Kodak T-7a

The same process as T-52 with two ingredients added to the bleach, each of them there to prevent a specific defect, and one number that Kodak changed between 1928 and 2006 without saying why. It is the best-documented sepia toner in this corpus and the only formula in the formulary whose maker printed it twice, seventy-eight years apart, and disagreed with itself.

⚠Read this before the formula: where the Level C work is

The formula is Level C, and the operation that puts it there is weighing the sulfide. The classification comes from sodium sulfide, 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 Stock Solution B.

The working baths are 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, or the weighing is avoided by buying a sulfide stock ready made, and the trays of diluted bleach and toner are then a Level B operation. Nothing on the sulfide page is disputed or softened by that; what it settles is where the Level C work happens.

This formula also has a feature T-52 does not, and it is not answered by any of the above: there is acid in the bleach, one bath away from a sulfide bath. That is Kodak’s own design, it is discussed honestly below, and it is the reason the two-minute wash between the baths is a step of the formula rather than a courtesy.

Bleach — Stock Solution A — the rehalogenating bleach, as a keeping stock

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	75 g	anhydrous
Potassium bromide	75 g	anhydrous
Potassium oxalate monohydrate	195 g	Kodak prints "potassium oxalate" and specifies no hydrate; the course reads an unqualified potassium oxalate as the monohydrate
Acetic acid (glacial)	40 mL of a 28% solution	28 per cent; Kodak's own footnote makes it from 3 parts of glacial acetic acid to 8 parts of water
Water	2000 mL, added	at 20 °C; Kodak gives water as an amount added at 20 °C rather than as a make-up volume: this stock has no stated final volume in either printing, and the strengths quoted on this page are computed against the two litres Kodak names.
A keeping stock, used at 1+1. Store it in a dark or yellow bottle; a ferricyanide solution photolyses to Prussian blue.

Toner — Stock Solution B — the sulfide redeveloper, as a keeping stock

Ingredient	Quantity	Form the source specifies
Sodium sulfide	45 g	anhydrous; the 1928 printing adds "not sulfite" in the formula line itself
Water	500 mL, added	at 20 °C; 90 g of sodium sulfide to the litre of water, which is the same stock strength Kodak printed in 1928. It is a storage form rather than a bath, because the salt deliquesces.
Nothing is toned in this. It exists so that the sulfide is weighed once.

Bleach Working Solution — the bath the print is bleached in

Ingredient	Quantity	Form the source specifies
Bleach — Stock Solution A	500 mL
Water	500 mL, added	at 20 °C
One part of stock to one part of water. Identical in the 1928 and 2006 printings.

Toner Working Solution — the bath the bleached print is redeveloped in

Ingredient	Quantity	Form the source specifies
Toner — Stock Solution B	125 mL
Water	375 mL, added	at 20 °C
One part of stock to three parts of water. The 1928 printing gives 130 c.c. of the same stock in 1.0 litre of water instead, which is 10.4 g/L rather than 22.5 g/L.

Used in this order — toning a thoroughly washed print

1. Bleach Working Solution — approximately 5 to 8 minutes — Until only a faint yellowish brown image remains, with agitation. The 1928 printing expects about one minute, and the end point it describes is slightly further on: until only faint traces of the half-tones are left and the black of the shadows has disappeared.
2. Water — at least 2 minutes — Cold running water. This is the step that keeps the acid bleach out of the sulfide bath, and it is the control every safety source for this process names.
3. Toner Working Solution — approximately 30 seconds — Until no further change occurs in the tone. The 1928 printing says the same in different words - about thirty seconds, until original detail returns.
4. Water — immediately — Rinse the print with water as soon as it leaves the toner.

Immerse a thoroughly washed print in the bleach, and agitate until only a faint yellowish brown image remains. Rinse the print in cold running water for at least 2 minutes. Immerse the print in the toning bath, and agitate until no further change occurs in the tone. Immediately rinse the print with water.

A fifth bath follows for fibre-base prints and is not part of this formula: 1 part Kodak Liquid Hardener to 13 parts water, or 2 parts Hardener F-5a to 16 parts water, for 2 to 5 minutes, eliminated if the print was fixed in a hardening fixer. Then a 30-minute wash at 18 to 24 °C for fibre-base papers, or 4 minutes for resin-coated.

