Kodak IN-21

An intensifier that can be washed off. Kodak Limited’s own closing sentence is that ordinary alkaline wash water will destroy the intensification, and Wall’s 1924 handbook says prolonged washing in running water removes it completely, generally first in patches. That is an unusual property for a photographic treatment and it is the key to what the deposit actually is.

☠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 workflow in outline, the image characteristics and the hazards. It gives no procedure: no mixing routine, no temperature, 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 negative, of a print, 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 intensifier this course can publish is IN-5, two pages earlier in the same handbook.

Solution A — the uranium stock

Ingredient	Quantity	Form the source specifies
Uranyl nitrate hexahydrate	20 g	printed as "uranium (uranyl) nitrate", with no hydrate stated; the course's entry is the hexahydrate
Acetic acid (glacial)	10 mL	glacial
Water	to make 500 mL	Half a litre, not a litre - both stocks of this formula are made to 500 c.c. That is 4 per cent w/v of the uranium salt and 2 per cent v/v of glacial acetic acid. No temperature is published.

Solution B — the ferricyanide stock

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	20 g
Water	to make 500 mL	4 per cent w/v. Kept in the dark, as any ferricyanide solution; the handbook does not say so here but says it of its other ferricyanide baths.
The same salt, at the same strength, as the ferricyanide half of a bleach - which is what it is, with a uranium salt waiting to catch what it makes.

Mixed in the ratio — the working intensifier

1 part Solution A + 1 part Solution B + 6 parts water

Use 1 part A, 1 part B and 6 parts of water.

Eight parts in all, so the working bath carries 5 g/L of the uranium salt, 5 g/L of potassium ferricyanide and 0.25 per cent v/v of glacial acetic acid. Equal weights are not equal moles - and how unequal cannot be settled from the page, because the hydrate is not stated. At 504.15 g/mol the hexahydrate gives 0.079 mol/L against the ferricyanide's 0.122 mol/L, a ratio of about 1 to 1.5; the anhydrous salt at 394.04 g/mol would give about 1 to 1.2. Wall says the ratio of uranium to ferricyanide is what decides the result, which makes that omission the most consequential silence on the page.

Purpose

To turn a weak negative into one that prints with great contrast. Kodak Limited’s header ranks it: “a simple intensifier giving, next to Formula IN-6, maximum intensification of negatives”. Only the quinone intensifier of the same handbook was reckoned stronger, and that one is built on chromium(VI).

The 1928 primer explains what “maximum” means here, and the explanation is not what a modern reader expects. A silver image can be very much intensified by toning it with uranium, it says, the reddish-brown uranium ferrocyanide having very great printing strength and converting a weak negative into one having a great effective contrast for printing purposes. The words printing strength and effective contrast are doing the work: the deposit is a coloured compound rather than more silver, and its power is in how strongly it absorbs the blue and ultraviolet light a printing paper responds to rather than in how dark it looks.

Recommended uses

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

Extremely thin and flat negatives, and nothing else. Wall’s verdict on the whole uranium method is that except for those it is not to be recommended, which is a striking judgement to find in a 1924 formulary that prints the formula anyway.

Where the negative would be printed soon. Kodak’s own warning that an alkaline wash destroys the intensification is a statement about permanence as well as about washing.

Where colour was acceptable. The result is reddish-brown by design, and Kodak Limited uses the colour as the criterion that the treatment has worked.

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

When another formula is preferable

• Kodak IN-5, the silver intensifier, in the same handbook: the only one of Kodak Limited’s five intensifiers whose every reagent has a written encyclopaedia entry at Level A or B, and the only one this course publishes as a formula to mix. It is proportional and it leaves image colour unchanged, which are two things this formula is not and does not.
• IN-1 and IN-4 are not alternatives. Mercury and chromium(VI) are Level D for reasons of their own, and swapping one Level D treatment for another is not a decision this course offers.
• A longer exposure. Wall’s own scope for the formula — extremely thin and flat negatives — describes a negative that was under-exposed, and an intensifier cannot put detail into a shadow that has none. What it produces from an empty shadow is a contrastier negative with the same empty shadow.
• For the toning use, not this page at all. Kodak Limited’s T-9 uranium toner is a different formula on the following page of the same handbook, with potassium oxalate, ammonium alum and hydrochloric acid in it, and it belongs to the course’s separate uranium toner entry, which is planned and not yet written. Uranium intensification and uranium toning are the same chemistry put to two purposes, and the course keeps them as two entries so that neither is published twice.

