Daguerre and the Daguerreotype: A Mirror With a Memory

Hold a daguerreotype at the wrong angle and you see your own face. Tilt it a few degrees and a picture appears, sharper than anything else made in the nineteenth century. Tilt it again and the picture turns inside out, highlights becoming shadows. It is the only photographic object in this course whose image is made of reflection rather than absorption, and understanding why is worth more than the date it was announced.

☠Level D: studied, never performed

The daguerreotype depends on mercury vapour, on iodine and later bromine and chlorine vapours, and in some later practice on potassium cyanide. It is classified Level D under this course’s safety rubric: described for its chemistry and its history, and never carried out.

So this page contains no procedure of any kind. No quantity, no temperature, no time, no apparatus drawing and no sequence detailed enough to follow appears on it, and the same restriction governs the process-atlas entry. Where a step has to be named for the history to make sense, it is named and nothing more. Part XXVI, which is the part devoted to processes the course studies and does not casually reproduce, owns the mercury chemistry, the halogen chemistry and the whole toxicological argument, in a lesson written for that purpose. No home variant is taught here, including the Becquerel process, which is named below and not described.

Daguerre before he was a photographer

Louis Jacques Mandé Daguerre was born at Cormeilles-en-Parisis on 18 November 1787 and died in 1851. He was not a chemist and never became one; Eder records that when Daguerre needed chemical advice in the early 1830s he got it from the chemist Jean-Baptiste Dumas, who placed his laboratory at Daguerre’s disposal.

What he was, was a professional manipulator of light. He trained under the scene painter Degotti, became expert in perspective and stage lighting, and worked with Prévost on panoramas. In 1822 he went into partnership with the painter Charles-Marie Bouton and opened the Diorama on 11 July of that year, in a purpose-built showroom in Paris: enormous translucent paintings, lit from in front and from behind, in which the light was changed while the audience watched so that a scene passed from day to night, or a peaceful valley filled with an avalanche. The Metropolitan Museum describes it as a popular Parisian spectacle of theatrical painting and lighting effects, and Eder records that it was one of the main attractions of the city.

That is the background a showman brought to Niépce’s problem, and it explains something about the result. Daguerre was not looking for a scientific record. He was looking for an image with the qualities of the Diorama — luminous, detailed, astonishing to a paying public — and the process he arrived at has exactly those properties and very few of the ones a scientist would have optimised for.

He wrote to Niépce in 1826, met him in Paris in 1827, and signed a ten-year association with him on 14 December 1829. Niépce died on 5 July 1833, and the partnership passed to his son Isidore. On 13 June 1837 a new contract was signed with Isidore, and Eder prints it: Daguerre gained the right to attach his own name alone to the new process, and in exchange Isidore got a document acknowledging that the process had been communicated to him, and that it reproduced objects sixty to eighty times more rapidly than the one his father had invented. Note the number. It is the earliest quantitative statement of the thing this page is about.

The plate

The daguerreotype is made on a sheet of copper faced with a thin layer of pure silver, polished until the surface is a mirror. The Library of Congress’s account of the medium describes exactly that: the silver-plated copper plate cleaned and polished until it looks like a mirror, then sensitised in a closed box over iodine until it takes on a yellow-rose colour.

The chemistry of that second step is one reaction, and it is the reaction Part I has been building towards. Iodine vapour meets metallic silver and converts the outermost layer of it into silver iodide, AgI — the same class of compound as the silver chloride Schulze, Scheele and Wedgwood worked with, and by a long way the least soluble of the three common silver halides. PubChem’s physical description gives it as a light yellow, odourless solid, gradually darkened by light, and puts its solubility at 2.8 × 10⁻⁶ g per litre at 25 °C, against 0.135 mg per litre for the bromide and 1.93 mg per litre for the chloride.

Two consequences follow, and both matter. The plate is not coated with anything: its own surface is converted, so there is no binder, no gelatin and no paper fibre between you and the image, which is why a daguerreotype resolves detail that no other nineteenth-century process approaches. And a salt that insoluble is correspondingly hard to wash out again, which is the problem the fixing section below runs into.

