Kodak T-1a

The other sepia toner. Where T-7a and T-52 bleach the image and rebuild it with a sulfide, this one converts the silver directly, in a single hot bath, using sulfur the bath makes out of its own hypo. No sulfide reagent is bought, weighed, stored or poured — and the reason that fact is on this page rather than in a footnote is that it is the one thing that distinguishes T-1a from every other route to a sepia print.

It does not make the process free of hazard, and the section below says exactly what it substitutes.

The hypo solution — the silver solvent, and the source of the sulfur

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	480 g	pentahydrated
Water	2800 mL, added	at 20 °C; Kodak gives water here as an amount added at 20 °C, not as a make-up volume; only the combined bath has one, and it is 4 litres.
Mixed thoroughly before anything else goes in. This is the largest solution by far and it is what the other two are added to.

The alum solution — the weak acid that frees the sulfur

Ingredient	Quantity	Form the source specifies
Potassium alum	120 g	dodecahydrated; the 1928 printing says "powdered potassium alum"
Water	640 mL, added	at 70 °C; Hot water: 70 °C in the 2006 sheet, "about 160 °F (71 °C)" in 1928. Alum dissolves slowly in cold water and this is the only warm step in the mixing.
Added to the hypo solution second. Kodak heads none of these three sub-solutions; the names used here are descriptive, and the 1928 primer's own phrase for the first two combined is "the hypo-alum solution".

The silver ripener — the silver charge that stops a fresh bath eating the highlights

Ingredient	Quantity	Form the source specifies
Silver nitrate	4 g or 4.2 g (as printed in the 1928 primer)	crystals; the 1928 printing gives 4.2 g, which is what its own 60 grains works out at
Sodium chloride	4 g or 4.2 g (as printed in the 1928 primer)	table salt; the 1928 printing gives 4.2 g
Water	64 mL, added	at 20 °C
Kodak footnotes the order in both printings: the silver nitrate must dissolve completely before the sodium chloride is added. The result is a milky white suspension and the whole of it, precipitate included, goes into the bath.

Mixed in this order — making the four-litre bath

1. Start with the whole of it of The hypo solution — The vessel that receives. Mixed thoroughly first.
2. Then add the whole of it of The alum solution — Added to the hypo solution.
3. Then add the whole of it, including the precipitate of The silver ripener — Added slowly to the hypo alum solution while that solution is stirred rapidly. The 1928 primer says the stirring is not optional: without it the bath goes a dirty grey or black.

Mix the hypo solution thoroughly; then add the alum solution; then add the silver solution, including the precipitate, slowly to the hypo alum solution while stirring rapidly. After combining the solutions, add water at 20 °C to make 4 L.

The final make-up to 4 litres is stated by both printings and is the only make-up volume in the formula; the schema records water per solution and has no field for a volume the combined bath is made to, so it is recorded here and in the page body. Against 4 litres the bath is 120 g/L of hypo, 30 g/L of potassium alum and 1 g/L each of silver nitrate and sodium chloride.

Purpose

To tone a print to cold brown or chocolate in one bath, without bleaching it first. Kodak’s 2006 description is that T-1a produces cold-brown to chocolate-brown tones; the 1928 primer sets it out as one of the two general methods of converting an image to silver sulfide — direct toning, with the hypo alum bath against bleaching and redevelopment — and explains why the trade used both. Hypo-alum toning is used on the large scale, it says, and bleach-and-redevelop for smaller quantities, because it is troublesome to use a bath that has to be heated.

That is still the trade-off. This bath takes an hour to bring up to temperature and holds it for a quarter of an hour per print; the bleach-and-redevelop route is over in ten minutes at room temperature. What this one buys is a bath that lasts, a process with nothing to time precisely, and no sulfide on the shelf.

Recommended uses

Any number of prints at once, in a tray that stays hot. The bath is reusable, improves with use, and is kept up to volume rather than replaced. That is a batch process, and it is what it was designed for.

