Talbot: Photogenic Drawing, the Calotype and the Negative-Positive Principle

A daguerreotype is one plate and there will never be another. The photogram you exposed on ordinary paper is one sheet, and it is quietly destroying itself. Niépce got as far as multiplicity, but only by leaving photography altogether and etching his plate so a printer could ink it. This page is about the man who made a photograph whose whole purpose was to make other photographs, and about why that turned out to matter more than anything else in the decade.

Como, October 1833: a bad draughtsman with an idea

William Henry Fox Talbot was born on 11 February 1800 and died at Lacock Abbey in Wiltshire on 17 September 1877; the Dictionary of National Biography records a twelfth wrangler at Cambridge, a Member of Parliament for Chippenham, and fifty years of papers on mathematics, physics, astronomy, chemistry and archaeology. In October 1833 he was on the shores of Lake Como with a camera lucida, and by his own account in The Pencil of Nature he was failing with it: when he took his eye from the prism, “the faithless pencil had only left traces on the paper melancholy to behold.”

He had tried the other way round on earlier Italian journeys, in 1823 and 1824 — a camera obscura throwing its image onto tracing paper laid on a pane of glass — and found that the pressure of the hand shifted the instrument, and that the detail defeated his patience. Then comes the sentence the whole medium turns on: how charming it would be if it were possible to cause these natural images to imprint themselves durably, and remain fixed upon the paper.

Note what he did next, because it is the difference between a wish and a research programme. He was in Italy and could not start, so he wrote the idea down together with the experiments he thought most likely to realise it. He had read that silver nitrate was peculiarly sensitive to light. He had never seen the experiment done, and says so — he did not know whether the action was fast or slow, “a point, however, of the utmost importance, since, if it were a slow one, my theory might prove but a philosophic dream.”

1834: the heresy of too little salt

He returned to England in January 1834 and worked through the obvious things, all of which failed. Silver nitrate brushed on paper: very slow. Silver chloride precipitated and spread on the paper while moist: no better, turning slowly to a darkish violet. Salt first, then silver nitrate: “almost the same as before”.

The discovery came out of sloppy brushwork. Sometimes the brush did not cover the whole sheet, and certain patches — generally near the edges of the brushed area — blackened much faster than the rest. Talbot reasoned that those borders had taken up less salt, tested it with a much weaker salt solution, and got a sheet whose whole surface turned black uniformly and rapidly. His own summary is the entire chemistry of the page: “a lesser quantity of salt produced a greater effect.”

⚗Why that is heresy, and why it works

Precipitating silver chloride is the reaction the silver salts page introduces:

AgNO₃ + NaCl → AgCl + NaNO₃

Silver chloride precipitated inside the paper fibres

Mike Ware works out what orthodox stoichiometry demands: to convert silver nitrate to silver chloride economically and completely, the silver nitrate needs just over one third — 34.4 % — of its own weight of salt. Prepare a paper at that ratio and, in Ware’s words, the attempt fails dismally: the surface goes lilac and then stops.

Talbot’s proportions are roughly one twentieth. Ware reads his Notebook P as implying about 1 % w/v salt, and reads “not saturated, but six or eight times diluted with water” as about 20 % w/v silver nitrate, against a saturated solution of around 150 % w/v. So every grain of silver chloride formed in the paper sits in a bath of unreacted silver nitrate, and Ware identifies that combination — the crystal plus the free silver around it — as the thing that prints out a strong image. The silver salts page explains why: photolysis stalls unless something takes up the liberated halogen, and excess silver ions in the wet paper let water do exactly that.

