Part XXVI Overview: What Level D Means and Why These Pages Have No Procedure

Two processes, twelve years apart. The salted paper print is 1839 chemistry, and Part XXII asks you to make one: salt the sheet, sensitise it with silver nitrate, print it in sunlight, tone it, fix it, wash it, at Level B, with gloves and a ventilated wet bench. Wet-plate collodion dates from about 1851, and no page in this course will ever ask you to pour a plate.

Nothing about the two dates explains that. What explains it is that one process needs a silver salt, a sheet of paper and sunlight, while the other needs a volatile flammable solvent poured by hand in a small dark room, a salting mixture that in period practice was frequently a cadmium salt, and — for a great deal of its working life — a fixer that killed people who let an acid reach it.

This part is the six pages where that reasoning is done out loud, on the processes where the answer comes out against reproducing them. It is not a list of prohibitions and it is not a chapter of warnings. It is the chemistry of the daguerreotype, of the cyanide darkroom, of wet plate, of the heavy-metal treatments and of the light-hardened colloids, taught properly, with the one thing missing that would let you attempt any of them.

The distinction the whole part rests on

A process is classified by the substances, energies and conditions it actually requires, not by its age. That sentence does more work in this course than any other safety statement, and it cuts both ways. Victorian chemistry is printed and practised across Parts XXI to XXV. Chemistry from living memory is refused: Kodak Limited was still printing a uranium toner and a mercury intensifier in 1949, and this course prints both as study entries with no procedure.

Four processes, sorted by what they need rather than by when they appeared

controls a reader at home can actually assemble

1. Photogenic drawing and salted paper, 1839 — practised in Parts I and XXIISilver nitrate on paper, a halide, sunlight, a thiosulfate fixer. The rubric puts it at Level B on the silver nitrate: splash protection, ventilation, eyewash within reach and collected waste are controls a domestic reader can assemble, so the course teaches it as a labLevel B
2. Cyanotype and the iron processes, 1842 onwards — practised in Parts XXI and XXIVIron(III) salts, an ultraviolet exposure, a water wash and a ferricyanide that is never acidified. Older than several of the processes this part refuses, and every practical page in those two parts sits at Level A or BLevel A to B
3. Wet-plate collodion, about 1851 — studied in this partDiethyl ether and ethanol poured by hand, a silver nitrate bath, cadmium salting salts in period practice, and a fixer that was frequently potassium cyanide. Twelve years younger than salted paper and out of reach in a way salted paper is notLevel D
4. The Kodak Limited mercury and uranium intensifiers, 1949 — studied in this partPublished as working formulas within living memory, in a manual that prints its own warning about the cyanide bath beside the formula. Age is no part of why they are refused; mercury(II) chloride, uranyl nitrate and free cyanide are the whole of itLevel D

The reverse case matters as much. Nothing here is being called dangerous because it is old, and nothing is being excused because it is famous. The daguerreotype is one of the two processes photography began with and it is still Level D; the cyanotype is three years younger than the daguerreotype and you will make one in Part XXI on a windowsill.

What Level D means, in the rubric’s own words

The course’s four levels are defined in src/data/safety-levels.json and rendered on the classification page. Level D’s summary there reads, in full:

Taught for its chemistry, history and significance. No actionable home procedure is given, because the original method depends on substances or conditions that are unacceptable outside a professional laboratory.

Its two criteria are “mercury vapour development, cyanide fixing or toning, uranium and cadmium compounds, and any process whose historical practice is documented mainly through the harm it caused”, and “processes for which a safer modern route exists and is taught instead, with the historical method described for understanding only”. Every page in this part meets the first criterion. Most of them meet the second as well, and where they do, the page names the modern route and the lesson that teaches it.

☠What a Level D page may contain, and what it may not

May, and on these pages must: the reactions, with mechanism; the historical workflow told as what people did; the image the process made and why the chemistry produced that image and not another; the hazards, from named sources with dates; where a safer modern route to a comparable result is taught in this course; and, where conservators or institutions work with the process under engineered control, a pointer to their published account.

