Wedgwood, Davy and the Problem of Permanence

Beginning at page 170 of the first volume of the Royal Institution’s journal, published in June 1802, is the first account anyone printed of making pictures with light. It carries, in the same few pages, a negative result that left the subject stalled for the next thirty years. Read carefully, though, the paper is not a failure report: it is an exceptionally clear statement of a problem, by two people who knew exactly what they had not solved and said so in print.

The paper, and who wrote which part of it

The full title is An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver, invented by T. Wedgwood, Esq., with Observations by H. Davy. Hunt cites the journal as the Journals of the Royal Institution; Ware and the Dictionary of National Biography both give it as the Journal of the Royal Institution of Great Britain, volume one, page 170. It matters that the title says invented by one man and observations by the other, because the two voices are distinguishable in the text and they were doing different things.

Thomas Wedgwood (born at Etruria Hall, Staffordshire, 14 May 1771; died at Eastbury, Dorset, 10 July 1805) was the third surviving son of the potter Josiah Wedgwood, in poor health for most of his short life, and the person who had the idea. Humphry Davy (1778–1829) was, at the time of writing, about twenty-three years old and at the beginning of the career that would make him president of the Royal Society and the founder of electrochemistry. Eder puts the ages of the two men at the time of publication at twenty-nine and about twenty-three, and is emphatic that although the work is routinely attributed to both, the credit for the conception belongs to Wedgwood.

⁇When were the experiments done, and was anyone earlier?

The date of the work. The publication is June 1802 and is not in doubt. When Wedgwood actually made the pictures is less clear: Mike Ware puts the first recorded contact images at around 1797 and describes the photograms as made about 1800, while the paper itself gives no dates at all. Wedgwood was in poor health throughout the period and the record is thin. This course therefore writes “the 1802 paper” for the document and “in the years before 1802” for the work, and does not give a year for the experiments.

The rival claim. Hunt reports that at about the same time the French physicist Jacques Charles proposed, in his Paris lectures, to make black profiles on a prepared paper by the action of light, and died without disclosing the preparation — adding that Charles’s own countryman the Abbé Moigno conceded that he left no authentic document to attest the discovery. Eder goes further and offers a deflationary explanation: Charles was librarian of the Institut de France, whose library carried the Journal of the Royal Institution, so he could simply have read Wedgwood and Davy.

What the course states. Wedgwood and Davy published the first account. Whether someone demonstrated something similar earlier and did not write it down is exactly the kind of question that cannot be settled, and the 1839 priority disputes in this part are full of it. An undocumented claim is not evidence, and it is also not a refutation; it is a gap.

🕰Reading a primary source: what to look for

This page quotes the 1802 memoir through Robert Hunt’s Researches on Light of 1854, which transcribes it at length. That is one remove from the original and the page says so wherever it matters. When you read any historical account of an experiment, four questions separate what was observed from what was concluded: what material was used; what was varied; what was measured, and with what; and what the authors themselves said they could not do. Wedgwood and Davy answer all four explicitly, which is rarer than it sounds.

What they made

The material is stated in the first line: white paper, or white leather, moistened with a solution of silver nitrate. Kept in the dark it does not change. In daylight it “speedily changes colour, and after passing through different shades of grey and brown, becomes at length nearly black”. They give the rate as a function of the light: two or three minutes in direct sun for the full effect, several hours in the shade. They give it as a function of colour too — red glass has very little action, yellow and green more, blue and violet “the most decided and powerful effects” — which is Scheele’s and Senebier’s ordering, arrived at independently and with the crudest possible instrument, a piece of coloured glass.

From that material they got four kinds of picture:

• Profiles. A shadow cast on the prepared surface leaves the covered part white while everything around it darkens. This is the photogram, and it is the same operation you carry out on the unfixed photogram page.
• Copies of paintings on glass. A painted glass panel laid on sensitised leather in the sun reproduces its light and dark passages. The paper notes that for this application the solution should be applied on leather.
• Objects that are partly opaque and partly transparent. Their examples are worth having: “the woody fibres of leaves and the wings of insects”, with sunlight passed through them onto the prepared leather. That is contact printing from a natural negative.
• Solar-microscope images. This one is Davy’s own contribution, marked as such in the text: he reports that the images produced by the solar microscope may be copied without difficulty on prepared paper, adding the condition that the paper must be placed only a small distance from the lens.