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. This is a two-stock, mix-at-use formula of exactly the kind R-4a is: two solutions that keep separately and a working bath made from them at the moment it is wanted, at one part of each to six of water. Kodak Limited publishes no keeping figure for either stock and none for the mixture.

🧮What one plus one plus six gives, and the number the source will not let you have

Eight parts in all. Per litre of working bath:

40 g/L ÷ 8 = 5 g per litre

Uranium (uranyl) nitrate

40 g/L ÷ 8 = 5 g per litre

Potassium ferricyanide

2% v/v ÷ 8 = 0.25% v/v

Glacial acetic acid

Equal weights, and Wall says the control is the ratio of the uranium to the ferricyanide — which is a molar statement, not a weight one. Potassium ferricyanide is 329.24 g/mol, so 5 g/L is 0.0152 mol/L. The uranium salt is 504.15 g/mol as the hexahydrate, giving 0.0099 mol/L and a ratio of about 1 : 1.5; as the anhydrous salt at 394.04 g/mol it would be 0.0127 mol/L and about 1 : 1.2.

Kodak Limited does not say which. So the one number Wall identifies as the control cannot be read off Kodak’s page, and this entry states the range rather than choosing an end of it. It is a good illustration of why the course records the hydrate on every entry where a source gives one — and says so when it does not.

Both stocks are made to 500 c.c. rather than to a litre, which is unusual in this handbook and worth noticing when reading the page: the strengths are 40 g/L, not 20.

The bath had a wash on either side of it and they were different washes. Kodak’s instruction before is a very thorough wash after fixing to remove all traces of hypo; the instruction after is to wash only briefly. That asymmetry is the formula’s most distinctive feature and it is explained under The mechanism.

Behaviour

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

Two to three minutes, and then it gets worse. Kodak Limited gives 2–3 minutes’ immersion as the point of maximum intensification and says that further treatment only increases fog. That is a ceiling with a named penalty rather than a target, and it is the only time on the page.

No temperature is published, and this entry does not supply one.

The colour is the check. “The intensified image should have a reddish-brown colour” is Kodak’s own test that the treatment has taken.

Hypo ruins it. The film or plate is washed very thoroughly after fixing to remove all traces of hypo before it goes in. A ferricyanide and a thiosulfate in the same bath are a Farmer’s reducer, so residual hypo does not merely dilute this treatment — it turns part of it into the opposite operation.

Water ruins it too, given long enough. Kodak: the usual alkaline wash water will destroy the intensification. Wall: prolonged washing in running water removes it completely, generally first in patches, and his own practice was to wash in dilute citric, oxalic or acetic acid instead, or in five changes of plain water at five-minute intervals.

No capacity and no keeping figure is published for either stock or for the mixture.

Image characteristics

Reddish-brown, by design. The colour is the deposit’s own and Kodak treats it as the criterion of success.

Printing strength rather than visual density. This is the property to hold on to. The 1928 primer’s phrase is that the reddish-brown uranium ferrocyanide has very great printing strength; a brown deposit can be modest to the eye and formidable to a blue-sensitive printing paper. A densitometer reading taken with a visual filter and one taken with a blue filter will not agree about this negative, which is not true of a silver one.

Great effective contrast, the primer’s own words, which is the same point again: what changed is what the paper sees.

Fog above the ceiling. Kodak says further treatment only increases fog — so the failure mode is not over-intensification but a rising base.

It is reversible, and that is the headline. Alkaline wash water destroys the intensification; prolonged washing removes it, first in patches. A treatment that a wash can undo is a treatment that decades of ambient humidity, alkaline mounting board and ordinary handling will also work on.

It leaves a signature. Uranium is detectable long afterwards, and the identification of uranium-treated photographic material is a conservation subject in its own right.

The mechanism

A ferricyanide bleach with something waiting to catch the product.

⚗Oxidise the silver, and precipitate the uranium on it

The first step is the step underneath every ferricyanide bath in this course, and the potassium ferricyanide page carries it as such: hexacyanoferrate(III) takes an electron from metallic silver and becomes hexacyanoferrate(II).

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

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

What is different here is what is in the bath to receive it. In R-4a the ferrocyanide meets thiosulfate and the silver leaves. Here it meets a uranyl ion, and the 1928 primer names the product: uranium ferrocyanide, reddish-brown, with very great printing strength.

No balanced equation for that step is printed here. The primer names the compound and neither it nor any other source in this corpus gives its formula or its stoichiometry, and the course does not write a plausible equation for a compound it has not read a formula for. What can be said is what the sources say: a uranium ferrocyanide is deposited, it is coloured, and it is what the intensification consists of.

