Talbot's gallo-nitrate of silver
Every developer in the rest of this book does one job. This one does three, and it is the same liquid each time. Brushed on an iodised sheet it is a sensitiser. Brushed on the same sheet after exposure it is a developer. Brushed on the finished negative months later it is an intensifier that brings back a picture the sun has faded, and sometimes reveals detail that was never visible in the first place.
It is also the first developer that was a liquid, and it keeps for minutes. Daguerre had been developing a latent image with mercury vapour since 1837 and publicly since 1839, so this is not the first development in photography; it is the first that could be poured, and the first that worked on paper.
| Ingredient | Quantity | Form the source specifies |
|---|---|---|
| Silver nitrate | 98 g | 9.8 per cent w/v, 0.577 mol/L — Ware's reading of Talbot's 100 grains of the crystallized salt in two imperial fluid ounces of distilled water, scaled by the course to a litre of finished solution. Talbot's exact figure works out at 9.77 per cent; Ware prints 9.8 and the table follows him |
| Acetic acid (glacial) | 140 mL | 14 per cent v/v of the glacial acid, which is what Talbot's "strong acetic acid" meant and what Cundell writes explicitly. Talbot's own proportion, one sixth of the volume of the silver solution, works out at 14.3 per cent of the finished liquid; Ware prints 14 and the table follows him |
| Water | to make 1000 mL | Talbot adds the acid to two fluid ounces of distilled water carrying the silver, about 56.8 mL, and states no final volume; the make-up to a litre is the course's construction of the same composition, and the arithmetic is under Mixing. He specifies **distilled** water, which matters here more than anywhere else on the page: tap water carries chloride, and chloride in a silver bath is silver chloride precipitated where it is not wanted. |
| Ingredient | Quantity | Form the source specifies |
|---|---|---|
| Gallic acid | 10 g | Approximately 1 per cent w/v — Ware's figure for a saturated solution of gallic acid in cold water, corroborated by Wall's independent 1912 entry giving the solubility as 1 per cent cold and 33 per cent boiling, and by PubChem's aggregate of about 1.1 g per 100 mL at 20 °C. **Talbot states no quantity at all**, only that "the quantity dissolved is very small", because the instruction is to saturate rather than to weigh |
| Water | to make 1000 mL | Talbot specifies **cold** distilled water, and the word is doing real work: gallic acid dissolves about thirty times better in boiling water than in cold, so a solution saturated warm would deposit crystals on cooling and would not be the solution he describes. |
Mixed in the ratio — The gallo-nitrate of silver as Talbot published it in 1841 — mixed at the moment of use
1 part Liquid A — Talbot's aceto-nitrate of silver + 1 part Liquid B — a saturated solution of crystallized gallic acid in cold distilled water
"When a sheet of paper is wanted for use, mix together the liquids A and B in equal volumes, but only mix a small quantity of them at a time, because the mixture does not keep long without spoiling. I shall call this mixture the Gallo-nitrate of silver."
The mixture is 4.9 per cent w/v (0.288 mol/L) silver nitrate, 7 per cent v/v acetic acid and 0.5 per cent w/v gallic acid, in Ware's reading. Every strength on this page halves on mixing, which is why the two stocks look twice as strong as the developer that is made from them.
Mixed in the ratio — Cundell's 1844 practice — the same equal volumes, measured and used at once
1 part Liquid A — Talbot's aceto-nitrate of silver + 1 part Liquid B — a saturated solution of crystallized gallic acid in cold distilled water
"When these solutions are about to be applied to the iodized paper, they are to be mixed together, in equal volumes, by means of a graduated drachm tube. This mixture is called the gallo-nitrate of silver. As it speedily changes, and will not keep for more than a few minutes, it must be used without delay, and it ought not to be prepared until the operator is quite ready to apply it."
The same ratio from a second, independent hand, and the reason it is recorded separately: Cundell puts a number on Talbot's "does not keep long". A few minutes, measured with a drachm tube, prepared only when the operator is ready. That is a working instruction where Talbot's was a caution.
Purpose
Section titled “Purpose”To flood a sheet of silver iodide with free silver ions, so that a halide deliberately made insensitive becomes, on Talbot’s own claim, more sensitive by a hundredfold than any photographic paper then described; and then, after exposure, to use the same free silver ions to build a visible image on the invisible trace the light left behind.
Two purposes, one liquid, and the economy of that is the whole design. It is worth separating them.
The sensitising purpose. Talbot’s iodised paper is built to be dead. Its silver iodide sits in an excess of iodide, carries adsorbed iodide and a negative surface charge, and — in Ware’s words — is “completely insensitive even to direct sunlight”. Brushing gallo-nitrate over it reverses that environment in seconds. Adsorbed iodide is displaced by adsorbed silver; the surface charge goes from negative to positive; and the crystal changes from what Ware calls fixed silver halide to what he calls sensitized silver halide. Ware states the causation directly: “the silver iodide already existing in the paper was made sensitive to light at this stage by the excess silver ions introduced in the ‘exciting’ solution”.
The developing purpose. A one-second exposure leaves a latent image of a few atoms of silver on some of those crystals, and nothing whatever that the eye can see. Washing the sheet again with gallo-nitrate makes it visible in seconds, and the mechanism is not the one a modern darkroom uses. A modern developer reduces the crystal the light has struck. This one reduces silver ions out of the solution and deposits them on the latent image speck as on a seed. That is physical development, and the difference is set out under The mechanism.
The third purpose, which nobody designed and which fell out of the second. Because the developer brings its own silver, it can go on adding density to a negative that has already been fixed and printed from. Talbot found that a faded calotype could be revived by another wash and a warming, “which causes all the shades of the picture to darken greatly, while the white parts remain unaffected”, and that new detail sometimes appeared which “had not before been seen, having been latent all the time”. No modern developer can do that, because a modern developer has only the silver already in the grain.