🧮Worked: how much silver chloride from a gram of salt

Take the stoichiometric case, so that you can see how far Talbot is from it. Relative molecular masses from PubChem: sodium chloride 58.44, silver nitrate 169.87, silver chloride 143.32.

n = m / M

Moles from mass

where n is the amount in moles, m the mass in grams and M the relative molecular mass in grams per mole. So 1.00 g of sodium chloride is 1.00 / 58.44 = 0.01711 mol. The equation above is one to one, so that consumes 0.01711 mol of silver nitrate — 0.01711 × 169.87 = 2.91 g — and yields 0.01711 mol of silver chloride, which is 0.01711 × 143.32 = 2.45 g.

Turn it round: 1.00 g of silver nitrate needs 1.00 / 169.87 × 58.44 = 0.344 g of salt, which is Ware’s 34.4 %. Talbot supplied about a twentieth, 0.05 g. That converts 0.05 / 0.344 = 0.15 g of the silver nitrate and leaves the other 0.85 g sitting in the paper unreacted — so roughly one part in seven reacts and six parts do not.

He was not making silver chloride efficiently. He was making a little silver chloride and deliberately leaving most of the silver nitrate behind, and the leftover is not waste but half the mechanism.

Photogenic drawing paper, in section

1. 1Cellulose fibres with gelatin sizing — the support and the binder are the same thing
2. 2Silver chloride crystals — formed in place where the two solutions met
3. 3Unreacted silver nitrate — about six parts in seven of what was brushed on; the halogen acceptor
4. 4Untreated paper below — the sensitive zone is shallow, which is why the sheet prints through

Drawn to show the arrangement, not to scale. Compare the layered structure of a modern resin-coated paper on the unfixed-photogram page: there the emulsion is a separate film sitting on a sealed base, and none of it is inside the paper.

By the spring of 1834 he was laying leaves, lace and other flat objects on the dried sheet under a sheet of glass pressed down tight, putting it in the sun until the open ground went dark, and carrying it into the shade. Ware notes that Talbot first called these contact images sciagraphs, shadow-drawings, before settling on photogenic drawings.

1835: the window, and the mousetraps

The camera was the hard case, and Talbot says why in one sentence: an hour or two in the camera obscura gave a marked outline of roof and chimneys against the sky and left the shaded parts blank or nearly so.

In the brilliant summer of 1835 he attacked it with repeated alternate washes of salt and silver and used the paper moist, and cut a bright-day camera exposure to ten minutes. The pictures, he adds, were “very pretty” but “quite miniatures”. Ware puts numbers on the miniature: the famous first negative is 36 × 28 mm, made in one of the small boxes his wife Constance later called mousetraps, with a lens of around f/4. Ware lists six conditions Talbot had to satisfy at once — excess silver, a small format, a wide aperture, patience, a bright subject rich in blue and ultraviolet, and some way of stopping the image changing — and observes that neglecting any one of them makes the attempt fail.

🕰Latticed Window at Lacock Abbey, August 1835

The National Science and Media Museum, which holds it, describes it as the earliest known surviving negative. It is a negative — the panes are dark and the leading light — and it is the size of a large postage stamp. Talbot told the Royal Society in January 1839 that in the summer of 1835 he made “a great number of portraits of a house in the country of ancient architecture”, several of which he exhibited that evening. The house was his own.

The Getty Conservation Institute’s atlas goes further, calling it the earliest photographic image on a paper substrate still in existence — but the same atlas, three paragraphs earlier, says the oldest photochemically produced images on paper still extant appear to be Hercules Florence’s pharmaceutical labels of 1833–34. Both sentences cannot be right as they stand. The museum’s narrower claim is the one this course repeats, and the many-inventors page takes Florence up.

Stabilising is not fixing, and Talbot knew the difference

Talbot’s two treatments came out of the same insight as his sensitiser, run backwards. If a deficiency of salt makes paper sensitive, an excess should make it dull. Ware’s chronology dates Talbot’s discovery of the excess-silver sensitiser and his first fixer, potassium iodide, to the same moment in June 1834, with a strong sodium chloride solution following on 8 February 1835.