May not, at any length, in any tone, under any heading: a quantity to weigh; a bath to mix; a sequence of steps; a temperature and a time to hold; a substitution offered as making it safer; or a sentence of the form “if you must do this, then…”. A partial procedure is worse than none, because it reads as permission while omitting the controls that would have mattered.

This is a build rule as well as an editorial one. npm run validate:labs fails the build if a page whose front matter says safetyLevel: D carries a Procedure, Preparation or Build heading, and the check is keyed to the safety level rather than to the page type precisely because every Level D entry in this course is a lesson or an assignment rather than a lab. The full argument lives once, on the Level D policy, and every page in this part cites it instead of restating it.

The prohibition governs the prose and not only the headings. A page that describes a mercury box in the second person, with the operations in order, has handed over a procedure whatever it calls the section. Where a step has to be named for the history to make sense, it is named and nothing more.

Where the letter is recorded

On the page’s own front matter, as safetyLevel: D, on every kind of page — lesson, assignment, process-atlas entry, formulary entry and chemical entry alike. Part I raised this as an open question when it reached the daguerreotype; the answer the course settled on is the one that the validator can act on, because a letter recorded only in the process atlas would leave the lesson unchecked. This overview carries the letter too. Every page in Part XXVI is Level D, this one included.

Why Level C exists, and where chromium(VI) actually sits

Level C is not a softer Level D. It is a real level with real pages behind it: work that a fume cupboard, a controlled waste route and a second competent person make acceptable, where the course gives the procedure to a reader who has those things. Level D is the level at which the course gives no procedure to anybody, because the controls that would make it reasonable are not ones a page can assume a reader has.

Chromium(VI) is the case where this course departs from its own rubric, and it says so rather than quietly reclassifying. The rubric’s Level C criteria name “chromium(VI) compounds, formaldehyde and similar substances in any quantity”, which places supervised dichromate work at C. This course presents the dichromated colloids at Level D anyway. That is a decision of the course, not a reading of the rubric, and it rests on three things: the course is written for a home darkroom and cannot assume a fume cupboard; potassium dichromate’s notified classification, aggregated from 491 reports across 19 notifications, carries twelve hazard statements including H340 for genetic defects, H350 for cancer and H360 for reproductive toxicity; and HSE’s EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium with the Carc and Sen annotations and a biological monitoring guidance value — which is to say that the recognised way of knowing whether the control worked is to test the worker’s urine. The complete ruling, including the chromium(III) that the course does permit as a gelatin hardener, is the chromium policy, and this part cites it rather than arguing it twice.

The map

Six pages. Five lessons and the assignment they exist to make possible. Each lesson takes one family, gives its chemistry in full, and stops where the procedure would start.

Page	What it settles	Level
The mercury daguerreotype	Halogen sensitisation, amalgam development, the mirror that reads as a picture, and the vapour that has no smell	D
Cyanide in the historical darkroom	Why cyanide fixed better than hypo, what an acid stop bath does to it, and where the hexacyanoferrates this course uses are and are not different	D
Wet-plate collodion	Nitrocellulose in ether, the silver bath, physical development, three objects from one process, and why the fire risk decides it	D
Uranium, mercury and the heavy-metal treatments	Intensifiers and reducers as a family, uranium as a process in its own right, and the metals named so that they can be recognised	D
Light-hardened colloids	The photochemistry that is not silver at all, from bitumen to dichromated gelatin, and the industry that came out of it	D
Assignment: assessing a historical process for hazard	Three period procedures classified by the reader, with the single deciding hazard named in each	D

What every page here has to deliver

The brief’s fifth rule is the one that governs this part, and it is a positive instruction rather than a prohibition: teach the history, the chemistry, the significance, the mechanism and the hazards, and classify the process as theoretical or specialist work. Five things, and a page that drops any of them has failed differently from a page that gives a procedure, but it has still failed.