Purpose

To convert a finished print into an image of silver sulfide, and to prevent, in the formulation itself, the two defects that most often spoil the attempt. Kodak’s 2006 description is one line: T-7a produces warm-brown tones similar to those produced by Kodak Sepia Toner, and it includes a bleach and a redeveloper.

The interesting part of this formula is not what it does, which is what every indirect sulfide toner does. It is why it has five ingredients where T-52 has three. The 1928 primer answers that directly and it is the most useful paragraph in the whole chapter: potassium oxalate is in the bleach because the blue iron salt is soluble in the oxalate, and acetic acid is in the bleach to prevent possible formation of blisters. Both are answers to failures, and both are formulation rather than technique.

Recommended uses

Warm-brown tones on a fully developed print. Kodak’s own comparison is with its packaged Sepia Toner, which it describes as giving warm brown tones on cold-tone papers and yellowish brown on warm-tone ones.

Where blue spotting has been a problem. If prints toned in a plain ferricyanide-bromide bleach keep arriving with blue specks, this is the formula that was designed against them. See The mechanism.

Where blistering has been a problem, for the same reason and by the other addition.

On negatives, as an intensifier. The 1928 primer’s redevelopment intensifier is this bleach followed by sodium sulfide, used on a negative rather than a print — perhaps the simplest method of intensification, in its words.

Where a hardened print is wanted afterwards rather than before. Kodak’s instruction is a non-hardening fixer before toning and a hardening bath after it, because a hardened emulsion is less receptive to the toner.

When another formula is preferable

• Where no acid may go near the ferricyanide, T-52, whose bleach is two salts and water. It has no protection against blue spots or blisters, and no acid in it either; that is the whole trade between the two formulas.
• Where the sulfide reagent is the objection, the hypo-alum bath T-1a, which arrives at a silver sulfide image without a sulfide being handled at all.
• Where the bleach is wanted on its own — for a redevelopment intensifier, for local reduction, or in front of somebody else’s toner — the rehalogenating ferricyanide-bromide bleach has its own entry.
• Where silver is to be removed rather than converted, Farmer’s reducer, which is a ferricyanide bath that does the opposite thing.

Mixing

Two stocks that keep, two working baths that do not, and no vessel that ever holds both chemistries.

1. Stock Solution A. Kodak prints the ingredients in this order in both printings and states no mixing order, so none is recorded in the data above. The order it prints is ferricyanide, bromide, oxalate, acetic acid, into water at 20 °C — and the acid last is the only sensible reading, because 40 mL of a 28 per cent acid going into two litres of an already-buffered solution is a different operation from acid meeting the dry salt.
2. Stock Solution B. 45 g of anhydrous sodium sulfide in 500 mL of water at 20 °C. The 1928 formula line prints the ingredient as “Sodium Sulphide (not Sulphite)”, which is worth repeating here for the same reason Kodak wrote it: the two names differ by one letter and the substitution does nothing at all.
3. The bleach working bath, 500 mL of A and 500 mL of water, mixed when wanted.
4. The toner working bath, 125 mL of B and 375 mL of water, mixed when wanted and discarded afterwards.

🧮What the stocks are worth, and where Kodak changed its mind

Stock A carries its four solids in two litres of water. Kodak states no final volume for it, so these are strengths against the water named, and the page says so rather than inventing a make-up figure:

75 g ÷ 2 L = 37.5 g/L ferricyanide and 37.5 g/L bromide

Bleach stock, per litre

37.5 g/L ÷ 2 = 18.75 g/L of each

Bleach working bath, at 1+1

Against T-52’s 50 g/L of each, this working bleach is about 38 per cent as strong — which is consistent with 2006’s five to eight minutes against a bath T-52 gives no time for at all, and inconsistent with 1928’s one minute for this same bath.

