Niepce and Heliography: Light That Hardens

A pewter plate 167 mm by 203 mm and about 0.15 mm thick, in the Harry Ransom Center at Austin, holds the earliest picture made in a camera that anybody can still go and look at. It contains no silver and no metal image at all. What is on it is hardened tar, and the parts of the tar that light never touched were washed away with a mixture of oil of lavender and a petroleum oil. Everything about that sentence points away from the chemistry the rest of this course is built on — which is exactly why it belongs here.

The problem Niépce was actually working on

Joseph Nicéphore Niépce was born at Chalon-sur-Saône on 7 March 1765 into a comfortable family with property in the surrounding villages, including Saint-Loup-de-Varennes, where the family estate at Le Gras stood. He was an inventor before he was anything else: with his elder brother Claude he designed an internal-combustion engine they called the pyréolophore, “a motor whose principle is air expanded by fire”, and obtained a patent for it on 20 July 1807 that protected it for ten years. The ten years that followed were spent trying, unsuccessfully, to make money from it.

His route into light-sensitive materials came through printing, not through chemistry. Lithography had arrived in France and Niépce took it up, and the practical obstacle was that he could not draw well enough to make the stones himself. What he wanted was a way to get an existing engraving onto a printing surface without an artist’s hand in between. Hold on to that, because it explains the shape of everything he did afterwards: his aim was a printing plate, not a picture. The camera views were a second line of work, and the one he found hardest.

By 1816 Claude had moved to Paris to promote the engine and Nicéphore was alone in Burgundy. The museum that holds his archive states his objective for that year plainly: to fix permanently, and without the intervention of a human hand, the images that form in the camera obscura.

1816: silver, and the wall he hit

He got images almost at once, and they had two faults he could not live with. They were negative — the museum’s chronology records that the values came out reversed, and Eder, reading the letters, notes that Niépce had to explain to his brother why the pictures looked so strange, with the objects also reversed left to right. And they were unfixed: they faded.

From Eder’s reading of the correspondence: on 5 May 1816 Niépce reports his first exposure in a miniature camera he had built himself, giving a negative image on white paper that he could not fix; Eder adds that he was most likely working with silver chloride. The museum’s chronology states the material directly as white paper coated with silver chloride, placed in the camera obscura, giving ephemeral differential effects. On 2 June he writes that it was impossible to find a substance of greater light sensitivity, and describes trying to fix images on metal plates with the aid of certain acids — finding that light did not noticeably influence the action of those acids. Letters of 16 June and 12 July show him continuing on stone, without success.

⁇Was it nitric acid?

The received account, repeated in most histories, says that Niépce tried to fix his 1816 silver chloride images with nitric acid and failed. The course could not confirm the specific acid from the sources it read. Eder, working directly from the Fouqué edition of the letters, says only “certain acids”, and describes the intent as etching the plate by the action of light rather than fixing a print. Nitric acid does appear in Niépce’s practice, unambiguously and repeatedly — but as the etchant that bites the metal where the resist has been washed away, which Hunt’s 1841 translation of Niépce’s own directions describes exactly.

What is not in dispute is the outcome, and the outcome is what the chemistry predicts. An acid strong enough to attack anything on that sheet attacks the image silver and leaves the silver chloride: the permanence page works through why. So this page states that Niépce tried acids as fixers, that they failed, and that the specific reagent is not settled by the sources here.

He kept looking. The letters of the same period show him trying an alcoholic solution of iron chloride that bleaches in sunlight and recovers in the shade; a paper coated with yellow iron oxide and fumed with chlorine; manganese dioxide; guaiacum resin, which turns green in light and then needs a stronger alcohol to dissolve it; and phosphorus, which he abandoned after burning his hands. Read that list as a research programme and its logic is clear. He is no longer looking for a substance that darkens. He is looking for a substance whose solubility light changes, because a solubility difference can be developed with a solvent and then used to protect a metal from acid. He had stopped trying to make a photograph and started trying to make a printing plate.

