Herschel: Hypo, Prussian Blue and the Words of Photography

Twenty years before anybody needed it, the answer to the permanence problem was sitting in print in the first volume of a Scottish journal, in a paper about a new class of salts, written by a man who had no interest whatever in pictures. This page is about how that happened, and about the two other things the same man did to photography: he named it, and — almost by accident, in a fortnight of 1842 — he handed it a process that needs no fixer at all.

1819: an accident with a jar

John Frederick William Herschel was born at Slough on 7 March 1792 and died at Collingwood on 11 May 1871. He was the son of the astronomer who discovered infrared, and he told people that light was his first love.

His paper “On the Hyposulphurous Acid and its Compounds” opens by admitting that the whole investigation was an accident:

Having set aside, for a few days, a solution of hydroguretted sulphuret of lime, I was struck by observing a bitterness in the liquid, when almost wholly decomposed and colourless, similar to that of sulphate of magnesia, which I could not account for.

He satisfied himself it was not magnesium sulfate, established that the liquid had lost its power to precipitate iron or copper as sulfides but still held lime bound to some acid, and worked outward from there. Most of the paper is crystallography and gravimetric analysis of one hyposulphite after another. But among the general characters of the class, before he has even started on the individual salts, is the sentence:

One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver, and retaining it in considerable quantity in permanent solution.

Muriate of silver is silver chloride. Twenty pages later, under Hyposulphite of Silver, he says it again in the form that the permanence page quotes, and adds the observations of a man playing with a genuinely strange solution: it tastes intensely sweet, a sweetness “surpassing even that of honey”, which he takes as evidence that the dissolution is not a simple one but involves mutual decomposition. Adding hydrochloric acid or a chloride causes no cloud, so the silver is not sitting there as free ion waiting to be precipitated back. And a coil of zinc wire dropped in “speedily separates the silver in a metallic state” — which is, incidentally, the first description in this course of silver recovery from a spent fixing bath, a subject Part XII returns to with Kodak’s figures.

Herschel was characterising salts. He had no photograph to try it on. Nobody did.

January 1839: a week’s work

Ware’s account is that Herschel took up photography in January 1839, stirred by the news of Talbot’s announcement, and had solved the problem of silver fixation within a week. His own paper is more precise about the trigger: his attention was first called to Daguerre’s concealed processes “by a note from Captain Beaufort, dated the 22nd of January last, at which time he was ignorant that it had been considered by Mr. Talbot, or by any one in this country.”

Ware dates the first thiosulfate fixing to 29 January 1839, and Herschel showed the result to Talbot on 1 February. The public statement came in a paper read to the Royal Society on 14 March 1839, whose title alone does two jobs at once — “Note on the Art of Photography, or the Application of the Chemical Rays of Light to the purposes of Pictorial Representation” — and whose argument is a specification before it is a recipe:

the author inquires into the methods by which the blackened traces can be preserved, which may be effected, he observes, by the application of any liquid capable of dissolving and washing off the unchanged chloride, but of leaving the reduced, or oxide of silver, untouched. These conditions are best fulfilled by the liquid hyposulphites.

That is the design brief the permanence page sets out, met. He also reports a second-best: pure water will “fix” the photograph by washing out the silver nitrate, but leaves a brick-red tint. Twenty-three specimens accompanied the paper, one of them a sketch of his telescope at Slough, “fixed from its image in a lens” — a camera picture, in other words, and of his father’s instrument.

The vocabulary of that title is worth pausing on. Chemical rays is Herschel’s term for the part of the spectrum that does photochemistry, and he meant it as a research instrument rather than a metaphor: Ware describes his whole photographic programme as an attempt to use light-sensitive coatings as probes for the regions beyond human vision: past the violet end into the ultraviolet Ritter found in 1801, and past the red end into the infrared his own father found in 1800. He was already in the habit of turning a chemical effect into a measurement; the DNB records his actinometer, described in 1825 in the Edinburgh Journal of Science, with which at the close of 1836 he made the first satisfactory measurements of direct solar radiation. Part XV builds the modern descendant of that instinct.

⁇The first photograph on glass

Herschel is regularly credited with a glass negative of the Slough forty-foot reflector, made in September 1839 and often called the first photograph on glass. The course could not verify that date from the sources it read.

