Uranium toner

The only toner in this formulary that intensifies the image while it colours it, and the only one whose colour a long wash in ordinary water will take off again. Both of those come from the same fact: what a uranium toner leaves in the paper is not silver at all.

☠Level D: this page is not a procedure

Uranyl nitrate is classified for historical study only, and the Level D policy governs what this page may contain. It may give the chemistry, the historical practice in outline, the image characteristics and the hazards. It gives no procedure: no mixing routine, no temperature, no times, and nothing below is addressed to a reader as something to do.

The composition is published because a formula that cannot be read cannot be identified, and identification — of a print, of a lantern slide, of a bottle in an inherited darkroom — is the use a reader has for this entry. The course asks nobody to weigh any of it, and no protective equipment list is offered, because the reason for the classification is not one that equipment answers. See Safety.

The toners this course can publish are T-7a for browns, T-21 for gold and T-26 for blues. The nearest thing to this one that a reader may lawfully and sensibly mix is none of them, and that is said plainly under When another formula is preferable.

Solution A — the uranium stock

Ingredient	Quantity	Form the source specifies
Uranyl nitrate hexahydrate	5 g	printed as "Uranium nitrate", with no hydrate stated; the course's entry is the hexahydrate
Water	500 mL, added	1 per cent w/v of the uranium salt. Wall prints a plain "Water 500 ccm" and no temperature. Half a litre, so a stock mixed with an equal volume of Solution B carries 5 g/L.
One salt in water and nothing else. On its own it tones nothing at all - it needs the ferrocyanide that only Solution B can make.

Solution B — the ferricyanide stock, held acid

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	5 g
Acetic acid (glacial)	14 mL	glacial
Water	500 mL, added	1 per cent w/v of potassium ferricyanide and 2.8 per cent v/v of glacial acetic acid. No temperature is published. A ferricyanide solution belongs in a dark bottle.
This is a bleach with an acid in it. Used alone it takes a print to silver ferrocyanide, which is exactly what the second of Wall's two routes does on purpose.

Mixed in this order — the single-bath route, the toner as one tray

1. Start with Solution A — the uranium stock; Wall lists A first and states no order of addition
2. Then add Solution B — the acid ferricyanide stock; "mix" is his whole instruction, so the order shown here is this table's and not his

Mix just before use.

Wall states no ratio and no order of addition. Two stocks each made to 500 c.c. read as the two halves of a litre, and that reading - equal volumes - is what every working strength on this page assumes and says so. Under it the bath carries 5 g/L of the uranium salt, 5 g/L of potassium ferricyanide and 1.4 per cent v/v of glacial acetic acid, and those first two figures do not depend on the total volume at all, only on the volumes being equal. "Just before use" is the keeping statement - the mixture is not a stock.

Used in this order — the two-bath route, which Wall offers as an alternative in the same sentence

1. Solution B — until bleached — The acid ferricyanide alone, which takes the silver image to silver ferrocyanide. Wall gives no duration and no temperature; the end point is visual.
2. Water — "Well washed". No time is published. This rinse is what keeps the ferricyanide out of the uranium bath, so that the deposit forms in the paper rather than in the tray.
3. Solution A — until the desired colour is obtained — The uranium stock alone, which finds the ferrocyanide already in the print. Again no duration, no temperature, and a visual end point.

Or the prints may be immersed in B until bleached, well washed, and then soaked in A until the desired colour is obtained.

Kodak's 1928 primer states this as the general alternative for the whole family - the metal salt used in the same bath as the ferricyanide, or the silver bleached to silver ferrocyanide first and the metal salt brought to it afterwards. It is the difference between a direct and an indirect toner, printed as two readings of one formula.

Purpose

To turn a silver print or slide brown or red, and to add density while doing it. Wall’s 1924 handbook gives the whole of it in one sentence: treatment of a silver image with a mixture of a soluble uranium salt and ferricyanide of potassium results in brown to red images, the colour being dependent on the ratio of the two salts and the duration of toning.

The second half of the purpose is the half that catches people out, and Wall puts it in the very next sentence: intensification also takes place; therefore, the prints should not be too intense at first. A uranium toner is not a colour applied to a finished print. It is an addition of material, and the material is coloured, so the print gets darker and browner at once and the correction has to be made at the enlarger before any toning happens.

Kodak’s 1928 primer comes at the same fact from the other end, in its chapter on intensification: a silver image can be very much intensified by toning it with uranium, the reddish-brown uranium ferrocyanide having very great printing strength, converting a weak negative into one having great effective contrast for printing. Toning and intensifying are one operation with two names, and which name a period manual used depended on whether the object in the dish was a print or a negative.

Recommended uses

What the sources record, as history. The course recommends none of them.

Bromide prints, where a red or a warm brown was wanted that no silver toner reached. This is the chapter Wall prints the formula in, and the reason uranium survived in the formularies as long as it did. A sulfide toner gives brown; a gold toner gives red-brown on a warm paper by a slow and expensive route; uranium gave red directly.

