The Toners We Study and Do Not Use

A lantern slide in a box of somebody’s grandfather’s things is red-brown, and much stronger than the others. A reprint of a 1924 formulary falls open at a toning bath whose first line is a uranium salt. A bottle in an inherited darkroom is labelled in a hand nobody can read. Each of those is a real situation, and in each of them the useful thing to know is not that the chemistry was dangerous but what it actually did to the silver, because that is what tells you what you are holding, how it will behave, and who to ask about it.

That is what this page is for. It takes four families of chemistry that acted on developed silver images, explains each one at the level of the reaction, says why photographers wanted it and what it cost them, and gives no procedure for any of it. Not a shortened procedure, not a safer variant, not a sequence with the quantities left out. None.

The reason is the hazard and only the hazard. Age is not the criterion and never has been in this course: the hypo-alum sepia bath T-1a was printed by Eastman Kodak in 1928, and Kodak Limited’s T-52 bleach is from 1949, and both are published here as routes a reader may take. What separates those from the baths below is not the decade they appeared in. It is that somebody reading them at home can assemble the controls the chemistry needs, and that for the four families below, nobody can.

Where this page stops

Three pages in this course own this material between them, and they divide it by what the substance is acting on, not by which element it is. Getting that boundary right is the difference between one argument made once and the same argument made three times in three slightly different forms.

Who owns which half of the same element

1. 1This page: the developed silver gelatin imageA print or a slide that has already been exposed, developed and fixed, and is then put into a bath containing uranium, cyanide, mercury or chromium(VI). The mechanism, the history and the image that resulted
2. 2Part XXVI: the toxicology, the objects and the whole processesThe full hazard treatment, the conservation handling of objects that contain these substances, and the historical processes in their entirety - the mercury-developed daguerreotype, cyanide in the collodion darkroom, the heavy-metal treatments
3. 3Part XXV: the same substances inside a sensitiserWhere mercury, uranium or dichromate is an additive in an iron or noble-metal sensitiser or clearing bath rather than a treatment applied to a finished silver image. Different chemistry, different page

So: the toxicology in depth, the care of a uranium-toned print in a collection and the daguerreotype’s mercury are Part XXVI’s, along with cyanide in the historical darkroom as a whole practice. Mercury inside a platinum sensitiser, and dichromate as a contrast agent in one, are Part XXV’s — and the mercuric sepia platinotype is already written there as a formulary entry. Neither is restated here.

Level D itself is defined once, on the Level D policy, and this page cites it rather than arguing it again. Two sentences of it are worth carrying in your head while you read: a Level D page may give the chemistry, the historical workflow in outline, the image characteristics and the hazards; it may not give quantities, a sequence of steps, a temperature and a time, a substitution that makes it safer, or a sentence beginning “if you must do this”. The prohibition covers the tone as well as the headings. Chromium is governed separately again, by the chromium policy, because the word covers two oxidation states and one of them is a hardener this course permits.

Uranium: the toner that worked

Of the four families here, uranium is the one that genuinely earned its place. It was not a curiosity at the edge of practice and it was not a lapse of care. It did two useful things at once out of two cheap bottles, and Kodak Limited was still printing a uranium toner as formula T-9 in 1949, headed brown to red tones in slides or films.

The mechanism, in one step and one gap

Uranium toning belongs to the family the gold, iron-blue and other metal toners page calls deposition, and it starts where every bath in that family starts. Ferricyanide takes an electron from metallic silver:

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

The oxidation that makes the ferrocyanide the toner needs

What decides whether that is a bleach, a reducer or a toner is what else is in the tray. Kodak’s 1928 primer runs the alternatives in one paragraph, and the uranium case is the one where a metal salt is waiting to catch the ferrocyanide the silver has just produced: uranium nitrate gives, in the primer’s words, the reddish-brown uranium ferrocyanide. The Getty Conservation Institute, looking at real prints eighty-five years later, identifies the same deposit on a toned platinotype as the uranium complex of the hexacyanoferric anion, from its carbon–nitrogen stretch at 2062 cm⁻¹ in the infrared, and confirms uranium in a 1960s silver gelatin print by X-ray fluorescence from its two major peaks at 13.64 and 17.22 keV.