And the purpose the course keeps this page for. This is where development in solution begins. The idea of development is Daguerre’s — the course’s own account of the many inventors puts him at 1837 with mercury vapour against Talbot at September 1840 with gallic acid — but mercury vapour cannot be poured into a dish, cannot be diluted, and cannot touch paper. What arrives here is a developer: a liquid with a reducing agent in it, a concentration, a keeping time and an end point the worker has to judge. Everything in Part VIII descends from that idea, and almost nothing in Part VIII resembles this particular liquid.
Recommended uses
Section titled “Recommended uses”Historical, every one of them, and they are worth listing separately because the same bottle appears at four different points in the process and does something different each time.
Exciting the iodised sheet. The published use: brushed over the marked side by candlelight, allowed to rest half a minute, dipped in water, blotted, dried cautiously at a distance from the fire — or, if the paper is to be used at once, left moist, which Ware says was preferred for its greater sensitivity. The Image Permanence Institute records the same sequence from the conservation side and notes that the exposure of one to ten minutes was made while the sheet was still damp.
Developing the exposed sheet. The same liquid, washed over again, with gentle warmth if the impression is weak. Talbot’s description of watching it happen is the best short account of development ever written by the person who invented it: “it is a highly curious and beautiful phenomenon to see the spontaneous commencement of the picture, first tracing out the stronger outlines, and then gradually filling up all the numerous and complicated details.”
Reviving a faded or under-exposed negative, described above and under The mechanism. Ware calls it “a crucially important means” and connects it directly to Talbot’s persistence with halide fixation.
Talbot’s direct-positive experiment, in which a sheet of excited calotype paper is deliberately darkened in daylight, dipped in the strong iodide bath until the discoloration apparently vanishes, exposed in the camera and then developed with gallo-nitrate. Hunt reproduces the specification. Note what it proves: the discoloration is only apparently removed, “for if the paper were dipped into a solution of gallo-nitrate of silver, it would speedily blacken all over”.
On surfaces that are not paper. Talbot’s third patent, as Hunt reports it, coats unglazed porcelain with albumen, sensitises it exactly as a sheet of paper is sensitised, and develops the image with gallo-nitrate. The same patent washes a warm steel plate with “an alcoholic solution of gallo-nitrate of silver, of moderate strength”, noting that “if the plate be cold, the sensibility is considerably lower”. Neither is published here as a formula — the alcoholic version has no stated strength — but they show how general the reagent was thought to be.
And in this course, as the first and clearest case of a developer that carries its own silver, to be read against D-76 at the other end of the century. Both make a negative. They have almost nothing else in common, and the comparison teaches more than either page alone.
When another formula is preferable
Section titled “When another formula is preferable”Almost always, and the list is arranged by what you are actually trying to do.
- To develop a modern film, D-76, or any chemical developer. A physical developer will not develop a gelatin emulsion usefully: the silver has to reach the latent image through the binder, and a modern grain already contains all the silver its density needs.
- To use a polyphenol developer today, PMK pyro. Pyrogallol is gallic acid minus one molecule of carbon dioxide, and PMK is what the family grew into once a bisulphite preservative and a metaborate alkali were available to control it.
- To make a photograph you can frame, the plain salting solution with the salted paper sensitiser, fixed with a thiosulphate fixer. That route is printing out and needs no developer, which removes every instability on this page at a stroke.
- To understand the material this excites, Talbot’s calotype iodised paper, which has to be read with this one; and Talbot’s photogenic drawing paper, which is what photography looked like before a developer existed.
- To make a developed image on paper with a published, repeatable behaviour, the silver chloride contact emulsion: a halide in a binder, on a support, with a speed and a fixing time.
- To stay inside the calotype family and get a sheet that keeps, the aceto-nitrate alone — solution A of this page, without solution B. John Adamson pointed out that stability was greatly increased by excluding the gallic acid, and Ware records that most photographers after 1846 adopted that as their preferred exciting solution. It is the same bottle with half the formula deleted.
- To develop a calotype without physical development at all, gallic acid alone, which is what Le Gray and Flacheron used and what Talbot himself tested privately. That is chemical development, and it gives a thinner, cleaner negative because no extra silver arrives.
- To fix anything, a plain hypo bath. This liquid is the opposite of a fixer and the two must never share a dish or a brush.
Mixing
Section titled “Mixing”Two bottles that must not become one until the last possible moment. That is the structural fact of this formula, and unlike most two-solution entries in this book the reason is not that the mixture is dangerous but that it is dying. Solution A keeps indefinitely. Solution B keeps a few days. Mixed, they keep minutes.
Talbot’s own words, and each of them is doing work. “Dissolve 100 grains of crystallized nitrate of silver in two ounces of distilled water; add to this solution one-sixth of its volume of strong acetic acid. Let this mixture be called A. Make a saturated solution of crystallized gallic acid in cold distilled water. The quantity dissolved is very small. Call this solution B. When a sheet of paper is wanted for use, mix together the liquids A and B in equal volumes, but only mix a small quantity of them at a time, because the mixture does not keep long without spoiling.”
Crystallized is a purity statement about the article as it was sold, not a hydrate: silver nitrate has none. Distilled is a chloride statement, and it is specified for both solutions here where it was specified for only one of the two on the iodising page. Cold is a solubility statement — gallic acid dissolves about thirty times better in boiling water than in cold, so a solution saturated warm will throw crystals as it cools and will not be the liquid described. One-sixth of its volume means one sixth of the silver solution’s volume, not of the finished mixture’s, and the arithmetic below confirms that reading against Ware’s independently published percentage. And only a small quantity at a time is the sentence that governs the whole formula.