Lantern slides and cine positives. Kodak Limited’s own header for T-9 is “uranium toner for brown to red tones in slides or films”, and its 1944 booklet adds “suitable for motion picture work”. A projected positive is looked through, so image colour is part of the picture rather than a refinement of it, and the whole family of ferricyanide-plus-metal-salt toners belongs to that trade.

Platinum prints. The Getty Conservation Institute’s platinotype atlas records A. Horsley Hinton’s procedure — uranium nitrate and acetic acid brought together with potassium ferricyanide and ammonium sulfocyanide — as one of the two most important ways of giving a fully processed black platinotype a reddish-brown tonality.

As a dye mordant rather than as a colour in itself. Kodak’s 1928 primer describes transforming the image into a mixture of uranium and silver ferrocyanides and then mordanting basic dyes onto it, and Kodak Limited printed the bath for that as formula T-17. Same chemistry, different object.

Not for anything that must last, and not for anything that must be washed properly. Those two constraints exclude every archival use between them, and both are constraints of the chemistry rather than of the course’s caution. See Image characteristics.

When another formula is preferable

• Kodak T-7a, the sulfide sepia toner, if what is wanted is brown. It converts the silver to silver sulfide, which is one of the most stable forms silver takes, instead of laying a soluble pigment beside it. It is the opposite trade to this one in every respect that matters: worse smell, better permanence.
• Kodak T-21 or T-26, if what is wanted is a red-brown or a blue by a route the course teaches. Both are gold toners and both are expensive, and neither is removed by water.
• A warm-tone paper and a warm-tone developer, which is the answer the course actually gives to “how do I get a warmer print” and which involves no toner at all. Image colour in a developing-out print is largely decided by silver particle size, and that is set in the emulsion and in the developer long before a toning tray is filled.
• Kodak IN-5, the silver intensifier, if what is wanted is density on a negative. It is proportional, it leaves image colour alone, and every one of its reagents has a written encyclopaedia entry.
• IN-21 is not an alternative to this page — it is this page’s other half. The two entries exist separately because a formulary indexes formulas and Kodak printed two of them; the arithmetic under The mechanism shows how little difference there is between the baths.

Mixing

This section gives no mixing procedure. What can be described without giving instructions is the shape of the thing, because the shape is where the chemistry shows.

It is a two-stock, mix-at-use formula, of the same kind as R-4a: two solutions that keep separately, and a working bath made from them at the moment it is wanted. Wall’s whole instruction for the mixture is three words — “mix just before use” — and that is a statement about the mixture rather than a convenience. What the two bottles hold is a uranium salt and a ferricyanide, and the whole point of the formula is that when those two meet in the presence of silver they make something insoluble. In a tray that something is the picture. In a bottle it is a precipitate.

Wall states no ratio and no order of addition, which is unusual and worth noticing. Both stocks are made to 500 c.c., and two equal stocks read as the two halves of a litre; every working strength quoted on this page assumes equal volumes and says so wherever it does. That assumption is safer than it looks, because the two salt concentrations in the mixed bath do not depend on the total volume at all — only on the volumes being equal. The quantity table has to print the two solutions in some order and prints them in Wall’s, which is A and then B; that is the table’s arrangement and not an instruction of his, and it is worth knowing that the course’s schema has no way to say “the source states neither a ratio nor an order” except in a note like this one.

🧮What equal volumes give, and the coincidence that follows

Each stock is 1 per cent w/v of its salt. Mix equal volumes and each is halved:

10 g/L ÷ 2 = 5 g per litre

Uranium (uranyl) nitrate

10 g/L ÷ 2 = 5 g per litre

Potassium ferricyanide

2.8% v/v ÷ 2 = 1.4% v/v

Glacial acetic acid

Now put Kodak Limited’s uranium intensifier beside it. IN-21 is two stocks at 4 per cent w/v used one part of each to six of water — eight parts in all — which is 5 g/L of the uranium salt, 5 g/L of potassium ferricyanide and 0.25 per cent v/v of glacial acetic acid.

The two salts are at the same concentration in both baths. One is printed in a London handbook of 1949 as an intensifier for negatives, the other in a Boston formulary of 1924 as a toner for bromide prints, and in the two reagents that make the image they are the same working solution. The acid differs by a factor of about 5.6, and the sources differ about the material and the purpose, and that is all.

This arithmetic is the course’s, on quantities both sources publish. Neither source draws the comparison.

Nothing in this formula tells the reader the order to dissolve things in, and that is a genuine difference from the Kodak relatives. Kodak Limited’s T-9 heads its directions “dissolve the chemicals in the order given”, and its T-17 mordanting bath is more explicit still: dissolve each chemical separately in a small volume of water, add the oxalic acid solution to the uranyl nitrate solution, and finally add the ferricyanide. In both, the ferricyanide goes in last and the uranium meets an oxalate or an acid first. The stated test in both is the same — the finished bath should be clear and pale yellow — and a bath that is not clear has already deposited some of what should have gone into the paper. Wall’s two-bottle arrangement reaches the same end by never putting them in one vessel at all until the moment of use.