Both leave the silver salt in the sheet. Ware records that Talbot and Herschel, who were precise about language, called the halide treatments fixing and called Herschel’s thiosulfate something else entirely — washing out — and that the modern habit of using “fixing” for the second and “stabilisation” for the first has blurred a distinction they intended. The chemistry of why each halide treatment fails, and the museum evidence of how the survivors have aged, belongs to Wedgwood, Davy and the problem of permanence, which owns it. What belongs here is Talbot’s own experience of the failure, which he printed:

after the lapse of some days the dark parts of the picture begin to fade, and gradually the whole picture becomes obliterated, and is reduced to the appearance of a uniform pale yellow sheet of paper.

That is iodide fixation failing, described by the man who invented it, and his verdict is that the process “must be considered as not sufficiently certain to be retained in use”. He kept using halide fixation anyway, for years after a better answer existed, and Ware gives the reason: hypo was costly and scarce at first, and — this is the part that surprises people — Talbot preferred the colour. A chloride-fixed photogenic drawing has lilac highlights and deep maroon shadows; a thiosulfate-fixed one has clear white highlights and a reddish-brown image. He liked the first.

January and February 1839: six weeks that decided the shape of the medium

Arago announced Daguerre’s invention to the Académie des sciences on 7 January 1839, and the daguerreotype page tells that side of it. Talbot, who had been sitting on five years of work and drafting an account he never sent, moved fast. Ware’s chronology gives the sequence, and the DNB corroborates the January dates:

Date, 1839	What happened
25 January	Faraday shows Talbot’s photogenic drawings at the Royal Institution
31 January	Talbot reads “Some Account of the Art of Photogenic Drawing” to the Royal Society — no working details
1 February	Herschel reveals thiosulfate “washing out” to Talbot, having first tried it on 29 January
19 February	Talbot writes to Herschel describing his own fixers: potassium iodide and sodium chloride
21 February	Talbot discloses the method to the Royal Society, in a letter to its Secretary
21 March	Talbot reports that silver bromide paper is very sensitive

The 31 January paper is worth reading for what it does not say. It states that the two difficulties which defeated Wedgwood and Davy were sensitivity and permanence; it claims both are solved; it mentions that some pictures have been left in sunshine for an hour without injury; it says the paper can be made visibly affected by full sunlight in half a second. It gives no recipe. The recipe came three weeks later, after the Fellows asked for it, and Ware notes that the 21 February letter describes photogenic drawing paper and fixation by sodium chloride or potassium iodide, with no mention of Herschel’s hyposulphite of soda at all — twenty days after Herschel had shown it to him.

September 1840: an exposure too short to see

Everything so far is a printing-out process: light does the whole job, you watch the picture arrive, and you stop when it looks right. James Reilly’s standard account of these papers puts the cost of that in one figure — printing out needs more light energy than developing out, “in some cases 100,000 times more” — and adds the consequences: you cannot enlarge onto such a paper, so every print is a contact print.

In the days following 20 September 1840 Talbot found the way round it. Ware dates the invention of the calotype to 23 September. Talbot described it to the Royal Society on 10 June 1841, in three stages he called iodising, exciting and bringing-out, using four chemicals and no more: silver nitrate, potassium iodide, acetic acid and gallic acid.

The calotype, from Talbot's own 1841 description

1. 1IodiseBrush the sheet with silver nitrate, dry it, dip it in potassium iodide for two or three minutes, rinse and dry. The result is a pale yellow coating of silver iodide that Talbot says is "scarcely sensitive to light" and keeps indefinitely in the dark.
2. 2ExciteShortly before use, by candlelight, brush on gallo-nitrate of silver: equal volumes of an acetic silver nitrate stock and a saturated solution of gallic acid, mixed fresh because the mixture does not keep. Rest half a minute, rinse, dry.
3. 3ExposeOne second in dull winter daylight is enough to leave a strong impression. Talbot gives one minute in summer at an aperture of one inch on a fifteen-inch lens, and one second at f/3 for a white plaster bust.
4. 4Bring outWash again with gallo-nitrate and warm gently. In a few seconds the exposed part begins to darken and finally grows entirely black, while the rest stays white. Stop when it looks right.
5. 5FixWash with water, blot, wash with potassium bromide (100 grains in eight or ten ounces), dip in water, dry. The negative stays transparent, "and that, therefore, there is no difficulty in obtaining a copy from it".