Why saturation rather than a weighing, and why it is not laziness. Gallic acid dissolves poorly in cold water — about 1 per cent, which Wall gives in 1912 and PubChem’s aggregate confirms at 20 °C. To weigh 10 g into a litre you would need a balance that reads to a tenth of a gram, and in 1841 the alternative was to tip a spoonful of powder into a bottle of water, shake it, and decant the clear liquid off the undissolved surplus. The result is reproducible to whatever precision the room’s temperature is reproducible, and the surplus at the bottom is a buffer that keeps it saturated as it is used. It is a good method. It is also the reason this formula cannot be scaled or standardised, because its second solution is defined by a physical property rather than by a number.
Why the mixture spoils, in one sentence. An oxidising silver salt and an easily oxidised polyphenol in the same beaker are a redox reaction waiting for an excuse, and light is the excuse. Hunt states the observation exactly, in 1844: “If we unite a solution of gallic acid and nitrate of silver, even in weak diffused light, it will be found that a precipitate is almost immediately formed, whereas the same solution will often remain clear for many hours in the dark.” That single sentence explains the candlelight, the small quantities, and the drachm tube.
What the acetic acid buys. Two things that pull in the same direction. Ware assigns it the role of inhibiting the spontaneous decomposition and spoiling of the excited paper. Wall, describing physical development in general seventy years later, says that the precipitation of silver by a reducing agent from a silver nitrate solution is “delayed by the presence of organic acid such as acetic or citric”. Two sources, two vocabularies, one effect: the acid slows the reaction down enough for it to be useful. Without it the mixture would be a precipitate before it reached the paper.
The paper is an ingredient and the schema cannot say so. Cundell insists on “a fine satin post paper, made by R. Turner, Chafford Mill”, chosen without flaw, watermark or the minutest black specks. Ware identifies Talbot’s own stock as John Whatman’s Turkey Mill. The gelatin sizing of an English sheet is not packaging: Ware’s section 23.3 gives it the job of retaining the sensitiser in the surface fibres and of influencing the colour and stability of the silver eventually formed. A French sheet sized with starch is a different substrate, and the same two solutions on it gave a different paper.
And the drying is a safety statement. Talbot dries the excited sheet “cautiously at a fire, holding it at a considerable distance therefrom”, and Cundell brings a hot iron within an inch or two of the same sheet to develop it. A piece of paper loaded with an oxidising nitrate, held to a fire, is exactly the combination the hazard classification of silver nitrate warns about. Nothing in this course asks anyone to repeat it.
Behaviour
Section titled “Behaviour”Its first behaviour is a hundredfold jump in speed, and it is the reason photography became possible out of doors. Talbot’s claim is that calotype paper is “sensitive to light in an extraordinary degree, which transcends a hundred times or more that of any kind of photographic paper hitherto described”, and he demonstrates it in the plainest possible way: cover half a sheet, give the other half one second of daylight “in dark cloudy weather in winter”, and the impression is strong. Ware’s independent estimate of the gain over photogenic drawing paper is the same order — about a hundredfold, turning an hour’s camera exposure into about a minute.
Its second behaviour is that the speed is invisible. Nothing appears. Talbot is careful about this and slightly delighted by it: 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”. It is the first time an exposure and a photograph are separate events on paper, and the second time in photography: the daguerreotype had made the same separation on a silvered plate, with mercury vapour, from 1837.
Its third behaviour is that it will not keep, at any stage, for long. For the excited sheet alone there are four published figures and the table above records all four rather than choosing between them: Talbot’s few hours, Hunt’s few hours, Cundell’s twenty-four, Ware’s one or two. The mixed developer keeps minutes. Solution B keeps days. Only solution A keeps.
Its fourth behaviour is the one a practitioner actually has to manage: it does not stop. Hunt sets out both failures in 1844, and they are the two ends of the same problem. “If it has not remained on the paper long enough, the opacity of the dark parts is not sufficient to ensure good positive copies; and if it remains too long, the light portions begin to darken, and, as this darkening proceeds with rapidity, the picture is soon rendered useless.” A modern developer exhausts, and a print left in the tray goes slowly wrong. This developer carries its own silver supply, so it has no natural end point at all — it simply starts fogging the highlights, and it accelerates while it does it. Talbot’s own instruction is to watch: “the artist should watch the picture as it developes itself, and when in his judgment it has attained the greatest degree of strength and clearness, he should stop further progress by washing it with the fixing liquid.”
Warmth is the accelerator, and it is the only control the worker has apart from time. Talbot warms gently before the fire and gets the image in a few seconds; without heat, a strong impression takes a minute or two. Hunt gives the reason in period terms and it is a good one: warmth speeds the change over the exposed parts, so the unexposed parts have less time to catch up, and “to preserve these parts quite transparent, it is therefore advantageous to accelerate the decomposition over the other parts by the aid of caloric”. That is a statement about the ratio of two rates, which is what development selectivity always is.
Two behaviours that Talbot reports as curiosities and that are not curiosities. In his experiment 3, a sheet moistened generously with gallo-nitrate darkens less than one moistened sparingly, and “the most rapid darkening takes place at the moment when the paper becomes nearly dry”; the edges of a moistened patch are more acted on than the middle. In his experiment 4, a sheet that is washed and dried after exciting is no longer visibly sensitive but has still recorded the exposure, which a later wash of gallo-nitrate brings out — “it receives a virtual instead of an actual impression from the light”. The first is a concentration effect at a drying edge and a warning about even coating; the second is the cleanest statement of the latent image anyone made in 1841.
What is not published. No capacity, in any source: this is a liquid brushed over a sheet and thrown away, and the question of how many sheets a litre will do was never asked. No pH, in any source, and the course will not compute one, because the bath contains a strong-acid silver salt, a weak acid at about 1.2 mol/L and a triprotic phenolic acid, and a calculated figure would be a guess dressed as a measurement. No temperature except “cold” for solution B and “warm” for the development.
