Kodak T-26

Kodak’s third gold formula on one sheet, and the one that behaves least like the other two. Same metal, a different sulfur ligand, four times the gold of GP-1 — and a toner that works from the highlights down instead of evenly across the scale.

It is also the formula that answers a question people often ask about sepia toning, and answers it the wrong way. Thiourea is the odourless alternative to a sulfide bath in modern practice. It is not the safer one.

⚠Read this before the formula

Level C, and it is the thiourea that puts it there. Its page gives the reasoning against the rubric: Level C’s first criterion names reagents classified as carcinogenic, as reproductive toxins or as sensitisers at the concentrations used, and thiourea is classified as two of those three — on some readings all three — while the second criterion, waste that cannot lawfully enter a domestic drain, is settled by its aquatic classification.

That page also states the course’s position in terms: this course gives no home procedure for thiourea toning, and the reason is a suspected carcinogen and reproductive toxin in an open tray. This page publishes the formula as chemistry and does not overturn that.

The sepia ruling of 5 September 2026 does not reach this formula, and it is worth saying why rather than leaving a reader to wonder. That ruling holds that the operation sets the level rather than the substance, so that a Level C solid weighed once outside the session leaves a Level B tray behind — which is what unblocks the sulfide route. The thiourea page’s objection is not only to the weighing: it names the reagent in an open tray, and no amount of routing around the balance changes what is in the tray. The ruling’s own closing sentence is that the encyclopaedia entries for sodium sulfide and thiourea are unchanged and correct, and this page takes it at its word.

Ingredient	Quantity	Form the source specifies
Chloroauric acid	40 mL of a 1% solution	printed as "gold chloride, 1% stock solution"; Kodak's footnote makes the stock by dissolving 1 gram in 100 mL of water, or buying it ready made
Thiourea	1 g
Tartaric acid	1 g
Sodium sulfate (anhydrous)	15 g	anhydrous
Water	937 mL, added	at 52 °C; Kodak gives the water as an amount at 52 °C and states no make-up volume for the bath. With the 40 mL of gold stock the liquid comes to 977 mL, and the strengths on this page are computed against that rather than against a litre nobody printed.

Purpose

To make a print blue, or rather bluish. Kodak’s description is unusually hedged for a product sheet and the hedging is the useful part: Blue Toner T-26 produces blue tones on some papers; warm-tone papers react well; neutral-tone papers change slightly to soft blue-black; cold-tone papers will not change. And the exact tone varies with the paper and the developer, with a soft grey-blue more common than a saturated blue.

Its second purpose, which Kodak gives its own section, is as the second half of a two-toner sequence: a sepia or brown toner first, then this bath, for red and orange.

Recommended uses

Warm-tone papers, which are the ones Kodak says react well.

Where the highlights are the subject. T-26 tones the highlights first and the shadows last, so a partially toned print is blue in the highlights and untoned in the shadows. That is a defect if you wanted an evenly toned print and a technique if you wanted a split one.

Red and orange, by way of a sepia toner. Kodak’s own multiple-toning route: sepia or brown first, wash thoroughly, then 15 to 30 minutes in this bath at 32 °C. Cold-tone papers yield a truer red and warm-tone papers an orange hue. The technique usually loses shadow density, so the sheet advises starting with a higher-contrast print.

Five prints at a time, not one. Kodak is explicit: tone prints one at a time and the first few will tone the most and the last few may not tone at all. Immerse up to five simultaneously for consistent results.

When another formula is preferable

• Where a gold toner is wanted without a Level C reagent, GP-1: the same metal with a thiocyanate ligand, at a quarter of the gold, mixed cold, ten minutes, and both of its ingredients at Level B.
• Where brown is wanted on a warm-tone paper, T-21 — though that formula is Level C for a different reagent, and the persulfate page points at GP-1 too.
• Where a print is to be protected rather than coloured, GP-1 again.
• Where blue is wanted on a cold-tone paper, nothing on this page: Kodak says plainly that cold-tone papers will not change.
• Where thiourea is being considered as a way around a sulfide toner, see Variants. It is not one.

