Uranyl nitrate hexahydrate

Every other substance in this encyclopaedia is hazardous for what it does chemically. This one is that too, but it is also the only radioactive reagent the photographic formulary ever contained — and, awkwardly for anyone who wants a simple story, the radioactivity is not what made photographers stop using it.

In photography

Uranium toning turned a print red. Wall’s 1924 handbook gives the reaction and the trade-offs in a paragraph: treating a silver image with a mixture of a soluble uranium salt and potassium ferricyanide gives brown to red images, the colour depending on the ratio of the two salts and on how long the toning runs; intensification takes place at the same time, so the print must not be too dense to start with; and the colour depends on the deposition of a uranium ferrocyanide, which is soluble in alkalis, so long washing in ordinary water will reduce the colour by dissolving the uranous salt. That last clause is the process’s whole character. A uranium-toned print is a print whose colour you can wash off.

⚗The chemistry

Eder worked out the scheme in 1876 and it applies to the whole ferricyanide family. Metallic silver reduces ferricyanide to ferrocyanide:

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

Silver reduces ferricyanide, the first step of every ferricyanide toner

The ferrocyanide so produced then forms an insoluble compound with whatever metal ion is in the bath, deposited exactly where the silver was — Eder demonstrated it for lead and stated that the same scheme operates in the darkening of silver images with uranium salts, giving a reddish-brown colour. The Getty Conservation Institute’s atlas identifies the deposit in surviving prints as the uranium complex of the hexacyanoferrate anion, detectable by a prominent C≡N stretch at 2062 cm⁻¹ in the infrared. The image is no longer only silver: it is silver plus a coloured, added pigment, which is why toning and intensifying happen together and cannot be separated.

The intensifier was the same chemistry aimed at a negative, and Wall is blunt about it: except for extremely thin and flat negatives, it is not to be recommended. The degree of intensification depends on the ratio of uranium to ferricyanide, and prolonged washing in running water will remove the intensification completely — generally, he notes, first in patches. An intensifier that washes away unevenly is not an intensifier so much as a delay.

It toned platinum prints too. The Getty atlas records A. Horsley Hinton’s procedure of 1897: uranium nitrate and acetic acid with potassium ferricyanide and ammonium sulfocyanide, toning a fully processed black platinotype to a deep brown or red-brown and reversible by washing in dilute ammonia. The uranium is easy to find afterwards, by its L-alpha line at 13.61 keV under X-ray fluorescence.

And it was, briefly, a printing process of its own. Vogel’s 1875 textbook describes nitrate of uranium as reduced by light to a sub-nitrate in the presence of organic bodies such as a paper support, giving an image so faint as to be scarcely perceptible, then made visible by plunging the sheet into a silver or gold solution, where the reduced uranium precipitates the metal as a brown or violet powder. The Encyclopaedia Britannica’s 1911 survey adds that uranium salts are acted on only by the light they absorb, the nitrate absorbing in the green-blue. Vogel’s verdict on the family is the shortest sentence here: uranium is too rare and too dear to be employed generally in photography.

Properties

A yellow crystalline solid — the colour comes from the uranyl ion, UO₂²⁺, and is why the salt was easy to recognise on a shelf. Towler’s 1864 description matches the compound this page is about: a yellow salt, very soluble, containing six equivalents of water which heat expels, and decomposing at higher temperature. He also gives two identifying reactions that turn out to be the photographic ones in disguise: the alkaline carbonates all throw down yellow precipitates from uranium solutions, and potassium ferrocyanide produces a red-brown precipitate.

Two of its properties belong to different worlds. Chemically it is an oxidising nitrate of a heavy metal, which is what the GHS classification describes. Physically it is radioactive, which the GHS classification does not address at all: NIOSH’s entry for soluble uranium compounds notes that the potential for cancer is a result of alpha-emitting properties and of radioactive decay products such as radon. The two hazards are regulated by different regimes and neither substitutes for the other.

!A gap this page does not paper over

The research record behind this entry carries no water-solubility figure at all — the field is empty — so the front matter of this page carries none either. Towler calls the salt very soluble in 1864 and that is the best the course can source. The record also rests on an unusually thin classification base, 39 company reports across two notifications, where the other pages here draw on hundreds. Both facts are worth knowing before quoting anything from this page.

Handling

The aggregated ECHA notifications classify it Danger: fatal if swallowed, fatal if inhaled, may cause damage to organs through prolonged or repeated exposure, and toxic to aquatic life with long lasting effects, each in 100 per cent of reports, with the oxidiser statement in 97.4 per cent. NIOSH treats soluble uranium compounds as a carcinogen with a recommended limit of 0.05 mg/m³ as uranium, an immediately-dangerous level of 10 mg/m³, and a symptom list — lacrimation and conjunctivitis, shortness of breath, cough and chest rales, nausea and vomiting, skin burns, red blood cells and casts in the urine — with the kidneys, liver, blood, lymphatic system and bone marrow among the target organs, and the potential for cancer noted as a consequence of the alpha-emitting properties and the decay products.

⚠Why Level D

The classification rubric names uranium compounds in the Level D criteria by name, alongside mercury vapour development, cyanide fixing and cadmium. The control it prescribes is that the page describes chemistry, workflow in outline, image characteristics and hazards and states explicitly that it is not a procedure. This page does: no concentration, no bath, no time appears above, and the ratios that set the colour are described as a relationship rather than given as numbers.

The finished print is a different question from the reagent, and the conservation literature answers it. The Getty atlas states that uranium-toned photographs are only slightly radioactive, that the radioactivity can be detected only with very sensitive probes, and that the amount is too low to damage the photographic image or to be considered a health hazard when handling such prints or putting them on display. A collection holding uranium-toned platinotypes is not the problem. A darkroom holding uranium nitrate is.

Waste

A spent uranium toner is three kinds of waste in one bottle: a heavy metal, an oxidiser and a radioactive material. The first two are ordinary hazardous waste, and the nearest WM3 code is 09 01 05*, bleach solutions and bleach fixer solutions, an absolute hazardous entry. The third is governed by radiological legislation outside the waste regime entirely, and satisfying the waste code does not satisfy it. The aggregated classification carries H411 in every report.

There is no domestic treatment. The chemistry that would take the uranium out of solution is the one Towler describes, precipitation by an alkaline carbonate — but a uranium precipitate is the same problem in a filter paper. Check your local regulations; they govern, and here they are likely to be two sets of them.

History

Uranium toning is older than it looks. Eder credits Selle, in 1865, with the first application of a mixture of potassium ferricyanide and uranium nitrate for intensifying and brown-colouring collodion negatives, and notes drily that the method met with little approval and that nobody investigated the reaction it depended on. That investigation came from Eder himself a decade later, by way of lead.

What kept uranium in the formularies was colour. Until selenium and gold toning matured, the ferricyanide-plus-metal-salt family was how a photographer got warm reds and browns; Wall prints half a dozen uranium recipes as late as 1924. What removed it was partly cost and rarity — Vogel’s objection in 1875, before anyone worried about radioactivity — partly the discovery that the tone washes out, and finally the twentieth century’s view of uranium compounds. Princeton’s environmental health guidance now lists uranium nitrate among the older, discarded intensifiers, in the same sentence as the mercury ones.