The primer’s own chemistry says the alum-and-hypo mixture will precipitate sulfur after a time; warming it shortens that time, which is ordinary activation energy behaviour and nothing peculiar to toning. Wall’s 1924 handbook proves the point from the other side by printing a different direct bath — Baekeland’s alum-sugar-hypo — whose best results come at about 32 °C and whose toning then takes thirty minutes. What does not follow is that Kodak’s formula may be run at a guessed temperature: T-1a has no published room-temperature version, its two printings agree on 49 °C and 12 to 15 minutes, and both put a hard ceiling only a few degrees above the working point. A thermometer is part of the formula.

Why there is silver nitrate and salt in a toner. This is the ingredient nobody guesses correctly, and the 1928 primer answers it directly: a fresh bath tends to weaken the print, eating out the highlights, and to prevent this a little silver must be added to the bath, preferably in the form of silver chloride. The reason is that the bath is also a fixer. Set the finished bath beside plain hypo and it is a fixing bath of about half the usual strength, deliberately acidified and held warm — and a fixing bath dissolves silver, starting with the finest and least protected deposits, which are the highlights. Charging the bath with silver of its own satisfies that appetite. The formula makes the charge in the beaker rather than buying it: silver nitrate dissolved completely first, then sodium chloride, which throws it down as a milky white suspension of silver chloride.

Ag⁺ + Cl⁻ → AgCl

The silver ripener, made in the beaker

The same reasoning explains the primer’s most counter-intuitive statement — that a hypo-alum bath which has been somewhat used works better than a fresh one. A used bath has taken silver from the prints that went before and is no longer hungry.

⚠What the hypo-alum route substitutes, rather than removes

It is worth being exact about what this bath buys. There is no Level C weighing anywhere in T-1a, and no sulfide reagent in the building — which is why the course classes it Level B outright.

What replaces it is a half-strength fixing bath, deliberately acidified, held at 49 °C for a quarter of an hour, and the equation above names the other product of that decomposition alongside the sulfur: sulfur dioxide, which has its own workplace exposure limit and to which many asthmatics are particularly sensitive. Trading a Level C reagent for a gas is a reduction in the hazard chain, not an elimination of it, and the same ventilation control applies.

Polysulfide and brown toners: converting without bleaching

The third member of the sulfide family is the polysulfide or “brown” toner, and it is the one most often chosen by people who want protection with as little colour change as possible. It is a direct toner — no bleach — but unlike hypo-alum it works at room temperature.

The reagent is old and its composition is not fixed. Wall’s 1912 dictionary gives potassium sulphide under its shop names — liver of sulphur, sulphuretted potash, potassium trisulphide — made by heating sulfur with potassium carbonate and pouring the melt onto slabs, and states plainly that it is of variable composition. That is not a gap in the source; it is a property of the material, and it is why this page gives no balanced equation for the conversion of the image. The course does not write a reaction for a reagent whose formula the reference works decline to fix.

The acid rule reaches it anyway, and a regulator has written that part down. The Environment Agency’s waste guidance assigns sodium and potassium polysulphides the same supplemental hazard statement as the simple sulfides — contact with acids liberates toxic gas — and prints a general equation for them in which two protons take one sulfur away as hydrogen sulfide and leave the rest of the chain behind. Indefinite composition is no protection at all: a polysulfide toner is a sulfide toner for every purpose in the section above, including the waste bottle.

What it does is well documented even though its composition is not. Kodak’s packaged Brown Toner converts the silver image to silver sulfide, is used as a dilute working bath, and tones in 15 to 20 minutes at 20 °C or 3 to 4 minutes at 38 °C with continuous agitation; Kodak’s handling note is that the solution is quite alkaline, so plastic tongs and gloves are called for. It is one of the two toners Kodak also recommends for negatives and slides for long-term keeping. ILFORD names polysulphide toners, alongside selenium, as especially recommended for its warm-tone fibre paper, and lists polysulphide among the toners that give a protective effect.

Three behaviours distinguish it in the tray, all from Moersch’s bench notes. It goes on toning in the final wash, which is the usual characteristic of polysulphide toners, so a print pulled at the colour you want will keep moving; the way to stop it is a bath of sodium sulfite after two or three changes of rinse water. Short times — 30 to 90 seconds — give cold browns, and longer times run redder. And used directly on a neutral or cold-tone paper it may produce almost no visible colour change at all while still doing its work: direct sulfur toning does not necessarily give a brown tone with any given paper, but the protecting effect on the silver is there, maximum black can increase considerably, and even short toning times give the highlight densities a stabilising effect. That last property is what makes it a protective toner rather than a colour treatment.