Quantities and times as Talbot published them in the Proceedings of the Royal Society in 1841. They are recorded here as history; the course does not ask you to make one, and the actionable salted-paper process waits for Part XXII, with the laboratory discipline of Part II in place.

The important sentence in the whole paper is the one about the invisible picture:

But the impression is latent and invisible, and its existence would not be suspected by any one who was not forewarned of it by previous experiments.

And in his closing experiments he says it again, in the language of a man reaching for a word that does not exist yet: dry paper “receives a virtual instead of an actual impression from the light, which it requires a subsequent process to develope.”

⚗What kind of development this is

Ware classifies it precisely, and the distinction runs through the rest of the course. Gallic acid alone can develop the image, using only the silver already in the sheet: that is chemical development. Talbot’s gallo-nitrate carries silver nitrate as well, so the developer deposits additional silver from solution onto the sites light created: that is physical development.

The practical consequence is one Talbot exploited and no modern printer can: a calotype negative worn out by repeated printing could be revived by another wash of gallo-nitrate and a gentle warming, and he reports that reviving one sometimes brought out details never seen before, “having been latent all the time”. A negative you can put more silver into is a negative you can repair. Part IV takes the latent image apart properly, and Part V returns to the two kinds of development.

The gain was about a hundredfold. Ware puts the calotype’s first camera exposures at about half a minute at f/4 in bright sun against the hour or more photogenic drawing needed, and Talbot claimed his paper’s sensitivity “transcends a hundred times or more that of any kind of photographic paper hitherto described”. Print-out on paper had reached its ceiling in 1835 and stayed there; development moved it in a fortnight.

The commercial consequence follows directly, and Ware names it: paper photography at last became a viable practice, “even for portraiture”, with a commercial potential that would eventually compete with and displace the daguerreotype. A sitter can hold still for half a minute. A sitter cannot hold still for an hour, which is why photogenic drawing produced buildings and leaves and no faces at all. Every portrait studio of the 1840s, on either process, exists because somebody found a way to stop light doing the whole job.

The idea the rest of the course rests on

The negative was there from the beginning, as a nuisance. Ware quotes Talbot’s Notebook M of 1835:

In the Photogenic or Sciagraphic process if the paper is transparent the first drawing may serve as an object to produce a second drawing, in which the lights and shadows would be reversed.

Ware’s chronology dates that thought to 28 February 1835 — before the window negative, and five years before Herschel supplied the words negative and positive for it in his 1840 paper, which the Herschel page owns.

Talbot explains the operation to a general reader in The Pencil of Nature, under Plate VII, a leaf: the first process gives a white image on a darkened sheet; that sheet is washed with a fixing liquid; it is then laid on a second sheet of sensitive paper, pressed into close contact and put in the sun, “this second process is evidently only a repetition of the first”. The second sheet comes out with the ground white and the image dark.

One original, many prints

1. 1Camera negativeExposed and developed once. Lights dark, shadows light, and laterally reversed.
2. 2WaxedTalbot patented hot beeswax treatment in 1843 to make paper negatives translucent, so that more light gets through and the fibres show less.
3. 3Contact frameNegative face down on sensitive paper, board below, glass above, pressed together with screws.
4. 4Printed out in the sunThree to thirty minutes on salted paper, judged by eye. Repeat as often as you like.
5. 5Salted paper printsPositive, right way round, and identical apart from the weather and the paper.