No balanced equation for that second step appears here, and the omission is deliberate and shared with the uranium toner entry. The sources name the compound; not one of them gives its formula or its stoichiometry. The course writes the iron version of the same step because a formula for Prussian blue is published, and refuses to write the uranium one because a plausible equation for a named compound is exactly what Rule 1 forbids. That is a gap in the record, not caution about the element.

Why it was wanted

Two reasons, and the second is the one usually forgotten.

The colour. Brown to dark orange-red, with the hue set by the ratio of uranium salt to ferricyanide and by the time. Kodak Limited’s T-9 note describes the tone passing from brown to red as the bath works, and records that mixing the uranium and iron toners in different proportions gave everything from reddish-brown to chocolate. On a lantern slide projected in a dark hall, a warm transmitted red of that intensity was worth having, and the two salts that produced it were ordinary shelf stock.

The density. Uranium toning intensifies. Nothing is removed: the silver is oxidised in place and stays in the emulsion as a white ferrocyanide, and the coloured uranium compound is added on top of it. Kodak’s 1928 primer is unambiguous about what that was worth — the reddish-brown uranium ferrocyanide has “very great printing strength”, and toning with uranium converts a weak negative into one with great effective contrast for printing. A bath that rescues a thin negative and colours a lantern slide is a bath people will keep on the shelf.

🕰It is older than the toner, and it was found by accident

Eder’s history credits Selle, in 1865, with the first application of potassium ferricyanide mixed with 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. The investigation came from Eder himself: working with Captain Victor Toth in 1875 on lead salts and ferricyanide, he found the general scheme, reported it to the Vienna Photographic Society on 14 December 1875, and stated that the same scheme operates in the darkening of silver images with uranium salts.

So the chemistry of every toner in the deposition family — iron-blue included, which this course does teach — was worked out on a metal the course will not let you touch, in service of an intensifier nobody uses. Lead nitrate carries that story.

Why it failed, which is the same sentence as why it worked

The deposit is stable in acid and destroyed by base. Every source that describes uranium toning says so in the same breath as describing it. Wall, 1924: the colour depends on the deposition of uranium ferrocyanide, “which is soluble in alkalis”, so long washing in ordinary water will reduce it. Kodak Limited, 1949, on T-9: washing should not be prolonged, “especially if the water is slightly alkaline, since the toned image is soluble in alkali”. The Getty’s platinotype atlas puts it at its bluntest — the toning can be reversed by washing the print in dilute ammonia.

Read that as a photographer rather than a chemist and it is devastating. Tap water is very slightly alkaline in most places. The conservation board a print is likely to be mounted on carries a calcium carbonate reserve by design, which is the same collision that the gold and iron-blue page works through for Prussian blue. A toner whose image is removed by its own wash water is a toner whose product has been under attack from the day it was made, and the attack takes the picture rather than only the colour, because in a uranium-toned image the colour is a large part of the density.

!What the course can and cannot tell you about surviving uranium-toned prints

The mechanism above predicts that uranium-toned prints in collections should commonly be faded, weakened and unevenly stained, and that is what the course would expect a conservator to report.

It is an expectation and not a finding. The corpus behind this page contains no conservation survey of the condition of uranium-toned photographs — the AIC Photographic Materials Group wiki carries a heading for uranium prints with no text behind it in the copy the course mirrored, and the Getty atlas characterises the material analytically rather than reporting its state. What the course holds is the alkali sensitivity, stated by three independent Tier 1 sources, and the fact that reversal by dilute ammonia is demonstrated. The step from there to “surviving prints are usually in poor condition” is an inference, it is labelled as one, and it is the permanence page’s kind of question rather than this page’s.

Why there is no procedure

In one sentence, because the full treatment belongs to Part XXVI and the substance record to uranyl nitrate: the salt is classified fatal if swallowed and fatal if inhaled, it is an oxidiser, and it is radioactive — a hazard which no GHS classification describes and which is regulated under a wholly separate regime, so that a page attempting to specify controls would be specifying them against only one of two hazards. Princeton’s environmental health guidance for photography reaches the same place as an instruction: do not use mercury, cyanide or uranium intensifiers.

The regulatory position is worth stating precisely rather than gesturing at, because “it’s controlled” is the kind of sentence that turns out to mean nothing when tested.