The sulfide is where the real disagreement lives. Stock B is 45 g in 500 mL, 90 g/L, in both printings:

90 g/L × 125/500 = 22.5 g per litre

Toner working bath, 2006

90 g/L × 130/1130 = 10.4 g per litre

Toner working bath, 1928

One formula number, one maker, two working strengths differing by a factor of 2.2. And the earlier of them lands within four per cent of Kodak Limited’s independent London formula T-52, at 10 g/L. Two of the three published figures agree; the outlier is the modern one. Nothing in the corpus says what the change bought, and this page does not guess — but a reader mixing T-7a should know which of the two they are mixing.

Behaviour

The bleach is slower than T-52’s and the end point is earlier. Kodak’s 2006 end point is only a faint yellowish brown image remains; its 1928 end point is further on — only faint traces of the half-tones left and the black of the shadows gone. Both are visual, and the 2006 sheet’s five to eight minutes is a time you can watch rather than a time you must catch.

The redevelopment is over in about thirty seconds in both printings, and this is the one thing the two agree on completely.

The print loses density, which Kodak states as a property of its sepia toners generally and answers with advice at the enlarger: develop fully, print slightly darker than normal.

Print processing before the toner decides more than the toner does. Kodak’s own guidance is unusually direct: improper fixing is probably the major cause of stains in toned prints; an exhausted fixer leaves insoluble silver compounds that form a dark yellow stain on meeting a toner, especially in borders and highlights; and prolonged fixing drives fixer into the paper base and makes sulfide-toned prints turn yellow. Air bubbles trapped between prints in the fixer show up later as round purple stains.

The bleached print between the baths is light-sensitive, being silver bromide in gelatin; Kodak’s own assessment of the risk, for its packaged version of this process, is that the effect is extremely small and may not be noticeable.

Image characteristics

Warm brown, and the warmth belongs to the paper. The 1928 primer’s rule governs here as it does on every sulfide toner page: silver sulfide runs from light brown to black according to its state of subdivision, and the subdivision is inherited. Its practical corollary is that the print should have been fully developed but not over-exposed.

Blue specks are the defect this formula exists to prevent, and their absence is the characteristic worth noticing. See The mechanism.

Blisters are the second, and the acetic acid is the answer to them.

The tone is stable and the print is more permanent than it was. That claim belongs to silver sulfide rather than to this formula, and its page carries the number behind it.

The mechanism

The two conversions are the same as any indirect sulfide toner’s and are set out on the T-52 page. What is specific to T-7a is why the bleach has four ingredients instead of two.

⚗The two extra ingredients, and the two defects they answer

Blue spots are Prussian blue, and the oxalate dissolves it. Every silver atom the bleach oxidises leaves a ferrocyanide ion behind. Iron(III) reaching the bath from a chipped enamelled tray, a steel clip or a pair of tongs finds that ferrocyanide and precipitates.

4 Fe³⁺ + 3 [Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃

The blue speck the 1928 primer warns about

The primer’s fix is the formulation rather than the technique: the tendency to form spots is reduced to a minimum by adding potassium oxalate to the bleach bath, since the blue iron salt is soluble in the oxalate. Oxalate is a chelating ligand for iron(III), and 195 g of it per two litres is a very large excess over any iron a tray could contribute. It does not stop the iron arriving; it stops the pigment staying.

Blisters are a swelling problem, and the acid answers it. The primer’s sentence is short — acetic acid is added also, to prevent possible formation of blisters — and it does not give a mechanism. What can be said without going beyond the source is that the bleach is mildly acid by design and that the print then meets a strongly alkaline sulfide bath, which is a large pH swing across a wet gelatin layer; a bath held on the acid side is the one Kodak chose.

The acid in a ferricyanide bath, honestly. Princeton’s guidance says flatly not to add acid to solutions containing potassium ferricyanide, and records cases of cyanide poisoning from acidifying Farmer’s reducer. Kodak’s own bleach nonetheless contains 40 mL of 28 per cent acetic acid per two litres. The potassium ferricyanide page sets out that disagreement in full, with the Environment Agency’s classification note on complex cyanides, and states the course’s position: it does not referee the dispute and takes the stricter control, because the cost of being wrong is asymmetric. This page does not restate that reasoning and does not soften it. What it adds is the specific fact that a mildly acidic ferricyanide bleach is standard manufacturer practice and that this is the formula it comes from.