🕰Where the iodine came from

Nobody arrived at silver iodide by accident. Humphry Davy — the same Davy who wrote the observations in the 1802 Wedgwood memoir — produced silver iodide and recognised that it was sensitive to light in 1814, and Eder points out that this is of particular interest to the history of photography precisely because both Niépce and Daguerre later worked with it.

And Niépce had already been fuming silvered plates with iodine by 1829, for a completely different reason: to darken the bare metal so that it made a better shadow against his pale bitumen. Robert Hunt, writing in 1841 while everyone involved was still alive, saw the connection and said that the practice appeared to have led the way to Daguerre’s process. The step that made the daguerreotype possible was invented for its cosmetic side-effect by a man who did not want its photographic one.

The discovery: an exposure too short to see

Here is the thing that changed photography, and it is not the mercury.

Everything in Part I up to this point has asked light to finish the job on its own — to build a quantity of image substance large enough for a human eye to see, unaided, while the exposure runs. That requirement is what put Wedgwood’s exposures in minutes of full sun and Niépce’s in hours or days, and it is what made the camera obscura impossible for both of them. Daguerre spent months, Eder records, exposing iodised plates in the camera and getting no picture at all. The plates came out looking exactly as they had gone in.

They were not blank. They carried a change too small to see — what this course, and everyone since, calls the latent image. The discovery was that such a change exists and can be found and made visible by a later chemical operation. Everything modern photography does rests on that sentence.

⁇The cupboard, and the broken thermometer

The story everyone tells: Daguerre put a batch of apparently blank exposed plates away in a cupboard, found a fully formed image on one of them days later, removed the contents of the cupboard one item at a time until only a dish of mercury was left, and so identified the agent. In the popular version a broken thermometer supplies the mercury.

Eder prints the story, and marks it as hearsay in his first words: Daguerre “is said to have been indebted to a curious accident”. His source is not Daguerre; it is Liebig, writing in the Cornhill Magazine, by way of Vogel’s 1878 textbook. The broken thermometer does not appear in Eder’s version at all, and the course could not find a source for it.

What the course states: that Daguerre found mercury vapour would develop an invisible exposure, and that the received account of how he found it reaches us at two or three removes and should be treated as folklore rather than as testimony. Anecdotes of this shape — the lucky accident, the single decisive object — are common in the history of chemistry and rare in laboratory notebooks.

Development, in one sentence

Daguerre found that a plate carrying an exposure far too short to darken it visibly could be made to yield a complete picture by exposing it to the vapour of mercury, which forms an amalgam with the silver in the places where light had acted.

That is the whole of what Part I says about it. The mechanism, the halogen chemistry that sets it up, the reason the amalgam forms where it does and not elsewhere, and the exposure routes and control measures that make the process Level D are owned by “The Mercury Daguerreotype: Iodine, Amalgam and a Mirror With a Memory” in Part XXVI. This page cites that lesson rather than restating any part of it, and the two are written to agree.

Amplification, and the end of the sensitivity problem

Now put the numbers side by side, because this is where Part I’s first problem gets solved.

Process	What light has to do	Reported exposure
Wedgwood on leather, 1802	Build a visible quantity of silver, unaided	Two to three minutes in direct sun in contact; the camera image never worked at all
Niépce, heliography	Harden enough bitumen to survive the solvent, unaided	Six to eight hours by his own account; about five days by Marignier’s reconstruction
Daguerre, 1839	Make an invisible change that something else will amplify	Three to fifteen minutes in the camera
Daguerre, after bromine acceleration, 1840–41	The same, on a more sensitive plate	Seconds to under a minute

The Library of Congress gives the 1839 figures as three to fifteen minutes, and records that changes to the sensitising, together with better lenses, soon brought the exposure below a minute. The 1837 contract’s “sixty to eighty times more rapidly” than Niépce’s process is the same claim made by the people who had measured it.