Image characteristics
Section titled “Image characteristics”The image is metallic silver deposited directly in the paper fibres, with no binder. The Photographic Materials Group catalogues photogenic drawings, salted paper prints and calotypes together on exactly that basis, and it is why every characteristic below is partly a property of the sheet.
The colour is a brown deepening to black during development, and a neutral grey when finished. Talbot watches the exposed part “begin to darken, and finally grow entirely black”; Hunt says the exposed portions “become brown”. Ware explains the difference between that and the warm reds of a printed-out image on the physics rather than the chemistry: physical development builds silver particles “relatively much larger than those of the print-out process”, and large silver particles scatter and absorb neutrally where colloidal ones give colour. This is the clearest place in the course where image colour is a consequence of particle size.
The tonal scale is long and, by contemporary account, unusually beautiful. Hunt’s 1844 judgement is an aesthetic one and is recorded as an aesthetic one: the pictures preserve “not only the bold outline of the object, but its minute and delicate details”, and the gradation of shadow “is often given in a really wonderful manner, the lights of the picture decaying in soft and almost invisible tints into the deepest shades”. He values them for a different reason from the daguerreotype’s: what they offer the artist is the truthful middle tone, and “the charm of colour alone is wanting, and this is compensated by the harmony of the whole”.
The failure mode of over-development is fog and a compressed range, not simply a dark print. This is the characteristic consequence of physical development and Ware supplies a documented case: Alexander Greenlaw’s very large Indian negatives “are very dark”, cannot be read at all by reflected light, and their “high density of base fog may have resulted from the process of physical development adopted by Greenlaw which deposited excess silver in the shadow values”. Greenlaw had to paint out his skies with gouache to get a printable negative. A developer that brings its own silver will put silver anywhere there is a nucleus, and an unexposed area of a hand-coated paper has plenty.
The base carries a stain, and the sources are explicit about it. Cundell’s objection to the published
strengths is precisely that they “are apt to stain, or embrown the paper”, and he recommends half strength
partly for that reason. Two things can be doing it: free silver nitrate left in the sheet, reduced by the
paper and its sizing; and the oxidation products of gallic acid, which are coloured, as anyone who has
seen an iron gall ink knows. The staining field of this entry is deliberately left unset rather than
answered. In this formulary that field means what it means on a
PMK page — a proportional image stain that adds printing density where the silver
is — and no source held by this course establishes whether a calotype carries one. What the sources do
establish is overall embrowning of the base, which is a different thing and is a fault rather than a
feature.
The negative prints slowly, and the reason belongs to the sheet underneath rather than to this liquid. Residual silver iodide leaves a yellow ground, and Hunt is blunt about the cost: the printing light “has to permeate a piece of paper, the yellow tint of which offers considerable interruption to those rays which are active in producing chemical change”. Yellow absorbs blue, and blue is what the printing paper needs.
The mechanism
Section titled “The mechanism”First, what free silver ions do to a crystal that was made insensitive on purpose
Section titled “First, what free silver ions do to a crystal that was made insensitive on purpose”The sheet this liquid meets is not neutral ground. Its silver iodide was precipitated in an excess of iodide, and Ware’s section 23.4 explains what that leaves behind: a precipitated silver halide is non-stoicheiometric at its surface, and which ion it adsorbs there depends on which ion was in excess in the liquid it was precipitated from. Iodide in excess gives adsorbed iodide and a net negative surface charge — Ware’s “fixed” silver halide. Silver in excess gives adsorbed silver ions and a net positive charge — “sensitized” silver halide.
Brushing on 0.288 mol/L silver nitrate swings that equilibrium hard the other way. The crystal is the same crystal; its surroundings are the opposite of what they were; and three things change together.
- The surface charge reverses. A negative surface repels photoelectrons back into the interior of the crystal, where the silver they make is trapped by the lattice and invisible. A positive surface attracts them outward, where silver specks can grow at the surface, free of the lattice — which is where a developer can find them.
- A halogen acceptor appears. Photolysis liberates halogen, and unless something removes it, it simply re-oxidises the silver that has just been made. Ware’s first listed outcome is exactly that: without an acceptor, “the halogen will react with the surface photolytic silver, reversing the reaction above and re-forming the halide.”
The acceptor Ware proposes for a Talbot sensitiser is water and silver(I) acting together, and it works by driving a disproportionation that would otherwise not happen:
- And in the particular case of iodide, the silver ion is the only thing that makes that reaction go. Ware gives the equilibrium constant of that step for each halogen: about 500 for chlorine, 0.01 for bromine, and 10⁻⁹ for iodine. Left to itself, iodine released by photolysis is simply not disposed of — which, Ware says, “explains why fixed silver iodide is so stable to light”. Free silver ions change it completely, because the silver iodide that forms is so insoluble that it drags the whole equilibrium across: “the presence of free silver(I) ions in the sensitizer environment will profoundly modify these equilibria in favour of the products … all three halogens are totally disproportionated by water in the presence of Ag⁺.” Ware prints the bromine case and says “and similarly for iodine”; written out for iodine, on his instruction, that is:
So the silver nitrate in this bath is not merely a supply of metal for later. It is the switch that makes the sheet photosensitive at all, and it is the reason the least sensitive of the three halides became the basis of the fastest process of its day.
What the gallic acid adds at the exciting stage, and why it was later removed
Section titled “What the gallic acid adds at the exciting stage, and why it was later removed”Ware is careful here and this page will be too: the gallic acid “increased this sensitivity still further, but its presence was not essential, as Talbot recognised.” So it does something, and the process works without it.