What it did find is the DNB’s notice of 1891, which states, on Abney’s authority, that “his reproduction in 1843 of an engraving of the Slough forty-foot reflector was the first example of a photograph on glass” — a different year, and a copy of an engraving rather than a camera negative. Ware separately records a camera negative of the same telescope annotated J.F.W.H. Photog. Feb 10 1839, which is on paper.

What the course states. Herschel made photographs on glass, and the DNB dates the first to 1843. The 1839 date is not asserted here, and a reader who finds a museum record settling it should treat that record as better evidence than either.

Why thiosulfate works where salt only slowed things down

Silver chloride is not insoluble because silver and chloride are welded together. It is insoluble because nothing on offer holds a silver ion more tightly than the crystal lattice does. Change that and the crystal dissolves. Thiosulfate holds silver very tightly indeed:

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

Fixing: silver chloride dissolved as the bis(thiosulfato)argentate complex

Read the products. The silver leaves the crystal as part of a negatively charged, freely soluble complex ion, and the chloride leaves as a free ion. Neither is photosensitive, and both wash out in water. Nothing is left behind that light can act on. Compare that with the two treatments Talbot used, where the halide stays in the sheet with its reactivity turned down: the failed candidates and their formation constants are worked through on the permanence page, and the practice on Talbot’s page.

Three answers to 'what do we do about the salt light did not touch?'

1. StabiliseStrong salt or potassium iodide. The unexposed silver halide stays in the sheet, converted or surrounded by excess halide so that it reacts slowly.a rate, not an end point
2. Wash outThiosulfate. The unexposed silver halide is complexed into solution and rinsed away, leaving only image silver in the paper.an end point
3. Wash out with plain waterThe cyanotype. The image substance is already insoluble and the unused sensitiser is already soluble, so water alone finishes the job.no fixer exists or is needed

All three are on this page or the two beside it, and the third is the reason Part XXI's process is the easiest in the course. Herschel invented the second and the third within three years of each other.

The halide matters, and it decides your fixing time

The three silver halides do not dissolve in hypo at the same rate, and the order is the same as the order of their insolubility in water: chloride goes fastest, bromide more slowly, iodide with real difficulty. Ware states it directly for the calotype: silver iodide “is only soluble with difficulty in thiosulphate solution, which needs to be heated, in consequence” — which is exactly why the patent Talbot took out on 1 June 1843 specifies a hot thiosulfate bath, and why it was for removing the yellow iodide tint from negatives rather than for making them safe.

Carry the consequence forward. A modern film emulsion is mostly silver bromide and a modern paper is chloride-rich; a film therefore needs a longer fixing time than a paper in the same bath, and a bromide-iodide film longer still. Part XI measures this properly, with a clip test, instead of quoting it.

⚗Three ways to get thiosulfate fixing wrong, all of them found in 1839

Ware lists the pitfalls, and each of them is a mistake somebody made at the time.

Fix before you wash and you get a stain. Thiosulfate meeting free silver nitrate throws down brown silver sulfide in the paper. A print-out paper is made with excess silver nitrate, so it has to be washed in plain water first. Herschel understood this; Ware suspects Talbot did not always attend to it, and that some of his dissatisfaction with hypo came from there.

Fix too long and you lose the picture. Print-out silver is colloidal and easily oxidised in the presence of thiosulfate. Too long in the bath, too strong a bath, or an oxidising impurity in the water, and density goes. There is a colour cost as well: the rich print-out black shifts to a weaker dull brown on fixing, which the Getty atlas records as the reason printing-out papers are deliberately overprinted.

Wash too little and you have built a slow bomb. Thiosulfate left in the paper attacks the image silver over years and converts it to silver sulfide, taking a rich brown down to a pale buff. That is the mechanism behind the faded plates of The Pencil of Nature. Part XI teaches washing and residual thiosulfate testing, and this is the page that tells you why it will be worth the trouble.

The words

Herschel had a classical education and enjoyed inventing terms. Ware credits him with photography itself, with negative and positive as nouns, with snapshot, and possibly with the verb to solarize.

The most quotable of them is the pair, and it arrives with an argument attached. In the paper received and read on 20 February 1840, paragraph 8 says only this:

To avoid much circumlocution, it may be allowed me to employ the terms positive and negative, to express respectively, pictures in which the lights and shades are as in nature, or as in the original model, and in which they are the opposite.