Where a cold, chocolate brown is wanted rather than a warm yellow-brown. Kodak’s own table puts hypo-alum at the cold end of the sepia range.

Where the print itself will choose the colour. The 1928 primer’s rule is unusually specific: blue-black images give cold chocolate tones and olive green images give warm sepia. On this route the paper and developer have more say than the toner does.

Where a sulfide reagent is not wanted in the building. See Safety, and read that section before treating this as the obvious answer.

When another formula is preferable

• Where a warm brown is wanted, the indirect sulfide route: T-7a or T-52, both of which give warmer tones than this bath.
• Where partial or split toning is the point, the indirect route again, because a bleach can be stopped half way and this bath cannot: there is no intermediate state to pull the print out of.
• Where a bath cannot be held at 49 °C — no tempered water bath, no thermometer, no space — this formula has no room-temperature version and should not be attempted at a guessed temperature. Both printings put a hard ceiling a few degrees above the working point.
• Where a single print is wanted this evening. Bringing four litres to 49 °C for one print is an hour of heating for fifteen minutes of toning.
• Where the print must not lose density. Kodak states plainly that this toner causes a loss of print density and contrast; the compensation has to be made when the print is exposed, which means deciding to tone before printing.

Mixing

Three solutions, in one order, into one vessel, and the order is the formula.

1. The hypo solution. 480 g of sodium thiosulfate in 2.8 L of water at 20 °C. Mix thoroughly.
2. The alum solution. 120 g of potassium alum in 640 mL of water at 70 °C. Add this to the hypo solution.
3. The silver ripener. 4 g of silver nitrate in 64 mL of water at 20 °C, dissolved completely, and only then 4 g of sodium chloride. The result goes milky white. Add the whole of it — precipitate included — slowly, to the hypo alum solution, while stirring that solution rapidly.
4. Water at 20 °C to make 4 litres.

!Four ways to get this wrong, all of them named by Kodak in 1928

The primer lists them, and each has the same visible symptom: the bath turns a dirty grey or black instead of milky white.

• Boiling water used for mixing. The alum water is hot, not boiling.
• The order of mixing changed.
• The bath not stirred while the white precipitate is added.
• The sodium chloride added before the silver nitrate has completely dissolved, which is footnoted separately in both printings.

The 2006 sheet disagrees about the symptom, and the disagreement is worth knowing rather than resolving: it says that when you combine the solutions a black precipitate may form and that it will not adversely affect the toning action of the bath if proper toning technique is used. The primer says a black bath is a wrongly mixed one. Both are Kodak. This page prints both and takes the 1928 reading as the safer working rule, because “milky white if correctly mixed” is a test a maker can use and “may form” is not.

🧮What four litres actually holds

The three solutions carry 2800 + 640 + 64 = 3504 mL of water and are then made up to 4000 mL, so about half a litre of the finished bath is the final make-up.

480 g ÷ 4 L = 120 g per litre

Hypo

120 g ÷ 4 L = 30 g per litre

Potassium alum

4 g ÷ 4 L = 1 g per litre of each

Silver nitrate and sodium chloride

The hypo figure is worth holding against the fixing baths: F-5 and plain hypo both run at 240 g/L or thereabouts, so this toner is a half-strength fixing bath that has been deliberately acidified and heated — which is exactly what it is chemically, and exactly what the 1928 primer’s explanation says.

The silver and the chloride are equimolar to within four per cent — 4 g of silver nitrate is 23.5 mmol at 169.87 g/mol, 4 g of sodium chloride is 68.4 mmol at 58.44 g/mol. They are not equimolar at all: the salt is in nearly threefold excess. That matters, because excess chloride is what keeps the silver chloride from redissolving in 120 g/L of hypo, and it is why Kodak insists the silver goes in first and the salt second.

