Uranium toned print

☠Level D: studied, never performed

Uranium is named explicitly in the Level D policy as one of the families the level covers, and the register that plans this atlas classifies this row D. It is, as the glossary puts it, the clearest case in the course of a process excluded on hazard rather than on difficulty or cost.

No procedure appears here: no quantity, no sequence, no conditions. Kodak IN-21 is in the formulary at the same level and for the same reason.

Uranium toning is the iron-blue toner’s chemistry with a different metal ion, and it produces the one result in this atlas that a printer can undo by accident: a print whose colour you can wash off. Uranyl nitrate’s page owns the substance.

The chemistry

The general ferricyanide pattern, with uranium in the blank. Eder worked the scheme out in 1876 and it applies to the whole family: metallic silver reduces ferricyanide to ferrocyanide, and the ferrocyanide so produced 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 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.

So 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. Wall’s 1924 handbook gives the whole trade-off in a paragraph: 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. Every other toner in this atlas leaves something the wash cannot reach; this one leaves something the wash removes, slowly, in ordinary water.

The intensifier version makes the point sharper still. 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.

Historical workflow

Fix, wash, tone by inspection in a bath of a soluble uranium salt with potassium ferricyanide, rinse carefully and not for too long, dry. The instruction that reads oddest to a modern printer is the one about the wash, and it is the direct consequence of the chemistry above.

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, which the uranotype covers.

The image

Brown to red, and the position in that range is set by the ratio of the two salts and by time. It is a colour the silver toners of the period could not easily reach, which is most of the explanation for why uranium stayed in the formularies as long as it did — Wall prints half a dozen recipes as late as 1924.

Denser than it went in. Toning and intensification are the same operation here, which means the print has to be made lighter in anticipation. Compare selenium, which also intensifies and where Kodak’s answer is to shorten development slightly, and sepia, which loses density and where the answer is to print darker. Three toners, three different corrections at the enlarger, all of them decided before the print is made.

Identifiable. Both the infrared C≡N stretch and the X-ray fluorescence line are published, so a uranium-toned object in a collection can be confirmed rather than guessed at. That matters more here than for any other toner, and the reason is in the hazards below.

Permanence

Poor, and unusually so, because the failure is a wash away. The uranium ferrocyanide is soluble in alkalis, so an alkaline wash aid, a buffered mount or simply a long wash in hard water attacks the colour. Hinton’s platinum toning was explicitly reversible by washing in dilute ammonia, which is a conservator’s convenience and a printer’s warning.

That places it at the bottom of this atlas’s toning cluster on evidence rather than on reputation. Sulfide converts the image to one of the most stable forms of silver known. Gold plates it with a metal that does not tarnish. Selenium has an observed increase in stability. Iron-blue offers no extra protection and is destroyed by alkali. Uranium offers no extra protection, is removed by water, and is radioactive.

Hazards

The one entry in this atlas where the hazard is radiological as well as chemical. Uranyl nitrate hexahydrate is photography’s one radioactive reagent, and its encyclopaedia entry is classified Level D on that basis. Princeton’s environmental health guidance now lists uranium nitrate among the older, discarded intensifiers, in the same sentence as the mercury ones.

Potassium ferricyanide carries the family incompatibility: never a concentrated acid, which may release hydrogen cyanide.

And the waste is the part that has no domestic answer. A bath carrying a soluble uranium salt and a hexacyanoferrate is not a stream a household waste route can accept, and the course’s disposal caveat applies with more force here than anywhere else: what the course can and cannot tell a reader about waste is jurisdictional, and this is a waste that needs a licensed route rather than a decision.

One consequence for a reader who owns such a print rather than making one. A uranium print in a collection is a radiological question and not only a conservation one, which is why the identification signatures above are worth knowing.

Why we study it and do not reproduce it

Because it completes the ferricyanide family. One reaction — silver reduces ferricyanide, and the ferrocyanide precipitates with whatever metal is in the bath — gives blue with iron, red with copper, reddish brown with uranium, and Eder demonstrated it with lead. A reader who has met three members of that family and not the fourth has an incomplete pattern.

Because it is the clearest case of exclusion on hazard alone. Nothing about uranium toning is difficult, and its results were admired. The course excludes it because of what the reagent is, and saying so plainly is more useful than quietly omitting a process the reader will meet in any period formulary.

Because what removed it from practice was not the hazard. Vogel’s verdict in 1875 — uranium is too rare and too dear to be employed generally in photography — predates any concern about radioactivity by two decades. What removed it was partly cost, partly the discovery that the tone washes out, and only finally the twentieth century’s view of uranium compounds. Three different reasons, arriving in that order, which is a useful corrective to the assumption that dangerous processes were abandoned because they were dangerous.

Part XXVI owns “Uranium, Mercury and the Heavy-Metal Treatments”, and Part XX owns “The Toners We Study and Do Not Use” at Level D. Neither part is yet written.