⚠What the law of England and Wales actually says, read on 6 September 2026

Regulation 12(1) of the Environmental Permitting (England and Wales) Regulations 2016 provides that a person must not, except under and to the extent authorised by an environmental permit, operate a regulated facility. Regulation 8 lists what a regulated facility is, and paragraph (e) of that list is a radioactive substances activity. Schedule 23, paragraph 11(2), defines that activity to include the case where a person who uses premises for the purposes of an undertaking keeps or uses radioactive material on those premises, disposes of radioactive waste on or from those premises, or accumulates radioactive waste on those premises.

Two honest limits on that reading. The activity as defined is addressed to someone using premises “for the purposes of an undertaking”, and whether a domestic darkroom is an undertaking is a question this course does not answer — the same unresolved question that the disposal ruling already hit in the waste regulations. And regulation 12(3) disapplies the requirement where a radioactive substances exemption covers the activity, so whether a particular quantity of a particular salt falls inside the regime is a matter for the regulator and not for a course page to compute. What is not in doubt is that a second regulatory regime exists, that it has nothing to do with photography or with waste chemistry, and that the household hazardous-waste route that takes your spent fixer is a waste route and does not reach it.

On purchase, this course makes no claim. No Tier 1 source establishing a purchase control on uranium compounds in the United Kingdom is in the corpus behind this page, so nothing is asserted about it. This is a reading of legislation, not legal advice; it covers England and Wales; it is dated; and local regulation governs wherever you are.

Cyanide: the thing in the room, not the thing in the toner

This section had to be rewritten against its own sources, and the correction is worth showing rather than hiding, because it is the sort of error the historical literature invites.

What the sources establish. Potassium cyanide was a fixer. Towler’s 1864 manual gives it as Formula No. 1 of three fixing solutions, ahead of hyposulphite of soda. Wall’s 1912 dictionary still lists it half a century later, with the reason it lost: cyanide of potassium is more powerful than hypo, “but its action on the image is so great as to deteriorate the half-tones occasionally”. It dissolves silver halide by complex formation, exactly as thiosulfate does, with a different ligand and a stronger grip — strong enough to keep going into the image itself.

Where cyanide acted on a developed silver image is the intensifier, not the toner. Kodak’s 1928 primer describes the Monckhoven intensifier, in which the image was bleached with mercuric chloride and then blackened in a bath of silver dissolved by potassium cyanide. It built density in the highlights and the same cyanide cut the shadows slightly, because it is a silver solvent while it is a source of silver. Raising the top of the curve and lowering the bottom is a very efficient way to add contrast, and Kodak Limited was still printing that bath as the high-contrast option of IN-1 in 1949.

⁇The correction: the gold-toning baths of the wet-plate era did not contain cyanide

This lesson’s manifest entry originally listed “cyanide in gold-toning formulas of the wet-plate era” as something to teach. Read against the sources, it does not hold, and the manifest has been corrected rather than the page written to it.

• Towler, 1864 — the manual of the wet-plate decade — gives six gold toning formulas. Every one is gold chloride, or the double chloride of gold and potassium, neutralised with carbonate of soda or with chalk; two of the six pair the gold with a second solution of nitrate of uranium. None contains cyanide. Towler’s cyanide of potassium is in a different chapter altogether, as Formula No. 1 of the fixing solutions.
• Wall, 1924 gives the working gold baths as alkaline, thiocyanate or combined, and the carmine-and-red-chalk baths as gold with ammonium sulphocyanide — that is, thiocyanate.
• Ware’s account of gold chemistry explains why a cyanide gold bath would be a poor toner even setting the hazard aside: the dicyanoaurate complex is so stable that the couple’s standard potential falls to about −0.6 V, far below silver’s, so the image silver could not reduce it at all.

Cyanide’s genuine connection to gold chemistry is real and is a different thing: cyanide is one of the four ligands Ware names as giving stable, water-soluble gold(I) complexes, alongside sulphite, thiocyanate and thiosulfate. That belongs to the gold page, where the complexing agent is shown to be the toner. This page found no cyanide gold-toning formula for a developed silver image in its corpus and therefore names none. Cyanide as a fixing and toning practice in its own right, across the whole of the collodion era rather than on a finished silver gelatin print, is a planned Part XXVI entry and not this page’s finding to make.

What actually killed people

Not the salt on its own, and not the toning bath. The acid.