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. So the acid in Solution A and the brief wash at the end are two halves of one instruction: the deposit is stable in acid and is attacked by base, and ordinary tap water is very slightly alkaline.

Why the silver is not what carries the image afterwards. Both this treatment and IN-5 add material to a silver image, which is the 1928 primer’s general definition of intensification. The difference is the material. IN-5 adds silver to silver and the negative remains a silver negative; this adds a coloured uranium compound whose behaviour — its colour, its printing strength and its solubility in alkali — belongs to the new compound and not to the old image.

Function of every ingredient

Uranyl nitrate, 20 g to the 500 mL of Solution A, 5 g/L in the working bath. The uranium, and the intensification itself: what the negative gains is a reddish-brown uranium ferrocyanide sitting on the silver, and this is where the uranium comes from. Wall names the control it belongs to — the degree of intensification depends on the ratio of the uranium to the ferricyanide — and demonstrates it with three ratios that give three different colours, from reddish through reddish-brown to brown. More uranium relative to the ferricyanide moved the result towards brown in his hands; less towards red. The form is not stated by Kodak and the difference between the hydrate and the anhydrous salt is about 28 per cent of the uranium, which is exactly the size of thing Wall’s ratio is sensitive to; the course records the gap rather than closing it. Its page carries the classification, the exposure limit, 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, 20 g to the 500 mL of Solution B, 5 g/L in the working bath. Both the oxidiser and half of the product, which is a dual role no other ingredient in this formulary has. As oxidiser it converts image silver to silver ion; as a source of ferrocyanide it supplies the anion of the coloured compound that is deposited. That is why it is not a catalyst and cannot be present in trace: every unit of deposit consumes it. More of it relative to the uranium moved Wall’s result towards red; less towards brown. It is Level B on its own — the same salt as the reducer R-4a — and it is not what makes this page Level D, but it brings its own absolute rule with it: no acid near a ferricyanide, ever, which sits oddly beside the acetic acid in Solution A and is dealt with under Interactions.

Acetic acid, 10 mL of glacial acid to the 500 mL of Solution A, 0.25 per cent v/v in the working bath. Kodak Limited publishes no function for it and none is invented here. What can be stated as fact is that Wall’s independently published uranium intensifier carries glacial acetic acid at the same 2 per cent of the stock, in both of its stock solutions, and that his washing instruction is a dilute acid rather than water — so two independent sources put the treatment in an acid environment throughout. What can be stated as reasoning, and is marked as reasoning: the deposit’s known enemy is alkali, on the evidence of Kodak’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, and the course does not claim to. It is Level B in its own right and the concentrated acid is the one substance on this page a reader might actually own.

Water, to make each stock up to 500 mL, and six parts of it in the working bath. Both stocks are made to half a litre, which is why their strengths are 40 g/L rather than the 20 g/L the printed weights suggest at a glance. The six parts of dilution water are the largest single component of the working bath and Kodak publishes nothing about what water — which, given that the treatment’s stated enemy is ordinary alkaline wash water, is a silence worth noticing.

Interactions

With hypo, destructively. All traces must be washed out before the treatment. A thiosulfate meeting this bath’s ferricyanide is Farmer’s reducer, which removes silver — so residual fixer turns part of an intensifier into a reducer. The instruction to wash very thoroughly after fixing is the most load-bearing sentence in Kodak’s directions.

With alkali, destructively and afterwards. Ordinary wash water destroys the intensification, per Kodak; prolonged washing removes it in patches, per Wall. This is an interaction with the darkroom’s most ordinary substance.

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 one. Solution A’s acetic acid is dilute and is in a separate bottle from the ferricyanide until the moment of use, and the same handbook that prints this formula prints the warning about ferricyanide elsewhere. The general rule stands and is not weakened by this formula’s existence: no concentrated or hot acid near any ferricyanide, and no acid into ferricyanide waste. The potassium ferricyanide page carries the reason.

With light, which turns a ferricyanide solution blue through the formation of Prussian blue.

With alkaline mounting materials, over years. A treatment that an alkaline wash removes is a treatment that an alkaline board will work on slowly. This is a conservation fact rather than a darkroom one.

With combustible material, against the solid uranyl nitrate, which NIOSH lists as its entire incompatibility entry and which is consistent with the oxidiser classification 97 per cent of notifiers carry.

Variants

Wall’s 1924 uranium intensifier is a different formula and a more informative one, because it publishes the control Kodak does not discuss. Its two stocks are 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 it publishes three ratios with three results — equal volumes for a reddish brown, 1 part of the first to 2 of the second for a reddish image, and 5 parts to 1 for a brown — adding that the colour also depends on how long each bath acts. Kodak’s IN-21 fixes one ratio and says nothing about the others.