⚗What amplification buys you

Silver salts and light establishes the size of the prize: making a speck of photolytic silver that a developer can find takes something like a million times less light than making one you can see with your eye. Printing out spends all of that; developing out spends almost none of it and lets chemistry make up the difference.

This is the single largest step change in the whole history of the medium, and it is worth being precise about what it did and did not do. It did not make the material more sensitive to light in any deep sense — the same photons do the same thing to the same crystals. It changed how much of the work light has to finish. That is why the daguerreotype’s exposure fell by a factor of tens overnight and then by another factor of tens as the plates were improved, while heliography, which has no amplification step at all, could never have got there by any amount of optical improvement. Part IV takes the latent image apart properly.

Fixing: hot salt, and then Herschel

A developed plate still carried the unchanged silver iodide, and Daguerre knew precisely what that meant. His 1839 manual states the purpose of the last operation in one clause: to remove from the plate the iodine which, if the picture were exposed too long to light, would go on decomposing and would destroy it. That is the permanence problem, named by a man who had read nobody’s paper on it.

His first answer was a hot saturated solution of common salt, and Eder is blunt about the result: an imperfect fixation which gave the plates a mottled appearance. It is the same chloride treatment, with the same weakness, that the permanence page works through — it modifies the residual halide far more than it removes it.

The second answer arrives in the same year and from outside. John Herschel had published in 1819 that freshly precipitated silver chloride dissolves in hyposulphite of soda with great ease and in large quantity; in 1839 he connected that to photographs, told Talbot, and Talbot was using it by 1 May. Eder records that Daguerre soon learned of it and abandoned the salt for hyposulphite of soda in the same year. The remarkable thing is that Daguerre’s own published manual already contains it: it offers a weak solution of pure sodium hyposulfite as an alternative to the salt solution and says plainly that the hyposulfite is preferable, because it removes the iodine entirely, which the salt does not always manage. Ware’s summary is that by the end of 1839, having adopted Herschel’s method, the process was enjoying widespread initial success. What thiosulfate actually does, and what it costs the image, belong to the Herschel page and to Part XI.

Gilding, accelerators and a lens: 1840 and 1841

Four improvements arrive in two years, and between them they turn a demonstration into an industry.

Gilding, 1840. A finished plate is desperately fragile. The AIC’s conservation page states it in the plainest terms available: an ungilded plate is so delicate that the image can be wiped off with a finger. The French physicist Fizeau found that treating the plate with a bath of hyposulphite of soda containing gold chloride greatly improved both its beauty and its permanence, and the treatment was adopted everywhere. Eder dates the invention to 1840; the AIC’s conservation page gives 1841 for the introduction of gold toning, and this page follows Eder without claiming the year is settled. The chemistry of the bath was worked out shortly afterwards by the Paris pharmacists Fordos and Gélis, who identified the double salt formed in it and called it hyposulphite of gold and sodium — later sodium aurothiosulfate, and sold in the trade as sel d’or. Eder notes that it subsequently became the basis of many combined toning-and-fixing baths for silver printing papers, which is Part XX’s material.

Bromine, 1840. John Frederick Goddard, lecturing at the Adelaide Gallery in London, published in a letter dated 12 December 1840 that using bromine in combination with iodine, instead of pure iodine vapour, considerably increased the sensitivity of the plate. Eder is careful to add that Goddard must share the credit with Franz Kratochwila in Vienna, who had the same result in September 1840 and published it in the Wiener Zeitung on 19 January 1841 — Kratochwila reported at least a fivefold gain, exposures of a few seconds, and portraits made on cloudy days in eight seconds. Goddard published first; Kratochwila did it first.

Chlorine, 1841. Claudet — the same Claudet who had bought the English patent rights — described iodo-chloride sensitising in May 1841, and read a paper before the Royal Society on 10 June 1841 comparing it with bromo-iodide.