The five obligations of a Level D lesson in this part

1. 1The chemistryWritten as reactions where the reaction is known, and as an honest gap where it is not. Bitumen has no formula in this course because its structure is not one anybody can write; dichromate crosslinking is stated with the step that is still argued over marked as argued over
2. 2Why it was usedEvery one of these processes solved a problem that photographers had. Cyanide was faster than the hypo of its period and left a whiter plate; uranium gave colour and density from two cheap bottles; the dichromated colloids gave pigment permanence when silver prints faded. Nobody used them out of carelessness
3. 3The historical workflow, in outlineWhat was done, in what order, told as history in the third person and the past tense. Enough for a reader to understand a period manual, a museum label or a conservator’s report. Not enough to follow
4. 4The image it madeThe property that a conservator identifies the object by, and the chemistry that produced it: a specular ground with scattering particles on it, a blue-and-ultraviolet-only tonal rendering, relief you can feel in raking light
5. 5Why modern knowledge changes the approachThe hazard data, from a named source with a date, read against what the period source believed; the control that would be needed; and whether a reader could assemble it. This is the step that produces the letter

Why the daguerreotype needed mercury, and what mercury does to a person

The daguerreotype’s sensitive layer is a thin skin of silver iodide grown on a polished silver surface by iodine vapour. There is no gelatin, no chemical sensitisation, no ripening and nothing at all like the crystal engineering of Part V. Left to finish the job by itself, that layer needs an exposure long enough to darken visibly, and the sensitivity problem that Part I spends its length on is precisely that such an exposure is too long for a camera.

Mercury is where that problem ends, and it is Part IV’s argument arriving before anybody had the vocabulary for it. At some point before the contract of June 1837 — no source this course has read dates it more closely, and the cupboard story is folklore rather than testimony — Daguerre found that a plate carrying an exposure far too short to darken it visibly could still be made to yield a complete picture by exposing it to the vapour of mercury. That is the discovery of development, and with it of the latent image. What the short exposure leaves behind is a latent image, and mercury vapour condenses preferentially where that latent image is, building a silver-mercury amalgam particle at each site. It is physical development — the image is built from material arriving out of the vapour phase, not by reducing the halide of the grain in place, which is what a developer in Part VIII does. The consequence is the daguerreotype’s whole appearance: an image made of particles that scatter light, standing on a mirror, which is why it reads as a positive from one angle and a negative from another.

What the metal does to a person is a separate fact, and it is the one that ends the discussion. Mercury has a measurable vapour pressure at room temperature, the historical process deliberately warmed it, and the vapour is invisible and has no smell — so nothing in the room tells the operator what is in the air. HSE’s EH40 gives mercury and its divalent inorganic compounds a long-term workplace exposure limit of 0.02 mg/m³ measured as mercury, with no short-term figure at all and a biological monitoring guidance value alongside it. NIOSH sets a recommended limit for mercury vapour of 0.05 mg/m³ with a skin notation and an immediately-dangerous-to-life level of 10 mg/m³, and its symptom list is largely neurological: tremor, insomnia, irritability, indecision, headache and lassitude, with the central nervous system and the kidneys among the target organs. A limit the regulator does not trust the air measurement alone to police is not a limit a domestic darkroom can work to.

!The variant that removes the mercury and does not remove the classification

The AIC records the Becquerel process of 1840, in which the exposed plate is brought out by red light instead of by mercury vapour. It needs no mercury and no bromine or chlorine sensitising, at the cost of plates around ten times slower, which makes portraiture difficult. Part I named it and left its classification to this part, and this part’s answer is that it stays at Level D: the mercury goes, the iodine fuming and the polished-plate work do not, and a variant that removes one of four vapour hazards has not removed the reason the process is here.

