Thiourea

Sepia toning by the classical route needs a bath of sodium sulfide, which smells of rotten eggs, gives off hydrogen sulfide and will fog any unexposed paper in the room. Thiourea does the same job without the smell, and Wolfgang Moersch states the consequence plainly: the odourless toners sold in shops are thiourea toners. What thiourea carries instead is a different problem, and the problem is on its label.

In photography

Sepia toning, as the second half of a bleach-and-redevelop. Kodak’s 1928 primer sets out the classical sequence: bleach the developed silver image in potassium ferricyanide with bromide, which converts image silver back to silver bromide, then treat the bleached print with sodium sulfide, which converts the silver bromide directly into silver sulfide. Wolfgang Moersch’s toning guide describes the thiourea baths as the odourless alternative to that sulfide step, and notes that the commercial odourless toners are thiourea toners. The two he prints are the Agfa formulas: 520, thiourea 5 g, potassium bromide 40 g and sodium hydroxide 3 g per litre, and 525, identical but with 15 g of sodium hydroxide.

What more or less of it does — and it is the alkali, not the thiourea, that steers the colour. Moersch’s rule is that the higher the pH, the darker the brown: 520 gives a delicate yellowish hue and 525 a stronger, darker one, and the matching table for his own MT3 toner runs from brown-black at 5 to 30 mL of alkaline activator per litre through medium brown, yellowish brown and dark yellow to bright yellow at 180 mL. Two other variables sit outside the toner. How far the print was bleached decides which tones are converted at all, so a brief, dilute bleach warms only the highlights. And the emulsion decides the hue: a high bromide content gives deep browns, while chloride-rich and mixed emulsions tone yellow whatever toner is used.

⚗What is sourced, and what is not

The product is silver sulfide. Kodak’s 1928 primer calls it a very insoluble compound of silver, states that a silver image or a silver halide treated with sulfur or a sulfide is at once transformed into it, and adds that experience shows this form of silver to be one of the most stable — which is why sulfide toning was the recommendation where permanence mattered. The primer also states that the colour ranges from light brown to black according to the state of subdivision, and that the toned image’s state of subdivision follows the untoned one, so exposure and development decide the hue before the toner is mixed.

How thiourea delivers that sulfur, this course has not verified. No source it holds states the mechanism, and the obvious guess — that alkali hydrolyses thiourea and releases sulfide — is a guess. What Moersch does establish is behaviour consistent with a slow reservoir rather than a free sulfide bath: the baths are long-lived, they are regenerated with sodium hydroxide rather than with more thiourea, and exhaustion shows as a print not fully toned in thirty seconds.

Gold toning is the other job, and there the chemistry is clear. Mike Ware gives the sel d’or reaction as a displacement in which silver reduces gold(I) held in a soluble complex, plating gold onto the image, and records that a similar reaction occurs with thiourea, “which is widely used for a blue gold toner”. His history of gold toning explains the role: gold(III) must be reduced to gold(I) and the gold(I) stabilised, and thiosulfate, thiocyanate and thiourea are the three sulfur-bearing molecules that do it. Kodak’s G-23 prints one such bath as Blue Toner T-26: a 1% gold chloride stock, thiourea 1 g, tartaric acid 1 g and anhydrous sodium sulfate 15 g in about a litre.

Properties

White or off-white crystals or powder that sink and mix with water. ChEBI describes the molecule exactly: urea with the oxygen atom replaced by sulfur, CH₄N₂S, relative molecular mass 76.12.

Solubility is the number that makes the toners possible. HSDB gives 142 mg/mL at 25 °C — one part in eleven — and a steep temperature curve, 117.5 g/L at 20 °C rising to 235.5 g/L at 40 °C. A 5 g/L toner is nowhere near saturation, which is why these baths hold their strength through many prints.

The pH is the property to be careful with. Moersch measures his alkaline thiourea toners at up to pH 13.4, warns that prints left too long in them soften the gelatin and dry to a dull surface, and sets a limit of one minute in the bath for that reason. Half the handling problem on this page is not the thiourea at all but the sodium hydroxide it is dissolved in.