In 1818 he found one.

Bitumen of Judea

Bitumen of Judea, or asphaltum, is a naturally occurring asphalt: a dark, brittle, tarry solid that Niépce already had in the workshop, both because lithographers use asphaltum varnish and because he and Claude had burned it as fuel in the pyréolophore. It is not a compound. Mike Ware, who is a chemist, puts it bluntly: the molecular structure of bitumen is so complex and variable as to constitute a chemist’s worst nightmare. There is no formula to write, and this page does not invent one.

Niépce’s own working directions survive, and Robert Hunt translated them in 1841. Powdered bitumen half-fills a wine glass; essential oil of lavender is added drop by drop until the bitumen is saturated, then more until it stands a few millimetres above; the mixture is covered and gently warmed until the oil has taken up the colouring matter of the bitumen. The resulting varnish is applied cold to a highly polished plate with a light roll of very soft skin, thin and even, giving a vermilion tint. The plate is dried on gently heated iron wrapped in paper — and Niépce adds that the operator should hold a small metal disc in front of his mouth while doing it, to condense the moisture of his breath, because damp ruins the coating.

After exposure, he writes, only a very faint outline is visible. The picture has to be brought out, and here is the step that makes heliography a different kind of process from everything else in Part I:

The bitumen resist, in section, at four stages

1. 1Coat — bitumen of Judea saturated with oil of lavender, rolled thin onto polished pewter, glass or silvered copper, and dried
2. 2Expose — a varnished engraving in contact; the ink lines shade the coating, the clear paper passes the light
3. 3Develop with a solvent — one volume of lavender oil to ten of white petroleum removes only what light did not harden; a water wash follows
4. 4Etch, if the object is a printing plate — nitric acid attacks the bare metal and cannot reach the metal the hardened bitumen protects

Drawn to show the relationships, not to scale: the bitumen layer is a thin film, not a slab. Light makes the coating stay, so the process is negative-working, and a solvent is doing the work a developer does in every silver process in this course.

Hunt’s translation of Niépce gives the solvent as one part by volume of essential oil of lavender to ten of oil of white petroleum, a mixture that is milky at first and clears in two or three days, and which is exhausted when it goes dark brown and opaque with dissolved asphaltum. The plate goes in; the operator watches by reflected light as the forms slowly unfold; the plate comes out, is drained vertically, and is then washed by pouring water down an inclined board. Ware’s account of the process gives the same operation in modern terms: photohardening where the light falls, and washing away the soluble asphalt in the shadows with lavender oil and petroleum.

☠Described, not offered

The directions above are here because they are the historical record and because the mechanism is unintelligible without them. They are not an instruction, and this page is not a procedure: no quantity is given beyond the ratio Niépce himself published, no time or temperature is given for the drying, and nothing here has been tested by this course.

A reproduction would put three things on a bench that Part I never handles. A volatile essential oil and a petroleum distillate, both organic solvents, worked in quantity and warmed. And, for an etched plate, nitric acid, whose harmonised classification under the European CLP regulation as reported by PubChem carries the signal word Danger with H272, may intensify fire; H314, causes severe skin burns and eye damage; and H330, fatal if inhaled. Each of those needs its own current safety data sheet and its own hazard assessment before anyone goes near it, and neither exists in this part. Where the course does return to light-hardened coatings, in Part XXVI, it does so with that assessment written first.

⚗What 'hardening' means, and what this course will not claim

Two things are certain and one is not.

Certain: the exposed bitumen becomes insoluble in the solvent that dissolves the unexposed bitumen. That is an operational fact, demonstrated every time the process works, and it is reproducible today — Jean-Louis Marignier has investigated and repeated Niépce’s work, and the modern analyses of the surviving plates find hardened bitumen where the image is.

Certain: this is negative-working in the printing sense and positive-appearing in the pictorial sense, which is a distinction worth keeping straight. Light makes the coating stay, so the areas that received most light end up covered. On a bare polished plate those covered areas are the matt, pale ones and the bared metal is dark, so the picture reads as a positive.