No temperature is published anywhere on this formula, in either solution or for the mixture, and none is supplied here.

Behaviour

Recorded from the sources so that the formula can be understood, not so that it can be run.

No time is published for either route. Wall gives end points and not durations: for the mixed bath the end point is the colour, and for the two-bath route it is “until bleached” and then “until the desired colour is obtained”. The only published duration anywhere in the uranium family is Kodak Limited’s for its own T-9 — maximum effect in about 10 minutes, the tone passing from brown to red in that time — and that belongs to T-9.

Two controls, and they interact. Wall names both in one clause: the colour depends on the ratio of the two salts and on the duration of toning. Kodak Limited’s T-9 demonstrates the second on its own, since its ratio is fixed and its tone still travels from brown to red over ten minutes. So a uranium toner has no single end point to aim at — it has a path through a colour range, and stopping is the decision.

It gets denser as it goes. This is the behaviour that separates uranium from every other toner in this formulary. Wall’s instruction is to make the print lighter than wanted before toning. Compare selenium, which also intensifies slightly and where the answer is to shorten development, and sulfide, which loses density and where the answer is to print darker. Three toners, three different corrections, all of them made before the print exists.

The bath is light-sensitive. Kodak Limited says so of T-9 in both printings and says it of the T-17 mordant too. A ferricyanide solution photolyses, and the product is Prussian blue — which on a print is a blue stain in the highlights. Kodak’s own troubleshooting note for the neighbouring iron toner names exactly that symptom and gives three causes for it, one of which is a bath that was not mixed correctly.

The wash afterwards is short, and deliberately so. Kodak Limited: wash for about 10 minutes, though the washing should not be prolonged, especially if the water is slightly alkaline, since the toned image is soluble in alkali. Wall says the same of the toner and is blunter about the intensifier — prolonged washing in running water removes it completely, generally first in patches — and his own practice for uranium work was to wash in 2 per cent citric acid, or 1 per cent oxalic or glacial acetic acid, rather than in water.

No capacity figure is published, and no keeping figure for either stock.

Image characteristics

Brown to red. Wall’s range for the toner; the Getty Conservation Institute, examining prints that survive, gives it as brown to dark orange-red. It is a colour the silver toners of the period did not easily reach, which is most of the explanation for why the formula stayed in the books.

Position in that range is chosen, not fixed. By the ratio of the salts and by time. Wall publishes three ratios with three results for his uranium intensifier, which is the same pair of reagents, and they run in a direction worth holding on to.

⚗Which way the ratio moves the colour

Wall’s intensifier stocks are uranium nitrate at 100 g/L and potassium ferricyanide at 40 g/L, and he publishes three mixtures:

• Equal volumes — a reddish brown image.
• 1 part uranium stock to 2 parts ferricyanide stock — a reddish image.
• 5 parts uranium stock to 1 part ferricyanide stock — a brown.

Converting those to molar ratios of uranium to ferricyanide, on the hexahydrate at 504.15 g/mol and potassium ferricyanide at 329.24 g/mol, gives about 1 : 0.61 for the reddish brown, 1 : 1.23 for the reddish and 1 : 0.12 for the brown. So: more ferricyanide relative to the uranium moves the result towards red; more uranium moves it towards brown.

Two things follow. The bath tabulated on this page, mixed in equal volumes, is about 1 : 1.53 — more ferricyanide-rich than any of Wall’s three, which is consistent with it being printed as a toner for reds and browns rather than as an intensifier. And Kodak Limited’s T-9, at 2.5 g of the uranium salt to 1.0 g of ferricyanide, is about 1 : 0.61 — the same molar ratio as Wall’s “reddish brown” mixture, reached independently in a different handbook a quarter of a century later.

The conversion is the course’s; the three colours and the three ratios are Wall’s, and the T-9 quantities are Kodak Limited’s. The hexahydrate assumption shifts every one of these numbers by the same factor, so the ordering survives even though the absolute molarities do not.

Denser than it went in, which is under Behaviour and is a characteristic as much as a behaviour.

Printing strength rather than visual density, on a negative. Kodak’s 1928 phrase is that the reddish-brown uranium ferrocyanide has very great printing strength. A brown deposit can look modest to the eye and be formidable to a blue-sensitive printing paper, so a densitometer reading through a visual filter and one through a blue filter will disagree about a uranium-toned negative in a way they do not about a silver one.

It is reversible, and that is the headline. The deposit is soluble in alkalis. Long washing in ordinary water reduces the colour; the Getty atlas records that Hinton’s uranium toning of platinum prints was reversed by washing in dilute ammonia, which is a conservator’s convenience and a printer’s warning in one sentence. Alkaline wash aids, buffered mount board and hard tap water all work in the same direction, slowly.

It leaves a signature that can be read off the object. Uranium’s L-alpha line at about 13.6 keV and L-beta at about 17.2 keV appear in an X-ray fluorescence spectrum, and the Getty atlas identifies the deposit itself as the uranium complex of the hexacyanoferric anion by a prominent C≡N stretch at 2062 cm⁻¹ in the infrared — a region relatively free of interference. This is the only toner in the formulary whose product can be named analytically from a finished print.