Hunt’s 1844 account is the period explanation and it is better than it looks. He observes that gallic acid carries on in the dark a change that light has begun — “whenever the salt used is sufficiently under the influence of light to undergo a change, however slight it may be, the gallic acid will carry on the action in the dark and without heat” — and calls it “a most delicate test for any change produced, either by luminous or calorific radiation.” Read in modern terms, a reducing agent sitting in the sheet at the moment of exposure will begin amplifying latent-image specks as fast as they form, which raises the effective sensitivity and, inseparably, fogs the sheet.
That inseparability is why the ingredient was abandoned. Ware records John Adamson’s finding that stability was greatly increased by excluding the gallic acid, and that “the recognition that gallic acid caused so much mischief increasingly persuaded most photographers after 1846 to adopt Talbot’s simple solution of aceto-nitrate of silver as their preferred method for exciting iodized paper”. Ware’s summary of what the calotype’s evolution actually achieved is worth putting beside the usual story of progress: exposures at the end of the calotype era “were just as lengthy — or even lengthier — … but the stability of the sensitized paper was greatly improved”. The profession traded back the speed this ingredient bought, on purpose, to get a material it could rely on.
Development, and why it is physical rather than chemical
Section titled “Development, and why it is physical rather than chemical”Wall’s 1912 dictionary draws the distinction against exactly this case, and his definition is worth having in full because it is contemporary with the last practitioners.
In chemical development, “the sensitive salt affected by light is itself reduced to the metallic state by certain chemicals. We have no silver nitrate slowly depositing silver on the film, but the sensitive silver salt itself is reduced.” In physical development, the plate is “covered with adherent solution of nitrate of silver”, a reducing agent is added, and “the nascent silver from the adherent nitrate solution is deposited on the light-affected places forming the image.” Wall adds a caution that dates his sentence precisely: “at present there is no proof that the sensitive salt is itself reduced.”
Talbot’s gallo-nitrate carries silver nitrate. By Wall’s own criterion the calotype is therefore physically developed, and Ware says so directly: development with gallo-nitrate is “now known as physical development”, while gallic acid alone “relies entirely on the silver pre-existing in the sensitized paper to constitute the final image — a process now called chemical development.”
The half-reaction underneath is the simplest in this book:
Ware makes the point that this half-reaction cannot run on its own. Pure silver nitrate is not photosensitive, “because the reduction half-reaction of the silver ion to metal has no possible accompanying oxidation half-reaction to provide the necessary electron: nitrate ions are fully oxidised and water requires a larger redox potential for its oxidation than is provided by the Ag⁺/Ag couple.” Something has to give up the electron. In this bath it is the gallic acid, and in a sheet of plain silver-nitrate paper it is the cellulose and the sizing.
Why the reaction is selective is the whole art. Silver ions are everywhere in the wet sheet, and gallic acid is everywhere in the wet sheet, and yet the picture appears only where the light fell. The reason is that reducing a silver ion to a silver atom in free solution is difficult — a single atom is unstable and falls apart again — whereas depositing a silver atom onto an existing speck of silver is easy, because the metal is already there to accept it. The latent-image speck is a catalyst and a seed at once. Development is therefore a race between the catalysed reaction at the specks and the uncatalysed one everywhere else, and every control the worker has — acid, dilution, warmth, and above all when to stop — is a control on that ratio. When the uncatalysed reaction finally gets going, that is fog, and Hunt’s “the light portions begin to darken, and, as this darkening proceeds with rapidity …” is a description of it starting.
Why this developer can intensify a fixed negative, and no modern one can
Section titled “Why this developer can intensify a fixed negative, and no modern one can”Because it brings its own silver. A modern developer applied to a finished negative has nothing to work with: the halide has been dissolved away by the fixer, and the only silver present is already metallic and already part of the image. Gallo-nitrate applied to a finished calotype has a bath of silver ions and a reducing agent, and an image made of metallic silver that is an excellent catalyst for depositing more of it. So the shadows thicken and the whites, having no nuclei, stay white.
That is a physical developer used as an intensifier, and it is why Talbot could report reviving faded negatives and finding detail “which had not before been seen, having been latent all the time” — those were specks too small to have been developed the first time, still present, and now given a second and longer chance. It is also, in Ware’s reading, part of why Talbot stuck with halide fixation rather than hypo: a bromide-fixed negative can be revived, and one dissolved out with thiosulphate has lost the option.
Function of every ingredient
Section titled “Function of every ingredient”Silver nitrate — 98 g/L in liquid A, 0.577 mol/L; 4.9 per cent w/v and 0.288 mol/L in the mixed gallo-nitrate.
What it is. AgNO₃, relative molecular mass 169.873, the one silver salt soluble enough in water to be useful, and by weight much the largest component of this formula. Talbot specifies the crystallized salt and distilled water.
Why it is here. For two entirely different jobs at two different stages, which is unusual and is the key to the page. At the exciting stage it is a surface modifier: its ions adsorb onto the silver iodide already in the sheet and convert it from Ware’s “fixed” state to his “sensitized” state, switching the sheet on. At the development stage it is a silver reservoir: it supplies the metal that is deposited onto the latent image.
What it does chemically. At the first stage, almost nothing except adsorb — no reaction is needed for the sensitising, only a change of environment. It also, at both stages, acts as the halogen acceptor that disposes of the iodine photolysis liberates, without which the latent image would be destroyed as fast as it formed. At the second stage it is reduced, one electron at a time, at the surface of the latent-image specks.
What follows photographically. The sensitivity of the sheet, its instability, and the maximum density available. Ware notes that the alternative — Guillot-Saguez’s method, which puts no silver in the sheet at the iodising stage — had to use a much stronger 7.1 per cent aceto-nitrate here to compensate, and could not be diluted at all “because it would have resulted in extremely poor negatives, lacking in silver density”. The silver in this bath is a density budget.