NIOSH describes potassium cyanide as a noncombustible solid, and then the sentence that matters: contact with acids releases highly flammable hydrogen cyanide. Its incompatibility line names acids, acid salts, chlorates and nitrates. Its recommended limit is a ceiling of 5 mg/m³ over ten minutes, not an eight-hour average, because the exposure this substance produces is not the kind you average; the concentration considered immediately dangerous to life or health is 25 mg/m³. A darkroom is a room that contains an acid stop bath, an acid fixer, and often acetic acid in a bottle. Kodak Limited printed the whole argument in five lines of its own 1949 handbook, beside its own formula: cyanide is a deadly poison, it reacts with acid to form poisonous hydrogen cyanide gas, and cyanide solutions should never be used in poorly ventilated rooms.

Towler, writing when cyanide was ordinary, records the chronic half of it in a passage that reads very differently now than it must have then. Silver stains can be washed off the hands with cyanide of potassium, he says, but doing so “might entail upon the operator incurable ulcers”; and the health of operators is much impaired, “especially in those large printing establishments where a number of females are employed in this department, who, by this continual manipulation in the two fluids, are frequently in a suffering condition.” Wall’s dictionary, half a century later, still has an entry for cyanogen soap — a soap containing potassium cyanide, sold for removing silver stains from the hands.

☠The rule that reaches the chemistry this course does use

Wall’s 1912 entry on cyanides carries a sentence that is not about cyanide at all, but about the ferricyanide sitting in the sepia bleach on the course’s own shelf: double cyanides such as the sulphocyanides and the ferrocyanides, “although apparently not actually poisonous themselves, should be regarded with extreme caution, as simple cyanides may be very readily produced from them under unexpected conditions.”

That is exactly right, and it is why the rehalogenating bleach the course does teach carries an absolute rule rather than a preference. Princeton’s guidance names the three conditions — heat, hot acid, strong ultraviolet — and records actual cyanide poisonings from acidified Farmer’s reducer. The incompatibility matrix carries the pair. A reader who takes nothing else from this section should take this: the reason ferricyanide and acid are kept apart in a darkroom that has never held a gram of cyanide is a nineteenth-century lesson still being paid for.

Mercury: where toning and intensification stop being different operations

Mercury is the family where the two words this part keeps apart turn out to name one operation.

The bleach is a displacement, and Kodak’s 1928 primer states the products: a silver image placed in a mercuric chloride solution forms a mixture of mercurous chloride and silver chloride, and the image goes white.

2 Ag + 2 HgCl₂ → 2 AgCl + Hg₂Cl₂

The bleach: two white solids where a black image was, and nothing yet removed from the sheet

Nothing has left the negative at that point and no density has been gained. What happens in the second bath decides both the colour and the density, and the arithmetic is the whole story: if the white image is redeveloped, both chlorides go back to metal, so — in the primer’s own words — to every part of silver an equal part of mercury has been added. One mercury atom for each silver atom, which the stoichiometry above confirms. If it is blackened with ammonia instead, the primer names a black mercury ammonium chloride and reports a higher degree of intensification still; the course prints no equation for that product, because the corpus names the compound and does not establish its formula.

Now put Wall’s 1924 print-toning chapter beside that. He gives a mercuric chloride and bromide bleach and then a choice of second baths — one for greyish-black, one for grey-violet, one for brown to violet-black, one for brownish-violet — and closes with the sentence that makes the point of this whole section: “It should be noted that all mercury toning gives intensification.”

⚗Why the two words collapse here and not elsewhere

A sepia toner converts the image: the silver ends as silver sulfide, density is lost, and the print must be made darker to compensate. A gold toner replaces it: silver dissolves out as gold arrives. Mercury does neither. The silver stays in the sheet as a halide, and a second metal is put in beside it — so the operation is additive by construction, and every colour it reaches arrives with density the print did not have.

That is what makes mercury the clean case of a general rule this part has been circling. Toning and intensification are the same family of operations aimed at different outcomes. Uranium is the other clean case: the same bath is printed as a toner (T-9) and as an intensifier (IN-21), and the only real difference is whether you wanted the colour or the contrast.