The mechanism

A ferricyanide bleach with something in the bath waiting to catch what it makes.

⚗Oxidise the silver, then precipitate the metal on it

Everything begins with one electron moving. Hexacyanoferrate(III) takes it from metallic silver and is reduced to hexacyanoferrate(II), leaving a silver ion where a grain of image silver was; the potassium ferricyanide page treats this as the reaction the whole family is built on.

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

The oxidation, which is where the ferrocyanide in the deposit comes from

Left to itself, that is a bleach: the silver ion and the ferrocyanide it just made come down together as silver ferrocyanide, and the black image goes pale. Kodak’s 1928 primer states it in exactly those terms — ferricyanide oxidises the silver image and forms silver ferrocyanide from it, so a negative put in a solution of ferricyanide is slowly bleached to silver ferrocyanide.

4 Ag + 4 [Fe(CN)₆]³⁻ → Ag₄[Fe(CN)₆] + 3 [Fe(CN)₆]⁴⁻

Solution B alone, which is what the two-bath route uses on purpose

What decides whether that bleach is a reducer, a sepia bleach or a toner is what else is in the tray. Kodak’s primer runs through the alternatives in a single paragraph: add hypo and the silver compound dissolves, which is Farmer’s reducer; add bromide and the silver ferrocyanide is converted to the more insoluble silver bromide, which is the rehalogenating bleach; add a salt of a metal that gives an insoluble coloured ferrocyanide and the silver ferrocyanide is converted into the ferrocyanide of that metal. Iron gives blue, copper gives red, and uranium nitrate gives the reddish-brown uranium ferrocyanide.

No balanced equation for that last step is printed here. The sources of this corpus name the compound — Kodak’s “uranium ferrocyanide”, the Getty’s “uranium complex of the hexacyanoferric anion” — and none of them gives its formula or its stoichiometry. The course writes the iron version of the same step, 4 Fe^3+ + 3 [Fe(CN)6]^4- → Fe₄[Fe(CN)₆]₃, on the rehalogenating bleach page because a formula for Prussian blue is published; it does not write the uranium one, because one is not. That asymmetry is not caution about uranium, it is the difference between a compound the course has read a formula for and one it has not.

Why this makes it a direct toner and an indirect one at the same time. Wall’s single sentence offers both routes, and Kodak’s primer confirms that both were general practice: the metal salt in the same bath as the ferricyanide, or the silver bleached first and the metal salt brought to it afterwards. The one-tray version is direct toning; the B-then-A version is indirect toning with no redevelopment step in it. Most toners are one or the other by nature. This one is either, from the same two bottles, and the reason is that the bleach product — silver ferrocyanide — is itself the intermediate the uranium reacts with.

Why it intensifies. Nothing is removed. The silver is oxidised in place and stays in the emulsion as silver ferrocyanide; the uranium ferrocyanide is added on top of it. Eder, who published the reaction scheme for this family in 1876, described the lead case as depositing silver ferrocyanide and lead ferrocyanide together, and stated that the same scheme operates in the darkening of silver images with uranium salts. Compare sepia toning, where the silver ends up as silver sulfide and the print loses density: there the image is converted, here it is added to.

Why an alkaline wash takes it off again. The uranyl nitrate page carries the fact and the source: Towler records that the alkaline carbonates all produce yellow precipitates from solutions of the salt, and that page draws the connection explicitly — it is the same reaction that makes a uranium-toned image fade in an alkaline wash. The deposit is stable in acid and attacked by base, ordinary tap water is very slightly alkaline, and the acid in Solution B, the short wash, and Wall’s habit of washing uranium work in dilute citric or oxalic acid are three expressions of one fact.

Why the whole thing is Level D anyway, which the mechanism does not touch, is under Safety.

Function of every ingredient

Uranyl nitrate, 5 g in the 500 mL of Solution A, 5 g/L in the mixed bath. The uranium, and the image the reader ends up looking at. What the print gains is a reddish-brown uranium ferrocyanide laid where the silver was, and this ingredient is the only source of the metal in it. It is also the ingredient that carries the control Wall names — the colour depends on the ratio of the uranium to the ferricyanide — and the direction is published: more uranium relative to the ferricyanide moves the result towards brown, less towards red. Its concentration therefore does two things at once and they are not independent, because raising it raises the amount of deposit as well as shifting its colour. The hydrate is not stated. Wall writes “uranium nitrate” and stops, exactly as Kodak Limited writes “uranium (uranyl) nitrate” and stops; the hexahydrate at 504.15 g/mol and the anhydrous salt at 394.04 g/mol differ by about 28 per cent in uranium for the same weighing, which is the size of thing the ratio is sensitive to. Its own page carries the classification, the exposure limits, and the sentence that matters most — that no GHS classification describes the radiological hazard and EH40 does not list it at all, so this is a substance with two hazard regimes of which the course can read only one.