Not certain here: the reaction. Marignier’s paper on the subject is titled “Asphalt as the World’s First Photopolymer”, and photo-induced cross-linking of the large aromatic molecules in the asphalt is the general shape of the explanation. But bitumen is a variable natural mixture with no single structure, this course has not read the peer-reviewed mechanism papers, and it will not paraphrase a title as though it were a mechanism. What you can state safely is the phenomenology: light insolubilises it, and no chemical amplification is involved — every photon that contributes has to do its own work on its own molecule.

Two families of photosensitivity

Everything in Part I so far has been about light reducing a metal salt. Heliography is the first member of the other family, and the split runs through the whole of photography.

	Light reduces a metal salt	Light hardens a coating
What light does	Frees an electron; a silver ion becomes a silver atom	Changes the solubility of an organic layer
What the image is made of	Metal, in nanoparticles or filaments	The coating itself, or whatever it later protects
What “development” means	A chemical reduction that amplifies an invisible change	A solvent that removes what light did not act on
Is there amplification?	Yes, once developing-out is invented: a few atoms become a whole crystal	No. What light did is all you get
Sign of the image	Darkens where light fell	The coating survives where light fell
Where it leads	The daguerreotype, the calotype, film, every silver process in this course	Photoresists, photolithography, photogravure, carbon and gum printing

The third row is the one that decides the nineteenth century. A silver halide crystal needs only a handful of silver atoms to become developable, and a developer then converts the entire crystal — which is why, as Silver salts and light puts it, printing out needs something like a million times the exposure a developed image needs. Bitumen has no such lever. Niépce could improve his lenses, his supports and his coating, and did all three; he could not change the fact that the process gives back only what the light put in.

Copying engravings, and the printing plate

The engraving copies came first and worked best, because contact printing gets the full strength of the sun and because a line engraving is exactly the kind of high-contrast subject a material with no tonal latitude can handle.

The museum’s chronology tracks the supports as he worked through them: glass plates in 1822, on which he made copies good enough to give away, including a portrait of Pope Pius VII; renewed attempts on lithographic stone in 1823, which he tried to etch so that the stone could serve as a printing matrix; copper in 1824 and 1825, where the etching finally worked and produced the plates now known by their subjects, among them a man leading a horse and a girl at a spinning wheel; and then in 1826 a switch to pewter, which he found gave much better results because the support was whiter. In May 1826 he named the invention: héliographie, sun-writing.

From 1825 he was corresponding with the Parisian engraver François-Augustin Lemaître, who pulled proofs from his plates and told him what was wrong with them, and with the optician Vincent Chevalier about lenses. That correspondence is the reason we can date the work at all.

🔬What the analysis found on the surviving plates

Three Niépce plates that went to the Royal Photographic Society’s collection in 1924, and the Le Gras plate at Austin, were examined by a team from the Getty Conservation Institute using X-ray fluorescence and infrared spectroscopy, and Ware summarises the results. The support of all four is “fine pewter”, an alloy of roughly 97% tin, 2% lead and 1% copper. The Le Gras image substance is confirmed as hardened bitumen. The Cardinal d’Amboise plate is also bitumen — but stereomicroscopy shows a deeply acid-etched metal surface underneath, so there the bitumen was doing its other job, as a photoresist for a printing plate, and photogravure ink pulls on paper are known from a nearly identical plate. Le Christ portant sa croix is only lightly etched and could not have been printed from.

The surprise was the fourth. Un Clair de Lune turns out not to be bitumen at all but a photohardened resinous gum whose infrared spectrum strongly resembles the physautotype — a process received history dates to 1832, five years after the plate. Ware notes the conflict and says that in the light of further research this episode may yet be rewritten. Analysis of the object is capable of overturning the documentary record, and here it may have done.