Handling

Thiourea is the first substance in this family with a harmonised classification rather than a notified one. Regulation (EC) No 1272/2008 gives it Warning, three pictograms and four statements: harmful if swallowed, suspected of causing cancer, suspected of damaging the unborn child, and toxic to aquatic life with long lasting effects. The 710 aggregated notifications on PubChem agree almost unanimously — 99.6% for the carcinogenicity statement, 99.7% for the aquatic one — and Japan’s NITE-CMC goes further, raising the signal word to Danger and adding skin sensitisation, respiratory irritation and organ damage on prolonged exposure. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit, and EH40 states that absence from its list does not indicate that a substance is without risk. Princeton University’s guidance calls thiourea a probable human carcinogen because it causes cancer in animals, requires local exhaust ventilation for toning solutions, and advises avoiding it wherever possible.

Why Level C. Thiourea is not a historical curiosity — it is in current products and current practice — so the rubric’s Level D does not fit. Level C does, on two of its criteria. The first names reagents classified as carcinogenic, as reproductive toxins or as sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control: thiourea is classified as two of those three, and on NITE-CMC’s and HSDB’s reading all three. The second names any procedure whose waste cannot lawfully enter a domestic drain, which H411 settles. The alkalinity alone would put the bath at Level B; the classification takes it past that.

⚠Safety

This course gives no home procedure for thiourea toning, and the reason is a suspected carcinogen and reproductive toxin in an open tray at pH 13. If you meet it in a supervised laboratory: engineered extraction, splash goggles, chemical-resistant gloves changed the moment they are contaminated, and no weighing of the powder outside extraction. HSE’s HSG262 is the reason gloves are not the whole answer — once a person is sensitised, tiny amounts of the allergen provoke a reaction and the only remedy is to prevent further exposure. Keep it away from oxidisers absolutely: CAMEO records violent decomposition with hydrogen peroxide and nitric acid, and explosion when thiourea is ground with potassium chlorate.

Waste

A spent thiourea toner is strongly alkaline and carries thiourea together with the bromide and silver it took from the bleached image; the ferricyanide bleach before it is a separate stream. Moersch’s instruction that a print be rinsed thoroughly before bleaching, because residual fixer turns the bleach into a reducer, is a processing rule that also keeps those streams apart.

The classification decides the route. H411 is toxic to aquatic life with long lasting effects, and the harmonised precautionary statements include P273, avoid release to the environment, P391, collect spillage, and P501, dispose of contents and container to an approved facility. There is no neutralisation step this course can offer: neutralising the alkali with acid leaves the thiourea behind, and acidifying a bath that has toned silver is the mixture Princeton’s guidance warns against. Bottle it, label it with the substance and the date, and take it to a licensed hazardous waste route — in England and Wales through the government’s household hazardous waste service finder. Check your local regulations; they govern.

History

Thiourea entered photography as a fixer that never caught on. Wall’s 1912 dictionary records that Dr Bogisch proposed thiocarbamide as a fixing agent for silver chloride, and that its real usefulness lay elsewhere: removing pyro and other developer stains, green fog and silver stains, for which Wall prints two clearing baths. He adds a curiosity — “thiocarbamide in the developer has a remarkable tendency to bring about reversal” — and notes that thiosinamine, allyl thiourea, was suggested by Liesegang as another chloride fixer and never came into general use.

That last molecule reappears on the other side of photography. Ware records, citing Sheppard’s paper of 1925, that Kodak’s research in the 1920s found traces of sulfur compounds in gelatin — mustard oil, allyl isothiocyanate and allyl thiourea — to be what sensitises a silver halide emulsion. The same class of molecule that tones a finished print by sulfiding its silver also, in parts per million, makes the emulsion fast in the first place.

The gold toners came next. Eder records that acid thiocarbamide gold toning baths were introduced by Hélain and Valenta; Ware dates Hélain’s discussion to 1902 and follows the line to D. J. Ruzicka in the 1930s, blue-toning with 0.03% gold chloride and 0.11% thiourea acidified first with sulfuric and later with citric acid. The use that has lasted is the sepia toner, and it lasted because the alternative smelled. The course has not found a source that dates its introduction. What the trade sold as an improvement in the darkroom’s air turns out, on the modern classification, to have been an exchange of one hazard for another.