Potassium ferricyanide, 5 g in the 500 mL of Solution B, 5 g/L in the mixed bath. Both the oxidiser and half of the product, which is a dual role almost nothing else in this formulary has. As oxidiser it converts image silver to silver ion; as the source of hexacyanoferrate(II) it supplies the anion of the coloured compound that is deposited. It is therefore consumed stoichiometrically rather than catalytically — every unit of deposit costs ferricyanide — which is why it cannot be present in trace and why exhaustion is a real limit rather than a slow drift. More of it relative to the uranium moves the colour towards red; less towards brown. Used alone, as the second of Wall’s two routes does, it is a plain bleach and takes the print to silver ferrocyanide with no colour in it at all. It is Level B in its own right — the same salt as R-4a and the sepia bleach — and it is not what makes this page Level D, but it brings its own absolute rule with it: no hot or concentrated acid near a ferricyanide, ever, which sits awkwardly beside the acid in the same bottle and is dealt with under Interactions.

Acetic acid, 14 mL of glacial acid in the 500 mL of Solution B, 2.8 per cent v/v of that stock and 1.4 per cent v/v of the mixed bath. Wall publishes no function for it and none is invented here. What can be stated as fact is that his own uranium intensifier carries glacial acetic acid at 40 mL/L in both of its stocks; that Kodak Limited’s IN-21 carries it in the uranium stock; that Kodak Limited’s T-9 carries hydrochloric acid instead and its T-17 mordant carries oxalic acid; and that Wall’s washing instruction for uranium work is a dilute acid rather than water. Five independently printed formulas in the same family are acid, and one of them is acid in the wash as well. What can be stated as reasoning, and is marked as reasoning: the deposit’s known enemy is alkali, on the evidence of Kodak Limited’s own closing sentence and of Towler’s observation on the uranyl page, and an acid bath is not an alkaline one. That is not the same as knowing why the acid is there. More of it does not obviously make a better toner, and it is the one ingredient here whose concentrated form a reader might actually own — glacial acetic acid is Level B and it is a burn hazard, not a nuisance.

Water, 500 mL added to each stock, and no temperature. Wall prints a plain “Water 500 ccm” for both. It is the reason the two salts sit at 1 per cent w/v in the bottles and 5 g/L in the mixed bath, and the reason those working figures depend only on the volumes being equal rather than on any total. The course records the volume as water added rather than as a make-up volume, because Wall uses his own “water to make” wording three times in the whole book and does not use it here; for a solution carrying 5 g of a salt the difference between the two readings is well under a per cent, and nothing on this page turns on it. The one thing worth saying about the water itself is what Kodak Limited says about the wash water — that a slightly alkaline supply attacks the result — which makes the choice of water a variable the sources notice and none of them specifies.

On the ingredients this formula does not have. Kodak Limited’s T-9 carries potassium oxalate and ammonium alum as well, and Wall’s uranium intensifier carries potassium oxalate in its ferricyanide stock. Those are discussed under Variants, where the formulas that contain them are. Their absence here is Wall’s, not an omission of the course’s.

Interactions

With hypo, destructively, and in both directions. Residual fixer carried into a ferricyanide bath is Farmer’s reducer, which dissolves the silver instead of toning it, and Kodak’s 1928 primer states exactly why: silver ferrocyanide is soluble in hypo. A print that has not been washed out before toning does not tone weakly, it reduces. Kodak Limited’s instruction for the neighbouring intensifier — wash very thoroughly after fixing, to remove all traces of hypo — is the most load-bearing sentence in that formula’s directions, and it applies here for the same reason.

With acid, in two directions at once, and this is the interaction to think hardest about. The deposit wants acid; the ferricyanide must never meet a hot or concentrated one. Potassium ferricyanide releases hydrogen cyanide on contact with hot or concentrated acid, and Princeton’s environmental health guidance records that cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid. This formula puts 2.8 per cent glacial acetic acid in the same bottle as the ferricyanide, and Kodak Limited’s T-9 puts hydrochloric acid in the same litre. The general rule stands and is not weakened by any of them: no concentrated or hot acid near a ferricyanide, and no acid stream into ferricyanide waste.

With alkali, destructively, and afterwards rather than during. Wash water, hypo clearing agents, buffered mounting board and alkaline storage enclosures all attack the deposit. This is an interaction with the most ordinary substances in a darkroom and in a store room, and it is why the permanence verdict on this treatment is what it is.

With light, against the ferricyanide, which photolyses to Prussian blue. Kodak Limited says of T-9 that it is light-sensitive and should be stored in the dark, and says the same of the T-17 mordant. On a print the symptom is blue in the highlights.

With iron, from a tray, a clip or a hard-water supply. Ferric ions and ferrocyanide make Prussian blue directly. This is the reason Kodak’s sulfide bleach T-7a carries potassium oxalate — the 1928 primer says the blue iron salt is soluble in the oxalate — and it is at least a plausible reason for the oxalate in T-9 and in Wall’s uranium intensifier. The course marks that as an inference: neither source states a function for the oxalate in a uranium formula.