🕰An 1839-to-1924 line that ends in a purple print

Wall’s 1924 handbook prints two liver of sulphur baths. Woodman’s is a piece of liver of sulphur about the size of a pea in 200 ccm of water; Vero’s is 7 g in a litre with a few drops of ammonia, of which Wall records that toning is very rapid and purplish-brown tones are obtained. Both are preceded by hardening in formaldehyde, which the course does not reproduce and which is the reason these are quoted as history rather than offered as procedure.

Bullock’s comparative result belongs beside them: with a given paper and a given black-and-white print, the hypo-alum and liver of sulphur processes give practically identical results, while the indirect methods tend to give yellowish prints. Two direct routes converge; the indirect one is the odd member of the family, and it is odd in a direction you can see.

What sulfide toning does to the print

Density and contrast fall, and you pay for that at the enlarger. Kodak states it for all three of its sulfide-forming toners — Sepia, Sepia II Warm and Brown Toner will reduce print densities, so develop prints fully and make them slightly darker than normal — and separately for T-1a, whose sheet says the toner will cause a loss of print density and contrast that you compensate by increasing exposure and development time. The decision to tone is therefore taken before the print is made, which is the single most practical consequence on this page.

🔬Why the density falls at all, and the one number that exists

Silver sulfide is not a weaker pigment because it is browner. It is a weaker pigment because it covers less. Mike Ware’s account, written for salt prints but general in its physics, quantifies the covering power of the two substances photometrically and gives the ratio: complete conversion of nanoparticle silver to silver sulfide causes a drop in optical density by a factor of about 30.

That number does not transfer to a silver-gelatin print, and Ware says why in the next sentence: in modern papers the higher concentration of silver and its larger particle size make their loss of density on sulphiding far less apparent. A developed gelatin print carries roughly ten times the silver of a salt print and carries it as micron-scale filaments rather than 10-nanometre spheres, so the covering-power penalty is much smaller — but how much smaller is a question no source read for this course answers with a figure.

So there is no published quantitative density loss for sulfide toning a developing-out print. Kodak describes it qualitatively and prescribes a qualitative compensation. The course therefore treats the size of the loss as something to measure rather than to cite, and the lab measures it on a step wedge against an untoned control from the same printing session.

The hue is mostly the paper’s, and only a little the toner’s. Four sources agree on this from four directions.

• The 1928 primer, mechanistically: silver sulfide runs from light brown to black according to its state of subdivision, and the state of subdivision of the toned image depends on that of the untoned image. The hue is inherited from the silver that was there. Its corollary is the printing instruction — a print for sulfide toning should be fully developed but not over-exposed.
• Moersch, by emulsion: a high bromide silver content promotes deep brown hues, while silver chloride emulsions and mixed emulsions with a high chloride content tone yellow, and this is irrespective of which toner is used and largely irrespective of the bleach. Since most current papers carry mixed emulsions, yellow is the default a modern paper drifts towards.
• Bullock, by emulsion speed: the faster the emulsion the more purple the colour, while slow papers tend to give yellowish tones.
• Kodak, for the hypo-alum bath specifically: blue-black images give cold chocolate tones, olive green images give warm sepia.

The developer you used weeks ago is still in the argument. Bullock found that with a given paper the degree of development exerts some action, although this may be masked by the character of the emulsion. Moersch is stronger and more recent: it is by no means irrelevant which developer is used before toning, and whether a warm or cold developer suits a particular case has to be found by trial. The chain is consistent with the mechanism — a developer that changes the size and shape of the silver deposit changes the state of subdivision the toner inherits — but the effect is small enough to be hidden by the emulsion, so it belongs in the notebook as a variable rather than in the plan as a lever.

And one deliberate lever that does exist. Bullock’s most usable finding is that dipping prints for about ten seconds in a 1 per cent w/v solution of sodium carbonate immediately before the sulfide bath tends to give more purplish tones, particularly with development papers and when a chloride bleach has been used. It is the same physical lever the thiourea toners use — alkali at the moment of conversion — applied to a sodium sulfide bath, and it costs one extra dish.