Kodak Limited’s T-9 uranium toner, on the following page of the same handbook: a single solution carrying uranyl nitrate, potassium oxalate, potassium ferricyanide, ammonium alum and hydrochloric acid, for brown to red tones in slides or films. It is the same chemistry aimed at colour rather than at printing strength, and it belongs to the course’s uranium toner entry - planned, not yet written - rather than to this page.

Kodak’s IN-6, the quinone-thiosulphate intensifier, which Kodak Limited’s own header names as the only one stronger than this. It is not published in this formulary at all, for two independent reasons: it is built on potassium bichromate, which the chromium policy forbids at any level, and one of its ingredients is Kodak’s own wetting agent, a proprietary product whose composition its maker does not disclose. A formula the course cannot state the composition of is not published as a formula.

No course variant is offered and none could be. There is no version of a uranium treatment without uranium.

Safety

Level D, from uranyl nitrate. Read that page and the Level D policy; this one does not restate the classification, the exposure limits or the first aid, and does not soften them.

Why this is Level D and not Level C, in the rubric’s own terms. The acute classification is severe — fatal if swallowed and fatal if inhaled, in every report — but the policy’s 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. Controls do not answer that; they cannot be specified against a hazard the page cannot characterise.

The second reagent brings its own absolute rule. Potassium ferricyanide releases hydrogen cyanide on contact with hot or concentrated acid, and this formula puts an acid in the same working bath. That is not a reason to distrust Kodak’s dilute acetic acid; 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 hazards would have to be controlled together.

Nothing on this page is a control and none is offered, because the classification is not a statement about how hard 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.

Kodak Limited publishes no keeping figure for either stock or for the mixture, and none is invented here. By the architecture — two stocks, diluted at use — both were evidently meant to keep.

A ferricyanide solution belongs in a dark bottle, which the handbook says of its other ferricyanide baths and not of this one.

Nothing on this page is stocked in a home darkroom. The uranyl nitrate page’s storage entry says so for the substance and this formula does not create an exception.

An old bottle of a uranium salt 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 and take the intensification off a treated negative. Towler’s observation on the uranyl nitrate page.

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

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

Combustible material, against the solid uranyl nitrate, per NIOSH.

Light, against the ferricyanide solution.

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 that. The uranyl nitrate page carries the position for the substance.

It would also be a ferricyanide waste, which means no acid stream may ever join it.

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 will not go 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 will meet it.

“The image never went reddish-brown.” Kodak’s own criterion failed, and the likeliest recorded cause is hypo left in the negative: the ferricyanide is spent on thiosulfate instead of on silver, and what happens instead of intensification is reduction.

“The fog rose.” Kodak names it: further treatment beyond 2–3 minutes only increases fog. The failure mode of overdoing this bath is a rising base rather than a runaway image.

“It went in the wash.” Expected. Kodak says ordinary alkaline wash water destroys the intensification; Wall says prolonged running water removes it completely, generally first in patches, and both are describing the same solubility.

“It came back patchy.” Wall’s own word, and it is the diagnostic one: a treatment that fails unevenly is being removed rather than failing to form.

“The negative reads dense but prints thin” — or the reverse. That is the printing-strength effect, not a fault. A coloured deposit and a silver deposit do not have the same ratio of visual to blue density.

“There is an old 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, using something you can hold. This formula’s whole claim is that a brown deposit has more printing strength than its visual density suggests. That claim can be tested without any uranium at all: read a sepia-toned print or a stained pyro negative through a visual filter and through a blue one, and see the two numbers disagree. Then read Kodak’s ranking of IN-21 again.

Draw the two intensifiers against each other. Sketch what a proportional silver intensifier (IN-5) does to a characteristic curve and what a treatment that adds a coloured compound and raises fog does to the same curve. They are different operations and the difference is visible on paper.

Find the hydrate question in your own sources. Kodak Limited writes “uranium (uranyl) nitrate” and stops. Take any three historical formulas you can reach and check how many state a hydrate for a salt that has one. The answer is the reason this course records the form on every ingredient line and says so when the source does not.

Assess it yourself, which is Part XXVI’s own assignment. The uranium case is the most interesting of the Level D families to assess, because the acute classification rests on the smallest sample the course cites, and the decisive hazard is the one no classification in the corpus describes. Write the assessment, then read Safety above and see whether you reached the same conclusion by the same route.