With another toner, on purpose. Kodak Limited prints its iron toner T-11 on the same page and ends it by saying that mixing the uranium and iron toning solutions in different proportions produces tones from reddish-brown to chocolate, and its 1944 booklet adds that analogous results follow from immersing in each solution successively for varying times. Both toners lay a metal ferrocyanide, so mixing them mixes two pigments in one gelatin layer.

With time and ordinary humidity, which is the interaction a conservator meets rather than a printer. A treatment an alkaline wash removes in an afternoon is a treatment an alkaline mount works on over decades.

Variants

Kodak Limited’s T-9, the uranium toner proper, printed in 1944 and again in 1949 with identical quantities. This is the best-corroborated uranium toner in the corpus and it is a single solution, not two. Per litre, made up to 1000 c.c.: uranium (uranyl) nitrate 2.5 g, potassium oxalate 2.5 g, potassium ferricyanide 1.0 g, ammonium alum 6.0 g, and hydrochloric acid (10 per cent solution) 5.0 c.c. The course’s copy of the 1944 booklet is an image scan whose text layer is unreliable, and the unit on that last line cannot be read from it; the 1949 printing gives c.c., and both printings give the avoirdupois column as 190 minims, which on Kodak Limited’s own scale of 80 ounces to 1000 c.c. works out at 4.95 c.c. So the acid is a volume in the source, not a weight, and two columns in two printings agree that it is. The chemicals are dissolved in the order given; the finished bath should be perfectly clear and pale yellow; it is light-sensitive and stored in the dark; the maximum effect comes in about 10 minutes with the tone passing from brown to red in that time; and the wash afterwards is about 10 minutes and no longer, especially in slightly alkaline water, because the toned image is soluble in alkali. The 1944 booklet adds that it suits motion-picture work and that it is convenient to keep 10 per cent stock solutions of the constituents for compounding a fresh bath quickly.

Why T-9 is not the formula tabulated at the top of this page, said plainly rather than left to be noticed: it contains ammonium alum, which has no page in this course’s chemical encyclopaedia and is not in the register of planned entries, and the formulary does not publish a formula containing an ingredient the reader cannot look up. Nothing about T-9 is withheld — its five quantities, its make-up volume, its mixing rule and its published time are all above — and the nearest written entry for what an alum does in a photographic bath is potassium alum, which is a different salt with the same anion and a comparable role. Kodak Limited states no function for any of T-9’s five ingredients, so what the alum is doing there is not established by the source; alum in a photographic bath is normally a hardener, and the course marks that as an inference rather than as Kodak’s statement.

Sedlaczek’s four baths, as Wall prints them. Wall gives four uranium toning baths on the same pages as the formula above and credits them to Sedlaczek, with the quantities given as volumes of 10 per cent solution: uranium nitrate with ammonium oxalate, hydrochloric acid and ferricyanide for browns; the same with potassium oxalate in place of the ammonium salt for warm browns, and a saturated solution of oxalic acid instead of the oxalate for redder tones still; uranium nitrate with Rochelle salt, tartaric acid and ferricyanide for rich browns; and uranium nitrate with Rochelle salt, saturated ammonia alum, tartaric acid and ferricyanide for reddish-browns. The second of them carries the same four reagents as Kodak Limited’s T-9, at about 1.8 times the strength and the same 2.5 to 1 weight ratio of uranium salt to ferricyanide. The course records these as Wall’s account of Sedlaczek and not as Sedlaczek’s own publication, which the authoring rules keep on the behaviour-only side of the line until the original is found; they are described here and not tabulated anywhere.

Wall’s uranium intensifier, on pages 137–138 of the same book. Two stocks — uranium nitrate at 100 g/L with glacial acetic acid at 40 mL/L, and potassium ferricyanide at 40 g/L with potassium oxalate at 10 g/L and the same acetic acid — and three published ratios giving three colours, which are the numbers the ratio callout above is built on. Wall’s own verdict on it is that except for extremely thin and flat negatives it is not to be recommended.

Kodak IN-21, the uranium intensifier of the 1949 London handbook, which has its own entry in this formulary. Two stocks at 4 per cent w/v used 1 + 1 + 6. Its working bath carries the same two salts at the same concentrations as this page’s formula mixed in equal volumes, and the entries are kept apart because the sources printed two formulas, not because the baths differ much.

Kodak Limited’s T-17 mordanting bath for dye toning, from the 1944 booklet: uranyl nitrate 5.0 g, oxalic acid 4.0 g and potassium ferricyanide 4.0 g, water to make 1000 c.c., used one part of stock to four of water. It is the same chemistry stopped early and put to a different purpose — Kodak’s 1928 primer explains that the image is transformed into a mixture of uranium and silver ferrocyanides, onto which basic dyes are then mordanted — and its instruction that mordanting much beyond a very slight tone gives inferior results is a useful reminder that the uranium deposit and the dye that clings to it are two different things.