The View from the Window at Le Gras

The camera views were the hard case, and only one survives. The Harry Ransom Center describes it as the earliest photograph produced with the aid of the camera obscura known to survive: a heliograph on pewter, 16.7 × 20.3 × 0.15 cm, made by coating the polished plate with bitumen dissolved in oil of lavender, putting it in a camera obscura at a second-storey window at Le Gras, and leaving it. What came out was an impression of the courtyard, the outbuildings and the trees.

What it looks like is not a photograph in any sense a modern eye finds comfortable. It is a small grey-brown plate on which pale, matt, hardened bitumen stands against darker bared pewter, and it must be lit and angled correctly to read at all: turn it and the image collapses into a blank sheet of metal. Niépce knew this and wrote about it. Describing two landscape attempts on glass, he says that viewed by reflection on the varnished side, and at a certain angle, the effect is remarkably striking, while by transmitted light it is confused and shapeless. The light and shade in the picture are not pigment or metal; they are a difference in how two surfaces scatter light. Nothing else in Part I behaves this way, and the next process that does is the daguerreotype.

The subject, in the Harry Ransom Center’s description, is the courtyard, the outbuildings and the trees outside the window. Whatever the duration of the exposure — and it is disputed — it was certainly long enough for the sun to move a good way across the sky, so the plate does not record the lighting of a moment. It records the sum of every lighting condition that occurred while the shutter, such as it was, stayed open. How long that was, and what year it happened, are both unsettled.

⁇The date and the exposure

The date. The Harry Ransom Center, which owns the plate, dates it 1827 in its object record and in its exhibition text. Ware also gives 1827. The Musée Nicéphore Niépce’s chronology places the Point de vue du Gras under 1827, among the views Niépce decided not to etch because the half-tones could not be rendered. Many older accounts, and a great deal of secondary writing, give 1826, and some give the range “1826 or 1827”. This course writes 1827, because that is what the holding institution states, while noting that the evidence is circumstantial in every version: there is no dated inscription, and the argument runs through the correspondence and through which support Niépce was using when.

The exposure. The traditional figure is about eight hours. It has a real basis: Niépce’s own directions, as Hunt translated them, give camera exposures of six to eight hours varying with the intensity of the light, and four to six hours for a contact copy of an engraving. Against that, Jean-Louis Marignier reconstructed the process and the camera — finding that before 1828 Niépce used a lens of 325 mm focal length and 81 mm diameter, which is f/4 — and concluded that a practical exposure was about five days. The Harry Ransom Center’s own text says “several days”. Ware reports Marignier’s figure and gives the estimated speed of the bitumen process as less than 10⁻⁶ ISO.

This course reports both and combines neither. Niépce’s six-to-eight hours is a working figure from a practitioner describing what he did; Marignier’s five days is a reconstruction under stated optical conditions. They are not the same claim, and the honest position is that the exposure is not settled.

✕A number you cannot get by dividing

It is tempting to reconcile the figures arithmetically: take Ware’s estimate of under 10⁻⁶ ISO, take a modern film speed, divide, and see which exposure comes out. Do not trust the answer. The three-to-four-hour contact-printing figure, the six-to-eight-hour camera figure and the five-day reconstruction come from different geometries — full solar beam in contact, versus a lens at f/4 — and the sub-10⁻⁶ ISO estimate is itself derived rather than measured, from a process for which the ISO speed criteria were never designed. You can use them to establish an order of magnitude, and that is all.

▷Do the order-of-magnitude comparison anyway, honestly

Estimate how much longer a bitumen camera exposure is than a modern film exposure of the same scene, and then write down, as a numbered list, every assumption you had to make to get there. A serviceable answer needs at least four of them: the film speed you assumed and where it came from; the aperture you assumed for each camera, given that Marignier’s reconstruction puts Niépce’s lens at f/4 and a modern lens is commonly stopped well down from its maximum; whether the ISO speed criteria mean the same thing for a print-out resist as for a developing-out emulsion, which they do not; and whether reciprocity holds over an interval of days, which it does not.