That last item is the key to the whole paper, and the reason it is not simply a list of successes.

What they could not make

Their stated objective was never the photogram. It was the camera:

The images formed by means of a camera obscura have been found to be too faint to produce, in any moderate time, an effect upon the nitrate of silver.

The next sentence says that copying those images was Wedgwood’s first object, that silver nitrate was suggested to him by a friend as a substance very sensitive to light, and that all his numerous experiments to that primary end proved unsuccessful.

Set the two results side by side and the reason is a matter of arithmetic, not of chemistry. In contact printing and under the solar microscope, the light reaching the sensitive surface is either the full beam of the sun or the full beam concentrated by a lens. In a camera obscura, the light from the scene is spread over the whole image and passes through an aperture that is a small fraction of the scene’s brightness — the ratio of hole diameter to screen distance sets it, and for the instruments of 1800 that ratio was small. Wedgwood was asking a material that needed minutes of direct sunlight to work in a few hundredths of that intensity. Davy’s condition on the solar microscope, that the paper must be close to the lens, is the same fact stated from the other side: he had found the one arrangement that concentrated enough light.

Why the leather beat the paper

The paper states, without explanation, that on leather the solution “is more readily acted on than when paper is used”. Everyone who has repeated the experiment has agreed. The interesting question is why, and the honest answer is that Wedgwood and Davy did not know.

The explanation this course carries is Mike Ware’s, who attributes it to the photohistorian Michael Gray. Silver nitrate is not appreciably light-sensitive by itself — that argument is made on Silver salts and light — and becomes so only when something is present that will accept the halogen or supply an electron. White kidskin of the quality used for these experiments was not tanned with tannic acid but tawed, cured with a solution containing aluminium chloride among other reagents. Residual chloride ions left in the leather react with the silver nitrate soaked into it to form silver chloride, in situ, in a matrix of protein. In other words, the leather was not a better support for silver nitrate; it was quietly converting the silver nitrate into a more sensitive salt.

Davy saw the second half of that argument without seeing the first. His observations include a direct comparison: in comparing the effects produced by light upon muriate of silver — silver chloride — with those upon the nitrate, “it seemed evident that the muriate was the most susceptible”, and both were more readily acted upon moist than dry. He adds that even in twilight, moist silver chloride spread on paper slowly changed from white to a faint violet, while the nitrate under the same conditions showed no immediate alteration. He had the more sensitive salt in his hands and identified it correctly. What nobody had was a reason to think that forming it inside the sheet, deliberately, would matter — which is what photogenic drawing paper does, and what makes it work.

Why the picture died

Now the sentence the paper is remembered for:

No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by Light have as yet been successful.

Read the rest of the paragraph, because it lists the attempts. They varnished the finished picture with a thin coating, which did not destroy its susceptibility to colouring. They washed it, and repeatedly: “even after repeated washings, sufficient of the active part of the saline matter will still adhere to the white parts of the leather or paper, to cause them to become dark when exposed to the rays of the sun.” Two rational strategies, both tested, both reported as failures. This is not an omission in the record; it is a negative result, published.

The mechanism is straightforward once you have photolysis. Wherever the shadow fell, the sheet is still fully loaded with unchanged, light-sensitive salt, and a room lit for human eyes still delivers photons at the wavelengths that salt absorbs. So the highlights go on darkening. The shadows cannot outrun them, because printing-out silver is self-limiting and they are already close to the ceiling. The picture is the gap between the two, and the gap closes from one side.

The picture, note, is never a substance. It is a difference between two areas of the same sheet, and a difference is spent when the two areas meet.

