Sulphide (sepia) toned silver gelatin print

Sepia toning is the only process in this atlas that replaces the image substance rather than plating it, thinning it or adding a pigment beside it. A finished silver print goes in and a silver sulfide print comes out, and the strongest permanence claim any manufacturer makes in this course is made about the result. Sodium sulfide’s page owns the reagent and Kodak T-7a and T-1a the two published routes.

The chemistry

Two baths, and the second one wins the silver on a solubility argument. Kodak’s sequence, unchanged between the 1928 primer and the 2006 toner sheet: bleach the developed print in potassium ferricyanide with bromide until only a faint yellowish image is left, rinse thoroughly, then put it in sodium sulfide, which converts the silver bromide directly to silver sulfide.

Silver sulfide is, in the primer’s words, a very insoluble compound of silver: treat a silver image or a silver halide with sulfur or a sulfide and it is at once transformed into it. The bleach has already put the image back into silver bromide, and the sulfide ion takes the silver from the bromide because it holds it far more tightly. From the OpenStax table of solubility products at 25 °C: silver chloride 1.6 × 10⁻¹⁰, silver bromide 5.0 × 10⁻¹³, silver iodide 1.5 × 10⁻¹⁶ — and silver sulfide 1.6 × 10⁻⁴⁹.

⚗The colour is decided before the toner is mixed

The 1928 primer states that silver sulfide runs from light brown to black according to its state of subdivision, and that the state of subdivision of the toned image follows that of the untoned one.

So exposure and development set the hue. A print should be fully developed but not over-exposed to tone well, and nothing you do in the sulfide bath will rescue a print that was wrong before it got there. That is the same rule that governs metallic silver itself, arriving one compound later: a toner does not choose the colour, it keeps the one the paper and the developer chose.

A second route reaches the same compound without a sulfide reagent. The hypo-alum bath, Kodak T-1a, gets its sulfur by acidifying hot hypo until it precipitates its own, at 49 °C — which is a different chemistry with different faults, and its own formulary page argues that reaching a silver sulfide image without handling a sulfide is not the same as being safe.

Historical workflow

Print, fix properly, wash, bleach, rinse thoroughly, redevelop in the sulfide, rinse, wipe, wash.

Two of those steps carry most of the failures, and Kodak’s guidance about them is unusually direct. Print processing before the toner decides more than the toner does: 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; prolonged fixing drives fixer into the paper base and makes sulfide-toned prints turn yellow; and air bubbles trapped between prints in the fixer show up later as round purple stains.

The rinse between the baths is not a formality. The bleach carries acetic acid, and where acid meets sulfide the product is hydrogen sulfide.

And the toner has a taxonomy of failure, which Wall’s 1912 dictionary gives and which is the most useful paragraph in the older literature on the subject. As the sulfide deteriorates in solution, hypo is formed; the first sign is a yellow-brown image, the next an apparent failure to act at all, and the last is that the bleached image gradually disappears, because the bath has become a fixing bath. A sulfide toner does not merely stop working. It starts dissolving your print.

The image

Brown, in a range set by the print rather than by the bath, from a cold chocolate on the hypo-alum route to the warmer browns of a sulfide redevelopment.

Density falls. Kodak states it as a property of its sepia toners generally and answers it at the enlarger: develop fully, print slightly darker than normal. The decision to tone is therefore taken before the print is made, which is true of surprisingly few processes in this atlas.

It is the one toned image that cannot easily be identified. The Getty atlas records brown sulfur toning as the commonest toning of the early twentieth century, used mainly on the matte-surface papers portrait photographers favoured between the wars — and then explains why analysis struggles with it. X-ray fluorescence does show sulfur in such a print, but the main source of that signal is not the Ag₂S of the image: it is the barium sulfate of the baryta layer, which is almost always present, and which makes the two contributions extremely difficult to tell apart. Selenium toning leaves a signature; sulfur toning hides inside the paper’s own.

Permanence

This is the strongest permanence claim in the course, and it is a manufacturer’s. Kodak’s 1928 wording is as strong as such a statement ever gets: where great permanency is required, prints should preferably be toned to a silver sulfide image, since experience has shown that this form of silver is one of the most stable. ILFORD’s current fibre-base warm-tone paper sheet still recommends polysulphide toners among the two it names as especially suitable, and lists sulphide (sepia) among the toners that give a protective effect.