The lens, 1840. Josef Petzval in Vienna calculated a portrait objective of far greater light transmission than anything then available; Voigtländer built the first one; and Anton Martin, at Petzval’s request, made the first portraits with it in May 1840. Eder dates the boom in portrait photography from that lens.

⁇Who made the first portrait?

Eder’s account credits John William Draper in New York, in the autumn of 1839, with the first photographic portrait on a daguerreotype plate: his assistant, face powdered with flour, sitting facing the sunlight for half an hour. Eder supports it with Sir David Brewster’s statement in the Edinburgh Review of January 1843 that he believed Draper was the first, portraiture having been thought impossible because neither Arago’s nor Dumas’s report on the invention mentioned it.

Other candidates are widely named, in particular Robert Cornelius in Philadelphia in late 1839. The course could not verify those claims from the sources it read and therefore does not adjudicate. What the evidence here supports is narrower and more interesting: that portraits were being attempted from the autumn of 1839 by giving enormous exposures and pushing the sitter’s tolerance to the limit, and that they only became a business after the plates were accelerated and Petzval’s lens arrived in 1840.

The image itself

Six properties, and all of them follow from the fact that the picture sits on the surface of a mirror.

It is a direct positive, and it is unique. No negative exists at any stage. The plate that was in the camera is the finished object, and there is nothing to print from. Copies were made by photographing the plate again, or by having an engraver copy it.

It is laterally reversed. A one-step process that puts the sensitive surface in the camera and then shows you that same surface must reverse the handedness of the scene, as Ware puts it: the direct photographic image is a mirror of reality, and to restore the true chirality it has to be re-photographed. The Library of Congress notes that a daguerreotype is laterally reversed unless the camera carried a mirror or a prism.

It is neither positive nor negative but ambivalent. This is the property everyone notices first and almost nobody explains. Ware’s account is exact: virtually none of the light falling on a daguerreotype is absorbed by it. The highlights are microscopic globules of silver amalgam, which scatter light diffusely; the shadows are polished silver, which reflects it specularly. Turn the plate so that the mirror throws a dark surrounding into your eye and the polished areas read as deep shadow and the picture is a positive. Turn it so that the mirror throws a bright source at you and the same areas read as highlights, and the picture inverts. Nothing changed but the geometry.

Why the same plate reads positive from one side and negative from the other

1. 1Bare polished silver — specular: it reflects the incoming light in one direction and shows you whatever it faces
2. 2Silver-mercury amalgam, the image — diffuse: globules from about 0.1 to 50 micrometres scatter light in every direction, so they look the same from anywhere
3. 3Eye off the mirror angle — no reflected beam reaches it; the silver reads black and the plate reads as a positive
4. 4Eye in the mirror angle — the whole reflected beam reaches it; the silver reads white and the plate reads as a negative

Virtually no light is absorbed anywhere on the plate, which is why a daguerreotype cannot be described as a pattern of densities the way a print can. It is a pattern of two different ways of returning light.

It resolves extraordinary detail. There is no grain in the sense a film has grain, no binder to scatter light and no paper fibre to interrupt the image; the AIC’s conservation page gives the image particles as ranging from 0.1 to 50 micrometres. The course has not verified any of the resolution figures that are quoted for daguerreotypes and does not repeat one.

It tarnishes, and it abrades. The exposed surface of the plate is the image, which is why the AIC’s guidance is that it must not be touched and why the historical convention is a cover glass held off the surface by a spacer with the edges sealed, the whole assembly in a case. Eder records Daguerre protecting his pictures in frames or cases under glass as early as 1839, after early plates kept in paper wrappings were damaged. Precisely which compounds the tarnish consists of is a conservation science question this course does not settle in Part I.

It is monochrome, and it is often not. Many surviving daguerreotypes were hand-coloured with dry pigment, and the AIC lists applied colour among the identification features.