Mixing

One bath, mixed warm, in one order, immediately before use.

1. Water at 52 °C, 937 mL.
2. Mix the 40 mL of 1 per cent gold chloride stock with the water.
3. Then, while stirring, add: thiourea 1 g, tartaric acid 1 g, sodium sulfate (anhydrous) 15 g.
4. Stir until the chemicals dissolve completely.

Kodak states no make-up volume and no final temperature. The bath is used at 20 °C or at 38 to 40 °C, so it is mixed hot and either worked hot or allowed to cool.

Mix it right before you use it. Kodak’s instruction, and the sheet adds a reason of its own: this is a fairly expensive toner to produce.

🧮What is in it, and how it compares to Kodak's other two gold baths

The liquid Kodak names is 937 + 40 = 977 mL. No make-up volume is published, so these are strengths against that:

40 mL × 1% = 0.4 g, in 977 mL = 0.41 g per litre

Gold chloride

1 g in 977 mL = 1.02 g per litre

Thiourea

1 g in 977 mL = 1.02 g per litre

Tartaric acid

15 g in 977 mL = 15.4 g per litre

Sodium sulfate

Kodak’s three gold formulas span a factor of four in gold: GP-1 at 0.10 g/L, T-21 at 0.119 g/L, and this at 0.41 g/L. That ranking is also the ranking of how much the colour moves — barely, brown, blue — which is the clearest evidence in the formulary for the general rule that a gold toner’s visible effect follows its gold concentration. It sits, too, exactly where Reilly puts a bath for glossy albumen: 0.4 to 0.5 g of gold chloride per litre.

The molar ratio of ligand to gold is the other number worth having. Thiourea at 76.12 g/mol gives 13.1 mmol/L; the gold, as the acid at about 339.8 g/mol, gives about 1.2 mmol/L. The ligand is in roughly elevenfold molar excess, which is what a bath needs to hold gold(I) in solution rather than drop it as metal.

And the sodium sulfate is fifteen times the weight of either active ingredient, which is why its unexplained presence is noted rather than passed over.

Behaviour

It is slow, and Kodak publishes how slow twice over. Eight to forty-five minutes at 20 °C; two to fifteen at 38 to 40 °C. A five-to-one range at each temperature is not vagueness; it is what a bath that works this gradually looks like when the times are published honestly.

Agitation barely matters. Kodak: the toning action is so gradual that only a small amount of agitation is necessary to prevent streaking. That is the opposite of every other formula in this batch.

The bath exhausts fast and unevenly across a run. The first few prints tone the most and the last few may not tone at all, which is why the sheet’s instruction is to put up to five prints in together.

Contrast and density appear to increase. Kodak’s compensation is at the enlarger and it goes the other way from every sepia toner’s: reduce the exposure slightly, rather than increase it.

A fine white precipitate appears if the solution is stored overnight, and Kodak says it does not interfere with the toning. The instruction is nonetheless to mix fresh.

It tones from the highlights down. This is the property to plan around. See Image characteristics.

Image characteristics

Grey-blue more often than blue. Kodak’s own words, and they are worth quoting because a page that promised a saturated blue would be over-selling a formula its maker under-sells.

Nothing at all on a cold-tone paper.

Highlights first, shadows last. Partial toning gives blue highlights against untoned shadows — which set against T-21’s uniform rate makes the pair of them the clearest demonstration in the formulary that the ligand, not the metal, decides how a toner moves across the tonal scale.

Apparent contrast and density both up.

Red or orange, over a sepia print. Kodak’s multiple-toning route, with cold-tone papers giving a truer red and warm-tone papers an orange.

The mechanism

A third sulfur ligand, and the same reduction as the other two gold toners.

⚗Why thiourea is in a gold bath at all

Gold in the salt you buy is gold(III), and silver oxidises only to silver(I), so a plain gold(III) bath spends three silver atoms for every gold atom deposited. The chloroauric acid page carries that argument and its equations and they are not repeated here.

Every useful gold toner works with gold(I), and Ware’s two routes to it are a sulfur ligand — thiosulfate, thiocyanate or thiourea — which both assists the reduction and stabilises the product, or a mild alkaline buffer, in which gold(III) slowly oxidises water over as much as twenty-four hours.