He adds direct and reversed for handedness, so that one can speak of a positive-direct or a negative-reversed picture, and notes that a camera on white paper impresses a negative-reversed photograph. But the paragraph before it is the one that matters, because it explains why he needed the vocabulary at all: the important line of inquiry, he writes, is the exact reproduction of indefinitely multiplied facsimiles of an original, “by which alone the publication of originals could be accomplished” — and this seemed the more deserving of attention because Daguerre’s pictures were understood to admit of no such reproduction. Herschel is naming the parts of a machine for making many pictures from one.

⁇Who first said 'photography'?

There are at least four claims, and they are not all claims about the same thing.

Herschel. Ware records his use of “photographic” in a letter to Talbot on 10 February 1839, and the same date on his first known camera negative, annotated J.F.W.H. Photog. Feb 10 1839. His Royal Society title of 14 March 1839 puts “the Art of Photography” into the record of the Society.

Wheatstone. Ware notes that Charles Wheatstone used “photographic” in a letter to Talbot on 2 February 1839, eight days earlier.

In print, first. Ware traces the first appearance of “photograph(y)” in print to a German provincial newspaper, the Vossische Zeitung, on 25 February 1839. Eder identifies the anonymous author, initialled “J. M.”, as the Berlin astronomer Johann von Mädler (1794–1874), and adds the qualification that decides the matter for practical purposes: an unnoticed newspaper piece by an anonymous contributor made no difference, “while Sir John Herschel’s mention of it made the word known to the whole world.”

Earlier still, in French, and unpublished. Boris Kossoy has argued that Hercules Florence, working in Brazil, used photographie in 1833 — the date Ware gives when reporting the claim. Florence published nothing, and his work was effectively unknown to historians until the 1970s; the Getty atlas confirms the surviving objects and dates his good results to 1833–34. The many-inventors page takes Florence up properly.

What the course uses. Herschel put the word into scientific circulation in the spring of 1839, and this course says that rather than “Herschel coined photography”. The course did not find a date for his first use of snapshot; Ware’s attribution is recorded here without one.

1842: from flower juice to Prussian blue

Between 1839 and 1842 Herschel spent a great deal of effort on a process that failed. Ware describes the anthotype, or phytotype: petals crushed in alcohol, the dye painted on paper, a print made by laying an engraving on it and letting sunlight bleach the exposed dye. It is positive-working, which he wanted, and it produces colour, which he wanted more — he told the British Association in 1841 that it held out “no slight hope of a solution of the problem of a photographic representation of natural objects in their proper colours.” It is also almost unusably slow, taking days or weeks, and there is no way to fix it: the same light that made the image goes on bleaching it. Ware notes that Herschel ran over a thousand such tests, and that the one durable result was a principle — a dye is bleached fastest by the light of its complementary colour.

In the spring of 1842 he changed tack from unstable organic dyes to deeply coloured inorganic ones, and the surgeon Alfred Smee sent him a sample of a salt Smee had learned to make cleanly by electrolysis: potassium ferricyanide, then called the ferrosesquicyanuret or red prussiate of potash. Herschel’s memorandum of 23 April 1842 is the moment:

Smee’s Red Ferrocyanate of Potash washed on paper gives it a fine pale green colour. … When the paper is thrown onto water the impression becomes stronger, loses its Violet ruddiness & turns to a fine prussian blue … This paper will prove valuable.

His diary that day is blunter: “Discovered the Photographic property of the Red Ferro sesquicyanuret of Potassm.” Ware’s comment is worth keeping: discovered seems the better word than invented.

Smee’s second gift did the rest. In a letter of 10 May 1842 he mentioned two salts newly “vamped up by the Chemists and Druggists” as iron tonics — the ammonio-citrate and ammonio-tartrate of iron — on the off-chance that dark solutions might be worth bleaching. Herschel tried the citrate photographically instead of medicinally, found it highly sensitive, and wrote back that it “has furnished me with an infinity of beautiful photographic processes”. Ferricyanide paper alone needs “half an hour or an hour’s exposure to sunshine”, which Ware calls optimistic; ferric ammonium citrate brings a contact print down to minutes.