Two further appearances are worth recognising rather than reproducing. Kodak’s 1928 primer prefers the chromium intensifier to the mercury one on the ground that mercury-intensified images are not as stable — a manufacturer choosing a chromium(VI) bath over a mercury one on permanence grounds, which is a sentence worth sitting with. And Wall’s 1912 platinum toning bath for bromide prints uses mercuric chloride as its tone control, a slight increase warming the tone and a decrease cooling it: the mercury is the adjustment and not an impurity, which is why a reader who meets that formula in a reprint should recognise it as belonging on this page rather than on a shopping list. The gold and iron-blue page flags the same bath from the other direction.

☠A period safety statement is a historical claim, not a safety source

Wall’s 1924 handbook makes a claim about mercuric chloride that a reader of a reprint will meet without any warning attached to it. Having correctly called the salt extremely poisonous when taken internally, he writes that “the absorption by the skin, even in the case of cuts and abrasions, is practically harmless”. That claim is false, and the modern record contradicts it in the strongest terms its vocabulary allows.

The notified classification aggregated from 218 reports on mercury(II) chloride gives signal word Danger with H310, fatal in contact with skin, in 100 per cent of them, alongside H300. NIOSH’s entry for mercury compounds other than the organo-alkyls carries the skin notation on both its limits, sets a ceiling of 0.1 mg/m³ for those compounds, and lists the central nervous system and the kidneys among the target organs, with tremor, insomnia, irritability and indecision among the symptoms.

The general rule this course applies: a historical manual is a Tier 1 source for what was done and never a source for whether it was safe. Rule 5 exists for this exact sentence.

The mercury-developed daguerreotype is a different subject and belongs to Part XXVI; nothing above is about it, and the chemistry is not the same chemistry.

Chromium(VI), and the bleach that replaced it

There is a single course-wide policy on chromium, it separates the two oxidation states, and it lives at the chromium policy. No page may restate it in its own words, so this one does not: chromium(VI) is never used at any level anywhere in this course, and chromium(III) as chrome alum is permitted at Level B as a hardener. What belongs here is the narrower question the policy does not answer — where hexavalent chromium actually turned up in work done on a developed silver image, which is a question worth asking before naming anything, because the answer is not where most people assume.

It was not, in this corpus, a toner. It was a bleach, and it appears in four places.

Chromium(VI) in the silver darkroom, as this course's sources record it

1. 1As the bleach of a sepia sequenceWall, 1924, in the sulphide-toning chapter: bichromate baths "have been recommended" as an alternative to the ferricyanide bleach, with formulas from Sedlaczek and from Blake-Smith - but they require much longer washing to clear the yellow bichromate stain, and over-washing the bleached print causes yellow tones
2. 2As the intensifier bleachKodak IN-4, a dilute bichromate and hydrochloric acid bath: the image was bleached, the yellow stain washed out, and the image then redeveloped. Kodak recommended it over the mercury intensifier for ease, certainty and permanence of the result
3. 3Inside another intensifier againKodak IN-6, the quinone-thiosulphate intensifier, whose Solution A is potassium bichromate in sulphuric acid
4. 4On the equipment rather than the imageKodak TC-1, the one-solution dish cleaner for silver and developer stains: potassium bichromate and concentrated sulphuric acid. Not applied to a photograph, but in the room, in a tray, in the hands

The chemistry is the reason it worked and the reason it is replaceable. Kodak’s 1928 primer classifies bichromate as “an oxidiser of the same type as permanganate or ferricyanide”, which is the whole mechanism in a phrase: a dichromate bleach and a ferricyanide bleach do the same job to the silver, for the same reason, and differ in what the oxidant is and what it costs you to keep it in the house. The potassium dichromate page carries the half-reaction and its standard potential; the IN-4 entry states plainly that nothing in this corpus establishes the stoichiometry of a dichromate oxidation of image silver in an acid bath, and declines to invent one. This page holds the same line, for the same reason.

The substitution exists and the course already teaches it. Everything a bichromate bleach did in a sepia sequence is done by the rehalogenating ferricyanide–bromide bleach, which is the bleach behind T-7a and T-52 — and Wall’s own objection to the bichromate version, that it needed far longer washing to clear its stain, means the substitution costs the print nothing.