The faults that only appear after toning

Toning does not usually cause the faults blamed on it. It develops them, in the photographic sense: a defect invisible on the untoned print becomes visible when the substance of the image changes. Kodak’s G-23 traces four of them to their origin, and every one is upstream of the toner.

What you see	Where it came from	Why it was invisible before
Dark yellow stain, worst in borders and highlights	An exhausted fixing bath, leaving insoluble silver compounds that washing cannot remove completely	The compounds are colourless until a toner reaches them
General yellowing of the whole print	Prolonged fixing: the paper expands, fixer penetrates the base and cannot be washed out	Fixer in the base changes nothing until sulfide or selenium meets it
Mottle in the base	An overconcentrated stop bath, or too little agitation in the first few seconds in the stop bath	Kodak states outright that the mottle will not be evident until the print is toned
Round purple stains	Air bubbles trapped between or under prints during fixing	Kodak gives the cause and not the mechanism; the course’s reading is a disc the fixer never reached, which looks clean on a washed print until a toner meets what is still in it

Two of those deserve expansion because they are counter-intuitive. An overconcentrated stop bath is not a stronger stop bath somebody mixed wrongly; Kodak’s cause is evaporation — a stop bath left in a tray for more than three days, or in a tank for more than a month, concentrates itself. And prolonged fixing is a fault, not extra insurance: Kodak’s ceiling is about ten minutes for fibre-base papers and two for resin-coated, and going beyond it drives fixer into the base where no wash will retrieve it.

Two more faults come from the toning sequence itself rather than from the print’s history.

A print carried wet from the fixer into the bleach loses its highlights. Moersch gives the mechanism exactly: remains of fixer thiosulfate make the bleach act as a reducer, redevelopment becomes impossible, and at least the highlights vanish irretrievably. That is the same bleach-against-reducer distinction from the bleach section, arriving inside the gelatin instead of in the tray.

A partly bleached print that is not fully converted is not a finished object. Moersch again: a print in which not-yet-toned metallic silver has been bleached to silver bromide is no longer archival, because that silver bromide would tone uncontrolled over time by gas action. It must either be dissolved by fixation or converted to silver sulfide by toning again. Partial bleaching is a legitimate and beautiful technique — it is how split toning by this route is done — but it carries an obligation to finish.

📓What to fix before you blame the toner

Kodak’s own summary is the one to keep: toning cannot disguise poor print quality, and a good candidate for toning has a full tonal scale with good detail in the highlights and shadows. The processing that gets you there is not this part’s invention — it is the archival sequence and fixer capacity from earlier parts, with three toning-specific amendments.

• Two-bath fixing, in a non-hardening fixer. Kodak does not recommend a hardening fixer for prints intended for toning, because it makes the emulsion less receptive; the hardening is deferred to a separate bath after the toner if it is wanted at all.
• ILFORD’s ordering rule: for toners that give a protective effect — sulphide (sepia), polysulphide, gold and platinum — run the optimum permanence fixing and washing sequence first, and then tone.
• Test rather than assume. The residual silver and residual hypo tests (ST-1 and HT-2) exist because the two faults they catch are exactly the two the toner will otherwise reveal for you, on a finished print.

Alternative route

A reader who cannot ventilate a room adequately cannot run a sodium sulfide bath indoors, and the course does not pretend otherwise. Three routes remain open, in descending order of how well they substitute.

Tone outdoors. Moersch’s own recommendation, and the most complete answer: in a darkroom with no ventilation and little supply of fresh air you should not use sulfur solutions, and the risk can be ruled out by toning outdoors, watering the print twice before returning indoors. The bleach, the rinse and the wash can all happen indoors; only the sulfide tray needs to be outside. This keeps the whole of the published chemistry and needs no change to the formula.

Buy the stock, or the whole toner, ready made. The Level C operation on this route is weighing a deliquescent solid, and it disappears if the sulfide arrives as a solution. A packaged sepia toner is the same two-bath process in bought form — Kodak’s own is a bleach and a redeveloper, described in G-23 alongside the mixed formulas — and the course’s policy on bought products applies to any of them: the maker publishes behaviour and dilutions, and the course teaches those rather than guessing at a composition nobody outside the maker knows.

