Silver sulfide

This is the compound that makes a sepia print permanent and the compound that yellows the highlights of an albumen print past saving. It is also what is on the back of an old teaspoon. One substance, three reputations, and the difference between them is only where in the paper it forms.

In photography

It is what sulfide toning produces. Kodak’s current toner sheet says so of three products in one column — Brown Toner, Sepia Toner and Sepia II Warm Toner all convert the silver image to silver sulfide — and the 1928 primer explains why the trade went that way: silver sulfide is very insoluble, so a silver image or a silver halide treated with sulfur or a sulfide is at once transformed into it. The two industrial routes are set out on the sodium sulfide page, which is the reagent’s own; this page is about what they leave behind.

The colour is not a property of the compound. The 1928 primer is precise: silver sulfide runs from light brown to black according to its state of subdivision, and the subdivision of the toned image follows the subdivision of the image it replaced. That is the same rule that governs metallic silver itself. A toner does not choose the colour; the exposure, the paper and the developer that made the original image chose it, and the toner keeps it.

Permanence is the claim, and it is an old one. Kodak’s 1928 wording is as strong as a manufacturer’s 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. The 2006 sheet generalises it — toning converts the image to an inert compound, and all its toners protect the image whether or not they change the colour.

⚗Forty orders of magnitude

Why the protection works is a solubility argument. 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 sulfide is roughly thirty-nine orders of magnitude below the chloride the course begins with.

2 Ag⁺ + S²⁻ → Ag₂S

The precipitation the numbers describe

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. Against a Ksp of 10⁻⁴⁹ there is almost nothing to work on. What the number does not say is that a toned print cannot fade: 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.

It is also the one toned image that cannot easily be identified. The Getty Conservation Institute’s 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.

The same compound is a defect. Reilly’s account of albumen prints is the other half of this page. Albumen binds silver so tightly that hypo does not remove it, so a small amount of silver remains in every area of a fixed albumen print, highlights included. Labile sulfur — from residual fixer, or from the atmosphere — converts that silver to silver sulfide, and that conversion is the immediate cause of the yellow highlight staining that disfigures so much nineteenth-century work. Reilly then names the cruelty of it: the sulfide stain is much more chemically stable than the colloidal silver of the image itself, so no known treatment removes the yellowing without destroying the print. Stability is not a virtue when the wrong thing is stable.

Properties

Ag₂S, relative molecular mass 247.80, CAS 21548-73-2; as a mineral it is acanthite. PubChem’s record is unusually bare — no physical description, no solubility figure, no classification — because this is a compound almost nobody manufactures. It is made in place, by everything from a toning bath to a London winter.

The one property that matters is the one that has already been given: it does not dissolve. The second is that it is not light-sensitive in any way this course uses, which is what separates it from the halides. And the third is that it forms from silver and sulfur with no help at all — Wall’s 1912 entry on the metal records that sulphurous vapours act on silver immediately, forming sulphides. Tarnish, sulfiding of a print and sepia toning are one reaction under three names, distinguished by whether it was wanted.

Handling

There is no classification to quote. PubChem holds no GHS block for this compound from any source, and EH40 has no entry for insoluble silver compounds — only metallic silver at 0.1 mg/m³ and soluble silver compounds at 0.01 mg/m³. EH40’s own paragraph 6 says that absence from the list does not indicate a substance is without hazard, and this page takes that seriously rather than reading the empty record as reassurance.

⚠Why Level A, and the one operation that is not

Against the rubric, silver sulfide as this course meets it is Level A. It is met as a product: a compound already inside a washed print, insoluble, immobile and bound in gelatin, raising no dust, no vapour and no splash. Ordinary darkroom controls cover it, and the hazards of the operation that made it belong to the toner, not to this.

The one operation that takes it out of Level A is making it. A sulfide toning bath is sodium sulfide, which the course classifies Level C, and the hazard is hydrogen sulfide. The specific mistake to avoid is chemical rather than procedural: acid meeting sulfide. Kodak’s own sheet forbids discarding a sulfide toner together with a stop bath or a fixer for exactly that reason, and the same rule applies in a tray as in a drain.

Waste

Almost all of this substance leaves the darkroom in the picture. The bottles that leave are the bleach and the toner, and they carry two problems: dissolved silver, which belongs in the silver-bearing stream and never in a sink, and sulfide, which must not meet anything acidic on the way out — Kodak’s instruction is to discard each solution separately and flush the drain between them. Check your local regulations; they govern, and they differ.

History

Sulfide toning was not invented. It was noticed. Reilly records that the first sulfur toning was the “old hypo” method of the 1840s and 1850s, in which a fixing bath was deliberately decomposed until free sulfur reacted with the silver image to give brownish silver sulfide — a practice that almost certainly began as an observation that exhausted baths toned prints more strongly than fresh ones. It was also, within a decade, known to be dangerous to permanence: prints fixed in a wholly exhausted bath could fade within weeks.

What survives complicates the verdict, and Reilly says so rather than tidying it away. Many sulfur-toned prints from the period are still in good condition — the Hill and Adamson calotypes among them — so under some circumstances the old method did give a stable sulfide image, though the odds were poor, and the practice that seems to have saved them was following the old bath with a fresh, strong one. Eighty years later Kodak was recommending the same end product by a route that no longer depended on letting the fixer rot.