What the week does to an unfixed photogram

• Open ground, exposed in the original printing
• Protected area, kept in room light
• Protected area, sealed in the dark

Show the numbers behind this plot

Three curves against days. The open ground starts dark, near 4.2, and creeps slowly to about 4.8 because printing-out silver is self-limiting. The area protected by the object starts near paper white at 0.2 and climbs steadily past 3 by day ten to about 4.5 by day twenty-eight. The vertical gap between the two, which is the image, falls from about four units to under half a unit. A third curve, the identical sheet sealed in an opaque envelope, stays flat at 0.2 throughout.

Series	Days in ordinary room light	Tone, 0 = paper white, 5 = darkest the paper reached
Open ground, exposed in the original printing	0.00	4.20
Open ground, exposed in the original printing	1.00	4.30
Open ground, exposed in the original printing	2.00	4.38
Open ground, exposed in the original printing	5.00	4.50
Open ground, exposed in the original printing	10.00	4.62
Open ground, exposed in the original printing	16.00	4.70
Open ground, exposed in the original printing	22.00	4.76
Open ground, exposed in the original printing	28.00	4.80
Protected area, kept in room light	0.00	0.20
Protected area, kept in room light	1.00	0.55
Protected area, kept in room light	2.00	0.95
Protected area, kept in room light	5.00	1.90
Protected area, kept in room light	10.00	3.00
Protected area, kept in room light	16.00	3.70
Protected area, kept in room light	22.00	4.20
Protected area, kept in room light	28.00	4.50
Protected area, sealed in the dark	0.00	0.20
Protected area, sealed in the dark	7.00	0.21
Protected area, sealed in the dark	14.00	0.22
Protected area, sealed in the dark	21.00	0.24
Protected area, sealed in the dark	28.00	0.25

The two lit curves converge from one side only: the shadows cannot outrun the highlights, because print-out silver is self-limiting and they are already close to the ceiling. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Their workaround follows directly, and it is the ancestor of every safelight in the rest of this course. The finished copy must be kept in an obscure place; it may be examined in the shade, but only for a few minutes; and by the light of candles or lamps, as commonly employed, it is not sensibly affected. A candle flame is a low-temperature thermal source, weak at the violet end of the spectrum where silver chloride does nearly all its absorbing, and generous in the red and infrared where the salt is close to blind. They had discovered that the useful question is never “is this light bright?” but “does this material absorb the wavelengths this light emits?”. Part XI’s safelight tables are the same question with numbers attached, and the unfixed photogram puts a coloured filter over the same material so that you can rank the answer yourself.

What a fixer has to do

Before reading how the nineteenth century solved this, it is worth writing the specification yourself. A treatment that makes a silver image permanent has to satisfy five conditions at once, and every failed candidate below fails at least one of them.

It must remove or deactivate the unexposed silver salt, which is the whole point. It must leave the image silver alone — and image silver is a nanoparticle metal with a very large surface area, which makes it far easier to attack than a lump of the same metal would be. It must be removable in turn, because a treatment that leaves its own reactive residue in the paper has traded one slow decay for another.

And two things it must not do. It must not make the sheet worse to look at — no strong stain, no opaque deposit in the highlights. It must not require conditions the picture cannot survive: no boiling, no strong acid, no solvent that dissolves the support.

⚗Two routes, and they are not the same route

There are exactly two chemical strategies, and confusing them causes most of the muddle in popular accounts.

Take the salt away. Convert the residual silver halide into something soluble and wash it out. The sheet ends up containing image silver and nothing else photosensitive. Mike Ware’s monograph calls this removal by chemical dissolution; Herschel, who found the reagent for it, called it washing out.

Make the salt sleepy. Leave the residual halide where it is, but change its chemical environment so that it no longer responds readily to light. The sheet is still loaded; it has simply been persuaded not to react quickly. Ware’s account calls this modifying the nature of the residual unexposed silver halide, and contemporary conservators call it stabilisation.

The second is not a weaker version of the first. It is a different operation with a different failure mode: a stabilised print does not last because its silver was removed, it lasts because its silver was slowed down, and slowed down is a statement about a rate, not about an end point.

