Mercury(II) chloride

For eighty years the answer to a thin negative was a white salt whose name was already a warning. Corrosive sublimate reached the darkroom because it does something no gentler reagent does as thoroughly: it converts a silver image into a pair of white chlorides that can be turned back into metal with mercury added.

In photography

The mercury intensifier was the standard one. Wall’s 1924 handbook calls it probably the most generally used of all intensifiers, and Kodak’s 1928 primer gives the chemistry in two sentences: a silver image placed in a solution of mercuric chloride reacts to form a mixture of mercurous chloride and silver chloride, and the bleached image appears white. What happens next decides the result. If the white image is redeveloped, both chlorides go back to metal, so that every part of silver now has an equal part of mercury beside it. If it is blackened with ammonia instead, Kodak says a black mercury ammonium chloride forms and the intensification is higher still.

⚗The chemistry

The bleach is a displacement, and it works because mercury(II) will take chloride from a metal that gives up electrons more readily than mercury does.

2 Ag + 2 HgCl₂ → 2 AgCl + Hg₂Cl₂

Bleaching a silver image with mercury(II) chloride

Two products, both white, both in the place the silver image occupied — which is why the plate goes opaque-white rather than clear. Density comes back only in the second bath, and the whole increase is material that was not in the negative before: the mercury is added, not merely rearranged. That is the difference between an intensifier and a developer, and it is also why a mercury-intensified negative has a permanence problem the original did not.

The practice around it was solubility and washing. Wall records that salt, ammonium chloride, hydrochloric acid or potassium bromide are added to the bleach for one purpose, to raise the solubility of the mercury salt, the bromide giving the greatest increase in density; that the plate must be free of hypo before it goes in; that bleaching runs until the image seen from the back is white; and that the mercury salt is held tenaciously by the gelatin and must be washed out with acidulated water or it will stain later. His list of blackening reagents runs to ten.

It also whitened, and browned. Archer’s own 1854 manual gives an acid corrosive-sublimate solution for whitening a collodion picture into a positive, warning that a weak collodion film will not survive it. Half a century later the same salt was doing something quite different in platinum printing: the Getty Conservation Institute’s atlas records that sepia platinotype papers carried mercury chloride in the sensitiser or the developing bath, where it retards the deposition of platinum, reduces the particle size and makes the image read browner. The atlas is careful to say that “mercury-toned” is not strictly correct — the effect is physical rather than a toning — and notes that the mercury remains detectable by X-ray fluorescence.

Properties

An odourless white crystalline solid, dense at 5.4 g/cm³, melting at 277 °C, and — the property that named it — slightly volatile at ordinary temperatures and able to be sublimed unchanged. Its water solubility is modest and rises sharply with heat, 6.9 g per 100 cm³ at 20 °C against 48 g at 100 °C, and unusually for a darkroom salt it dissolves well in alcohol and appreciably in ether.

Two of its properties are photographic facts in disguise. CAMEO records that it is decomposed by sunlight, which is why Wall told photographers to keep the bleach in the dark; and CAMEO’s incompatibility list includes albumin and gelatin, which is Wall’s observation seen from the other end. A substance that binds protein does not rinse out of an emulsion, and the acid wash in the historical procedure exists for exactly that reason.

Handling

The aggregated ECHA notifications classify it Danger: fatal if swallowed, causes severe skin burns and eye damage, causes damage to organs through prolonged or repeated exposure, and very toxic to aquatic life with long lasting effects, each in 100 per cent of 218 reports across 12 notifications; suspected of causing genetic defects and of damaging fertility or the unborn child in 99.5 per cent; fatal in contact with skin in 26.6 per cent. CAMEO calls it extremely toxic. NIOSH lists four exposure routes — inhalation, skin absorption, ingestion and contact — with tremor, insomnia, irritability and weight loss among the symptoms and the kidneys and central nervous system among the target organs, and HSE’s EH40 gives mercury and its divalent inorganic compounds, naming mercuric chloride, a limit of 0.02 mg/m³ as mercury over eight hours.

☠What the 1924 handbook believed

Wall’s handbook gets this wrong in a single sentence, and the sentence is worth reading because it shows how confidently a period source can state something false. Having called the salt extremely poisonous when taken internally, he writes that absorption by the skin, even in the case of cuts and abrasions, is “practically harmless”. It is not: the aggregated classification carries fatal in contact with skin, NIOSH names skin absorption as an exposure route, and EH40 attaches a biological monitoring value because the dose that matters is the one already in the body.

No page in this course should be read as reproducing what a historical manual asserts about safety. Historical sources are authority for what was done; they are not authority for whether it was acceptable.

⚠Why Level D

Two criteria of the classification rubric apply. The first is Level C’s: a reagent classified acutely toxic where a fume cupboard and a specialist waste route are the recognised controls, which a home darkroom does not have. The second is Level D’s: a safer modern route exists and is taught instead. Intensification is an unusual case, because what replaced the mercury intensifier is mostly not another intensifier — it is a better-exposed negative, a different paper grade, and, where a real increase in density is wanted, selenium. Princeton’s environmental health guidance lists the mercuric chloride intensifier, Monckhoven’s intensifier and the mercuric iodide route together as older, now discarded intensifiers, and discarded is the right word.

This page therefore gives chemistry, workflow in outline and hazards, and no procedure: no concentration, no bath sequence, no time. Nothing above could be worked from.

Waste

The historical practice generated exactly the waste stream modern rules exist to stop. Wall’s sequence has the bleached plate washed in several changes of acidulated water precisely to strip mercury out of the gelatin, and that rinse water carried the mercury away — in the 1920s, down a sink.

Today a mercury-bearing bleach falls under the Environment Agency’s WM3 entry 09 01 05*, bleach solutions and bleach fixer solutions, absolutely hazardous with no assessment needed, and mercury is separately controlled in every waste regime the course has read. GOV.UK’s household guidance sends hazardous waste from a home to a collection point, never to the bin or the drain. The course has found no published domestic treatment that renders a mercury solution fit to discharge, and offers none. Check your local regulations; they govern.

History

Mercury(II) chloride is the reason people associate mercury with early photography, and the association is only half right. The daguerreotype’s mercury is a different substance: the plate was developed over elemental mercury heated to vapour, and the conservation literature notes that the Becquerel process, needing no mercury development at all, is what modern practitioners use instead.

Its strangest appearance is the earliest. In 1840 Herschel reported that washing a picture over with a weak solution of corrosive sublimate obliterates it completely, leaving paper so white that the nicest examination detects no trace and which is thereafter insensible to light — and that brushing it with a neutral hyposulphite instantly revives the image in all its force, as often as one pleases. Robert Hunt, a year later, found the same salt whitening the darkened parts of a photographic paper into a negative that hypo then turned positive. Neither man was intensifying anything; both had found the reaction the intensifier would later be built on.

By the time Kodak was selling Eastman Tested Mercury Bichloride, the salt had become ordinary — a bottle on a shelf, described in the primer as virulently poisonous and used anyway, because nothing else gave that much density. What ended it was not a better intensifier but a change in what is acceptable to keep where people work.