Why cyanide fixed better than hypo, and what it did to the people who used it

Fixing is complex formation, and Part XI taught it with thiosulfate as the ligand. Cyanide is the same chemistry with a different ligand, and a more avid one:

AgBr + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + Br⁻

The fixer this course teaches: the bis(thiosulfato) complex

AgI + 2 CN⁻ → [Ag(CN)₂]⁻ + I⁻

The fixer the wet-plate darkroom often preferred: the dicyanoargentate complex

Hardwich’s 1864 manual introduces potassium cyanide as the salt most frequently employed in fixing and calls it a most energetic agent in dissolving the insoluble silver salts, far more so than hyposulphite of soda — and adds the property that decided the choice for a plate that had to be finished quickly: the soluble double salt it forms is not decomposed by dilution with water, so a cyanide-fixed plate needs less washing than a hypo-fixed one. For collodion positives it also left a whiter picture, which on an ambrotype or a tintype, where the highlights are the image, is not a small thing. The same speed is why the bath does not stop at the unexposed halide: too concentrated, or left too long, and it whitens and then dissolves the middle tints. The full evidence, and the correction that the nineteenth century’s so-called cyanide toning bath was a thiocyanate rather than a cyanide, is in the formulary entry for cyanide fixing and toning.

Two separate things then killed people, and they are worth keeping apart because the controls against them are different.

The acid. Any acid reaching a cyanide salt or its solution liberates hydrogen cyanide gas at once, and a darkroom is a room with an acid stop bath and an acid fixer standing within arm’s reach of the sink. Kodak Limited printed the warning beside its own 1949 mercury-intensifier formula, in the manual that also printed the 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. EH40 gives hydrogen cyanide 0.9 ppm long-term and 4.5 ppm over fifteen minutes with the Sk notation for skin absorption.

The salt itself, through the skin, over years. The aggregated ECHA notifications classify potassium cyanide Danger with H300, H310 and H330 — fatal if swallowed, fatal in contact with skin, fatal if inhaled — each in 99.8 per cent of 575 reports, and NIOSH lists skin absorption among its four exposure routes. Towler’s Silver Sunbeam of 1864 describes cyanide being used to wash silver stains off the hands, warns that this might entail upon the operator incurable ulcers, and records that 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 continual manipulation in the toning and fixing baths are frequently in a suffering condition”. That is a period observation rather than an epidemiological study, and the cyanide lesson is careful about which of the commonly repeated stories can actually be traced. It is still the clearest surviving statement that the people running the baths were being harmed and that it was noticed at the time.

⚗The boundary that matters to your own darkroom

Potassium ferricyanide is not free cyanide. It is hexacyanoferrate(III), a coordination complex whose cyanide is bound to iron, and this course uses it at Levels A and B in rehalogenating bleaches and in Farmer’s reducer. That is a real chemical distinction and not a comforting one — but it has conditions attached, and Princeton’s environmental health and safety guidance for photography states them: potassium ferricyanide will release hydrogen cyanide gas if heated, if hot acid is added, or if exposed to strong ultraviolet light, and cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid. That is the origin of a rule you have already met in Parts XX, XXI and XXVIII: a ferricyanide bath is never acidified, never mixed with stop bath, never heated and never left in the sun. The incompatibility matrix carries the pair.

What made collodion practical, and what ether does in a small room

Collodion is nitrocellulose dissolved in diethyl ether and ethanol. Archer’s own account records that gun-cotton is not soluble in pure ether free from alcohol, so the alcohol is a necessary part of the solvent and not a diluent. Poured onto glass, the mixture leaves a tough transparent film that will hold silver halide without gelatin; immersed in a silver nitrate bath, it forms the halide inside the film; and it has to be exposed and developed while still wet, because a dried collodion film stops letting the processing solutions in. That is the whole reason the process is called wet plate, and the reason a photographer working away from a studio had to carry a darkroom with them.

What it bought was decisive. Glass gave a negative with none of the paper fibre of a calotype, and one negative made any number of prints, which a daguerreotype never could. Within a decade it had displaced both of the processes photography began with — on sensitivity, cost and reproducibility together, which is the collodion lesson’s argument to make rather than this page’s.