For the intensifier the answer is different but it also exists, and it was already in the same handbook. Kodak’s 1928 primer names three materials that were deposited on a silver image to intensify it — silver, mercury and a chromium compound — and the first of the three is the one this course publishes. IN-5, the silver intensifier for proportional intensification of transparencies, is written at Level B because every reagent in it has an encyclopaedia entry at Level A or B. So of Kodak’s own three routes to the same effect, one is a carcinogen, one is a mercury salt, and one is silver added to a silver image. The course takes the third, and says plainly that IN-5 works on a transparency rather than on a print.

⚠Why the substitution is not a close call

NIOSH’s entry for chromic acid and chromates marks it a carcinogen and sets the recommended limit at 0.001 mg/m³ as chromium, which is fifty times below its own recommended limit for mercury vapour and among the lowest figures anywhere in this course. Its incompatibility list is the contents of a workroom: paper, wood, sulfur, aluminium, plastics. Nasal septum perforation is on the symptom list, and the lung is on the target-organ list as a cancer site.

Set that beside the alternative, stated fairly rather than flatteringly. Potassium ferricyanide is not innocuous: its aggregated notified classification carries H302, H315, H319, H335, H411 and — in about one notification in seven — H361, suspected of damaging fertility or the unborn child. What it does not carry is any carcinogenicity or sensitisation statement, and its one absolute rule, never with acid, is a rule a reader can actually keep. Where a substitution costs the print nothing and removes a carcinogen, refusing to take it would not be respect for historical practice; it would be a worse print made more dangerously.

Lead, and the metal this course cannot find

Two footnotes belong here, because a reader working through period formularies will meet the first and may wonder about the second.

Lead was the metal the whole deposition family was discovered on. Eder and Toth’s 1875 work on ferricyanide with lead salts gave silver ferrocyanide and lead ferrocyanide together, and it was from that experiment that the general scheme was written down and then applied to uranium and to iron. As a toner it offered what the lead nitrate page calls a palette no other process could match from one bleach, the colour chosen in a second bath. As a process Wall’s own verdict was that the results were not very satisfactory, the whites tended badly to stain, and intensification came whether it was wanted or not. It is Level D, the argument is on the lead nitrate page, and the unusual feature of that argument is that the historical method was also simply not very good.

Thallium is sometimes named in the same breath as these metals, and this page names no thallium toner, because no thallium toning formula for a developed silver image appears anywhere in the corpus behind it. That is a gap in the evidence rather than a verdict on the chemistry, and it is recorded here so that a reader who meets the claim knows what the course has and has not seen. Where thallium does appear in this course’s plans it is a cyanotype toner, alongside lead and nickel, and it belongs to Part XXVI as historical study with no working procedure — a different image, a different pigment and a different page. What can be said is what a regulator says about the element: HSE’s EH40 gives soluble thallium compounds a long-term limit of 0.1 mg/m³ with the Sk notation for skin absorption. A forum or a list is where a question like this starts; it is never where it is settled.

The same scale, with selenium and thiourea on it

The risk in a page like this one is that Level D starts to read as a list of frightening names. It is not. It is a statement about controls, and the clearest way to show that is to put the substances this course does teach on the same scale as the ones it refuses.

Four substances from this part, on one rubric

controls a domestic reader can assemble

1. Selenium toner, as a bought dilute liquid — Level BHARMAN calls its own product a toxic chemical, states that it is toxic if swallowed and may cause sensitisation by skin contact, and asks for a well-ventilated area, preferably with extraction, gloves, eye protection and hazardous-waste collection. Every one of those is something a reader can actually do, and the maker names themtaught, with a lab
2. Elemental selenium, the raw element — Level DSame element, opposite verdict, and the reason is the operation rather than the substance: reaching a selenosulfate from the powder means a fusion with caustic soda or a boil with sodium sulfide, of a solid classified toxic by two routes, beside photographic materials. EH40 sets selenium and its compounds at 0.1 mg/m³studied only
3. Thiourea, in an open tray at pH 13 — Level CSuspected of causing cancer and of damaging the unborn child on the harmonised European classification. The 5 September ruling that the operation sets the level does not rescue it, because the objection is to the reagent in the tray and not only on the balancesupervised lab
4. Uranium, cyanide, mercury, chromium(VI), lead — Level DNot because the statements are worse than thiourea's in every case, but because no set of controls a domestic reader can assemble makes the procedure reasonable — and, for uranium, because one of its two hazard regimes cannot be characterised from anything this course can readthis page

Look at the top two rows together, because they are the same element. Elemental selenium is Level D in this course and a bought selenium toner is Level B, and nobody is being inconsistent. The level is assigned to the operation, exactly as the course owner ruled on 5 September 2026 when the sepia lab was unblocked: what makes the raw element unacceptable is the preparation, and a safer route to the identical print exists and is taught. Kodak Limited’s own 1949 selenium formulas T-55 and T-56 are published in this course as study entries for precisely that reason, while the experiment that measures selenium’s effect on the print curve runs at Level B out of a bottle.