Hinton’s uranium toner for platinum prints, recorded in the Getty Conservation Institute’s platinotype atlas: a solution of uranium nitrate and acetic acid brought together with solutions of potassium ferricyanide and ammonium thiocyanate, toning a fully processed black platinotype to a deep brown or red-brown, and reversible by washing in dilute ammonia. Wall prints a formula of that shape too, used very dilute — a small measure of the uranium stock into a litre of water, then the thiocyanate, then the ferricyanide — but the course’s copy of Wall garbles the unit on the ferricyanide line, so the reagents and the order of addition are recorded here and the quantities are not.

A uranium toner for printing-out paper, also in Wall: uranium nitrate with thiosinamine, for what he calls the red chalk or Bartolozzi colours, printed deeply and given a salt bath before toning and a thorough wash and fix after. Thiosinamine — allylthiourea — has no entry in this encyclopaedia, so the formula is named and not tabulated.

Uranium with iron together, which Wall gives from Blake-Smith and from Eder for blueish-green tones, both of them uranium nitrate with an iron ammonium citrate and a ferricyanide. And Kodak Limited’s T-9-plus-T-11 mixtures, already noted under Interactions, which run the other way, towards chocolate.

No course variant is offered and none could be. There is no version of a uranium toner without uranium, and the substitutes are not variants of this formula but different chemistry with a different mechanism: sulfide converts the silver, gold plates it, selenium converts it. Each has its own entry.

Safety

Level D, and the uranium is why. The classification comes from uranyl nitrate, whose own page owns the GHS record, the occupational limits and the first aid; the Level D policy owns the rule those facts are read against. Neither is repeated here, and nothing below qualifies either of them.

Why the classification is D and not C, in the rubric’s own terms. The acute GHS classification is severe — fatal if swallowed and fatal if inhaled, in every report — but the reasoning does not rest on that alone, and the substance’s own page is careful to note that the sample is 39 reports across 2 notifications, the smallest behind any classification this course cites. The decisive point is the one the Level D policy states: none of the GHS record covers the radiological hazard. No GHS classification describes it, EH40 does not list it, and it is regulated under an entirely separate regime everywhere the course has looked. A substance with two hazard regimes of which the course can read only one is a substance the course does not put in a reader’s hands, and controls cannot be specified against a hazard the page cannot characterise. Princeton’s environmental health guidance reaches the same practical conclusion from a different direction and states it as an instruction: do not use uranium intensifiers, and they are especially hazardous to the kidneys.

The second reagent brings its own absolute rule, and this formula makes it sharper than most. Potassium ferricyanide releases hydrogen cyanide on contact with hot or concentrated acid, and every uranium toner in the sources holds an acid — glacial acetic here, hydrochloric in T-9, oxalic in T-17. Princeton adds that strong ultraviolet light will do it too, and records actual poisonings from acidified Farmer’s reducer. That is not a reason to distrust a dilute acetic acid in a cold bottle; it is a reason to note that a formula which is Level D for its uranium also carries the ferricyanide rule, and that the two would have to be controlled together.

The finished print is a different question from the bath, and the sources answer it separately.

⚠What the Getty measured, and what that statement does and does not cover

The Getty Conservation Institute measured a 1960s uranium-toned silver gelatin print with a survey meter and published the figure: about 117 micro-REM per hour, measured about 1 cm above the print, against a natural background of about 10. Its own conclusion is that this level is well above background but not high enough to cause health issues in occasional handling; that the uranium is well embedded in the gelatin layer of the photograph; and that handling can therefore be carried out using standard conservation procedures.

That is a statement about a washed, dried print in a hardened gelatin layer, made by conservators about objects in a collection. It is not a statement about the salt, the bath, a spill or the waste, and this page does not extend it to any of them. The substance is fatal if swallowed and fatal if inhaled on its own classification, it is an oxidiser, and its radiological control is a separate regime again. The distance between “an object you may hold” and “a reagent you may weigh” is exactly the distance between a conservation guideline and a darkroom procedure, and it is why this page prints the first and refuses the second.

Nothing on this page is a control and none is offered, because the classification is not a statement about how difficult the controls would be. It is a statement that a page cannot assume them.

Storage

What the sources record, and what the course’s own pages say.

The mixture does not keep. “Mix just before use” is Wall’s whole instruction, and it is the only keeping statement he makes about this formula. What is in the mixed bath is a uranium salt and a ferricyanide in the same water, which is a precipitate looking for a nucleus.

No keeping figure is published for either stock, and none is invented here. By the architecture — two bottles, combined at use — both were evidently meant to keep separately.

A ferricyanide solution belongs in a dark bottle, and the uranium bath belongs in the dark too. Kodak Limited says so of T-9 in both printings, and of the T-17 mordant.

Nothing on this page is stocked in a home darkroom. The uranyl nitrate page’s storage entry says so for the substance, and it adds the part that has nothing to do with photography: possession of uranium compounds is subject to radiological regulation independent of anything written here. The solid is also an oxidising nitrate, and NIOSH’s entire incompatibility line for it reads “combustibles”.

An old bottle of a uranium salt in an inherited darkroom is not a find. It is a hazardous-waste question with a radiological component, and it is a question for a professional and for the local authority rather than for a course page.