Kodak Limited’s London bath T-51 reaches the same place by a different route and it is instructive to compare: 200 g/L of hypo, 44 g/L of alum, and 0.5 g/L of silver nitrate dissolved in ammonia rather than precipitated with salt, plus 1 g/L of potassium iodide. Half the silver, a different halide, and a bath two-thirds again as strong. Neither maker explains its choice.

Behaviour

It must be hot and it must not be hotter. 49 °C (120 °F) is the working temperature in both printings. The 1928 primer says the bath works best between 49 and 52 °C and that above 54 °C there is danger of blistering and bleaching of the image; the 2006 sheet forbids anything above 49 °C outright and forbids more than 20 minutes there. Kodak Limited’s T-51 sets its own ceiling at 60 °C. A tray in a tempered water bath is the published method and a thermometer is not optional.

A fresh bath is worse than a used one, and this is the formula’s most surprising property. The primer: a bath lasts for a long time, and as a general rule a hypo-alum bath which has been somewhat used works better than a fresh one. A fresh bath tends to weaken the print, eating out the highlights — because 120 g/L of hypo in a warm tray is a fixing bath, and a fixing bath dissolves silver. The silver ripener exists to give that appetite something else to eat.

Sediment forms and comes off on the print. Both printings call for a rinse in warm water and a wipe with a soft sponge before washing. Kodak Limited says the same about T-51.

Density and contrast fall. Kodak’s instruction is to compensate at the enlarger by increasing both the exposure and the development time, which means the decision to tone is taken before the print is made.

Blisters are the characteristic failure and heat is the cause. Wall’s 1924 handbook adds the rule the Kodak sheets do not: when hot toning baths are used, the prints should be allowed to cool before washing, otherwise blisters are very likely to form.

Image characteristics

Cold brown to chocolate, at the cold end of the sepia range, and the print that went in decides where in that range it lands.

The colour is inherited, not imposed. The 1928 primer’s general rule for silver sulfide — that it runs from light brown to black according to its state of subdivision — has a specific form for this bath: the colour of the final tone is related directly to the colour of the original black and white image, blue-black giving cold chocolate and olive green giving warm sepia.

The highlights are where a bath’s condition shows. Eaten highlights mean an under-ripened bath. This is the only toner in the formulary whose characteristic fault is loss rather than stain.

Toning is all-or-nothing. There is no bleached intermediate and no way to stop half way with a clean division between converted and unconverted areas, so the split-tone effects available on the indirect route are not available here.

The mechanism

The bath makes its own sulfur, and the 1928 primer explains it in a paragraph that is worth following closely because everything else on this page follows from it.

⚗Why acid plus hypo plus heat is a toner

Add an acid to a solution of hypo and it tends to precipitate sulfur. A solution of alum in water is weakly acid. So alum added to plain hypo — with no sulfite present to defend it, which is the difference between this bath and every fixing bath in the formulary — goes turbid in time and precipitates sulfur.

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

What acid does to thiosulfate

The primer’s own formulation of the useful state is careful: 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. That is a description of a bath poised on the edge of decomposing rather than one that has decomposed, and it is why a bath that has stood and been used is better than a fresh one.

A print in that bath, at about 49 °C, has its silver image converted directly to silver sulfide.

2 Ag + S → Ag₂S

Direct toning, with no halide in between

Why there is silver in the toner. Because the same bath is also a fixer. At 120 g/L of hypo and 49 °C, a fresh bath will dissolve image silver — the primer says it eats out the highlights — and the cure is to give the bath a silver charge of its own so that it is not hungry. The primer specifies the form: a little silver must be added, preferably in the form of silver chloride. That is what the third solution makes.

Ag⁺ + Cl⁻ → AgCl

The ripener, made in the beaker rather than bought

The milky white suspension is silver chloride, and the excess sodium chloride around it is what keeps it from simply redissolving in all that hypo.

Why the order of addition is the formula. Silver nitrate meeting a hot, concentrated, acidified thiosulfate solution directly — rather than as a chloride suspension stirred in slowly — gives silver sulfide in the beaker instead of in the print. That is the dirty grey or black bath the primer warns about, and it is why the salt goes in before the silver goes anywhere near the hypo.