Talbot's own workflow. The waxing step is his, dated by the patent of 1 June 1843; Le Gray's waxed-paper process of 1851 is a different and later thing, in which the paper is waxed before sensitising, and this course does not conflate them.

His own claim for it is in the caption to the last plate of The Pencil of Nature:

The number of copies which can be taken from a single original photographic picture, appears to be almost unlimited, provided that every portion of iodine has been removed from the picture before the copies are made.

Read the proviso as chemistry, because it is. He explains it himself: solar light and a trace of iodine acting together, though neither alone, decompose silver and form a colourless iodide. An iodide-fixed negative that has to survive thirty printing exposures in full sun is a negative being slowly bleached by its own fixer. That is the practical reason the negative-positive principle eventually needed Herschel’s thiosulfate and not Talbot’s halides — and Ware notes that Talbot’s own Reading prints, more than ten thousand of them, were thiosulfate-fixed even while he was still halide-fixing his negatives.

📷What a reproducible original actually buys you

The Getty atlas illustrates the point better than any argument: its figure 4 is six versions of Talbot’s The Open Door laid out together on a light table, on sheets carrying several different watermarks. Six of the same picture, and not one of them is the original.

Nothing else in Part I can do that. A daguerreotype is unique; Herschel’s own 1840 paper says so, noting that Daguerre’s pictures were understood to admit of no reproduction, and treats the possibility of “indefinitely multiplied fac-similes” as the more important line of inquiry precisely because it alone makes publication possible. Every printing technology the rest of this course touches — enlarging, contact printing, editions, the very idea that a photograph has a negative somewhere — descends from this.

The patent, and what it cost

Talbot took out four patents in the decade after the calotype, and Ware lists them: No. 8,842 of 8 February 1841, “Improvements in obtaining Pictures or Representations of Objects”; No. 9,753 of 1 June 1843, which covers hot thiosulfate fixation and the waxing of paper negatives; and two more, in 1849 and 1851.

The 1841 patent did immediate damage that had nothing to do with money. Ware records that the Royal Society refused to print the calotype paper in its Philosophical Transactions on the ground that Talbot had already made the method public in his patent specification; after weeks of angry correspondence and Herschel’s intervention it compromised on the less prestigious Proceedings, and Talbot issued the text privately as a pamphlet. For three years afterwards, Ware writes, the process “stagnated, transfixed by the patent”, because the patent inhibited publication of any improvement.

🕰The Scottish exception

There is one place the patent did not reach. Ware states it plainly: “The patent did not hold sway in Scotland, however, where Talbot had not registered his claim, so it was from here that the first, tentative chemical advances were made.”

The St Andrews group — Dr John Adamson, the chemistry professor Arthur Connell, William Holland Furlong and Sir Hugh Lyon Playfair, around Sir David Brewster, who was Talbot’s old friend and persuaded him to share information before publication — found their difficulty was not the calotype but fixing the positive prints, and did not get far until they gave up Talbot’s halides for Herschel’s hypo. John Adamson taught his younger brother Robert, who by May 1843 was the first successful professional photographer in Scotland; Brewster introduced him to the painter David Octavius Hill. Between 1843 and 1846 Hill and Adamson made more than three thousand images, and Ware describes their salt prints as notable for rich colour, high density and good stability — comparable with the best Talbot made himself.

Ware’s tentative explanation for that stability is deflating and probably right: not a secret ingredient but dilute hypo and scrupulous washing. Cundell’s 1844 account recommends a 2.5 % fixing bath where the manuals of the 1850s specify 10 to 30 %. At Reading, Henneman washed each batch of twenty-five prints for ten minutes in each of three baths of hot water, and Schaaf traces the severe sulphide fading of The Pencil of Nature prints mainly to residual thiosulfate and to Reading’s water supply. The same chemical, used two ways, gives you Hill and Adamson or gives you a faded book.