The candidates, and why each one failed

Reagent	What it does to the residual silver chloride	Which condition it fails
Water, repeatedly	Removes free silver nitrate; leaves the chloride, which is essentially insoluble	Does not remove the salt. Wedgwood and Davy’s own tested failure.
Varnish	Encapsulates the surface	Does not remove or deactivate anything; light passes straight through. Their second tested failure.
Strong sodium chloride solution	Converts free silver ions to more chloride; dissolves a little as [AgCl₂]⁻; leaves most of it in place, less reactive	Stabilises rather than fixes. Highlights fog to a lilac veil.
Potassium iodide solution	Converts chloride and free silver to silver iodide, which is very insensitive without a halogen acceptor	Stabilises rather than fixes. The image silver is then attacked by excess iodide, and the yellow iodide ruins printing.
Aqueous ammonia	Dissolves silver chloride as the diammine complex [Ag(NH₃)₂]⁺	Genuinely removes it, but is far weaker against bromide and iodide, is volatile and alkaline, and had already been done in 1777 without anyone noticing.
Nitric acid	Dissolves the image	Attacks precisely the wrong thing.

Strong salt solution

The reasoning is elegant and it half works. If a deficiency of chloride makes the paper more sensitive, an excess should make it less. Immerse the finished print in strong brine and two things happen. Any free silver ion is precipitated as more silver chloride:

Ag⁺ + Cl⁻ → AgCl

Excess chloride mops up the free silver

and a small proportion of the silver chloride itself goes into solution as a chloro-complex:

AgCl + Cl⁻ → [AgCl₂]⁻

Chloro-complexes, first step: real, and much too weak to be useful

[AgCl₂]⁻ + Cl⁻ → [AgCl₃]²⁻

Second step, where the chloride is in large excess

Ware is explicit that this second reaction is not sufficient to be useful in its own right, and the formation constant says why. OpenStax’s table gives the equilibrium Ag⁺ + 2 Cl⁻ ⇌ [AgCl₂]⁻ a formation constant of 1.8 × 10⁵. That is a real complex, but a modest one, and it is competing against a salt that is already among the least soluble things in the darkroom.

So the highlights of a salt-treated print still contain a great deal of silver chloride — now surrounded by excess chloride ions instead of excess silver ions, and much less light-sensitive in that state. Under strong light with some ultraviolet in it, some of it still reduces. The characteristic result is a rapid, slight deposition of silver in the highlights that Talbot called “pale lilac” and others have described as dull purple or mauve, followed by a much slower fogging that gives the impression of a light-fast image. That impression is the trap.

Potassium iodide

Iodide is the more aggressive of the two halide treatments, and it converts rather than dissolves. Free silver ions become silver iodide; and because silver iodide is even less soluble than the chloride, iodide will convert the chloride too:

AgCl + I⁻ → AgI + Cl⁻

Conversion of the residual chloride to the iodide

Silver iodide is quite insensitive to light in the absence of a halogen acceptor, so a print treated this way does not fog in the way a salt-treated one does — Ware notes that this is probably why the treatment was favoured at the outset. It fails at the other end. In the presence of excess iodide ions, the nanoparticle silver of the image is readily oxidised back to pale silver iodide by the oxygen of the air, faster in humid conditions and faster still in light. So an iodide-treated picture fades in the dark. On top of that, the residual silver iodide is yellow, and yellow absorbs exactly the blue and violet light you would use to print through the negative.

Ammonia

Ammonia is the one that genuinely dissolves the salt, and it was demonstrated a quarter of a century before Wedgwood picked up his first sheet of leather. Silver ion is a soft Lewis acid and ammonia a good ligand for it; the diammine complex is far more stable than the chloro-complex, with a formation constant OpenStax gives as 1.7 × 10⁷ for Ag⁺ + 2 NH₃ ⇌ [Ag(NH₃)₂]⁺, and that is enough to pull silver chloride into solution:

AgCl + 2 NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻

Silver chloride dissolving as the diammine complex

Carl Wilhelm Scheele performed exactly this operation in 1777, on silver chloride that had been blackening in the sun for a fortnight: the ammonia dissolved the unchanged chloride and left the black product behind. He was testing what the black substance was, and the significance passed him by; that story belongs to Silver salts and light, which owns it.