More or less of it. Less is what everybody eventually chose. Cundell halved it in 1844 and, by Ware’s account, proposed ten- to forty-fold dilution in 1846; Greenlaw’s late developer used 0.2 per cent aceto-nitrate, which is about one twenty-fifth of the strength printed above. What is bought by that is stability and cleanliness; what is paid is sensitivity and maximum density. More of it gives a faster and denser sheet that stains, embrowns and fogs, which is precisely Cundell’s complaint. And Talbot’s own experiment 3 warns that quantity applied is not the same variable as strength: a sparingly moistened sheet darkened more than a generously moistened one, and the fastest darkening of all came at the moment the sheet was nearly dry.
What it interacts with. Everything on this page, mostly destructively. It is reduced by the gallic acid it is deliberately mixed with, which is why the mixture dies. It is precipitated by any halide, including the chloride in tap water and the iodide bath next door. It is an oxidiser, and the sheet carrying it is dried at a fire. And it stains skin, wood and cloth black over the following hours.
Acetic acid — 140 mL/L of the glacial acid in liquid A, 14 per cent v/v; 7 per cent v/v in the mixed gallo-nitrate, which is roughly 1.2 mol/L — the course’s arithmetic from the specific gravity of 1.05 that NIOSH gives for the glacial acid, not a figure any source prints for this bath.
What it is. CH₃COOH, relative molecular mass 60.05, a weak acid, miscible with water in all proportions, and — in the glacial form Cundell names explicitly — a corrosive, flammable liquid that freezes at 16.7 °C. Talbot writes “strong acetic acid”; Cundell writes “glacial acetic acid” for the same solution at the same strength, which is how the course knows what Talbot meant.
Why it is here. To buy time. It is the only ingredient in this formula whose job is to make a reaction slower rather than faster.
What it does chemically. Two effects that reinforce each other, and the sources describe them from two different directions. Ware: the role of the acetic acid “was to inhibit the spontaneous decomposition and spoiling of the excited paper”. Wall, defining physical development in general: the precipitation of silver from a silver nitrate and reducing-agent mixture is “delayed by the presence of organic acid such as acetic or citric”. A phenol is a far weaker reducing agent in acid than in alkali — every developer in Part VIII is built on that fact, in the opposite direction — so holding the bath strongly acid suppresses the reduction everywhere and leaves the catalysed reaction at the latent-image specks as the fastest thing happening.
What follows photographically. A developer with a usable working life instead of a beaker of black precipitate, an excited sheet that lasts hours instead of minutes, and a bath that develops selectively instead of everywhere at once.
More or less of it. No source varies it independently — Cundell keeps the same one-sixth proportion while halving everything else, and Guillot-Saguez’s aceto-nitrate uses acid too. The course therefore does not claim to know what happens if it is changed alone, and says so. What can be said from the chemistry is the direction: less acid means faster reduction, shorter keeping and more fog; more acid means a slower, cleaner, less sensitive bath, up to the point where the acid itself becomes the handling problem.
What it interacts with. The gallic acid, whose reducing power it suppresses. The paper, which it acidifies and which will hold that acidity into storage. And, as a hazard, alkalis and oxidisers — see Incompatibilities.
Gallic acid — a saturated solution, approximately 10 g/L in liquid B and 0.5 per cent w/v (about 0.029 mol/L) in the mixed gallo-nitrate.
What it is. 3,4,5-trihydroxybenzoic acid, C₇H₆O₅, relative molecular mass 170.12, a white hygroscopic polyphenol obtained in Talbot’s day, as Wall records, “by fermentation from powdered galls” — the growths a wasp induces on an oak, and the raw material of European iron gall ink for a thousand years before this. It dissolves about 1 per cent in cold water and 33 per cent in boiling.
Why it is here. It is the developing agent: the electron donor. It is also the smallest component of the formula by a wide margin, at about a tenth of the silver nitrate by weight — and, because the two relative molecular masses happen to be almost identical, 170.12 against 169.873, a tenth by moles as well. That coincidence is what makes the ratio under Mixing so easy to read.
What it does chemically. It reduces silver(I) to silver(0), and does so far faster at the surface of an existing silver speck than in free solution, which is what makes an image rather than a fog. Its own oxidation products in an acid silver bath are not established by any source held by this course, and the page will not name one; what is established is that they are coloured, which is where part of the base stain comes from, and that they precipitate silver out of a mixed bottle within minutes in light and within hours in the dark.
What follows photographically. The latent image becomes visible, and this is the moment photography stops being printing out. Hunt’s 1844 verdict is worth quoting in full because it is a contemporary judgement of importance rather than a modern one: “The discovery of the extraordinary property of the gallic acid, in increasing the sensibility of the iodide of silver, was the most valuable of the numerous contributions which Mr. Talbot has made to the photographic art.”
More or less of it. It cannot be increased: the solution is saturated, so the only way to add gallic acid is to warm the water, and a warmed saturated solution deposits its surplus on cooling. It can be decreased — by dilution, which Cundell recommends, or by removing it from the exciting solution entirely, which Adamson recommended and which most of the trade adopted after 1846. Both give a slower and much more reliable material. Removing it from the developer as well leaves nothing that develops, which is why the opposite simplification — gallic acid alone, no silver — is the other viable route and gives chemical rather than physical development.
What it interacts with. Silver nitrate, which it reduces, which is simultaneously the formula’s purpose and the formula’s shelf-life problem. Light, which starts that reduction. Warmth, which accelerates it. Iron, which gives the blue-black of iron gall ink and will stain any solution mixed in a steel vessel. And its own oxidation by air, which is why solution B keeps only a few days.