Function of every ingredient

Sodium thiosulfate pentahydrate, 480 g, so 120 g per litre of finished bath. Not the solvent here but the reagent: it is the substance the sulfur comes out of. Warmed and mildly acidified with no sulfite to protect it, thiosulfate decomposes and gives free sulfur, and that sulfur is what tones the print. More would make a stronger fixing bath as well as a stronger sulfur source, and the fixing action is the one you do not want. Less slows the toning and reduces the bath’s appetite for image silver at the same time. Note the form: the pentahydrate at 248.19 g/mol is what both printings specify, and 480 g of the anhydrous salt would be a substantially stronger bath.

Potassium alum, 120 g, so 30 g per litre. The acid. Its solution is weakly acid, and that weak acidity is the whole trigger for the sulfur. It has a second job that neither printing states and that this page therefore offers as a reading rather than a fact: potassium alum is the hardener of Kodak’s own acid hardening fixing baths, and a print about to spend fifteen minutes at 49 °C benefits from a hardened gelatin. More alum would acidify further and precipitate the sulfur faster — which is the direction of a bath that goes off on the shelf rather than in the tray. The dodecahydrate is what Kodak specifies; the 1928 printing adds “powdered”, which is about dissolving rather than composition.

Silver nitrate, 4 g, so 1 g per litre — 4.2 g in the 1928 printing. The ripener’s silver. Without it the bath’s first prints lose their highlights to a hot half-strength fixing bath. It is never used as silver nitrate: it is converted in the beaker to silver chloride before it goes anywhere near the hypo. More silver has not been published as a control by any source read here; Kodak Limited’s T-51 uses half as much by a different preparation. Its own page carries the hazard, and the relevant point for this formula is that the solid is weighed once, in a small quantity, and immediately converted.

Sodium chloride, 4 g, so 1 g per litre — 4.2 g in the 1928 printing. Table salt, and it does the one job the primer names: it precipitates the dissolved silver nitrate as silver chloride, which is the form the primer says the silver should preferably take. At nearly threefold molar excess over the silver it also leaves free chloride in the bath, which is what stops the freshly made silver chloride redissolving in 120 g/L of hypo. The order matters more than the quantity: both printings footnote that the silver nitrate must dissolve completely before the salt is added.

Water, at three temperatures and in four places. 2.8 L cold for the hypo, 640 mL hot for the alum, 64 mL cold for the ripener, and then whatever it takes to reach 4 litres. The hot water is the only one whose temperature is load-bearing, and the primer warns that boiling water spoils the bath. The final make-up is the only volume either printing states for the bath as a whole; the schema records water per solution, so it is written here as well as in the combination note above.

Interactions

With heat, which is the formula’s own instruction and also its limit. 49 °C is the working point; both printings forbid going higher; the failure is blistering and staining of the print.

With sulfite, which is not in it and must not be. The 1928 primer’s explanation turns on alum being added to plain hypo without any sulfite present. Sulfite is what defends thiosulfate against acid in every fixing bath in this formulary; a hypo-alum toner that has picked up sulfite is a fixing bath that will not tone.

With a fixing bath carried in on the print, which is one route by which that sulfite arrives. Kodak’s instruction is a thoroughly washed print.

With acid, more than is already there. The bath is deliberately on the acid side and it is a thiosulfate bath at 49 °C. Additional acid pushes it further towards decomposing in the tray, which is a sulfur dioxide problem rather than a photographic one. See Safety.

With the tray and the water bath. Kodak’s general toning guidance forbids metal trays for any toner: only unchipped enamel, hard rubber or plastic.

With drying and washing while hot. Wall’s rule: let the print cool before washing, or blisters are very likely.