Why the wash between the baths is chemistry. Acid in a sulfide bath makes hydrogen sulfide. This bleach contains acid. Kodak’s own step 2 is therefore not a rinse but a control: at least two minutes in cold running water, and Princeton’s guidance names exactly this case — rinse thoroughly after an acid bleach before the toner.

Function of every ingredient

Potassium ferricyanide, 75 g in two litres of Stock Solution A, so 18.75 g/L in the bath. The oxidiser, and a one-electron one: the moles present set how much silver the bath can bleach. More is faster and less even; less is slower and more controllable, and this bath is already the weaker of the two published sepia bleaches. Its own page carries this formula’s principal hazard, which is what it does with an acid rather than what it does with silver.

Potassium bromide, 75 g in two litres. The halide, and the reason this is a toner’s bleach rather than a reducer: it catches the oxidised silver as silver bromide in place. Less and some silver comes down as the ferrocyanide, which redevelops badly. More begins to dissolve silver bromide in its own right, which costs image intensity.

Potassium oxalate, 195 g in two litres — more than the other three ingredients together. It is not part of the bleaching chemistry at all. It is there to keep iron(III) in solution, so that the ferrocyanide the bleach generates cannot make Prussian blue on the print, and the 1928 primer says so in one sentence. It is also the reason this bleach can be used in a tray whose enamel has seen better days, where T-52’s cannot. A note on the form: Kodak prints “potassium oxalate” and specifies no hydrate; the course reads that as the monohydrate, and the difference is not negligible — 195 g of the anhydrous salt would carry about 11 per cent more oxalate than 195 g of the monohydrate, at 166.22 against 184.23 g/mol. Since the oxalate is present in enormous excess for its job, the difference is unlikely to matter here, and this page says that as a judgement rather than as a fact from a source.

Acetic acid, 40 mL of a 28 per cent solution in two litres, so 10 mL/L in the working bleach. Present to prevent blistering, per the 1928 primer, and for nothing else. Note the form carefully: this is not glacial acid, and Kodak supplies the arithmetic in a footnote — three parts of glacial acetic acid to eight parts of water gives approximately 28 per cent. Putting 40 mL of glacial acid here instead would be roughly three and a half times the intended acid in a ferricyanide bath, which is the one mistake on this page that is a safety mistake and not a photographic one. Less risks blisters; more is not a control anybody has published a reason for.

Sodium sulfide, 45 g in 500 mL of Stock Solution B. The redeveloper: it supplies the sulfide ion that converts the bleached silver bromide to silver sulfide. Kodak specifies the anhydrous salt, and the 1928 formula line adds “not sulfite” inside the formula itself. More in the working bath tones faster and buys nothing that has been published; less tones more slowly and, past a point, incompletely — and the working strength is exactly what Kodak changed between its printings. The classification, the exposure limits and the incompatibilities are on its own page and are not repeated here.

Solution A, 500 mL of it in every litre of the working bleach. Kodak’s own line for the bleach bath is a volume of its own stock, not a list of salts, and it is recorded that way for the same reason Kodak wrote it that way: the reader mixes one bottle and measures out of it, and the strength of the bath is a property of that bottle.

Solution B, 125 mL of it in every 500 mL of the working toner. The same reasoning one bath along — and here the line matters more, because it is the line whose numbers changed. Writing “22.5 g of sodium sulfide” in its place would hide the fact that the change Kodak made was to the dilution, not to the stock.

Water, five times over. Twice as the solvent of a stock, twice as the diluent of a working bath, and once — between the bleach and the toner — as a bath in its own right with a published time of at least two minutes. That fifth appearance is the safety control of the whole sequence.

Interactions

With acid, which this formula contains by design. See The mechanism. The rule that follows is that no further acid is introduced: no stop bath in the run, no acid fixer poured into the bleach’s waste, no acid cleaner on a tray that has held it, and never a warm bleach.

With iron. Protected against, uniquely among the bleaches in this formulary, by the oxalate. The 1928 primer still adds the belt to the braces: particular care should be taken not to use trays with any iron exposed.

