Mercury

This is a study entry. It exists so that a reader who meets a daguerreotype in a museum, or a mercury box in a nineteenth-century manual, knows what was in the room and what it was doing. It gives the chemistry, the hazard data and the historical role in full and no quantity, sequence, temperature or apparatus at all, because the course classifies mercury development at Level D and a Level D page is not a procedure at any length or in any tone.

In photography

Mercury vapour was the first developer. The daguerreotype plate is silvered copper, polished to a mirror and fumed with iodine until its outermost skin is silver iodide, and an exposure short enough to be useful in a camera leaves that surface looking exactly as it did before. Mercury made the difference visible: vapour from a warmed pool of the liquid metal condenses on the plate and combines with the silver where light had acted. The Library of Congress’s account of the medium and the AIC conservation wiki’s process page both describe the plate in those terms, and the wiki says what the image is made of — the highlights are a silver and mercury amalgam, the darks bare reflective metal, and the image particles run from about 0.1 to 50 micrometres.

That is the whole operation, and it is strange beside every other process here: a metal vapour alloys with a metal surface, selectively, and the alloy is the picture.

⚗Amalgamation, and a mechanism the course will not assert

An amalgam is an alloy of mercury with another metal, and mercury forms them readily: NIOSH’s own incompatibility list names calcium with the reason in brackets, amalgam formation, alongside lithium, rubidium, sodium carbide and copper.

What the course can state is the phenomenology. Where light had acted on the silver iodide, mercury condensed and an amalgam formed; where it had not, it did not; and once the unchanged silver iodide was removed, the amalgam remained as the light parts of the picture. Eder records the period arguments about why, and they do not agree. Arago in 1839 gave a chemical account, silver iodide reduced by light to metallic silver which then absorbs the mercury; Donné, the same year, a physical one, the exposed layer going powdery so that mercury reaches the metal beneath while the unexposed layer resists; Moser in 1842 argued from his breath pictures that condensation alone explained it, and showed that a fully exposed plate could be developed with steam; Choiselat and Ratel in 1843 proposed a silver subiodide.

Four accounts, from people with the plates in front of them, and no agreement. This course does not adjudicate between them and does not substitute a modern mechanism it has not read. What is not in doubt is that mercury goes where the light went.

The halogen vapours belong to the same operation. Iodine made the plate sensitive; bromine and later chlorine made it fast enough for a portrait. Eder records the reverse effect too, from Gaudin in 1841 and from Shaw and Percy two years later: exposing a plate that already carries a latent image to any of those vapours destroys it, though it will take a new exposure and develop from that. A daguerreotype darkroom held four vapour hazards at once, three halogens and one metal.

The salt has a career of its own. Mercury(II) chloride was photography’s standard intensifier for eighty years after the daguerreotype had gone. It is a different substance with a different classification and its own entry.

Properties

The properties that matter here are physical rather than chemical, and there are three.

It is a liquid metal: silver-white, heavy at a specific gravity of 13.6, odourless, freezing at −38 °F and boiling at 674 °F, and non-combustible.

It has a vapour pressure at room temperature. NIOSH gives 0.0012 mmHg. That is a small number and the most important one on this page, because it is not zero and because the liquid usually sits in an open vessel. The vapour has no colour and no smell, so nothing about the room reports it, and the historical process deliberately warmed the pool.

It does not dissolve in water. NIOSH and the ILO-WHO safety card give the solubility as none; HSDB puts it at 0.28 micromoles per litre at 25 °C, adds that it dissolves in nitric acid and, to some extent, in lipids. A metal that will not go into water and will go into fat has a route into a body that intuition does not predict.

Handling

The harmonised CLP classification is signal word Danger: H330, fatal if inhaled; H360D, may damage the unborn child; H372, damage to organs through prolonged or repeated exposure; and H400 with H410 for the aquatic environment. The 570 aggregated ECHA reports behind it put H410 at 100 per cent and H330 at 98.6 per cent, and add a tail the harmonised entry does not carry.

NIOSH sets a recommended limit for mercury vapour of 0.05 mg/m³ over eight hours with a skin notation, a ceiling of 0.1 mg/m³ for other mercury compounds, and an immediately-dangerous-to-life level of 10 mg/m³ as Hg, with all four exposure routes — inhalation, skin absorption, ingestion and contact. Its symptom list is mostly neurological: tremor, insomnia, irritability, indecision, headache and lassitude, alongside cough, chest pain and breathlessness, stomatitis and salivation and loss of weight, with the eyes, skin, respiratory system, central nervous system and kidneys as target organs.

HSE’s EH40 gives mercury and divalent inorganic compounds, measured as mercury, a long-term limit of 0.02 mg/m³ and no short-term figure at all — and then does something it does for only a dozen substances in the list: it sets a biological monitoring guidance value, 20 µmol of mercury per mol of creatinine in urine, sampled at random. A limit measured in the air says what the control must achieve; a limit measured in the worker says the regulator does not trust the air measurement alone.

⚠Why Level D

Mercury is named in the first line of the classification rubric’s Level D criteria, and the Level D policy makes the argument in full and is cited here rather than restated. What this page adds is why the controls cannot be assembled at home: the hazard is a vapour that leaves an open liquid at room temperature, is invisible and odourless, and is regulated at a concentration a domestic reader has no way to measure. Every other Level D substance here announces itself somehow — cyanide by the acid two trays away, dichromate by its colour. This one announces nothing.

The second D criterion applies too. The AIC records the Becquerel variant of 1840, in which the exposed plate is brought out by red light rather than by mercury vapour and which needs no mercury and no bromine or chlorine sensitising, at the cost of plates about ten times slower, making portraits difficult. The course names it and does not teach it either.

The one operation that would take this entry out of Level D does not exist. No quantity is small enough and no bench arrangement simple enough: the substance would have to stop having a vapour pressure.

Waste

Nothing here produces a waste stream, because the course produces no mercury waste. What a reader may still meet is a broken article — a thermometer, a switch, a lamp. Under WM3, metallic mercury is 16 03 07*, an absolute hazardous entry, and 20 01 21* covers fluorescent tubes and other mercury-containing waste from households. Harmonised and notified classifications alike carry H400 and H410, so the aquatic route is closed as firmly as the drain, and GOV.UK’s household guidance routes hazardous waste from a home to the council’s collection service. Check your local regulations; they govern.

History

Mercury entered photography in the 1830s and left it, as a developer, within twenty years. Eder marks his account of the discovery as hearsay — Daguerre “is said to have been indebted to a curious accident” — and the course treats the cupboard story as folklore, as the Part I lesson does. The consequence is solid: an exposure too short to see could be found and amplified afterwards, and the whole of developing-out photography follows from it.

The retreat had nothing to do with health. Wet collodion, from the early 1850s, gave a negative that could be printed many times, and the unique, laterally reversed, fragile mirror could not compete. Mercury went out with the process rather than being driven out of it.

What survives is the object. The AIC records that ungilded plates are so vulnerable that an image can be wiped off with a finger, which is why they are sealed behind a cover glass over a spacer. The amalgam a nineteenth-century operator made in a warmed box is still in collections, still scattering light from particles a tenth of a micrometre across.