The hazard that decides it is not primarily toxicity. It is fire, and the numbers are unusually stark. NIOSH gives diethyl ether a flash point of −49 °F, which means there is no room temperature at which the space above the liquid is too cold to burn; an explosive range from 1.9 to 36.0 per cent in air, which is unusually wide; and a note that it is a gas above 94 °F, which is 34 °C. The vapour is heavier than air, so it travels along a bench or a floor to find an ignition source rather than waiting to be found. NIOSH adds a storage behaviour that no GHS block carries: ether tends to form explosive peroxides under the influence of air and light, and those peroxides concentrate in the residue as the liquid evaporates, so the hazard belongs to the bottom of an old bottle. Set that beside an open pour, in a small dark room, and the case does not need the rest of the process to be made.

The rest of the process supplies it anyway: a corrosive silver nitrate bath, period salting mixtures that frequently contained cadmium salts, a fixer that was often cyanide, and a varnishing step that applies heat to an alcohol-borne resin. Cadmium bromide is the entry that shows why a single classification is not an assessment: the ECHA aggregate reads mildly, signal word Warning with H302, H312 and H332, while the NITE-CMC blocks in the same PubChem record carry H340 and H350, and EH40 sets cadmium and its compounds at 0.025 mg/m³ with the Carc annotation. Where the aggregate and the regulator disagree that sharply, the course takes the stricter reading and says which it took.

!Modern wet plate is a real practice, and this is what that means here

Wet plate is worked today, safely and well, by people who were taught it in person, with cadmium-free salting, thiosulfate fixing, forced ventilation and no ignition sources, in a workshop rather than a domestic room. The course says so plainly rather than pretending otherwise, and the collodion lesson points at a supervised workshop and at suppliers’ published instructions read as manufacturer documents. What it does not do is print a shortened version of those instructions, because a reader following a summary has the process without the room it belongs in.

The metals, and the chemistry that is not silver at all

Two more families complete the part, and each is here for its own reason.

The heavy-metal treatments are what a photographer did when a negative had to be made printable after the fact, in an era with no variable-contrast paper and no second chance at a plate. Intensification and reduction are one idea: convert the image silver into something else, or deposit a second material on it, so that density or colour changes. Mercury(II) chloride bleaching and then blackening, uranyl nitrate with ferricyanide laying down a red-brown uranium ferrocyanide, chromium bleaches, and — in the cyanotype literature rather than the silver one — lead, thallium and nickel walked into a Prussian blue lattice. Uranium is the one to understand, because it carries two hazard regimes and the course can only read one of them: its notified classification gives H300 and H330 in 100 per cent of reports — but from 39 reports across 2 notifications, much the smallest sample behind any classification this course cites — with a NITE-CMC block adding suspected genetic defects and possible carcinogenicity, and none of that describes the radiological hazard, which no GHS classification covers and which EH40 does not list, because it is regulated separately. Princeton’s guidance states the practical position in one line: do not use mercury, cyanide or uranium intensifiers, and uranium intensifiers are radioactive and especially hazardous to the kidneys. A hazard the sources cannot characterise is a hazard against which no control can be specified.

The light-hardened colloids are the part of the course where the photochemistry is not silver at all. No latent image, no development amplification, no fixer: light changes the solubility of a colloid, and the picture is what survives the wash. Niépce’s bitumen is the first of them, by a mechanism the lesson states as an open question rather than repeating the nineteenth century’s word for it; Mungo Ponton found in 1839 that paper soaked in bichromate of potash is powerfully and rapidly acted on by sunlight and needs only water to fix; Talbot’s photoglyphic engraving patent of 29 October 1852 added the sensitivity of dichromate mixed with gelatin or gum, and carbon printing, gum bichromate, collotype, the Woodburytype and photogravure all descend from it. That is most of the photographically illustrated printing of the nineteenth century, which is why the family cannot be left out — and chromium(VI) is why none of it is performed here.

Reading a period source: fact, interpretation, and the sentence that is neither

This is the part of the course where the brief’s seventh rule earns its place, because the historical literature on these processes is full of confident statements that modern toxicology contradicts.