Why, then, is photography not fixed with ammonia? Three reasons, and they are all practical. Its grip on silver is far weaker than the reagent that eventually won, so the bath is slow and easily exhausted. It works much less well on silver bromide and hardly at all on silver iodide, which are the halides that fast photography would be built on. And it is a volatile, alkaline, pungent solution that swells gelatin and attacks paper sizing. It is the right idea in the wrong molecule.

Nitric acid

Nitric acid is the instructive failure, because it is a plausible move that gets the chemistry exactly backwards. Acids dissolve things; silver chloride is a stubborn deposit; therefore attack it with an acid.

But silver chloride is essentially untouched by nitric acid, while silver metal dissolves in it readily — that reaction is how silver nitrate is made in the first place, and it is the basis of the whole of Part I’s chemistry. Scheele’s own test is the cleanest demonstration: he took the black residue left after the ammonia treatment, dissolved most of it in nitric acid, and precipitated horn silver from the resulting solution. Something that dissolves in nitric acid and comes back as silver chloride when you add chloride is silver metal. Which means nitric acid removes the image and leaves the salt: the precise inverse of fixing.

Niépce tried acids on his own light images around 1816 and abandoned silver as a result. His page owns that episode. What belongs here is the general point: the reason the attempt failed is not that the acid was too weak or too strong, but that the image and the unwanted salt are different substances with opposite solubilities, and any reagent that cannot tell them apart in the right direction is useless however vigorous it is.

The one nobody reached for

In 1819, in the first volume of the Edinburgh Philosophical Journal, John Herschel published a long paper on the hyposulphurous acid and its compounds. In among the crystallography and the gravimetric analyses is this:

Muriate of silver newly precipitated is soluble in all the liquid hyposulphites, and, as has been before observed, in that of soda, with great ease and in large quantity.

That is the sentence. It was in print, in a well-read journal, seventeen years after Wedgwood and Davy said they had nothing, and twenty years before anybody applied it to a photograph. Herschel was not thinking about pictures; he was characterising a new class of salts. What the complex is, why it is so much stronger than any of the candidates above, what a fixing bath actually does to an emulsion and what it costs the image are all taught on the Herschel page, which owns them. The only claim this page makes is the historical one: the answer existed, published, for two decades before the question was asked in a form that could reach it.

!The moral, and it recurs

Twice in this part, the solution to the outstanding problem was already in the literature. Scheele dissolved unexposed silver chloride off an image in 1777 and did not notice. Herschel dissolved it more effectively in 1819 and had no image to try it on. What was missing both times was not the chemistry but the connection between two communities who were not reading each other. When you meet a problem in your own darkroom, the question “has someone in a neighbouring field already solved this?” is not idle.

What the survivors look like now

The distinction between stabilising and fixing is not academic, and the evidence sits in museum drawers. Ware records that halide-stabilised photogenic drawings survive to the present day — the technique was good enough to keep pictures for the better part of two centuries — and describes what survival looks like in each case. Chloride-stabilised sheets have highlights veiled in a slight purple or lavender fog of photolytic silver, which continues to deepen with light exposure and can eventually obliterate the detail it sits on. Iodide-stabilised ones have gone the other way: the mid-tone detail fades toward the uniform primrose-yellow of silver iodide, and it does so in the dark as well as in the light.

Two different chemistries, two different failure modes, both slow, and both visible in the object itself. This is why conservation departments care which treatment was used on a nineteenth-century print: the answer tells them which way the picture is going to fail and therefore how to store it. It is also why the vocabulary matters. Ware notes that Talbot and Herschel were precise about the distinction and that the later drift of the word “fixing” to mean only the thiosulfate route — with “stabilisation” left for the other one — has blurred what they meant. This course keeps them apart, as the terminology table in the authoring guide requires.

Why 1802 was not photography

Two things were missing, and the 1802 paper names them both without quite realising it has done so.