🕰Four processes in one summer, and a collective name

Ware’s table of Herschel’s 1842 siderotypes — from the Greek sideros, iron — is the family tree of half the alternative-process world:

Process	Image substance	Where it leads
Cyanotype	Prussian blue	Blueprint; Part XXI
Argentotype	Silver	Kallitype, Van Dyke brown; Part XXIV
Chrysotype	Gold	Ware’s modern chrysotype
Kelainotype	Mercury	Nowhere; it defeated him

He named the chrysotype in imitation of Talbot’s calotype and said so in a letter, calling it “one of the most striking and magical effects which has yet turned up in photography”. It was a commercial failure for a century and a half. The one he thought least of is the one that changed drawing offices worldwide.

His paper “On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on some new Photographic Processes” was accepted on 15 June 1842 and partly read to the Royal Society on 16 June, with 43 specimen sun-prints attached. Article 219 coins the name for the whole class: any process “in which cyanogen in its combinations with iron performs a leading part, and in which the resulting pictures are blue”. Kyanos is Greek for a deep blue.

Why water is enough

Article 223 of that paper is, essentially, the recipe still sold in kits: paper washed with a mixture of ammonio-citrate of iron and ferrosesquicyanate of potash in about equal proportions, exposed, then “thrown into water and dried”, giving a negative blue picture.

The process runs in two steps that Herschel himself separated experimentally. In article 210 he left the ferricyanide out altogether, exposed the plain citrate paper for four or five seconds — an effect “quite imperceptible to the eye” — and only then, in the shade, washed it with the ferricyanide, at which point a strong blue appeared exactly where the sun had fallen. He concludes that the light acts on the iron, and that the cyanide salt is “a mere precipitant on the nascent compounds resulting from that influence”.

Cyanotype, in two steps and a rinse

1. 1Light reduces the ironUltraviolet and blue light acting on an iron(III) salt of an organic acid transfer an electron from the acid to the metal, giving iron(II). The colour change is slight and the product is only mildly stable, so the second step has to follow.
2. 2Iron(II) meets ferricyanideThe iron(II) reacts with hexacyanoferrate(III) to give Prussian blue, which is highly insoluble and intensely coloured. Ware notes that the intermediate rearranges instantly by internal electron transfer, so the pigment is ferric ferrocyanide however you made it.
3. 3Water carries the rest awayUnreacted ferric ammonium citrate and potassium ferricyanide are both freely soluble; Prussian blue is not. Washing removes everything light did not use and leaves the image behind.
4. 4Air finishes itPrussian white formed by over-exposure is colourless and re-oxidises to Prussian blue over some hours in air, which is why the print deepens as it dries.

The mechanism in outline, from Ware's Cyanomicon and the Getty atlas. Part XXI does the photochemistry properly, with the ligand, the quantum yield and the reasons the classic sensitiser misbehaves.

That third step is the whole point of this page’s title. A cyanotype needs no fixer because its image substance is already insoluble and its unused sensitiser is already soluble. A salted paper print needs a fixer because the exact opposite is true: the image silver is insoluble and so is the silver halide beside it, and no amount of water will tell them apart. Herschel solved the same problem twice by two completely different routes, and the second route is the one you can hand to a beginner.

🔬What Prussian blue is, and why it is that colour

Ware’s account: Prussian blue is ferric ferrocyanide, and it is unusual among iron compounds in containing the metal in two oxidation states at once. The deep blue comes directly from that mixed valence — electrons hop from the iron(II) to the iron(III) by absorbing red light, and what is left of white light after the red is removed is blue.

The Getty atlas dates the pigment’s first preparation to Heinrich Diesbach in Berlin between 1704 and 1710, and its use in oils and watercolours to about 1730. It was already a hundred and thirty years old, and a standard artists’ colour, before anyone made a photograph out of it.

Two of its weaknesses will matter to you in the next hour. Alkali destroys it. Ware reports Holtzman’s finding that a buffer at pH 9.4 — no more alkaline than a saturated solution of calcium carbonate, the buffer commonly built into archival papers and boards — completely decolourises it in one to ten minutes, and that 0.25 mol/L sodium carbonate at about pH 10.7 does it in under half a minute. Light fades it, reversibly. Pure Prussian blue is stable to light, but where an oxidisable impurity such as citrate is trapped in the lattice the excited pigment can pick up an electron and become colourless Prussian white — which air then oxidises back. A faded cyanotype recovers in the dark, which is a sentence you can write about almost nothing else in this course.