Take the hypo-alum route instead, which needs no sulfide reagent at all — at the cost of a bath held at 49 °C, a longer session, and sulfur dioxide in place of hydrogen sulfide. It is not a ventilation-free option; it is a different bath with different requirements, and the section above says what they are.

What the course does not offer as the accessible alternative is thiourea, even though it is the one that removes the smell entirely. The reason is in the classification: replacing a gas with a workplace exposure limit by a suspected carcinogen and reproductive toxin in an open alkaline tray is not an accessibility improvement, and a page that offered it as one would be trading the reader’s health for their convenience.

Where the actionable procedure lives

This page is chemistry. The procedure is the next page: Lab: bleaching and redeveloping a sepia print, at Level B, and every control it carries is a consequence of something argued above rather than a list copied from a manual — the two-minute rinse from the acid-and-sulfide rule, the separated waste bottles from Kodak’s disposal instruction, the ventilation check from Princeton and Moersch, the sealed paper box from the 1928 primer’s fogging paragraph, and the ready-made or once-weighed stock from the course owner’s ruling on where the Level C operation sits.

The quantities are in the formulary and are not repeated here: T-7a for the American bleach with oxalate and acid, T-52 for the London bleach without them, T-1a for the hypo-alum bath, the rehalogenating bleach on its own, and T-56, the 1949 sulphide-selenium variant, which is T-52’s bleach with selenium powder dissolved into the sulfide stock and which the course publishes at Level D as chemistry only.

Recap

Sulfide toning converts the image to silver sulfide by two routes that meet at the same compound. The indirect route oxidises the silver to a halide with a ferricyanide-bromide bleach and then exchanges the halide for sulfide, a swap driven by thirty-six orders of magnitude of solubility and therefore over in seconds; all its control lives in the bleach. The direct routes reach the metal itself — hypo alum by acidifying hot thiosulfate until it precipitates its own sulfur, polysulfide by a reagent of variable composition at room temperature — and give colder, less yellow results.

A published bleach’s extra ingredients are answers to named defects: oxalate keeps stray iron in solution so it cannot make Prussian blue spots, acetic acid prevents blisters. The acid is also the reason the rinse after the bleach is a step of the formula.

Every sulfide bath obeys one rule. Acid plus sulfide gives hydrogen sulfide, whose smell stops warning you before the concentration stops mattering, and which fogs every unexposed material in the room on the way past. Waste is separated, ventilation is a control, and the session is separated from printing in space and in time.

What you get is a print with less density and less contrast than the one that went in, in a hue decided mostly by the paper’s emulsion and by how the print was exposed and developed, and in a substance Kodak calls one of the most stable forms of silver — a claim the archival-evidence lesson tests rather than repeats. What you also get is an honest report of your own fixing and washing, whether or not you asked for one.

Check your understanding

Question 1. A print goes from an acid stop bath, through a five-minute wash, into a T-7a bleach, and straight from the bleach into the sulfide toner. Which control has been dropped, and what is the consequence?

• The wash after the stop bath is too short; the print will stain yellow
• The two-minute rinse between bleach and toner; the acetic acid in the T-7a bleach is carried into an alkaline sulfide bath
• Nothing has been dropped, because the bleach is only mildly acid
• The print should have been hardened before bleaching, or the emulsion will blister

Show the answer and why

Answer: The two-minute rinse between bleach and toner; the acetic acid in the T-7a bleach is carried into an alkaline sulfide bath

T-7a carries 28 per cent acetic acid in its bleach stock by design, so the bleach is deliberately acid. Kodak prints the rinse as step 2 of a five-step formula and Princeton gives the rule in general terms: rinse thoroughly after an acid bleach before the print enters the toner. Acidified sulfide releases hydrogen sulfide, and the print is the carrier. Note the design consequence too: Kodak Limited T-52 bleach has no acid in it, which is one honest reason to prefer it.

Question 2. Why is redevelopment in the sulfide bath complete in about thirty seconds when the bleach before it takes five to eight minutes?