Thiourea is the row that shows Level D is not simply “the worst things”. Its harmonised classification carries a suspected-carcinogen statement and a suspected developmental-toxicity statement — as serious in kind as several statements on this page — and it is Level C rather than D, because it is current practice, in current products, and a supervised laboratory with engineered extraction is a real place where it is handled properly. Level C means somebody can do this properly, and it is not you at home. Level D means no set of controls a reader can assemble makes it reasonable — and where institutions or conservators do work with these substances under controlled conditions, the course points at their published accounts instead of reproducing them.

!The four questions that put a substance at Level D

This is the test the pages above are actually applying, and it is worth being able to run yourself, because Part XXVI will ask you to.

1. What are the hazards, from a named source with a date? Not a reputation, not a forum, not a period manual’s reassurance — a classification, an exposure limit, a safety data sheet.
2. What control addresses each one, and can a domestic reader assemble it? A fume cupboard is a control. “Be careful” is not.
3. Is any hazard one the sources cannot characterise? Uranium’s radiological half is the example: you cannot specify a control against a hazard you cannot read.
4. Is there a route to the same photograph that this course can teach? Where the answer is yes, the historical method is described and not performed — and the page says the omission was a decision, not an absence of evidence.

Carrying this forward

Five things from this page are worth holding.

1. Uranium toning worked, and its failure mode is its mechanism. A uranium ferrocyanide deposited on an intact silver ferrocyanide image gives colour and density from two cheap bottles — and is taken off again by alkali, which includes ordinary wash water, buffered board and dilute ammonia.
2. Cyanide’s place in this chemistry was the fixer and the intensifier, not the toning bath, and what made it lethal was the acid already standing in the darkroom. The rule that keeps ferricyanide away from acid today is the surviving half of that lesson.
3. Mercury shows that toning and intensification are one operation with two names. Nothing is removed, a second metal is added atom for atom, and every colour arrives with density.
4. Chromium(VI) was a bleach, and the ferricyanide bleach the course teaches does the same job better. Where a substitution costs the print nothing and removes a carcinogen, taking it is not squeamishness.
5. Level D is about controls, not about names. The same element sits at Level B in a bottle and Level D in a jar, and a substance the course teaches under supervision carries statements as grave as some it refuses outright.

And the three situations this page opened with, answered. The red-brown lantern slide might be uranium-toned and might equally be sepia; nothing about the colour alone settles it, and what does settle it is the pair of X-ray fluorescence peaks the Getty publishes, which is a laboratory’s job and not a darkroom’s. What follows either way is that it should not be washed. If it is uranium-toned, the Getty’s own measurement is why a conservator handles the object with ordinary care rather than alarm. The formula in the reprint is now recognisable as something to read and put down. And the unlabelled bottle is neither a find nor a keepsake: it is a hazardous-waste question, in the uranium case with a radiological component the waste regime does not cover, and it belongs to a professional and to your local authority rather than to a tray.

Where this goes next: the permanence claims that keep surfacing here — that a uranium image is attacked by alkali, that a mercury-intensified image is less stable than a chromium-intensified one — are evidence claims of exactly the kind the archival-evidence page exists to weigh. The toxicology, the whole processes and the conservation of the objects themselves are Part XXVI’s. And when Part XXVI asks you to assess a historical process for hazard yourself, the four questions above are the ones to run.

Check your understanding

Question 1. A period manual describes a toner as a mixture of a soluble uranium salt and potassium ferricyanide, and notes that the image should not be washed for long, especially in slightly alkaline water. What is the chemical reason for that instruction?