The number you get is worth much less than the list. The point of the exercise is that the list is where all the uncertainty lives, and a figure quoted without it — of the kind you will find in most popular accounts of this plate — is a figure you cannot check.

The plate reached Austin by a long route. Niépce took heliographs to England in 1827 and left several with his host at Kew, Francis Bauer; three of them came to the Royal Photographic Society’s collection in 1924, and this one was separated from the group. The Harry Ransom Center purchased it in 1963 as part of the Gernsheim Collection, after what Ware describes as a five-year search by the photohistorians Helmut and Alison Gernsheim. The frequently quoted rediscovery date of 1952 the course could not verify from these sources, and does not assert.

Why heliography could not become photography

Four reasons, in descending order of how fatal they are.

No amplification. Every other objection follows from this one. The image is what the light built, photon by photon, with no chemical multiplier behind it.

Exposure measured in hours to days. No portrait is possible at that speed, and Niépce never made one. Nor is any subject that moves, or any light that changes — which is why the sun appears on both sides of the Le Gras courtyard. Even after Niépce got better lenses from Chevalier in 1828, Eder records exposures still lasting all day.

A viewing problem, not just a recording one. The picture depends on the angle you hold it at and the light you hold it in. It cannot be handed round, framed on a wall or reproduced by eye.

No tonal scale in the printing plate. This is the one that hurt him most, because printing was the goal. The museum’s chronology records repeatedly that he could not render the half-tones: the acid either bit or did not, and a continuous-tone camera view has no way of instructing it. In 1827 he gave up etching his camera views for exactly this reason. Turning a continuous tone into something a printing press can carry is a nineteenth-century industry in itself, and Part XXVI, on light-hardened colloids, is where this course picks the thread up.

England, Daguerre, and the last five years

In 1827 Claude fell seriously ill in England and Niépce went to him, discovering on arrival that his brother’s reported decade of work on the engine did not exist. Needing money, he spent four months trying to interest London’s scientific societies in heliography, and wrote for the purpose a Notice sur l’héliographie, dated at Kew in December 1827. It went nowhere. Eder’s account is specific about why: because the method was not disclosed, the Royal Society declined to hear the paper and never printed it. Niépce would not give up the secret and the Society would not take an undisclosed process, and both positions were reasonable.

Back in France, on Lemaître’s advice, he moved from pewter to silver-plated copper, and in 1829 he took a step whose consequence he did not live to see. To improve the contrast of the finished plate — because the picture is the difference between bright metal and pale varnish, and brighter metal makes a worse dark tone — he exposed the bared silver to iodine vapour, darkening it, and then dissolved the bitumen off with spirit of wine. Hunt, writing in 1841, saw where that led and said so: the practice “appears to have led the way to Daguerre’s beautiful process”. By 1829 Niépce was getting good results this way, and the exposure was still three to four hours in the sun.

On 14 December 1829, at Chalon-sur-Saône, he signed a ten-year association with Louis Jacques Mandé Daguerre, a Parisian painter and proprietor of the Diorama, who was charged with perfecting the invention. In June 1832 the two of them, working together in Burgundy, produced a different process they named the physautotype, in which the sensitive coating is not bitumen but the residue left when oil of lavender is evaporated, dissolved in alcohol and laid on in an extremely thin wash. Ware records Marignier’s finding that this residue is a resin — colophony, chiefly abietic acid — that light-hardens without any inorganic sensitiser, and that the resulting image is visible by differential light-scattering, rather in the manner of the daguerreotype. Daguerre’s own note, which Hunt prints, argues for the lavender-oil residue over bitumen on exactly these grounds, while conceding that even it is not entirely unaffected by prolonged sun.

Niépce died suddenly on 5 July 1833, aged 68, without having made his invention public. His son Isidore inherited the partnership, and where that goes is the daguerreotype’s page.

What heliography actually became

It is easy to file Niépce as a false start. He was not one; he was the start of a different line, and that line is enormous.