The clearest example sits in Wall’s Photographic Facts and Formulas of 1924, beside the mercury intensifier. Wall states that mercuric chloride is extremely poisonous when taken internally, but that absorption by the skin, even in the case of cuts and abrasions, is “practically harmless”. The first half of that claim is correct and the second half is a period assertion that the current classification does not support: mercury(II) chloride’s notified classification carries H310, fatal in contact with skin, in 100 per cent of 218 reports.

What to do with a sentence like that is a teachable operation rather than a matter of taste. It is evidence of what was believed, and every page in this part uses period sources that way. It is not evidence of what is true, and no page in this part uses a period source that way for a hazard claim. The same discipline separates a documented death from darkroom folklore, a manufacturer’s selling claim from a measurement, and a conservator’s observation of a real collection from a plausible mechanism nobody has tested.

🕰The honest statement that belongs at the front of this part

Some of this chemistry was practised for decades by people who did not know what it was doing to them, and some of it by people who did. Both are in the record. Kodak Limited printed its cyanide warning in 1949 beside the formula it was warning about; Towler in 1864 described operators in a suffering condition and went on describing the process. The harm is part of the subject rather than a footnote to it, and a page here that told the chemistry without it would be telling less than half of what happened.

The skill this part exists to teach

The point of five lessons that give no procedure is the sixth page, which asks you to reach the same verdicts yourself on processes nobody has classified for you. Period manuals are freely available in reprint and online, and a reader who meets an unfamiliar formula needs a method rather than a list of forbidden words.

The assessment, as the assignment sets it out

1. 1List every substance the procedure namesIncluding the ones named only in passing, and including the products of any step that mixes two of them. Period nomenclature has to be translated first: sulphocyanide is thiocyanate, bichromate is dichromate, and hyposulphite is thiosulfate
2. 2Find each one’s current classification and exposure dataA GHS classification, a workplace exposure limit or a safety data sheet, from a named source, with the date you read it. Not a reputation, not a forum, and not the period manual’s own reassurance
3. 3Identify the energies and the waste streamsHeat, flame, ultraviolet, mains electricity; and what is left in the tray afterwards, and whether any lawful domestic route exists for it
4. 4Identify the failure modes, and what each would do to a personNot what the procedure does when it works. What a splash, a spill, a wrong bottle, an acid in the wrong tray or an unventilated hour would actually produce
5. 5Classify on the worst single item, and name which item it wasThe level is set by the highest criterion met, never by an average, and an assessment that does not name the deciding substance or step has not finished. Then ask what control would move it down a level, and whether a reader could assemble that control

The second half of each assessment is the historical half, and it is what turns the exercise into photography rather than paperwork: why was the procedure used, what problem did it solve, and what solves that problem now? For most of the processes in this part the modern descendant is not another bath at all. It is a paper grade, a different developer, or a decision made at exposure — which is the course’s actual recommendation in place of intensification, and the assignment requires the modern route to be named with the lesson slug that teaches it.

The objects, and what to do with one

Several of these processes made things that are now in collections, in dealers’ cases and in family houses: daguerreotypes in sealed packages, ambrotypes and tintypes with fragile varnish, uranium-toned lantern slides, mercury-intensified negatives that have faded unevenly. A reader is much more likely to meet one of those than to meet a formula.

The general answer is nearly always the same and it is not satisfying: keep it dry, dark and unhandled, do not clean it, do not open a sealed package, and take conservation advice before doing anything else. An ungilded daguerreotype was so delicate that the image could be wiped off with a finger — which is why the gilding step Part I describes was adopted everywhere — and even a gilded plate sits inside a sealed package of plate, mat, cover glass and case that is part of the object. A varnished collodion plate’s surface is the object in the same sense. Historic plates and cases may also carry residues of the chemistry that made them, which is a conservator’s judgement rather than a reader’s and which the daguerreotype lesson settles against a named institution’s published advice. Where the object is not a picture but a bottle — an unlabelled jar in an inherited darkroom — it is never opened, never smelled and never poured away; it is a hazardous-waste question for the local authority or, if the label suggests cyanide, for the emergency services. GOV.UK routes household hazardous waste to the council’s collection service rather than to the bin or the drain, and what may lawfully be done differs between authorities and changes. Check your local regulations; they govern.