With hypo. In the bleach it makes a reducer; forming inside an old sulfide stock it makes a fixer, which is Wall’s terminal failure mode for this process; carried in on a badly fixed print it makes stains.

With a hardening fixer, before toning. Kodak recommends against one, because a hardened emulsion is less receptive to the toner, and defers the hardening to a bath after the toning.

With unexposed photographic materials. The 1928 primer’s rule for the whole class: no photographic material should be stored in a room where sulfides are kept or where sulfide toning is done.

With resin-coated paper. Kodak’s general toning guidance is to keep toning times to a minimum on RC paper so the solution does not penetrate the edges, and to leave a wide border to trim if toning runs past eight to ten minutes.

Variants

T-7a as Kodak printed it in 1928. The two stocks and the bleach working bath are identical. The redeveloper is not: 130 c.c. of Stock No. 2 in 1.0 litre of water, which is 10.4 g/L against the 2006 sheet’s 22.5 g/L. The bleach time is not either: about one minute, to a further end point. The hardening bath is 1 part of the F-1a hardener to 16 parts water for five minutes, against 2006’s 1 part of Kodak Liquid Hardener to 13 parts water or 2 parts of F-5a’s hardener stock to 16 parts. These are two printings of one formula that do not agree, and the course prints both rather than choosing one and calling it T-7a.

Kodak Limited’s T-52, the London formula for the same process: a plain ferricyanide-bromide bleach at 50 g/L of each salt, no oxalate, no acid, and a working sulfide bath at 10 g/L. Its own page sets out what it gains and gives up.

The bleach on its own, which the formulary carries as a separate entry in its plainest published form.

A course variant is not offered. There is nothing to make safer by reformulating: the objections are the sulfide reagent, which no dilution removes, and the acid in the bleach, which is the ingredient preventing blisters. A version without the acid already exists and is T-52.

Safety

Level C, from sodium sulfide, on three criteria set out on its page against the rubric. Read that page and the potassium ferricyanide page before this section is of any use; neither is restated here.

Where the Level C work is. Under the ruling of 5 September 2026 the operation sets the level rather than the substance: weighing 45 g of a deliquescent Level C solid into Stock Solution B is a Level C operation, done once and outside the session under the controls the sulfide page states, or avoided by buying a sulfide stock ready made. Diluting that stock 1 + 3 and toning a print in it is a Level B operation. This entry is at Level C because it publishes both halves of the formula.

⚠What this formula adds to the sulfide hazard

An acid bleach, immediately before a sulfide bath. The bleach carries 10 mL of 28 per cent acetic acid per litre; the toner is strongly alkaline; and the print goes from one to the other carrying whatever it holds.

The controls are Kodak’s own and Princeton’s, and both name the same step: rinse thoroughly after the acid bleach before the toner — at least two minutes in cold running water, which is step 2 of the published procedure and not an optional tidying. To that add the sulfide page’s controls, which are extraction rather than an open window, splash goggles, chemical-resistant gloves, separate tongs and dishes for the two baths, and no unexposed paper or film in the room.

And one arithmetic error to avoid: Kodak’s 28 per cent acetic acid is not glacial acid. Substituting glacial for it, volume for volume, puts about three and a half times the intended acid into a ferricyanide solution.

Storage

Stock A in a dark or yellow bottle, because a ferricyanide solution photolyses to Prussian blue.

Stock B sealed, dated, and nowhere near an acid. Kodak publishes no keeping figure for either stock in either printing, and none is invented here. What Wall’s 1912 dictionary says about the class — that a dissolved sodium sulfide keeps unreliably, decomposes to hypo, and eventually dissolves the bleached image rather than toning it — is his statement and the test for it is under Troubleshooting.

Neither working bath is stored. Both are mixed when wanted; the toner is discarded after use.

Not in the same room as photographic materials.

Label both stocks with the formula, the solution letter, the strength and the date, and label them against each other: a bottle of bleach stock and a bottle of sulfide stock are both clear and both about to be diluted by very different amounts.