⚠One incompatibility to carry into the next page

Potassium ferricyanide is a hexacyanoferrate: the cyanide in it is bound to iron, not free. PubChem’s aggregated ECHA classification carries the signal word Warning, with H319 causing serious eye irritation on 62.8 % of notifications and H302 harmful if swallowed on 25.3 %, and H411 toxic to aquatic life with long-lasting effects on 45.3 %. Ware rates it a low toxic hazard, and adds the sentence this course carries forward: “It is incompatible with concentrated acids, which may release deadly hydrogen cyanide gas.”

That is why the next page tells you never to let cyanotype paper, or the water it was rinsed in, meet an acid, a bleach or a heat source. The control is avoidance, and avoidance is easy, because nothing in the assignment calls for any of the three. Ware’s own assessment of the traditional process is that a normal exposure and wash will not release any significant or perceptible amount of hydrogen cyanide; the course states the incompatibility anyway, because a reader who improvises with lemon juice or household bleach has left the process the assessment was made for.

Anna Atkins, and a claim worth stating carefully

Anna Atkins (1799–1871) was the daughter of John George Children, the Royal Society’s Secretary, who was in the chair at the February 1839 meeting where Talbot disclosed photogenic drawing, and who lived with her from 1840. The Herschels moved to Kent the same year, thirty miles away; the Herschel daughters treated Anna as an aunt. When the 1842 paper appeared, a copy went to Children directly, and Ware thinks it likely she picked the process up more informally as well.

She had a problem the process fitted exactly. Seaweeds are the awkward case in botanical illustration — too fine and too tangled to draw reliably — and a dried, pressed alga laid on sensitised paper is its own negative. Her prefatory note says so, with the modesty of someone who signed the book “A. A.”:

The difficulty of making accurate drawings of objects as minute as many of the Algae and Confervae, has induced me to avail myself of Sir John Herschel’s beautiful process of Cyanotype, to obtain impressions of the plants themselves, which I have much pleasure in offering to my botanical friends.

Photographs of British Algae: Cyanotype Impressions was issued privately in parts from 1843 to 1853. Ware puts the total at roughly 420 plates or more across twelve fascicles, varying between copies, and estimates that the edition demanded the hand-printing of over five thousand cyanotypes. The National Science and Media Museum gives over 400 photographs and a ten-year run from 1843. Every plate carries its Linnaean binomial in Atkins’ own handwriting, written on a translucent slip and printed with the specimen — the label is part of the image, not a caption under it.

⁇The first photographically illustrated book

Three different claims circulate, and they are settled by reading the wording rather than the dates.

Ware records that Atkins distributed more than a dozen copies of the first fascicle in October 1843, and that the first part of The Pencil of Nature did not appear until June 1844: “British Algae therefore takes precedence in the claim to be the first photographically illustrated book, albeit a privately-published one.” The Getty atlas agrees, calling her three volumes the earliest examples of books illustrated with photogenically produced images.

The DNB, writing in 1898, calls The Pencil of Nature “the first book ever illustrated by photographs produced without any aid from the artist’s pencil” — a formulation that quietly excludes a privately circulated part-work.

Eder asserts something else again: that Talbot’s Sun Pictures in Scotland (1845) “was the first book illustrated with photographs”, which cannot be right on his own chronology, since he dates The Pencil of Nature to 1844 on the previous page.

What the course uses. Atkins’ first fascicle, October 1843, is the earliest photographically illustrated book this course can date from its sources, and the qualifier privately issued belongs in the sentence every time. The National Science and Media Museum’s own wording — the first photographically illustrated book in Britain — is a model of how to make the claim without overreaching.

Was it science or art? Ware quotes Robert Hunt’s 1848 review, which praises the plates for “the extreme fidelity with which even the most attenuated tendrils of the marine plants are copied”, and then recommends the process “particularly to ladies, and to those travellers who, although not able to bestow much attention or time on the subject, desire to obtain accurate representations of the botany of a district.” Ware calls the compliment double-edged, and it is. The plates are botanical records made to a taxonomic standard; they are also arranged on the sheet by somebody making decisions about placement, negative space and scale that no taxonomy required. Both things are true, and the question the assignment page asks you is which of the two you are doing when you lay a fern on a sheet of blue paper.