• Prolonged washing dissolves residual thiosulfate, which would otherwise protect the deposit
• The coloured deposit is a uranium ferrocyanide that is attacked by alkali, so wash water removes the image itself
• The gelatin swells in alkali and releases the uranium as a soluble complex
• Alkali converts the uranium compound to a more soluble oxide that migrates into the highlights

Show the answer and why

Answer: The coloured deposit is a uranium ferrocyanide that is attacked by alkali, so wash water removes the image itself

Wall in 1924 and Kodak Limited in 1949 both attach the warning directly to the solubility of the deposit in alkali, and the Getty atlas demonstrates the extreme case on a toned platinotype: dilute ammonia reverses the toning outright. The reason it costs so much on a uranium-toned image in particular is that this is a deposition toner rather than a conversion toner - the colour is a large part of the density, so losing the colour loses the picture, not just the hue.

Question 2. Kodak's 1928 primer says that redeveloping a mercuric-chloride-bleached image adds to every part of silver an equal part of mercury. Which fact about the bleach makes that a one-to-one ratio rather than some other number?

• Mercuric chloride is used in twofold excess, so half of it is wasted
• Two silver atoms and two mercury(II) ions give two silver chlorides and one mercurous chloride, which is two mercury atoms for two silver atoms
• The bromide added to the bleach doubles the mercury taken up
• Ammonia blackening deposits a second mercury atom per silver atom

Show the answer and why

Answer: Two silver atoms and two mercury(II) ions give two silver chlorides and one mercurous chloride, which is two mercury atoms for two silver atoms

The bleach is 2 Ag + 2 HgCl₂ → 2 AgCl + Hg₂Cl₂. Mercurous chloride is a dimer, so one formula unit carries two mercury atoms, and the two silver atoms that started the reaction are matched by two mercury atoms in the white image. Redevelop and both metals come back, so the negative now contains as many mercury atoms as silver atoms. That is why mercury toning always intensifies: the second metal is added to the sheet rather than exchanged for the first, which is the opposite of what a gold bath does.

Question 3. Why does this course keep an absolute rule against acid contacting the potassium ferricyanide in its own sepia bleach, when it never handles cyanide at all?

• Acid decomposes ferricyanide to ferrocyanide, which stains the print blue
• The rule is inherited convention with no chemical basis in a cold, dilute bath
• Complex cyanides can release hydrogen cyanide under conditions of heat, hot acid or strong ultraviolet, and poisonings from acidified Farmer's reducer are on record
• Acid precipitates Prussian blue in the tray, which contaminates the fixer

Show the answer and why

Answer: Complex cyanides can release hydrogen cyanide under conditions of heat, hot acid or strong ultraviolet, and poisonings from acidified Farmer's reducer are on record

Wall's 1912 dictionary states the principle before anyone had the modern vocabulary for it: double cyanides are not themselves acutely poisonous, but simple cyanides may be very readily produced from them under unexpected conditions. Princeton's guidance names the three conditions and records real poisonings from treating Farmer's reducer with acid. It is the clearest case in the course of a rule that looks like superstition until you know the mechanism, and then looks like the minimum.

Question 4. A reader argues that since thiourea carries a suspected-carcinogen classification and elemental selenium does not, thiourea should be Level D and selenium should not. Where does the argument go wrong?

• It is correct; the course has the two levels the wrong way round
• Selenium's classification is in fact more severe, so the premise is false
• The level is assigned to the operation and the available controls, not to the worst hazard statement: thiourea is handled properly in supervised laboratories, while the route from selenium powder to a usable toner is a fusion or a boil that no home controls address, and a bought bottle reaches the same print
• Suspected-carcinogen statements are always treated as less serious than acute toxicity statements

Show the answer and why

Answer: The level is assigned to the operation and the available controls, not to the worst hazard statement: thiourea is handled properly in supervised laboratories, while the route from selenium powder to a usable toner is a fusion or a boil that no home controls address, and a bought bottle reaches the same print

The rubric grades what you have to do and what would have to be true for it to be reasonable. Level C says somebody can do this properly, with engineered extraction and a second competent person, and that somebody is not a reader at home; Level D says no assemblable set of controls makes it reasonable, or that a safer taught route to the same result exists. Selenium proves the point twice over, because the same element sits at Level B as a dilute bought liquid and Level D as a powder - and the difference between those two rows is the operation, exactly as the 5 September 2026 ruling on sepia toning stated it.

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