Kodak’s sheet is a complete set of the first route: thiosulfate in T-21, thiocyanate in GP-1, thiourea here. The gold-borax toner is the second route.

Why this one tones the highlights first and T-21 does not. No source read for this course explains it, and this page will not invent an explanation. What can be set out is the observation on both sides — Kodak states that T-21 tones highlights and shadows at a uniform rate and that T-26 tones the highlights first and the shadows last — and the fact that the two baths differ in the ligand and in the gold concentration and in almost nothing else. That is a well-posed question with no published answer, which is why it appears under Experiments.

Why the bath dies across a run of prints. There is 0.4 g of gold salt in the litre, and a print takes what it takes. Kodak’s remedy is to share one bath’s gold between five prints simultaneously rather than let the first print have it all.

What the sodium sulfate is doing is not established. It is fifteen times the weight of the gold’s ligand and Kodak assigns it no role. Two readings are available and neither is adopted here: an inert salt raising the ionic strength of the bath, or a bulking agent for a packaged powder that survived into the published formula. No source read for this course discusses sodium sulfate in a gold toner, so the entry says what it is and where it sits, and stops.

What the tartaric acid is doing is only slightly better established. Kodak gives no reason for it either. What can be said from the sources is that it puts the bath on the acid side, and that a gold bath’s condition is read from its colour: Reilly’s rule, on the chloroauric acid page, is that an acidic, inactive bath keeps a yellow colour while a working one is colourless. A deliberately acid gold bath is therefore a slow one, which fits a toner Kodak describes as so gradual that agitation hardly matters. That is a reading, offered as one.

Function of every ingredient

Chloroauric acid, 40 mL of a 1 per cent stock, so about 0.41 g per litre. The gold, and the substance the colour is made of. It is four times GP-1’s concentration and more than three times T-21’s, and the ranking of those three matches the ranking of how much each moves the colour. Kodak prints it as “gold chloride”; the course’s reading of that name in the photographic literature is this substance and not gold(III) chloride proper — the two are 50 and 64.93 per cent gold respectively, so a gram of the wrong one is about thirty per cent more gold than intended. More would tone faster and further; less is what happens to the bath as prints go through it, and it is why the last print in a run may not tone at all. Kodak’s footnote about the solid stands here as on the other gold pages: it is deliquescent and liquefies rapidly in a normal room atmosphere, so it lives tightly stoppered in a dry place.

Thiourea, 1 g, so about 1.02 g per litre. The sulfur ligand, and the reagent that sets this page’s safety level. Its job is Ware’s: it takes gold(III) to gold(I), where one silver buys one gold instead of three, and holds the gold in solution so the bath does not drop metal into the tray. At about elevenfold molar excess over the gold it is present in the quantity that job needs rather than in a catalytic trace. More would reduce faster and, by the general behaviour of these ligands, dissolve more silver alongside; less risks an under-reduced bath that bleaches rather than tones. Its own page carries the classification, the exposure information and the course’s refusal to publish a home procedure for it, and none of that is restated or softened here.

Tartaric acid, 1 g, so about 1.02 g per litre. The acid. Kodak states no function for it. What is observable is that it is present in exactly the same weight as the thiourea, that it puts the bath on the acid side, and that an acid gold bath is by Reilly’s rule a slow one — which is consistent with a toner whose published times run to forty-five minutes. More or less is not a control any source read here publishes.

Sodium sulfate, 15 g anhydrous, so about 15.4 g per litre. The bulk of the formula by weight, and the ingredient this entry cannot explain. Kodak assigns it no role, no other source read for this course discusses it in a gold toner, and it is chemically unreactive towards everything else in the bath. Under Rule 6 the honest form is this: its quantity, its form and its position in the mixing order are recorded, and its function is not established. It is not a trace, it is not an accident of transcription, and supplying a mechanism for it would be exactly the kind of plausible sentence the course exists to avoid.