!About this page's title

“A mirror with a memory” is usually credited to Oliver Wendell Holmes, writing about photography in the Atlantic Monthly in 1859. The course has not read that essay and cannot vouch for the wording or the date, so the phrase appears here as an apt description rather than as a quotation. It earns its place either way: the two halves of it name the two properties this section has spent five paragraphs on, and no other object in this course is both at once.

1839: the announcement, the pension and the patent

The chronology matters because it is quoted wrongly so often.

Date	Event
6 January 1839	The Gazette de France publishes a notice of the invention, without details
7 January 1839	Arago reports the invention to the Académie des sciences in Paris
8 March 1839	The Diorama burns down, taking Daguerre’s laboratory, his written records and most of his early work
14 June 1839	Preliminary agreement between Daguerre, Isidore Niépce and the Minister of the Interior, Duchâtel
3 July 1839	Arago presents the commission’s report to the Chamber of Deputies, which passes the bill
30 July 1839	The upper chamber passes it, 237 votes to 3
14 August 1839	Miles Berry applies for English patent No. 8,194 on Daguerre’s behalf
19 August 1839	Arago gives the full description at a joint session of the Académie des sciences and the Académie des beaux-arts

The terms of the purchase were a life pension of 6,000 francs a year to Daguerre and 4,000 to Isidore Niépce, half of each reverting to their widows, in exchange for a sealed package containing the history and a complete description of the invention, which Arago was to verify and which was not to be opened until the bill passed. Eder prints the agreement and the law.

!'Free to the world', with two asterisks

The phrase belongs to the occasion and is not quite true.

Asterisk one: England. Five days before the process was disclosed, Miles Berry filed an English patent for it as “a communication from a foreigner residing abroad”, naming Daguerre and Isidore Niépce as the inventors. Eder calls this a mark of Daguerre’s well-developed business acumen. The rights were bought by Claudet, and the AIC’s summary of the episode is that the French government offered the process patent-free to the entire world except England. Anyone in Britain who wanted to make daguerreotypes had to pay.

Asterisk two: the hardware. The Metropolitan Museum’s account notes that although Daguerre was required to reveal, demonstrate and publish detailed instructions for the process, he retained the patent on the apparatus needed to practise it. The chemistry was a gift; the camera was for sale.

This is the first time in the history of the medium that a technical disclosure and a commercial position are managed separately, and it will not be the last. Talbot’s patents, which arrive shortly afterwards and are argued about on his own page, are the other half of the story.

Why the course studies it and does not perform it

Three of the four reagents that make a daguerreotype have workplace exposure limits, which is a convenient way of saying that people who work with them professionally are monitored.

HSE’s EH40 lists mercury and its divalent inorganic compounds, measured as mercury, at a long-term limit of 0.02 mg per cubic metre of air; bromine at 0.1 ppm long-term and 0.2 ppm short-term; and iodine at a short-term limit of 0.1 ppm. Potassium cyanide, which entered later daguerreotype practice for fixing and cleaning, carries an aggregated GHS classification on PubChem as fatally toxic by every route. And EH40 carries a warning worth quoting in any course like this one: the absence of a substance from the list does not indicate that it is safe.

Those figures are not a hazard assessment. They are the reason one is needed, and the reason it is not attempted here: an exposure limit implies monitoring, and monitoring implies equipment and competence a domestic darkroom does not have. One property is worth stating because it is the one that defeats intuition — mercury is a liquid that gives off vapour at ordinary room temperature, and that vapour has neither colour nor smell, so nothing about the room tells you what is in it. The toxicology, the exposure routes, the controls a properly equipped laboratory would need and the full argument for the classification all belong to Part XXVI.

The Becquerel process is named here and not taught. It is a variant, listed by the AIC as dating from 1840, in which the exposed plate is developed by red light instead of by mercury vapour; the AIC records that it needs no mercury and no bromine or chlorine sensitising, and that the plates are around ten times slower in consequence, which makes portraiture difficult. It still requires iodine. Whether and how the course would ever classify it is Part XXVI’s decision, not this page’s.

