Potassium cyanide

The wet-plate photographer had two fixers and chose between them on speed and image colour. One was hypo. The other dissolved silver halide faster, left a whiter picture, and killed people. Hardwich’s 1864 manual introduces it as the salt most frequently employed in fixing, and then, four sentences later, as highly poisonous and to be used with caution. Both sentences are true, and the second is why this page carries no procedure.

In photography

It fixed, and it fixed hard. Hardwich states the property plainly: a solution of potassium cyanide is a most energetic agent in dissolving the insoluble silver salts, far more so than hyposulphite of soda. The salts are converted into cyanide and exist in solution as a soluble double salt which, unlike the double sulfocyanide, is not decomposed by dilution with water — so a cyanide-fixed plate needs less washing than a hypo-fixed one, because the complex it leaves behind does not fall apart in the rinse.

⚗The chemistry

Fixing is complex formation, and cyanide forms an exceptionally stable one. Two cyanide ions take a silver ion out of the lattice and hold it in solution:

AgI + 2 CN⁻ → [Ag(CN)₂]⁻ + I⁻

Cyanide fixing: the dicyanoargentate complex

Hardwich’s comparison is the useful one: this is a far more energetic solvent of the insoluble silver salts than hyposulphite of soda is, and the double salt it forms — unlike the double sulfocyanide — is not decomposed by dilution. Speed and stability together are why it does not stop at the unexposed halide, which is the warning he attaches. If the solution is too concentrated, or left on too long, the middle tints first whiten as the image silver itself is converted to silver cyanide and then begin to dissolve. He adds a detail no modern fixer needs: the plate must not be exposed to strong light while the cyanide is on it, because light favours that conversion.

Why anyone accepted the risk was the look of the picture. For collodion positives — ambrotypes and tintypes — Hardwich notes that hyposulphite contains sulfur and is somewhat unstable, so the image is liable to be superficially darkened, while cyanide is free from that objection and always produces a whiter picture. The Image Permanence Institute’s booklet on negatives records the same difference as a process variation with a visible signature: fixing in hypo gives a darker, brownish grey image colour, fixing in cyanide a lighter, milky tan. The conservation wiki’s ambrotype entry still describes the developed plate as fixed in hypo or potassium cyanide, because both are in the historical record and both are found in surviving objects.

Hardwich did not recommend it for negatives, and the reason is instructive: negative images dissolve in a fixing agent more easily than collodion positives do, so without much care the cyanide lowers the intensity and whitens the surface — especially, he notes, when fixing outdoors with the sun on the plate. He tells the reader to prefer sulfocyanides or hyposulphite as safer in the hands of a beginner, meaning safer for the negative. A century and a half later the sentence reads differently.

A second use survived longer. Kodak’s 1928 primer describes the Monckhoven intensifier — bleach with mercuric chloride, then blacken with silver dissolved in potassium cyanide — as a very powerful method used chiefly by photo-engravers, where the cyanide cuts the shadows very slightly while the highlights intensify, giving a great increase in contrast. Princeton’s environmental health guidance lists it among the older, now discarded intensifiers.

Properties

A white granular or crystalline solid, hygroscopic to the point that NIOSH notes it absorbs moisture from the air and forms a syrup, and freely soluble at 716 g per litre at 25 °C — which is why a fixing bath could be made strong enough to work in seconds. NIOSH gives the faint, almond-like odour that everyone quotes; the safety card gives the correction that matters, that the dry solid is odourless, so smell is not a detector.

The property that governs everything else is what happens with acid. CAMEO classes it a basic salt and records that it reacts with acids of all kinds to generate poisonous hydrogen cyanide gas; NIOSH says the same in its fire entry. Even dissolving it in neutral water releases some of the gas.

CN⁻ + H₃O⁺ → HCN + H₂O

Any acid converts cyanide to hydrogen cyanide

CAMEO’s air-and-water entry puts the timescale on it: in acidic water, dangerous amounts of hydrogen cyanide form at once, and in its excess-water test half the maximum theoretical yield of the gas appeared within twelve minutes. Set that beside the working chemistry of a collodion darkroom, where the silver bath is held at pH 4 to 5 with nitric acid, and the hazard is not hypothetical: it is two trays apart.

Handling

The aggregated ECHA notifications classify it Danger, and the three acute-toxicity statements arrive together — fatal if swallowed, fatal in contact with skin, fatal if inhaled — each in 99.8 per cent of 575 reports across 30 notifications, with a third of notifiers using the combined H300+H310+H330 form. CAMEO’s health entry calls it super toxic and puts the probable human oral lethal dose below 5 mg per kg of body weight.

NIOSH sets a ceiling of 5 mg/m³ as CN over ten minutes, an immediately-dangerous-to-life level of 25 mg/m³, and four exposure routes including skin absorption; the symptoms it lists are asphyxia, weakness, headache and confusion, nausea and vomiting, breathing first faster and then slow and gasping, with the cardiovascular system, central nervous system, thyroid and blood as target organs. HSE’s EH40 gives cyanides other than hydrogen cyanide, cyanogen and cyanogen chloride a workplace exposure limit of 5 mg/m³ as CN over eight hours, and lists potassium cyanide by name with a skin notation.

⚠Why Level D

The classification rubric names this one in its own words. Level D covers “mercury vapour development, cyanide fixing or toning, uranium and cadmium compounds, and any process whose historical practice is documented mainly through the harm it caused”, and its control is that the page describes chemistry, workflow in outline, image characteristics and hazards and states explicitly that it is not a procedure. This one does. No concentration, no bath, no time appears above.

The second D criterion applies too: a safer route exists and is taught instead. Thiosulfate fixing does the same job by the same kind of chemistry, and every modern wet-plate supplier sells it that way — Bostick & Sullivan’s wet-plate kit ships a rapid fixer described on the sheet as the alternative to what it calls deadly cyanide-based fixers. Nothing about the picture requires the cyanide; only the last few per cent of speed and a slightly lighter image colour do.

Waste

A spent cyanide fixer is a silver-bearing solution and a cyanide-bearing solution at once, and the two halves pull in opposite directions. Silver recovery from ordinary fixer is a mature, published practice; cyanide destruction is not something a darkroom does. Worse, the reflex that serves other photographic waste — acidify, then precipitate — is the most dangerous thing that can be done to this one, because acid liberates hydrogen cyanide at once. Under WM3, entry 09 01 04*, fixer solutions, is an absolute hazardous entry, and the aggregated classification carries H400 in 99.8 per cent of reports. GOV.UK’s household guidance routes hazardous waste from a home to a collection point. The course has found no published domestic treatment and offers none. Check your local regulations; they govern.

History

Cyanide fixing arrived with the collodion positive and stayed for the working life of the wet plate. It was never the only fixer: the Image Permanence Institute records that both cyanide and hypo continued in use for negatives throughout the collodion era, with cyanide chosen for speed. What it left behind is identifiable — a lighter, milky-tan image colour that a conservator can read off a surviving plate.

The retreat began early and from inside the practice. Hardwich was already steering beginners away from it in 1864 on the grounds that it eats the image; the toxicity was stated on the same page but was not, in his account, the deciding argument. That order has since reversed. Modern wet-plate practitioners work as the collodion manuals describe with the fixer swapped for a thiosulfate bath, and the pictures are indistinguishable to anyone but an analyst — which is the cleanest demonstration of why a Level D substance is studied rather than used. The chemistry mattered; the specific reagent did not.