Incompatibilities

Concentrated or hot acid, and stop baths, with the ferricyanide. The bleach’s own 28 per cent acetic acid is the formula’s, and no more is added.

Acid of any kind, with the sulfide, which is the reaction that makes hydrogen sulfide.

Oxidisers, with the sulfide.

Hypo, into the bleach and inside an ageing sulfide stock.

Iron and steel, less dangerously here than in any other bleach in this formulary, thanks to the oxalate — but the 1928 primer still says to keep exposed iron out of the trays.

Hardening fixers, before toning, on photographic rather than safety grounds.

Unexposed film and paper, in the room.

Developer, in either direction.

Waste

Three streams that never meet. The spent bleach carries ferricyanide, ferrocyanide, bromide, oxalate and acetic acid; the spent toner is strongly alkaline and carries sulfide, bromide and a little silver; the wash water carries traces of both.

Kodak’s own instruction is the one to remember, and it is a prohibition: do not discard sulfide-type toners with stop baths or fixing baths, because the combination generates hydrogen sulfide gas — which, the same sheet notes, will fog unexposed paper and film and oxidise unprotected silver images. The reflex of neutralising an alkaline waste with acid is exactly the forbidden move.

The bleach’s waste is acidic already and stays in its own bottle, away from the sulfide and away from any further acid.

Follow the silver-bearing waste SOP, the general chemical waste SOP and the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Blue spots, in the formula that was designed to prevent them. Either the oxalate was omitted or the bath is old and heavily loaded with ferrocyanide. Check the weighing first: 195 g is the largest quantity in the formula and the easiest to mis-scale.

Blisters at the edges or across the sheet. The acid was omitted, or glacial acid was used and the print met a much sharper pH swing than intended, or the baths are at different temperatures.

The bleached print will not tone, or tones a weak yellow-brown. The sulfide stock has decomposed. Wall’s three stages apply and only the first two are recoverable by mixing fresh stock.

The bleached image disappears in the toner. Wall’s third stage: the stock has become enough hypo to fix. There is no remedy.

Yellow stain in the highlights and borders. Fixing, not toning. An exhausted fixer, or over-fixing, or both.

Round purple stains. Kodak names this one exactly: air bubbles trapped between or under prints during fixing, which only show up after selenium or sulfide toning.

Mottle in the print base. Kodak names this one too, and it is also not the toner: an over-concentrated stop bath — a tray left more than three days, or a tank more than a month — or insufficient agitation in the first few seconds in the stop bath.

The bleach takes a minute rather than five, or eight rather than five. Both are published Kodak figures for this bath; work to the end point rather than the clock.

A smell of rotten eggs. Leave and ventilate, then find the acid.

Experiments

Settle Kodak’s disagreement with itself. Mix the toner working bath both ways — 125 mL in 500 mL and 130 mL in 1130 mL — and tone identical bleached prints in each to the point where no further change occurs. Record the time, the colour and the maximum density. Two of the three published sulfide strengths in this formulary agree at about 10 g/L and the modern one is the outlier; nothing in the corpus says what doubling it bought, and this is a one-evening experiment that would say.

Prove the oxalate. Two bleaches from the same stock, one with a deliberate trace of an iron salt added, on identical prints; then repeat with a plain ferricyanide-bromide bleach at matched strength. The 1928 primer’s claim is that the oxalate keeps the blue iron salt in solution. It is a claim you can photograph.

Find the bleach’s real end point. Strips pulled at one, two, four, six and eight minutes, all toned together. Kodak’s two printings differ by a factor of five to eight on this number; your paper will have one answer and it will not be either of theirs by accident.

Test the blister claim. Kodak’s acid is there against blisters and gives no mechanism. Bleach twin prints in the stock with and without its acetic acid, at the same dilution, and put both through the same sulfide bath and the same wash. If blisters are a pH-swing effect this is where they appear.

Compare the two formulas at matched strength. T-7a’s bleach diluted to T-52’s 50 g/L, or T-52’s diluted to T-7a’s 18.75 g/L, with everything else held. What is left when the strengths match is the oxalate and the acid, which is exactly the comparison this page is about.