Water, 937 mL at 52 °C. The bath is mixed hot because the solids dissolve better hot, and then worked either at 20 °C or at 38 to 40 °C. Kodak states no make-up volume, so the finished bath’s volume is not published and every strength on this page is computed against the 977 mL of liquid the sheet names.

Interactions

With fixer carried in on the print. Reilly’s warning for gold toners, on the chloroauric acid page, is that the toner is ruined by even a trace of fixer. Kodak’s own instruction is a thoroughly washed print.

With a sepia or brown toner, deliberately. Kodak’s multiple-toning sequence is sulfide first, thorough wash, then this bath, for red and orange. The wash between is not optional and the sheet says so.

With metal. Ware records that these gold compounds attack most metals; Kodak’s general toning guidance forbids metal trays for any toner. Glass and plastic throughout.

With oxidisers, and this is the thiourea’s rule. Its page records violent decomposition with hydrogen peroxide and nitric acid, and explosion when thiourea is ground with potassium chlorate. Nothing oxidising near the solid.

With light and organic matter, both of which reduce dissolved gold.

With warm-tone papers, in a way that changes the colour. Moersch’s observation about thiourea toners generally is that with warmtone papers the colour turns yellowish owing to the pH of the toner. That is his product rather than this one, and it is recorded as his.

Variants

Kodak GP-1 and T-21, the same maker’s other two gold formulas, on the same sheet, with thiocyanate and thiosulfate in place of the thiourea. Their pages set out how differently the three behave.

The gold-thiocyanate and gold-borax toners used by the printing-out processes, at 0.1 and 0.16 g/L of gold, both mixed cold from published stocks. See gold-thiocyanate and gold-borax.

Moersch’s thiourea toners, and why they do not solve the sulfide problem. This is the variant worth spelling out, because it is the one people reach for. Modern practice offers thiourea toners as the odourless alternative to a sodium sulfide bath, and Moersch’s own toning guide puts the case in a sentence: where the use of sodium- or polysulfide toner is no advantage over odourless thiourea when toning indirectly, the choice is easy. He also records a real photographic difference — toning in sodium sulfide goes beyond the bleached areas and will affect the shadows with increasing time, while a thiourea toner cannot tone the remaining metallic silver but only the areas where it finds a silver salt — which makes thiourea the more controllable reagent for split toning.

None of that is a safety argument, and it must not be read as one. Sodium sulfide is Level C and thiourea is Level C, on different criteria: the sulfide for acute toxicity, corrosivity and its waste; the thiourea as a suspected carcinogen and reproductive toxin with an aquatic classification. Odourless is not the same as safe, and in this particular pair the odour is the warning the other substance gives you. A course that swapped one for the other on hazard grounds would have moved the problem, not solved it.

No course variant is offered. There is nothing to reformulate: the ligand is the reagent that carries the classification, and a gold toner without a sulfur ligand is the alkaline route, which is a different formula and has its own page.

Safety

Level C, from thiourea, on two of the rubric’s Level C criteria. Read that page: it carries the classification, the sources behind it, the exposure argument and the course’s refusal to publish a home procedure. None of it is restated or softened here.

The gold is Level B and its page carries its own controls: nitrile gloves whenever the solid, the stock or the bath is handled; splash goggles for the solid and the stock; plastic rather than metal for spatulas and stirrers; hands washed before eating.

⚠The two things this formula is most likely to be got wrong about

First, that odourless means safer. It does not. Thiourea’s classification is the reason this page is Level C, and it is a classification a sulfide bath does not carry — a suspected carcinogen and reproductive toxin, against the sulfide’s acute toxicity and corrosivity. Two different hazards, both at the same level, and the one that does not smell is the one whose harm is slow.

Second, that a small quantity is a small hazard. There is one gram of thiourea in this bath. The control its page asks for is engineered extraction and no weighing of the powder outside it, and that control is about the powder and the tray, not about the total mass. HSE’s HSG262 is the reason gloves are not the whole answer for a sensitiser, and the carcinogen and reproductive-toxin statements are not concentration-managed at a darkroom bench.

Keep it away from oxidisers absolutely: its page records violent decomposition with hydrogen peroxide and nitric acid, and explosion when thiourea is ground with potassium chlorate.