Looking at daguerreotypes

The last thing on this page is an exercise you can do today, and it needs a screen rather than a darkroom. Several major collections publish daguerreotypes under open-access terms — the Library of Congress’s Prints and Photographs Division and the Metropolitan Museum’s open-access collection among them — and a screen shows you a good deal even though it cannot show you the one property that makes the object what it is.

Find three, and write a structured paragraph on each: the tonal scale, from the deepest shadow to the brightest highlight, and where in that scale most of the picture sits; the surface, including any tarnish, abrasion, cover-glass deterioration or applied colour; the handedness, looking for lettering, buttons, partings and rings that betray lateral reversal; and the condition, described in terms of what has changed rather than of how good it looks.

Then note what the reproduction cannot give you. A photograph of a daguerreotype has to choose one angle, and in choosing it, throws away the ambivalence that the section above spent five paragraphs explaining. What you are looking at is one frame of something the object does continuously in the hand.

Recap

• Daguerre was a showman and a lighting specialist, not a chemist, and the process bears the marks of it: a spectacular object rather than a convenient one.
• The plate is silver on copper, polished to a mirror, its surface converted to silver iodide by iodine vapour. There is no coating and no binder, which is why the detail is so fine.
• The discovery that mattered is the latent image: an exposure too short to see, made visible afterwards by a chemical operation. Mercury vapour was that operation, and Part XXVI owns it.
• Amplification cut the exposure from hours to minutes and then to seconds, and closed Part I’s sensitivity problem. Nothing optical could have done it.
• Fixing was a hot salt solution first and hyposulphite of soda within the year, on Herschel’s chemistry of 1819. Fizeau’s gilding of 1840 made the fragile plate durable.
• The image is a direct positive, unique, laterally reversed, and readable as positive or negative according to the angle, because it is a contrast between scattering amalgam and mirror-polished silver rather than between densities.
• It was announced on 7 January 1839 and described on 19 August, bought by the French state for two pensions and given to the world — except England, where it was patented five days before the description was published, and except the apparatus, which Daguerre kept.

Check your understanding

Question 1. Which single innovation in the daguerreotype closed the sensitivity gap that had defeated Wedgwood and Niepce?

• The use of a polished silver surface, which reflects more light onto the sensitive layer
• The use of silver iodide rather than silver chloride, which is intrinsically far more sensitive
• Development: an exposure too short to produce a visible change is amplified afterwards by a chemical operation
• The Petzval portrait lens, which admitted far more light than earlier lenses

Show the answer and why

Answer: Development: an exposure too short to produce a visible change is amplified afterwards by a chemical operation

The others all helped, and the lens genuinely mattered for portraits, but only development changes the kind of problem being solved. Printing out asks light to build a visible quantity of image substance, which needs roughly a million times more exposure than making a developable speck. Development asks light only to make the invisible speck and lets a chemical reaction supply the rest, so exposure falls by orders of magnitude at a stroke. Heliography, which has no amplification step, could not have reached camera exposures by any amount of optical improvement.

Question 2. Why does the same daguerreotype look positive at one viewing angle and negative at another?

• The image silver changes colour with the angle of illumination
• The image areas scatter light diffusely while the bare plate reflects it specularly, so what the mirror is facing decides how the shadows read
• The cover glass acts as a partial mirror and superimposes a reversed image
• The plate is coated with a thin film that produces interference colours

Show the answer and why

Answer: The image areas scatter light diffusely while the bare plate reflects it specularly, so what the mirror is facing decides how the shadows read

Virtually no light is absorbed by a daguerreotype, so the image cannot be a pattern of densities in the way a print is. Highlights are microscopic globules of silver amalgam that scatter light in every direction and therefore look the same pale tone from anywhere. Shadows are polished silver, which behaves as a mirror and shows you whatever it happens to be facing: hold something dark opposite it and the shadows go black, hold something bright and they go white and the picture inverts. This is why the cased presentation with its mat and its viewing angle is functional rather than decorative.