What this part owns, and what it does not

Four boundaries, drawn so that the same argument is made once rather than four times in slightly different words.

• Part I owns the 1839 history, the priority disputes and the invention narrative. Its daguerreotype lesson tells that story and hands the mercury chemistry, the halogen chemistry and the whole toxicological argument to this part by name.
• Part XX owns uranium, cyanide, mercury and chromium(VI) as they act on a developed silver gelatin image — the toning bath applied to a finished print. This part owns the toxicology in depth, the objects, and the whole processes.
• Part XXV owns the same substances inside a sensitiser — mercury in a platinotype, lead as a contrast addition, dichromate as a contrast agent. Different chemistry, different page.
• This part owns whole processes: the ones where the mercury, the cyanide, the solvent or the dichromate is not an additive to something else but the process itself.

The formulary sits across all four, and it carries the evidence so that the lessons do not have to reprint it. Its entries marked safetyLevel: D are study entries: written to the formulary’s own format, with the composition given as history and no mixing order, no working strength and no procedure. Cyanide fixing and toning, the uranium toner, the Kodak Limited mercury and uranium intensifiers, the mercuric sepia platinotype, Herschel’s hydrargyro-cyanotype and the lead, thallium and nickel cyanotype toners are the ones this part cites most. Beside them sit the earliest formulas in the course, which Part I owns — Daguerre’s 1839 process outline, Niépce’s bitumen coating and Schulze’s scotophorus mixture.

Wedgwood and Davy’s silver nitrate on leather is the instructive exception, and it is worth looking up while the distinction is fresh. It is the first photographic formula ever published, from 1802, older than everything on this page — and it carries Level B, not Level D, because one part of silver nitrate in about ten of water brushed onto a piece of leather is chemistry a reader can control. The age of a formula never was the question.

What you need first

Less equipment than any other part of this course, and more reading.

Part IV, for the latent image and for development as amplification, without which mercury development is a curiosity rather than the first solution to the sensitivity problem. Part III, for complex formation, which is what both fixers on this page are doing. Part XI, for what a fixer is actually for and what a residue of it costs. And Part II, which is the one that makes this part usable rather than merely readable: the hazard vocabulary, how to read GHS codes and a safety data sheet, and the incompatibilities that explain why an acid two trays away is a hazard and not a housekeeping detail.

The chemical encyclopaedia carries the full hazard record for every substance named in this part, each with its sources and the date they were read. The lessons quote the figure that decides the argument and link to the entry for the rest, so that the pages stay about photography.

Alternative route

There is no alternative route to give, because there is no route. Nothing in this part is mixed, coated, exposed, developed or poured. Every page is reading, chemistry and hazard arithmetic, and the assignment is written work with a printable worksheet. A reader with no darkroom, no ventilated bench, no budget and no ability to handle chemicals at all can complete this part in full, and will finish it holding the one thing that makes the rest of the course’s practical work legible: the reason a level is assigned, and the ability to assign one.

That is worth saying plainly, because it inverts the usual accessibility note. Everywhere else in this course, the reader without a facility is the one being accommodated. Here they are at no disadvantage whatever.

The pages of this part

Part26 of 28Level3 — AdvancedPages6Estimated time5.8 hoursHighest safety levelLevel D

0 / 6 lessons in this part completed

• LessonThe Mercury Daguerreotype: Iodine, Amalgam and a Mirror With a MemoryD55 min
• LessonCyanide in the Historical Darkroom, and the Ferricyanide That Is Not CyanideD45 min
• LessonWet-Plate Collodion: The Process That Made the Nineteenth Century Look Like ThatD55 min
• LessonUranium, Mercury and the Heavy-Metal TreatmentsD50 min
• LessonLight-Hardened Colloids: Bitumen, Dichromate and the Pigment ProcessesD50 min
• AssignmentAssignment: Assessing a Historical Process for HazardD90 min