• The sulfide bath is far more concentrated than the bleach
• The bleach has to penetrate the gelatin, while the toner works only on the surface
• The bleach is a redox reaction with a rate; the redevelopment is a precipitation whose driving force is a solubility difference of about thirty-six orders of magnitude
• Redevelopment is faster because the bleached print is thinner

Show the answer and why

Answer: The bleach is a redox reaction with a rate; the redevelopment is a precipitation whose driving force is a solubility difference of about thirty-six orders of magnitude

Silver bromide has a solubility product of 5.0 x 10 to the minus 13 and silver sulfide 1.6 x 10 to the minus 49. Once a sulfide ion reaches a grain of silver bromide the exchange is effectively irreversible, so the step is limited only by diffusion into the gelatin. That also explains a practical fact: there is no controlled partial redevelopment, so every effect that depends on converting only part of the image has to be produced by stopping the bleach instead.

Question 3. A darkroom worker leaves the box of unexposed paper on the shelf during a sulfide toning session, on the ground that it is sealed inside its black bag. What is the specific risk the 1928 Kodak primer names?

• None, provided the bag is intact; the hazard is to the person, not to the paper
• A very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, which is why no photographic materials should be stored where sulfide toning is done
• The alkaline vapour raises the paper contrast grade
• The paper will absorb moisture from the warm bath and cockle

Show the answer and why

Answer: A very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, which is why no photographic materials should be stored where sulfide toning is done

Kodak states it as an inventory rule rather than a warning, and gives the mechanism: the gas sulfides the silver halide of the stored material directly. Kodak G-23 extends the same point to finished work, saying that hydrogen sulfide gas fogs unexposed paper and film and will oxidise unprotected silver images in negatives and prints. The control is separation in space and in time, not packaging.

Question 4. Kodak hypo-alum sepia toner T-1a contains a small charge of silver nitrate, added before an equal weight of sodium chloride. What are they for?

• They supply extra silver so the toned image is denser than the untoned one
• They form silver chloride, which is the substance actually converted to silver sulfide
• They charge the bath with silver so that a fresh bath does not dissolve image silver out of the highlights
• They buffer the bath so it does not decompose before it reaches working temperature

Show the answer and why

Answer: They charge the bath with silver so that a fresh bath does not dissolve image silver out of the highlights

The 1928 primer answers it directly: a fresh bath tends to weaken the print, eating out the highlights, and to prevent this a little silver must be added, preferably in the form of silver chloride. The bath is a fixing bath of about half the usual strength held at 49 degrees Celsius, and a fixing bath dissolves silver, starting with the finest deposits. It is also why the primer says a somewhat used bath works better than a fresh one: it has already taken silver from earlier prints.

Question 5. Two prints from the same negative and the same session are sepia toned identically. One is on a bromide paper, one on a chloride-rich modern mixed emulsion. What should you expect, and why?

• Identical tones, because the toned substance is silver sulfide in both cases
• The bromide paper deeper brown and the chloride-rich paper yellower, because the hue follows the state of subdivision of the silver the toner inherits
• The chloride-rich paper deeper brown, because chloride grains are finer
• A difference only if the two papers were developed in different developers

Show the answer and why

Answer: The bromide paper deeper brown and the chloride-rich paper yellower, because the hue follows the state of subdivision of the silver the toner inherits

Kodak 1928 gives the mechanism: silver sulfide runs from light brown to black according to its state of subdivision, and the toned image inherits the subdivision of the untoned one. Moersch gives the modern observation: a high bromide silver content promotes deep brown hues while chloride and mixed emulsions tone yellow irrespective of the toner. Bullock adds emulsion speed, the faster emulsion giving the more purple result. The developer matters too, but Bullock notes its effect can be masked by the emulsion, so it is the weaker of the two variables.

Question 6. A toned print shows round purple stains and a dark yellow edge stain that were not visible before toning. What is the correct diagnosis?

• The toner was exhausted and should be replaced
• The bleach was too strong and attacked the borders first
• Air bubbles trapped in the fixer left unfixed spots, and an exhausted fixer left insoluble silver compounds in the borders; the toner revealed both
• The print was washed for too long after bleaching

Show the answer and why

Answer: Air bubbles trapped in the fixer left unfixed spots, and an exhausted fixer left insoluble silver compounds in the borders; the toner revealed both

Both are fixing faults that Kodak G-23 traces explicitly. Air bubbles trapped between or under prints during fixing later produce round purple stains in prints toned with selenium or sulfide toners; an exhausted fixing bath leaves insoluble silver compounds that washing cannot remove and that form a dark yellow stain on meeting a toner, especially in borders and highlights. The fixes are agitation and two-bath fixing, and both happen before the print ever reaches a toner. The general rule is that toning reveals processing faults rather than causing them.

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