The solubility argument behind it is the forty-orders-of-magnitude one above. Read the number as a direction of travel rather than as a promise: every reaction that threatens a silver image — oxidation by peroxides in a mount, attack by residual thiosulfate, dissolution by a bleach — has to get silver into solution first, and against a solubility product of 10⁻⁴⁹ there is almost nothing to work on.

Two things the number does not say. The gelatin, the paper and the mount all have their own failure modes. And a partial bleach leaves untoned silver behind that is exactly as vulnerable as it was before — so conversion has to be complete for the claim to be complete.

There is an irony worth carrying out of this entry. The compound this process makes deliberately is the same compound that ruins an albumen print by accident, where labile sulfur finds the silver the fixer could not remove. The difference between the treatment and the disease is whether the conversion was chosen and complete.

Hazards

⚠The sequence is the hazard

The dangerous thing about sepia toning is not the sulfide alone; it is that the bath immediately before it is acidic. Kodak’s own T-7a bleach contains acetic acid, and the print carries some of it. Sodium sulfide plus acid gives hydrogen sulfide, and Kodak’s own safe-handling note is that sulfide-type toners are not discarded with stop baths or fixing baths because the combination generates hydrogen sulfide gas — which can fog unexposed paper and film and will oxidise unprotected silver images in negatives and prints.

The rinse between the baths, and the separation of the waste streams, are both that one reaction.

Level C, on the rubric’s own criteria as sodium sulfide’s page argues them: a reagent classified toxic in contact with skin by nearly every notifier; a unanimous eye-damage statement, so a failure mode that is a burn rather than a spoiled print; and waste that cannot enter a domestic drain. Level D would be wrong, because this is not a historical curiosity — Kodak printed T-7a as a current formula in 2006 and ILFORD still recommends the toner class.

⁇Level B or Level C? The manifest and the encyclopaedia disagree

The course’s curriculum manifest classifies Part XX’s sepia lab, “Bleaching and Redeveloping a Sepia Print”, at Level B. The sodium sulfide encyclopaedia entry argues Level C for the reagent, and Kodak T-7a is published in the formulary at Level C.

The two have not been reconciled, and this atlas does not reconcile them. It records the higher of the two here, because a process entry that quoted the lower one without saying the other exists would be making a safety claim the corpus does not support. The direct hypo-alum route, T-1a, is published at Level B and reaches a silver sulfide image without a sulfide reagent being handled — which may be what the manifest’s Level B refers to, and the course cannot tell from the manifest alone. Part XX is where it must be settled.

Where the course teaches it

Part XX, Toning Chemistry, in a lesson and the lab that follows it.

Sulfide and Sepia Toning: Bleach, Redevelop, Convert takes the two routes to one compound apart separately: the indirect route bath by bath, what each ingredient of a published bleach is answering, the sulfide redeveloper and the salt it is made from, thiourea as the odourless alternative, the direct hypo-alum route in which the bath makes its own sulfur, and the polysulfide toners that convert without bleaching at all. It also states the rule that governs every sulfide bath ever mixed, and the faults that appear only after toning.

Lab: Bleaching and Redeveloping a Sepia Print is the Level B practice, and its design is the reason it is worth doing rather than reading: three identical prints, one bleached to the endpoint, one stopped a third of the way and one left alone, measured patch for patch before and after — so that the density a sulfide toner costs is your own number rather than a manufacturer’s adjective.

Two further pages of the part bear on this entry directly. What a Toner Actually Does to a Silver Image is where conversion is placed as one of three routes and where the vocabulary on this page comes from. Toning, Permanence and What the Evidence Actually Supports weighs the sulfide protection claim against selenium’s and gold’s, and is careful about what kind of statement each one is: Wall’s 1924 recommendation is there, labelled by Wall himself as accumulated trade experience rather than as measurement.

The chemistry and the formulas are where they were. The sodium sulfide and silver sulfide encyclopaedia entries carry the substances, and four published toners carry the weights — T-7a and T-52 on the sulfide route, T-1a on the hypo-alum one, and T-56, which dissolves selenium into the sulfide stock and is published at Level D for that reason.