The DNB records the end of the story: in 1852, at the request of the presidents of the Royal Society and the Royal Academy, Talbot threw his discoveries open, keeping only portrait-taking for sale to the public; and in December 1854 he tried and failed in the courts to enforce his patent against a photographer the DNB names Sylvester Laroche, whose collodion development he held to infringe it. Ware refers to the same action as brought “against Laroche” in 1854, and notes that Talbot’s answers under cross-examination are themselves technically interesting.

⁇Reade and gallic acid

Joseph Bancroft Reade is regularly credited with the discovery of chemical development, and the DNB says outright that Talbot’s priority claim for the calotype “directly conflicts” with Reade’s.

What Reade did, on Eder’s account, is soak writing paper in a decoction of nutgalls, coat it with silver nitrate, and use the moist paper at once in a solar camera; he exhibited pictures made this way at the Royal Society in April 1839. Eder judges the work “extremely deficient” for the purpose of the claim, because Reade saw the tanning substance only as an accelerator of ordinary blackening and did not recognise development of a latent image at all; Eder adds in a footnote that Reade obtained images only with silver nitrate and gallic acid, whereas Talbot’s development applies to a latent image in a silver halide.

A second, separate Reade claim concerns hypo. Eder reports the story that Reade found Herschel’s 1819 solubility observation in Brande’s Chemistry in 1839, had an apothecary make him some sodium hyposulphite, used it to fix silver chloride paper and told Herschel — who, on this account, had been using the ammonium salt and adopted Reade’s sodium one. Eder’s own verdict on it is the one this course adopts: “There is at this time no way of verifying this belated story.”

What the course states. Gallic acid was in the air in 1839, and Ware notes that Herschel, Reade and others had all seen its sensitising effect on print-out papers. What Talbot has, and the sources this course read do not give Reade, is the latent image in silver iodide and a procedure for developing it. The course therefore credits Talbot with the calotype and treats the broader priority claim as unresolved, which is what the DNB, writing in 1898, also does.

The Pencil of Nature and the photograph as a printed object

Ware’s chronology dates the founding of the Reading establishment, under Nicolaas Henneman, to January 1844, and the first fascicle of The Pencil of Nature to June 1844. Six fascicles appeared between 1844 and 1846 carrying twenty-four plates, every print made by hand, in sunlight, and pasted in.

Talbot wrote a caption for each, and read together they are the earliest sustained attempt this course knows of to say what photographs are for. Plate III, a shelf of china, proposes the inventory and then the courtroom exhibit: if the mute testimony of the picture were produced against a thief, “it would certainly be evidence of a novel kind”. Plate XXIII proposes the facsimile of an old master drawing, “preserved from loss, and multiplied to any extent”. And Plate VI, The Open Door — a broom against a doorway, sunlight on rough stone — proposes something else again: that a photograph can take as its subject the sort of thing “where ordinary people see nothing remarkable”, on the authority, he says, of the Dutch school of painting.

He is also honest about the medium’s difficulty, in the remarks that precede the plates: the copies are almost facsimiles as to design, but vary in tint, because each is separately formed by a sun whose strength is “exceedingly variable even in serene weather” and on papers whose sizing differs between batches by secrets of the trade. Asked which tint was best, several persons of taste offered nothing approaching unanimity, so he printed them as they came.

Looking exercise

Find, in an open-access museum collection, a Hill and Adamson salted paper portrait of the 1840s and a daguerreotype portrait of the same decade, and write a paragraph on each under four headings: detail, tonal scale, surface and what the process makes easy.

The Getty atlas will tell you what to expect from the salt print: a matte, sunken-in appearance, because the image silver is in the paper rather than on it; a tonality between light brown and reddish brown on prints made before gold toning became usual after 1847; and, on a light table, enough translucency to read the watermark. Expect it to be softer than the daguerreotype, and know why: the atlas notes that a print made from a paper negative is less sharp because the individual fibres of the negative scatter the light passing through them, and that well-waxed negatives reduce the effect without abolishing it. The daguerreotype has no fibres, no binder and no grain between the image and the eye, and it shows.