Incompatibilities

Alkalis, which precipitate the uranyl salt from solution and take the toning off a finished print. Towler’s observation, on the uranyl nitrate page.

Thiosulfate, in the bath or carried in on the print, which turns the ferricyanide half into a reducer.

Hot or concentrated acids, against the ferricyanide. Hydrogen cyanide.

Strong ultraviolet light, against the ferricyanide, for the same reason, per Princeton.

Iron, from trays, clips or the water supply, which gives Prussian blue in the highlights.

Combustible material, against the solid uranyl nitrate, which is NIOSH’s whole incompatibility entry for it and is consistent with the oxidiser classification.

Developer, in either direction, as for every bath in the darkroom.

See incompatibilities.

Waste

Nothing here is generated in this course, because the process is not carried out.

A uranium-bearing waste is not an ordinary photographic waste. It carries a chemical hazard the course can read and a radiological one it cannot, and there is no dilution, neutralisation or precipitation described on this page that would change either. The uranyl nitrate page carries the position for the substance, including the aggregated classification as toxic to aquatic life with long-lasting effects.

It would also be a ferricyanide waste, which means no acid stream may ever join it — and this bath already contains an acid, which is a complication rather than a permission.

Where such a solution exists, it is a matter for a licensed hazardous-waste route and a professional, and the disposal ruling explains why the course goes no further than that. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

There is no procedure to troubleshoot. What follows is what the sources record going wrong, kept because it explains the chemistry and because a reader working from a period manual, or looking at an old print, will meet it.

“The print went pale instead of red.” The ferricyanide worked and the uranium did not arrive. The recorded cause is residual hypo: silver ferrocyanide is soluble in thiosulfate, so an unwashed print gets Farmer’s reducer instead of a toner.

“The highlights are blue.” Prussian blue, from iron or from light on the ferricyanide. Kodak Limited’s note for the iron toner on the same page gives three causes for blue highlights — the slide was fogged during development, the bath was stale, or it was not mixed correctly — and the last two are the ones that belong to the bath.

“The bath is cloudy and the tone is weak.” The deposit is forming in the tray rather than in the paper. Kodak Limited’s stated test for both of its uranium baths is that the solution should be perfectly clear and pale yellow, and its stated defence is the order of addition, with the ferricyanide going in last.

“It came out darker than I printed it.” Expected, and not a fault. Wall’s instruction is that the prints should not be too intense at first, because intensification takes place along with the toning.

“The colour faded in the wash.” Expected. Kodak Limited: do not prolong the washing, especially if the water is slightly alkaline, since the toned image is soluble in alkali. Wall says prolonged running water removes the intensified version completely, generally first in patches — and in patches is the diagnostic word, because a result that fails unevenly is being dissolved rather than failing to form.

“An old print in the collection has gone patchy and orange-brown.” Not a darkroom question. Both the X-ray fluorescence lines and the infrared C≡N stretch are published, so a uranium-toned object can be confirmed rather than guessed at, and the uranium toned print page carries the identification and the conservation position.

“There is a bottle of uranium nitrate in the darkroom I inherited.” Not a troubleshooting question. See Waste.

Experiments

No experiment on this page involves making or using this formula.

Separate visual density from printing density with something you can hold. The claim underneath both this toner and its intensifier twin is that a coloured deposit has more printing strength than its visual density suggests. That can be tested with no uranium at all: read a sepia-toned print or a stained pyro negative through a visual filter and then through a blue one, and watch the two numbers disagree. Then read Kodak’s 1928 sentence about printing strength again.

Watch the ferricyanide family change metal. Kodak’s 1928 paragraph says one reaction gives four different outcomes according to what shares the tray — hypo for a reducer, bromide for a sepia bleach, an iron salt for blue, a copper salt for red, a uranium salt for reddish brown. Three of those are formulas this course publishes. Put R-4a, the rehalogenating bleach and T-7a side by side and write out what each one adds to the same first step. The uranium column is the one you can complete on paper and must not complete in a tray.

Do the ratio arithmetic yourself, and then argue with it. Wall publishes three ratios and three colours for the uranium intensifier. Convert them to molar ratios, line them up against Kodak Limited’s T-9 and against this page’s bath, and see whether the ordering you get matches the one in Image characteristics. Then ask what the unstated hydrate does to your answer — and satisfy yourself that it moves every number and no ordering.

Find the missing function. Kodak Limited prints five ingredients in T-9 and assigns a role to none of them. Take any three historical formulas you can reach and count how many explain what each ingredient is for. The answer is why this formulary’s longest required section is the one headed Function of every ingredient, and why a page is allowed to say that a source states no function rather than inventing one.

Assess the case yourself, which is Part XXVI’s own assignment. Uranium is the most interesting of the Level D families to assess, because the acute classification rests on the smallest sample the course cites, the decisive hazard is one no classification in the corpus describes, and a conservation body has published a measurement saying that finished prints may be handled normally. Write the assessment from those three facts, then read Safety above and see whether you reached the same conclusion by the same route.