Sensitivity. A material that needs two or three minutes of direct sun to darken cannot record a camera image at all. Everything the paper achieved, it achieved by putting the sensitive surface into the strongest light available — the direct beam, or a solar microscope’s concentrated beam. That gap is closed by development: expose briefly to make an invisible change, then amplify it chemically. Daguerre is where that first works.

Permanence. The image, once made, is a difference that keeps eroding. That gap is closed by Herschel’s thiosulfate.

The last sentence Davy contributed to the memoir is the fairest summary anyone has written of the state of the art in 1802: nothing but a method of preventing the unshaded parts of the delineation from being coloured by exposure to the day is wanting, to render this process as useful as it is elegant. He was wrong only in the word nothing — the sensitivity problem was equally fatal, and he had reported it himself two paragraphs earlier. But as a statement of what to work on next, it is exact, and it is why this course organises the whole part around three problems rather than a list of inventors.

Recap

• The 1802 memoir is Wedgwood’s invention with Davy’s observations, published in the first volume of the Journals of the Royal Institution in June 1802, and this page reads it through Hunt’s 1854 transcription.
• It succeeded at contact printing and at the solar microscope, and failed at the camera obscura, for a reason that is about light intensity rather than about chemistry.
• Leather beat paper because the tawing process left chloride in it, so the silver nitrate became silver chloride in place. Davy identified the chloride as the more sensitive salt without drawing that conclusion.
• The picture died because the unexposed salt stayed in the sheet, so the highlights kept darkening until they met the shadows. Candlelight was safe to view by because a candle is weak where silver chloride absorbs.
• A fixer must remove or deactivate the residual salt, spare the image silver, and be washable out. Salt and iodide only deactivate; ammonia genuinely dissolves but too weakly and only for the chloride; nitric acid dissolves the image instead.
• Herschel had published, in 1819, that freshly precipitated silver chloride dissolves readily and copiously in hyposulphite of soda. Nobody connected it to a picture until 1839.

Check your understanding

Question 1. Wedgwood and Davy copied images from a solar microscope without difficulty but could not record a camera obscura image at all. What does that pair of results establish?

• That silver nitrate is sensitive to the light of a lens but not to daylight
• That their failure was a matter of the intensity of the light reaching the sensitive surface, not of the chemistry of the material
• That the camera obscura they used was optically faulty
• That the solar microscope produced ultraviolet which the camera obscura did not

Show the answer and why

Answer: That their failure was a matter of the intensity of the light reaching the sensitive surface, not of the chemistry of the material

The same material, the same operator and the same season gave a picture in one arrangement and nothing in the other. What differed was how much light per unit area arrived: the solar microscope concentrates the direct solar beam through a lens, while a camera obscura spreads the light of a whole scene over the whole image and admits only the fraction its aperture passes. Davy even states the operating condition - the paper must be placed only a small distance from the lens - which is the same fact written as an instruction.

Question 2. Why was white leather more sensitive than white paper when both were soaked in the same silver nitrate solution?

• Leather is smoother, so the solution formed a thinner and more even layer
• The protein in leather is a stronger reducing agent than cellulose
• The leather was tawed with a preparation containing chloride, so the silver nitrate was converted in place to the more sensitive silver chloride
• Leather retains more moisture, and moisture is the only variable that matters

Show the answer and why

Answer: The leather was tawed with a preparation containing chloride, so the silver nitrate was converted in place to the more sensitive silver chloride

This is Michael Gray's explanation as reported by Mike Ware, and it turns a puzzling observation into an ordinary one. Fine white kidskin was cured by tawing rather than tanning, using a solution containing aluminium chloride; residual chloride ions in the leather meet the silver nitrate and precipitate silver chloride inside the material. Davy independently reported that silver chloride is the more sensitive of the two salts, so the pieces were all on the table - what was missing was the idea of forming the chloride in the sheet deliberately, which is what photogenic drawing paper does.

Question 3. A photogram is treated with a strong solution of ordinary salt and then dried. A year later its highlights have gone a pale lilac. What happened, and what does it tell you about the treatment?