Then note the thing the reproduction cannot show you: one of those two objects existed in a single copy, and the other existed in as many as anybody wanted.

Recap

• Talbot’s problem in 1833 was that he could not draw, and his idea was to make the camera obscura image imprint itself. He wrote it down before he could test it.
• The 1834 discovery is counter-intuitive: less salt gives more sensitivity, because what prints out a strong image is silver chloride surrounded by unreacted silver nitrate.
• The Latticed Window negative of August 1835 is the earliest known surviving negative, and it is 36 × 28 mm because a small format concentrates scarce light.
• Talbot’s fixers, salt and potassium iodide, stabilised rather than removed. He knew it, published the iodide failure himself, and went on using halides for years because he preferred the colour.
• The calotype of September 1840 is the first developed latent image on paper: a one-second exposure brought out by gallo-nitrate of silver, worth about a hundredfold in speed, and developed physically, so a tired negative could be revived.
• Printing-out and developing-out is the distinction the whole course runs on; Reilly puts the exposure penalty at up to 100,000 times.
• The negative-positive principle — one original, many prints — is the reason this route won, and it is the only 1839-era process that made publication possible.
• The 1841 patent slowed British photography for three years by blocking publication, and Scotland, where Talbot had not registered it, produced the best salt prints of the decade.

Check your understanding

Question 1. Talbot found that paper prepared with a weaker salt solution was far more sensitive than paper prepared with a strong one. Why?

• Weak salt solution penetrates deeper into the paper, so more silver chloride forms
• A deficiency of salt leaves unreacted silver nitrate around each silver chloride crystal, and that free silver lets the reaction continue instead of stalling
• Strong salt solution dissolves the silver chloride as a chloro-complex as fast as it forms
• Weak salt solution leaves the paper more acidic, and acid increases the sensitivity of silver chloride

Show the answer and why

Answer: A deficiency of salt leaves unreacted silver nitrate around each silver chloride crystal, and that free silver lets the reaction continue instead of stalling

The crystal alone is not the sensitive material; the crystal plus its surroundings is. Photolysis in a silver halide releases a halogen atom, and if nothing takes that halogen up it attacks the fresh silver and the reaction reaches a standstill at a barely visible grey. Excess silver ions in damp paper let water carry the chlorine away, so the darkening keeps going. Ware calls the crystal-plus-free-silver combination the chemical secret of photogenic drawing paper, and it is why the stoichiometrically correct ratio of one third by weight fails completely.

Question 2. 1.00 g of sodium chloride (M = 58.44) is soaked into a sheet of paper and then treated with an excess of silver nitrate. What mass of silver chloride (M = 143.32) forms?

• 0.41 g
• 1.00 g
• 2.45 g
• 5.85 g

Show the answer and why

Answer: 2.45 g

1.00 / 58.44 = 0.01711 mol of chloride; the reaction is one to one, so 0.01711 mol of silver chloride forms; 0.01711 × 143.32 = 2.45 g. The product weighs more than the reactant because a chlorine atom has swapped a sodium partner (23) for a silver one (108). Note what the calculation cannot tell you: it is the stoichiometric case, and Talbot was not working stoichiometrically. He supplied salt at about one twentieth of the silver nitrate's weight where this reaction wants about one third, so roughly six parts in seven of his silver nitrate never reacted at all - and that leftover is what makes the paper sensitive.

Question 3. Talbot fixed a photogenic drawing in a bath of potassium iodide. Some days later the dark parts of the picture had faded and the sheet was turning a uniform pale yellow. What has happened?