Look at the British patent abridgments for photography for 1877 to 1883, fifty years after his death, and the class is full of specifications that are recognisably his process: a metal plate coated with bitumen or asphaltum dissolved in benzene or turpentine, exposed under a negative in a printing frame, the unaffected coating cleaned off with a solvent, and the bared metal then etched with acid or built up in a galvanic bath. Change the resin, change the solvent, keep the logic, and you have photolithography and photogravure — and, a century later, the manufacture of every integrated circuit ever made, by exactly the sequence in the flow diagram above.

The other thing to take from this page is a habit of mind. Niépce did not solve the problem he set out to solve. He solved a neighbouring one, and the neighbouring one turned out to matter more than the original. That happens often enough in this subject to be worth expecting.

Recap

• Niépce’s goal was a printing plate made without an artist, which is why he cared about solubility rather than about darkening.
• In 1816 he made unfixed, negative silver chloride paper images in a camera and abandoned silver when acids failed to fix them. Which acid is not settled by the sources here; that acids fail for chemical reasons is settled.
• Bitumen of Judea, dissolved in oil of lavender and coated on polished metal or glass, becomes insoluble where light strikes it. Development is a solvent, not a reducer, and there is no amplification: what light did is all you get.
• That makes heliography the founding member of the hardening family of processes, whose descendants are photoresists, photolithography and photogravure, as against the metal-salt family that produced every silver process in this course.
• The View from the Window at Le Gras is on pewter, is dated 1827 by the institution that owns it, and took either six to eight hours or about five days depending on whose account you take. Both figures are reported here and neither is adopted.
• The process could not make a portrait, could not render half-tones for etching, and had to be held at the right angle to be seen at all. Niépce’s own move to iodine-darkened silver in 1829 is the bridge to what came next.

Check your understanding

Question 1. In heliography, what is the "developer" and what does it do?

• A reducing agent that converts exposed bitumen into metallic silver
• A solvent mixture that dissolves away the bitumen light did not act on, leaving the hardened bitumen behind
• An acid that etches the plate wherever light fell
• A hardening bath that fixes the exposed bitumen so that it resists the later etch

Show the answer and why

Answer: A solvent mixture that dissolves away the bitumen light did not act on, leaving the hardened bitumen behind

Niepce's "developer" is one volume of essential oil of lavender to ten of oil of white petroleum, and it is a solvent operation with no redox chemistry in it at all. That is the deepest difference between this family of processes and the silver ones: a silver developer amplifies an invisible change by reducing whole crystals, multiplying the effect of the original exposure by a very large factor, whereas a solvent can only reveal a difference light has already finished making. Nitric acid does appear in the workflow, but later and for a different purpose - etching the bared metal to turn the plate into a printing matrix.

Question 2. The View from the Window at Le Gras was exposed for somewhere between several hours and several days. What does that imply about the lighting recorded on the plate?

• Nothing: a long exposure records the average scene faithfully
• The plate records the sum of every lighting condition that occurred during the exposure, so its shadows and highlights are not the lighting of any single moment
• The plate records only the brightest moment of the exposure
• The plate records only the first few minutes, after which the bitumen is saturated

Show the answer and why

Answer: The plate records the sum of every lighting condition that occurred during the exposure, so its shadows and highlights are not the lighting of any single moment

A photographic exposure integrates: every photon that arrives while the plate is open contributes, whenever it arrived. Over hours the sun moves through a large arc and over days it repeats the whole cycle, so a wall lit at nine in the morning and a wall lit at four in the afternoon can both be recorded on one plate, and neither shadow pattern belongs to a moment you could have stood and watched. This is worth carrying forward: the same integration is what lets the pinhole camera in Part VII record a river as a smooth mass, and what makes a person who walks through a long exposure disappear.

Question 3. What single property of silver halide photography does bitumen lack, and what follows from that lack?