A note on the reagent Talbot offers as a substitute and rejects. He writes that “instead of employing a solution of crystallized gallic acid for the liquid B, the tincture of galls diluted with water may be used, but he does not think the results are altogether so satisfactory.” A tincture of galls is an alcohol extract of the same oak galls, containing gallotannins as well as gallic acid — the material the course files under tannic acid. It is not published as a variant here because no source gives a strength for it, and because Talbot’s own verdict on it is negative.
Interactions
Section titled “Interactions”Silver nitrate and gallic acid — a deliberate incompatibility, timed. This is the only formula in the book whose two solutions are combined because they react with each other, and whose instruction is therefore to combine as little of them as possible as late as possible. Everything else on this page is a consequence: the two bottles, the drachm tube, the candlelight, the few minutes.
Acetic acid and gallic acid — the brake. The acid does not stop the reduction, it slows it, and the whole formula lives in the window that slowing opens. Take the acid out and the mixture is a precipitate before it reaches the brush; put more in and the developer becomes too sluggish to bring an image out before the sheet dries, which Cundell says is itself a fault — “if the paper be allowed to dry before washing off the gallo-nitrate, the lights sink and become opaque”.
Silver ions and the iodide already adsorbed on the sheet. The sensitising interaction, described under The mechanism. It is a competition for the crystal surface, and this bath wins it by weight of numbers.
Silver ions and the paper itself. Ware’s section 23.1 records that paper impregnated with silver nitrate alone can be printed on, the reducing partner being “the organic matter — cellulose of the paper, or the starch or gelatin sizing agent”. That reaction does not stop just because a better one is available: it is a source of base fog and of the embrowning Cundell complains of, and it is worse the more silver is left in the sheet. Cundell’s habit of washing the excess gallo-nitrate off immediately in several changes of water is a direct attack on it. That his twenty-four hours of keeping, against Talbot’s few hours, follows from the washing is the course’s reading and not Cundell’s claim; he reports the keeping and the washing and does not connect them.
Warmth and every reaction here at once. Heat accelerates the wanted reduction at the specks, the unwanted reduction in the background, the oxidation of the gallic acid by air, and the drying of the sheet. Hunt is aware of the trade and states it as a ratio problem; Cundell manages it by keeping the sheet moving in front of the iron so that no part dries before the rest.
Gallic acid and iron. Not a photographic interaction but a practical one with a thousand years of precedent: gallic acid and iron(III) give the blue-black of iron gall ink. Glass or porcelain only.
Gallo-nitrate and thiosulphate, in either direction. A trace of hypo carried backwards on a wet hand or a shared dish dissolves silver halide and leaves a dead patch that will not appear until development. A trace of gallo-nitrate carried forwards into the fixing bath throws silver out of it. In any silver process contamination travels upstream at least as often as downstream.
And an interaction with the finished photograph that this formula creates and does not resolve. Talbot’s proviso in The Pencil of Nature is that a negative will yield an almost unlimited number of copies “provided that every portion of iodine has been removed from the picture before the copies are made”. Iodine and sunlight together attack image silver where neither alone does much. The developer is not the cause, but the negative it makes is the thing at risk, and the same developer is the repair.
Variants
Section titled “Variants”None of the variants below is published as a formulary entry of its own. They are here because Ware treats the exciting solution and the developer as two of the four axes on which the whole calotype family varies, and a page about Talbot’s version that did not name the others would misrepresent how the process was actually worked.
Talbot’s own unpublished simplification. Ware records from notebook Q that Talbot “also tested simpler, somewhat slower versions, in which the exciting solution was aceto-nitrate of silver without added gallic acid, and the developing solution was gallic acid alone, but these procedures did not see publication.” Both of the two great later simplifications were therefore in the inventor’s own notebooks before anyone else proposed them, and he chose speed over stability: Ware reads the complexity of the published formula as arising “from Talbot’s quest to maximise the sensitivity in order to minimise exposure times in the camera”, and says that “in effect he was prepared to sacrifice a degree of chemical stability for the advantages of shorter exposures.”
Cundell, Philosophical Magazine, May 1844 — halve everything. The first published modification, and the most consequential. Cundell restates the formula at its original strength in different measures — fifty grains of silver nitrate to the ounce with one sixth of its volume of glacial acetic acid, which is arithmetically Talbot’s solution A — and then says plainly that “for many purposes these solutions are unnecessarily strong, and, unless skilfully handled, they are apt to stain or embrown the paper”, and that where extreme sensitiveness is not required “they may with advantage be diluted to half the strength, in which state they are more manageable and nearly as effective.” He also tightens three things Talbot left loose: five to ten seconds of contact rather than half a minute, an immediate and thorough wash rather than a single dip, and a stated life for the mixture of a few minutes. His sheet then keeps a day.
Cundell again, August 1846 — ten to forty times. Ware records a further paper in which the gallo-nitrate “could be diluted anywhere between ten and forty times”. The course has not read it and reports it as Ware reports it. Ware’s comment on the direction of travel is the one to keep: “by trading off sensitivity against more certain results, Cundell recommended the very opposite of what Talbot had sought to achieve.”
John Adamson — leave the gallic acid out of the exciting solution. Ware records Adamson’s finding that stability was greatly increased by doing so, and that after 1846 most photographers used plain aceto-nitrate of silver — solution A of this page — to excite the paper, keeping the gallo-nitrate for development only. This is the single largest change ever made to the formula and it consists of deleting an ingredient from one of its two uses.
Guillot-Saguez, Paris, 1847 — all the silver at this stage. Having iodised his paper with potassium iodide alone and no silver at all, Guillot-Saguez had to put all the silver in here, using a strong 7.1 per cent aceto-nitrate with no gallic acid. Ware notes the trade-off precisely: dilution “was not an option here because it would have resulted in extremely poor negatives, lacking in silver density”. Development was still carried out with Talbot’s gallo-nitrate.