• The image silver has dissolved in the iodide bath and washed away
• The residual silver chloride has darkened until it caught up with the image
• The image silver has been converted back to pale silver iodide, and the yellow is the residual iodide
• The paper sizing has yellowed with age and obscured the image

Show the answer and why

Answer: The image silver has been converted back to pale silver iodide, and the yellow is the residual iodide

Iodide treatment converts rather than removes: the residual chloride becomes silver iodide, which is very insensitive, so an iodide-fixed sheet does not fog the way a salt-fixed one does. The failure is at the other end. In the presence of excess iodide the nanoparticle image silver is readily oxidised back to silver iodide, which is pale primrose-yellow, so the picture fades towards the ground rather than the ground darkening towards the picture. Talbot published exactly this observation in The Pencil of Nature and called the process not sufficiently certain to keep.

Question 4. A calotype negative has been used to make thirty prints and has grown too faint to print from. Talbot could restore it. How, and why is that impossible with a modern film negative?

• By re-fixing it in fresh hypo; a modern negative is already fully fixed
• By washing it again with gallo-nitrate of silver and warming it, which deposits fresh silver from solution onto the image; modern development is chemical, using only the silver already in the emulsion
• By bleaching and redeveloping it; modern negatives cannot be bleached without destroying the gelatin
• By waxing it again; a modern negative is on a base that will not take wax

Show the answer and why

Answer: By washing it again with gallo-nitrate of silver and warming it, which deposits fresh silver from solution onto the image; modern development is chemical, using only the silver already in the emulsion

Talbot's developer carries silver nitrate as well as gallic acid, so it can lay down additional silver on the existing image: physical development. That is a solution reservoir the image can draw on again later, which is why he could "revive" a worn negative and sometimes see detail that had never appeared. A modern developer is a chemical developer: the only silver available is what is already in the crystal, so once the crystals are developed and the rest is fixed away there is nothing left to build with. Intensification, in Part XIX, is the modern attempt to get some of that back.

Question 5. Talbot wrote that the number of copies from one negative "appears to be almost unlimited, provided that every portion of iodine has been removed from the picture before the copies are made". Why the proviso?

• Residual iodide makes the negative brittle, so it tears in the printing frame
• Residual iodide is yellow and slows printing, but does no lasting harm
• Light plus a trace of iodine together decompose silver, so an iodide-fixed negative is slowly bleached by its own printing exposures
• Residual iodide transfers to the print and stains it

Show the answer and why

Answer: Light plus a trace of iodine together decompose silver, so an iodide-fixed negative is slowly bleached by its own printing exposures

Talbot gives the reason himself: solar light and a minute portion of iodine acting together, though neither separately, decompose the silver and form a colourless iodide. Every print you pull costs the negative some image. The yellow tint is a real nuisance too, because it absorbs exactly the blue and ultraviolet that print the paper, but the bleaching is the one that ends the edition. This is the practical argument that pushed paper photography from halide stabilising to thiosulfate fixing, and Talbot's own Reading prints were thiosulfate-fixed.

Question 6. The 1841 calotype patent did not extend to Scotland. Which piece of evidence best shows that this mattered technically, rather than only commercially?

• Hill and Adamson made more than three thousand images between 1843 and 1846
• The Royal Society refused to publish the calotype paper in its Philosophical Transactions
• The first chemical improvements to Talbot's process came from the St Andrews group, whose salt prints survive in notably better condition than Talbot's own
• Talbot relaxed the patent in 1852 at the request of two learned societies

Show the answer and why

Answer: The first chemical improvements to Talbot's process came from the St Andrews group, whose salt prints survive in notably better condition than Talbot's own

Volume alone would only show commercial freedom. The technical evidence is that the improvements came from where the patent did not run: the St Andrews photographers abandoned Talbot's halide fixing for Herschel's hypo, Cundell published a dilute 2.5 % fixing bath in 1844, and the prints made that way have survived far better than the Reading prints, whose fading Schaaf attributes to residual thiosulfate and poor washing. A patent that blocks publication does not just restrict who may practise a process; it stops the process improving, which is exactly what Ware says happened for three years.

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