Variants

Kodak Limited’s T-51, the London hypo-alum bath, 1949. A different formula for the same process and it is worth setting out in full, because it makes different choices at every point. 200 g of hypo in 1000 c.c. of hot water; 44 g of ordinary potassium alum stirred in; boiled for two or three minutes and cooled to about 65 °C. The ripener is not made with salt: 0.5 g of silver nitrate in 15 c.c. of water, with .880 ammonia added drop by drop with vigorous stirring until the precipitate first formed just redissolves, stirred into the hypo-alum mixture — followed by 1 g of potassium iodide in another 15 c.c. of water. Prints are fixed, briefly rinsed, soaked for 10 minutes in a saturated solution of potassium alum, rinsed again, and toned at not more than 60 °C. Twice the hypo, one and a half times the alum, half the silver as an ammonia complex rather than a chloride, an iodide the American formula has no counterpart for, and a pre-hardening bath. It is not a variant of T-1a and T-1a is not a variant of it; they are two makers’ formulas for one process.

Wall’s 1924 direct baths. Baekeland’s alum-and-hypo, ripened with a 10 per cent silver nitrate solution after being heated to 50–60 °C on each of three days; Artura’s, ripened with silver nitrate and salt solutions in the way T-1a is; and an alum-gold-hypo bath that adds gold chloride to the same architecture. The figures in the scan this course holds are not legible with enough confidence to reproduce as formulas, so they are described and not printed. What they establish is that the silver ripener is not a Kodak idea but the standard practice of the period.

A course variant is not offered. The two published quantities that differ between Kodak’s own printings — 4.0 against 4.2 g — are both recorded in the data above as the alternative forms they are, rather than being averaged into a third figure that no maker ever printed.

Safety

Level B. Two of the rubric’s Level B criteria apply: the handling of silver nitrate, which it names explicitly, and a procedure that generates vapours.

⚠What this formula substitutes, and what it does not remove

It is tempting to read T-1a as the safe sepia toner, because no sodium sulfide is bought, weighed, stored or poured, and that reagent is what puts the indirect toners at Level C. The first half of that reading is correct and the second half is not.

What this bath is, chemically, is a half-strength fixing bath that has been deliberately acidified and then held at 49 °C for a quarter of an hour at a time. The sodium thiosulfate page records what that combination does, with its source: Princeton University’s guidance states that sodium thiosulfate on heating or long standing in solution can decompose to form sulfur dioxide, and that darkroom ventilation exists partly to control it. EH40 sets sulfur dioxide at 0.5 ppm over eight hours. This formula asks for heat and acid at once, on purpose, because that decomposition is the toning mechanism.

So the honest statement is: the hypo-alum route trades a Level C reagent for a warm acid thiosulfate bath that gives off a gas with a workplace exposure limit. That is a real reduction in hazard — sulfur dioxide announces itself at concentrations far below harm, where hydrogen sulfide deadens the sense of smell that would warn you — but it is a substitution and not an elimination, and a page that called this the safe sepia toner would be wrong.

Controls: ventilation that actually moves air across the tray, not a cracked window; splash protection for a hot bath; a thermometer, because the ceiling is only five degrees above the working point; gloves and eye protection while the silver nitrate is weighed and dissolved; and nothing acidic added to a hot thiosulfate bath for any reason.

The thermal hazard is ordinary and real. Four litres at 49 °C in a tray in a water bath is a scald risk and a spill risk before it is anything else.

Silver nitrate stains skin and everything else, and its own page carries the classification and the handling. Here it is weighed once, in four grams, and converted immediately.

Storage

The bath is stored and reused, which is unusual in this formulary. Kodak publishes no shelf life for it and none is invented here. What both makers say instead is behavioural: the 1928 primer, that a bath lasts a long time and a used bath works better than a fresh one; Kodak Limited, for T-51, that the bath can be used repeatedly, may be kept up to its original bulk by the occasional addition of fresh solution, and is discarded when it ceases to tone satisfactorily. Those are the two published statements and they agree.