• Sensitivity to blue light, so bitumen could only be used in direct sunlight
• Chemical amplification of the exposure, so the exposure cannot be shortened by any subsequent processing
• The ability to form a negative, so no copies could be made
• Solubility in water, so the plates could not be washed

Show the answer and why

Answer: Chemical amplification of the exposure, so the exposure cannot be shortened by any subsequent processing

A silver halide crystal becomes developable once a few silver atoms have formed at a sensitivity speck, and a developer then reduces the whole crystal, so a very small amount of light can be turned into a large amount of image. Bitumen has no such lever: each molecule that hardens does so because a photon acted on it, and the solvent development can only remove what did not harden. That is why heliographic exposures are measured in hours and days, why a portrait was impossible, and why the process could never have become the basis of popular photography however good the optics got.

Question 4. The Cardinal d'Amboise plate and the Le Gras plate are both bitumen on pewter. What did microscopy show that distinguishes them, and what does it tell you about Niepce's purpose?

• The Le Gras plate is deeply etched, showing it was intended for printing
• The Cardinal d'Amboise plate is deeply acid-etched under the bitumen, showing the bitumen was serving as a photoresist for a printing plate
• Neither is etched; both were intended only to be looked at
• The Cardinal d'Amboise plate contains silver, showing a hybrid process

Show the answer and why

Answer: The Cardinal d'Amboise plate is deeply acid-etched under the bitumen, showing the bitumen was serving as a photoresist for a printing plate

Stereomicroscopy in the Getty Conservation Institute study found a deeply acid-etched metal surface beneath the bitumen of the Cardinal d'Amboise plate, and ink pulls on paper are known from a nearly identical plate, so that object is a photomechanical printing matrix rather than a picture. The Le Gras plate is not etched: it is the picture itself. The same coating on the same alloy served two entirely different ends, which is the clearest possible demonstration that Niepce's programme was about printing and that the camera views were the harder, secondary problem.

Question 5. In 1829 Niepce began fuming the bared silver of his silvered-copper plates with iodine vapour. What was he trying to achieve, and why does it matter to the rest of this part?

• To sensitise the plate to light, which is what iodine does in the daguerreotype
• To darken the bare metal so that it read as a deeper shadow against the pale bitumen, improving the contrast of the finished picture
• To fix the image by converting residual bitumen to a stable compound
• To etch the plate without acid

Show the answer and why

Answer: To darken the bare metal so that it read as a deeper shadow against the pale bitumen, improving the contrast of the finished picture

In heliography the image is a contrast between two surfaces, hardened bitumen and bare polished metal, and polished silver is a poor shadow because it reflects. Fuming it with iodine darkened it and deepened the tonal range. Niepce's purpose was cosmetic, not photographic; the silver iodide he was forming was a by-product he did not want for its light sensitivity. Hunt, writing in 1841, noted that the practice appeared to have led the way to Daguerre's process, in which the very same silver iodide layer is the sensitive material and the whole point.

Question 6. A modern conservator finds that a plate catalogued as an 1827 bitumen heliograph has an infrared spectrum matching photohardened resinous gum, not bitumen. What is the correct conclusion?

• The plate is a later forgery, because that material did not exist in 1827
• The catalogue date is wrong, because the physautotype was invented in 1832
• Either the received date of the physautotype or the date of this plate needs re-examination, and the analysis has raised a question the documents alone could not
• Infrared spectroscopy cannot distinguish these materials, so the result should be disregarded

Show the answer and why

Answer: Either the received date of the physautotype or the date of this plate needs re-examination, and the analysis has raised a question the documents alone could not

This is what happened with Un Clair de Lune, and Ware states the conflict without resolving it: the plate came from the 1827 gift to Francis Bauer, and its spectrum resembles the physautotype, which received history attributes jointly to Niepce and Daguerre in 1832. Both dates rest on evidence, so the analysis does not simply overturn one of them; it shows that the documentary chronology has a gap in it. The general lesson is worth more than the particular case: physical analysis of an object is independent evidence, and when it disagrees with the paper record the right response is to name the disagreement rather than pick the more familiar answer.

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