Question 3. Daguerre's 1839 manual states the purpose of the final operation as removing the iodine, which would otherwise go on decomposing in light and destroy the plate. Which earlier problem is he describing?

• Reciprocity failure
• The permanence problem: unexposed light-sensitive salt left in the picture continues to react
• Chemical fog from an exhausted developer
• Solarisation of the highlights

Show the answer and why

Answer: The permanence problem: unexposed light-sensitive salt left in the picture continues to react

It is the same problem Wedgwood and Davy stated in 1802 and could not solve, arriving at a completely different process by a completely different route. Any process that leaves unchanged light-sensitive material in the finished object has an image with a limited life, because the residue keeps responding to whatever light the picture is shown in. Daguerre's first answer, a hot saturated salt solution, only partly worked, for the same reason chloride treatment only partly worked on paper. Herschel's thiosulfate is what actually solved it, and Daguerre's own manual already recommends it as preferable.

Question 4. What was the practical consequence of an ungilded daguerreotype plate, and what did Fizeau's 1840 treatment change?

• The image faded in light; gilding made it insensitive to further exposure
• The image could be wiped off with a finger; gilding with a gold chloride bath improved both durability and appearance
• The plate tarnished within days; gilding sealed it against the atmosphere permanently
• The image was too low in contrast; gilding acted as a chemical intensifier only

Show the answer and why

Answer: The image could be wiped off with a finger; gilding with a gold chloride bath improved both durability and appearance

The AIC's conservation page states the fragility literally: an ungilded modern plate can have its image wiped off by a finger, because the image consists of loose particles sitting on a polished surface with nothing binding them. Fizeau's bath of hyposulphite of soda containing gold chloride was adopted almost universally because it addressed durability and tone together. The double salt formed in that bath was identified shortly afterwards by Fordos and Gelis and became, under the trade name sel d'or, the ancestor of the combined toning and fixing baths used for printing papers later in this course.

Question 5. The daguerreotype is often described as having been given free to the world. In what two respects is that not accurate?

• It was patented in France, and the plates could only be bought from one supplier
• An English patent was filed five days before the public description, and Daguerre retained the patent on the apparatus
• The pension had to be repaid, and the process was kept secret for a year
• Only the fixing step was published, and the sensitising was withheld

Show the answer and why

Answer: An English patent was filed five days before the public description, and Daguerre retained the patent on the apparatus

Miles Berry applied for English patent number 8,194 on 14 August 1839 as a communication from a foreigner residing abroad, naming Daguerre and Isidore Niepce; the rights were then bought by Claudet, and British practitioners had to pay. Separately, the Metropolitan Museum notes that although Daguerre was obliged to reveal, demonstrate and publish the process, he kept the patent on the equipment needed to work it. The generous gesture was real and so were both exceptions, which is why the course states the nuance rather than the slogan.

Question 6. Why does this course describe the daguerreotype in detail and give no procedure for it?

• Because the materials are no longer commercially available anywhere
• Because the process depends on mercury vapour and halogen vapours whose control requires a professional laboratory, so the course teaches it as chemistry and history and classifies it Level D
• Because the process is protected by patent
• Because the results are inferior to those of processes taught later in the course

Show the answer and why

Answer: Because the process depends on mercury vapour and halogen vapours whose control requires a professional laboratory, so the course teaches it as chemistry and history and classifies it Level D

Safety overrides historical authenticity: a procedure that needs engineering controls a domestic darkroom cannot provide is taught as chemistry and history rather than as an instruction, and that is what Level D means. The workplace exposure limits give a sense of the scale of the problem - mercury and its divalent inorganic compounds at 0.02 mg per cubic metre long-term, bromine at 0.1 ppm, iodine at 0.1 ppm short-term - and mercury vapour in particular is invisible, odourless and released from a liquid at room temperature. The full argument, and the classification of the mercury-free Becquerel variant, belong to Part XXVI.

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