The bath may be worked at 38 to 40 °C, which is a warm open tray of a Level C reagent and is the condition under which the ventilation argument matters most.

Storage

Mix it right before you use it. Kodak’s instruction and the only shelf-life statement it makes.

A fine white precipitate appears overnight, which Kodak says does not interfere with toning — a statement about the precipitate rather than a licence to keep the bath.

The 1 per cent gold stock is the thing that keeps, and it keeps in the dark, away from organic matter, tightly stoppered and dry. Label it with its strength.

The thiourea solid is stored to its own page’s instructions, away from oxidisers and away from anything that will be weighed on the same balance without cleaning.

Nothing metal touches any of it.

Incompatibilities

Oxidisers, with the thiourea. Its page records violent decomposition and, with a chlorate, explosion.

Acids, beyond the tartaric acid the formula contains, and especially with any thiocyanate or sulfide bath sharing the bench.

Metals, with the gold.

Light and organic matter, with the gold stock.

Fixer, carried in on the print or the tongs.

Sulfide toners, in the waste bottle, though not in sequence on the print — Kodak’s own multiple-toning route puts one after the other with a thorough wash between.

Developer, in either direction.

Waste

A gold-bearing, thiourea-bearing stream, and the thiourea is what decides its route. Its page records the aquatic classification that makes this waste something a domestic drain cannot lawfully take, and that is one of the two criteria that put the formula at Level C in the first place.

Not acidified, not combined with a sulfide toner’s waste, not combined with any oxidising waste.

Collect it, label it as gold- and thiourea-bearing, and follow the general chemical waste SOP and the disposal ruling; where the bath has toned prints it is a silver-bearing stream as well. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Recorded from the source for the sake of understanding the formula, not as instructions for running it.

Nothing happened. Cold-tone paper, most likely — Kodak says plainly that they will not change. Otherwise a bath that has already given its gold to earlier prints.

The first print went blue and the fourth did not. Expected, and Kodak names it: prints toned one at a time exhaust the bath unevenly, and the remedy is to immerse up to five together.

Blue highlights and untoned shadows. Partial toning. Kodak names this as the normal appearance of an incompletely toned print in this bath, because it works from the highlights down.

Streaking. More agitation, though only a little: the sheet says a small amount is enough because the action is so gradual.

A grey-blue rather than a blue. Kodak’s own expectation. A soft grey-blue is more common than a saturated blue, and the exact tone varies with the paper and the developer.

The print looks darker and more contrasty than it did. Expected; Kodak’s compensation is to reduce the print exposure slightly next time.

A fine white precipitate in a stored bath. Kodak says it does not interfere with the toning. The instruction is still to mix fresh.

The bath went yellow and stopped. Reilly’s rule for a gold bath: yellow is inactive.

Experiments

For a laboratory with the controls thiourea’s page asks for.

Answer the question this page could not. A step wedge through T-26 and an identical one through T-21, both pulled at the same fraction of full toning, both read step by step and plotted. Kodak states that one tones the whole scale uniformly and the other tones the highlights first; nothing in this corpus explains why, and the two curves are the first evidence anyone would need.

Separate the ligand from the concentration. T-26 at Kodak’s gold and T-26 diluted to GP-1’s 0.1 g/L, on the same paper. If the highlights-first behaviour survives the dilution it belongs to the thiourea; if it does not, it belongs to the gold concentration.

Test the sodium sulfate. The formula as printed against the formula with the sodium sulfate omitted, same paper, same time. Kodak gives it no function and no source read for this course explains it; a controlled pair of prints would say whether it does anything visible at all.

Reproduce the red. Kodak’s multiple-toning sequence — sepia or brown, thorough wash, then this bath for 15 to 30 minutes at 32 °C — on a cold-tone and a warm-tone paper. The sheet predicts a truer red on one and an orange on the other, and predicts a shadow density loss on both.

Time the two temperatures against each other. Eight to forty-five minutes at 20 °C, two to fifteen at 38 to 40 °C. Establish where in each range your paper sits, and whether the colour reached by the fast route is the same colour as the slow one.