From Herschel’s blue to the blueprint

Herschel patented none of it. Ware notes the consequence twice over: plagiarists had a free run, and the process spread with nothing to slow it. The Getty atlas dates the first commercial cyanotype paper to 1872, made by Marion et Cie in Paris as papier ferro-prussiate; the first commercial blueprint machine, built in Switzerland, reached the United States at the Philadelphia Centennial of 1876; and from roughly 1870 to about 1950 the cyanotype and its variants were the main way engineers and architects copied plans, until diazo processes replaced them. The word outlived the technology by seventy years and now means a plan of any kind.

The photographic revival came from the other direction: the Getty atlas dates the rediscovery of cyanotype by contemporary photographers to the 1960s, as an alternative to commercial silver gelatin materials, and notes drily that silver gelatin will itself soon be an alternative process.

Part XXI owns the chemistry, the sensitiser formulas, the toning and the conservation. The next page hands you paper somebody else has coated and asks you to make three pictures with it.

Recap

• Herschel’s 1819 paper on the hyposulphites, written about salts and not about pictures, records that their solutions dissolve freshly precipitated silver chloride readily and in quantity.
• In January 1839 he connected the two within a week, told Talbot on 1 February, and published the principle on 14 March: dissolve and wash off the unchanged halide, leave the image silver alone.
• Thiosulfate works because it takes silver into a soluble complex ion, so nothing photosensitive is left. Chloride dissolves fastest, bromide slower, iodide only with heat — which is why film fixes more slowly than paper.
• Fixing has three classic failure modes: staining if free silver nitrate is not washed out first, density loss from over-fixing print-out silver, and slow sulfiding from thiosulfate left in the paper.
• He gave the medium photography (in circulation, if not in print, first), and negative and positive in 1840, in a paragraph arguing that multiplied facsimiles are what make publication possible.
• In April 1842 he found that ferricyanide paper prints blue, and in May, on Smee’s suggestion, that ferric ammonium citrate makes it fast. The 1842 paper names the cyanotype and three sibling iron processes.
• A cyanotype needs no fixer because the image is insoluble and everything else is soluble. A silver print needs one because the image and the unused salt are both insoluble.
• Anna Atkins’ Photographs of British Algae, from October 1843, is the earliest photographically illustrated book this course can date — privately issued, over four hundred plates, every label in her own hand.

Check your understanding

Question 1. Why does thiosulfate remove unexposed silver chloride from a print when a saturated solution of sodium chloride does not?

• Thiosulfate is a stronger acid, and acids dissolve silver salts
• Thiosulfate binds silver ions into a soluble complex ion, so the crystal dissolves and washes out; chloride binds them far too weakly to compete with the lattice
• Thiosulfate reduces silver chloride to metallic silver, which then washes away
• Sodium chloride works equally well but is much slower

Show the answer and why

Answer: Thiosulfate binds silver ions into a soluble complex ion, so the crystal dissolves and washes out; chloride binds them far too weakly to compete with the lattice

Solubility is a competition. Silver chloride is insoluble because nothing on offer holds Ag⁺ better than the lattice does; two thiosulfate ions do, so the silver leaves as a soluble anion and the chloride leaves as a free ion, and both rinse out. Chloride does form complexes, but weak ones, so a strong brine leaves most of the halide in the sheet with its reactivity merely damped. That is the difference between removing the salt and slowing it down, and it is the difference between a print that stops changing and one that changes slowly.

Question 2. A calotype negative fixed in a plain room-temperature hypo bath still shows a yellow tint in its clear areas after twenty minutes. What is the tint, and what would shift it?

• Silver sulfide from thiosulfate attack; a longer wash
• Residual silver iodide, which dissolves in thiosulfate only with difficulty; a hotter, stronger bath
• Stained gelatin from the gallic acid developer; a stop bath
• Oxidised silver nitrate; a preliminary rinse in plain water

Show the answer and why

Answer: Residual silver iodide, which dissolves in thiosulfate only with difficulty; a hotter, stronger bath

Silver iodide is the least soluble of the three halides in water and the hardest to complex out with thiosulfate, so it needs heat. Talbot's patent of 1 June 1843 specifies exactly that: a hot or boiling thiosulfate solution to take the yellow tint of silver iodide out of a calotype, which mattered because the yellow absorbs the blue and ultraviolet light you print with. Note the order of difficulty is the same as the order of insolubility in water: chloride, bromide, iodide.