Store it covered, labelled and dated, and expect sediment. A hypo-alum bath that has stood is not spoiled; it is ripened.

Keep it away from sulfite of any kind, which is the one contaminant that stops it working.

Do not store it hot and do not repeatedly re-heat what is not going to be used. Every hour at temperature is an hour of decomposition, and the same decomposition that tones the print eventually exhausts the bath.

Incompatibilities

Sulfite, which defends the thiosulfate the formula needs to decompose. Photographically fatal, not hazardous.

Additional acid, in a hot thiosulfate bath. Sulfur dioxide.

Metal trays and tanks, per Kodak’s general toning guidance: unchipped enamel, hard rubber or plastic only.

Heat above the published ceiling — 49 °C in 2006, 54 °C in 1928, 60 °C for Kodak Limited’s own bath. Blistering, staining and uneven toning.

Unexposed photographic materials, as a general darkroom rule near any silver nitrate and any bath that sheds sulfur.

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

Waste

A silver-bearing thiosulfate stream, and one of the more concentrated ones in the course. The bath holds 120 g/L of hypo, its own gram per litre of added silver, and whatever silver it has taken out of prints — and because a fresh bath actively dissolves image silver, an early bath takes more than a mature one.

It is not acidified, neutralised or mixed with anything. Adding acid to a spent hypo-alum bath is the sulfur dioxide reaction on purpose, in a bottle.

It does not go in with a sulfide toner’s waste, for the reasons the sodium sulfide page gives, and it does not go in with a ferricyanide bleach’s waste either.

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

Troubleshooting

The bath went grey or black on mixing. One of the four causes the 1928 primer names: boiling water, the order changed, no stirring while the ripener went in, or the salt added before the silver nitrate had dissolved. Note that the 2006 sheet takes a different view of a black precipitate; the mixing rules are cheap to follow either way.

The highlights have been eaten away. An under-ripened bath, or one whose ripener was omitted or mis-measured. This is the failure the silver is in the formula to prevent.

Blisters. Too hot, or the print washed while still hot. Both printings give the ceiling and Wall gives the cooling rule.

Stain, or non-uniform toning. The 1928 primer names both as symptoms of a bath above 54 °C.

Sediment or scum on the dried print. Expected: rinse in warm water and wipe with a soft sponge before washing, which both makers specify.

Nothing is happening. The bath is cold, or it has picked up sulfite, or it is finally exhausted. Take the temperature first — it is the commonest of the three by a wide margin.

The print is much lighter than it was. Expected, and it is answered at the enlarger next time: Kodak’s own advice is to increase both exposure and development for prints intended for this bath.

A sharp, choking smell over the tray. Sulfur dioxide from the bath doing what it is designed to do, faster than the ventilation is clearing it. Turn the extraction up and the heat down.

Experiments

Ripen a bath and watch the highlights come back. Mix the bath without the silver ripener, tone a strip, then add the ripener and tone its twin. The 1928 primer says a fresh unripened bath eats out the highlights. This is that claim, in an afternoon, with a densitometer.

Find your paper’s temperature window. The same print toned at 43, 46, 49 and 52 °C for 15 minutes. Kodak gives one working temperature and a ceiling; where the tone stops improving and where the blisters start are properties of your paper.

Test the inheritance rule. The 1928 primer says blue-black images give cold chocolate and olive green images give warm sepia. Print the same negative on a cold-tone and a warm-tone paper, tone both in the same bath at the same time, and photograph the pair together. This is the cheapest demonstration in the formulary that a toner does not choose the colour.

Compare the two sepia routes on one negative. Four identical prints: two through this bath, two through T-7a. Read the density loss and the colour of each. Kodak sells both because they do not give the same result, and no source in this corpus shows them side by side.

Watch a bath mature. Read the maximum density of a toned step wedge on the bath’s first day, tenth print and fiftieth print. The claim that a used bath works better than a fresh one is one of the few claims in this formulary that is about a bath’s history rather than its composition.