• The image silver has oxidised; the treatment removed the silver but not the image
• Residual silver chloride is still present and has been slowly reduced by light; the treatment stabilised the sheet rather than fixing it
• The salt itself has crystallised out and is scattering light
• The paper has yellowed with age and the effect has nothing to do with the treatment

Show the answer and why

Answer: Residual silver chloride is still present and has been slowly reduced by light; the treatment stabilised the sheet rather than fixing it

A chloride bath precipitates free silver ions as more silver chloride and dissolves only a small proportion of the chloride as [AgCl2]-, whose formation constant of about 1.8 x 10^5 is far too modest to strip the sheet. What it does achieve is to surround the remaining crystals with excess chloride instead of excess silver, which makes them much slower to react. Slower is not stopped: strong light with some ultraviolet in it still deposits nanoparticle silver in the highlights, and small silver particles read as violet. That is the difference between stabilising and fixing, in one visible symptom.

Question 4. Nitric acid was tried as a fixer and failed. Which statement best explains why the attempt was doomed rather than merely badly executed?

• Nitric acid is too dilute to attack a solid, whatever its concentration
• Nitric acid dissolves silver metal readily and silver chloride hardly at all, so it removes the image and leaves the light-sensitive salt
• Nitric acid converts silver chloride to silver nitrate, which is even more sensitive
• Nitric acid destroys the paper before it can reach the silver

Show the answer and why

Answer: Nitric acid dissolves silver metal readily and silver chloride hardly at all, so it removes the image and leaves the light-sensitive salt

A fixer has to discriminate between two silver-containing substances in one sheet and remove exactly one of them. Nitric acid discriminates perfectly well - in the wrong direction. Scheele had already demonstrated both halves: he dissolved the black photolytic product in nitric acid and recovered horn silver from the solution by adding chloride, which is how he proved the black product was metallic silver. Any reagent with that selectivity is a bleach, and Part XX uses reagents of exactly this kind deliberately, to bleach a print before redeveloping it.

Question 5. Herschel published in 1819 that freshly precipitated silver chloride dissolves with great ease and in large quantity in hyposulphite of soda. Why did that not immediately solve photography's permanence problem?

• Because hyposulphite of soda could not be manufactured in useful quantities until the 1830s
• Because Herschel was studying a class of salts, not making pictures, and the people making pictures were not reading that literature
• Because the reaction only works on freshly precipitated chloride and not on an exposed print
• Because Herschel kept the result secret until 1839

Show the answer and why

Answer: Because Herschel was studying a class of salts, not making pictures, and the people making pictures were not reading that literature

The paper is a piece of inorganic chemistry: crystal forms, analyses, the behaviour of a newly characterised acid and its salts. The sentence about silver chloride is an incidental property, not a proposal. Wedgwood was dead by 1805 and the people who might have connected the two bodies of work were in different rooms. The lesson generalises: a solved problem in a neighbouring field is invisible until somebody reads both literatures, which in 1839 Herschel himself finally did.

Question 6. A conservator examines two photogenic drawings from the 1830s. One has lavender-fogged highlights; the other has mid-tones fading towards a uniform pale yellow, and has been kept in a dark box throughout. What treatment was each most likely given?

• Both were fixed with thiosulfate; the difference is the paper stock
• The first was stabilised with chloride, the second with iodide
• The first was stabilised with iodide, the second with chloride
• The first was fixed with ammonia, the second was never treated at all

Show the answer and why

Answer: The first was stabilised with chloride, the second with iodide

The two treatments fail in opposite directions and leave different signatures. Chloride stabilisation leaves silver chloride in the highlights, which slowly photolyses to nanoparticle silver and reads as a purple or lavender veil - a light-driven failure. Iodide stabilisation converts the residue to silver iodide and leaves excess iodide in the sheet, and in that environment the image silver is oxidised back to pale yellow silver iodide by atmospheric oxygen, which happens in the dark as well as in the light. The dark storage in the question is the clue that distinguishes them.

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