Question 3. Herschel exposed paper coated with ferric ammonium citrate alone for a few seconds, saw nothing, then washed it in the shade with potassium ferricyanide and got a strong blue where the sun had fallen. What does the experiment establish?

• That the ferricyanide is the light-sensitive component and the iron salt is a stabiliser
• That the light acts on the iron salt, and the ferricyanide is only a precipitant applied afterwards
• That the two salts must be mixed before coating or no image forms
• That the blue image is produced by heat rather than by light

Show the answer and why

Answer: That the light acts on the iron salt, and the ferricyanide is only a precipitant applied afterwards

Splitting a reaction into stages and showing that only one of them needs light is the cleanest way to locate the photochemistry, and Herschel says so in article 211, having "succeeded in separating the final action ... into two distinct steps or stages, the photographic influence being confined to the first, and the ferrosesquicyanate acting as a mere precipitant". The light reduces iron(III) to iron(II) using the citrate as the electron donor; the ferricyanide then finds the iron(II) and makes Prussian blue. It is also a working method, and Part XXI uses it.

Question 4. A finished, washed and dried cyanotype and a finished, washed and dried salted paper print are both left in the same drawer for a decade. Which is more likely to be attacked by the mounting board it is stored on, and why?

• The salt print, because residual thiosulfate reacts with board sizing
• The cyanotype, because archival board is buffered with an alkali and alkali destroys Prussian blue
• Neither; both are chemically inert once dry
• The cyanotype, because the iron salts remaining in the paper continue to react

Show the answer and why

Answer: The cyanotype, because archival board is buffered with an alkali and alkali destroys Prussian blue

This is the case where the standard conservation precaution is the wrong precaution. Archival boards are buffered with calcium carbonate to protect paper against acid, and Ware reports Holtzman finding that a pH 9.4 buffer — the pH of saturated calcium carbonate — completely decolourises Prussian blue in one to ten minutes. Cyanotypes want unbuffered enclosures. The salt print does have a residual-thiosulfate problem, but that is a consequence of how it was washed, not of what it is stored on.

Question 5. Which statement about the word "photography" is best supported by the sources this course read?

• Herschel coined it, in his Royal Society paper of 14 March 1839
• Mädler coined it, in the Vossische Zeitung of 25 February 1839
• Florence coined it, in Brazil in 1833
• Several people reached it independently in 1833-39; Herschel is the one who put it into scientific circulation

Show the answer and why

Answer: Several people reached it independently in 1833-39; Herschel is the one who put it into scientific circulation

Wheatstone used "photographic" in a letter on 2 February 1839 and Herschel on 10 February; the first appearance in print that Ware could trace is an anonymous newspaper piece of 25 February, which Eder attributes to Mädler; and Kossoy argues for Florence's unpublished French use in 1833. Eder's own comment is the useful one: the newspaper use was noticed by nobody, while Herschel's made the word known everywhere. "Who said it first" and "who made it the word we use" are different questions, and the second is the one with a clear answer.

Question 6. Herschel spent more effort on the anthotype than on the cyanotype and abandoned it. What was the defect he could not engineer around?

• The plant dyes were too expensive to obtain in quantity
• The images were negative, and he wanted a positive-working process
• The image is made by light bleaching the dye, so any light that could show the picture also continued to destroy it, and no fixer exists
• The exposures required a spectrum rather than ordinary sunlight

Show the answer and why

Answer: The image is made by light bleaching the dye, so any light that could show the picture also continued to destroy it, and no fixer exists

The anthotype was positive-working and in colour, which is what he wanted, and Ware records over a thousand tests. Its defect is structural rather than practical: the image is the surviving dye, so the process has no end point and there is nothing to remove or convert to stop it. Compare the cyanotype, where the image substance is an insoluble pigment and the light-sensitive material washes out in water. Whether a process can be stopped is a better question to ask of it than how fast it is.

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