Le Gray, Flacheron and the waxed-paper school — gallic acid alone. Ware’s Table 1 shows the developer splitting into two families, and this is the other one: a saturated 1 per cent gallic acid solution with no silver, which is chemical rather than physical development and relies entirely on the silver already in the sheet. It gives a thinner, cleaner negative, no intensification and no revival.
Greenlaw, India, published 1869 — the two families reconciled. Gallic acid at 1 per cent with a very dilute 0.2 per cent aceto-nitrate: gallic acid development with just enough silver to build density. That is Talbot’s formula diluted about twenty-five-fold on the silver side, and Ware records that “Greenlaw’s process” was still being cited as the best of calotype practice in 1912.
Talbot’s second patent, No. 9,753 of 1 June 1843. Hunt lists its nine claims. Two bear on this page. The third is io-gallic paper: iodised paper simply washed with gallic acid, which “will keep in a portfolio” and is made sensitive later by a wash of silver nitrate — the two components of the gallo-nitrate separated in time instead of in bottles, one of them stored in the sheet. The fourth goes further: iodised paper washed with a mixture of twenty-six parts of saturated gallic acid to one part of the usual silver solution “can then be dried without fear of spoiling, may be kept a little time, and used without further preparation”. That inverts the whole proportion of this page — gallic acid in vast excess and silver as the trace — and buys keeping with it.
Talbot’s third patent — gallo-nitrate off paper altogether. Hunt describes albumen-coated unglazed porcelain sensitised with gallo-nitrate exactly as paper is, and a polished steel plate coated with albumen and potassium iodide, then washed while still warm with “an alcoholic solution of gallo-nitrate of silver, of moderate strength”, with the note that “if the plate be cold, the sensibility is considerably lower”. No strength is given for the alcoholic version, so it is recorded and not published.
Safety
Section titled “Safety”Level B, and the level is set by the silver nitrate rather than by anything exotic.
Silver nitrate governs. Its harmonised classification under Regulation (EC) No 1272/2008 is signal word Danger, with H272 (may intensify fire; oxidiser), H314 (causes severe skin burns and eye damage), and H400 and H410 (very toxic to aquatic life, with long-lasting effects). Liquid A is a 9.8 per cent solution, which is strong enough to burn an eye and certain to stain a hand: the stain appears hours after the splash and has to wear off with the skin. Sealed splash goggles rather than safety glasses, nitrile gloves, an apron, and eyewash within reach before the bottle is opened. The regime is in the silver nitrate handling SOP and the full classification, with the argyria warning, is on the chemical’s own page.
Glacial acetic acid is the second hazard and is under-rated because it smells like vinegar. PubChem’s aggregate of the ECHA notifications gives Danger with H226 (flammable liquid and vapour), H314 and H318. International Chemical Safety Card 0363 asks for a face shield or eye protection when it is handled, and notes that the pure acid is a solid below 16.7 °C — which is why a bottle can be found frozen in an unheated darkroom and why warming it to pour is a temptation to resist near a flame. Add acid to water and never the reverse. The 7 per cent v/v in the finished bath is a much milder thing than the bottle it came from; the bottle is what the hazard statements describe.
Gallic acid is a powder, and that is what sets its handling. The aggregated notifications give Danger, with H319 from about three quarters of notifiers, H315 from about three quarters, H335 from about seven in ten, and H318 — causes serious eye damage from about one in five. It is hygroscopic, and it is a light powder that travels. Weigh it over a tray, in still air, with extraction or an enclosure; once it is a made-up saturated solution, the handling is much easier.
What is not a hazard here, and why. There is no toxic gas, no heavy metal beyond silver itself, no chromium, no cyanide and no mercury. Nothing in this formula is heated in normal use, and the two solutions are mixed at room temperature with no exotherm worth the name. The oxidiser and the flammable liquid are in the same bottle, which sounds worse than it is at 14 per cent v/v in water — but it is a real reason to keep the concentrated acid and the solid silver nitrate on different shelves, which is under Incompatibilities.
Storage
Section titled “Storage”Solution A keeps and is the only thing here that does. An acidified silver nitrate solution should be kept in amber glass, capped, cool and dark, in a vessel that has never held food, labelled with its strength and its date per the labelling SOP. Silver nitrate solutions are decomposed by light faster than the solid is, and the acid does nothing to prevent that.
Solution B keeps a few days and Cundell says so. “The gallic acid solution will not keep for more than a few days, and only a small quantity, therefore, should be prepared at a time.” A polyphenol in water takes up oxygen from the air; the solution discolours as it does, and a discoloured solution is a partly spent one. The solid keeps far better than the solution, provided the jar is closed, because gallic acid is hygroscopic and an open jar gains water.
The mixture keeps minutes and is not stored at all. Talbot: mix only a small quantity at a time, “because the mixture does not keep long without spoiling”. Cundell: “it ought not to be prepared until the operator is quite ready to apply it.” Hunt supplies the reason and the one mitigation there is — darkness. A mixture of gallic acid and silver nitrate “will often remain clear for many hours in the dark” but precipitates almost immediately in weak diffused light.
The excited sheet keeps somewhere between one hour and three months, and the spread is the honest answer. Talbot says three months in a press is common but not uniform and recommends a few hours; Hunt says two or three months has been found but the sheet is often ruined by spontaneous change in the dark; Cundell, who washes his sheets thoroughly, says twenty-four hours at least “preserving all its whiteness and sensibility”; Ware, reading the practice as a whole, says an hour or two before it fogged. Four figures across a factor of two thousand, from four people who all worked the process. What they agree on is that it is unpredictable, and unpredictability is why the iodised sheet exists as a separate, storable stage.
The finished negative. The Photographic Materials Group treats photogenic drawings, salted paper prints and calotypes as one storage problem: a stable temperature between 18 and 30 °C to avoid embrittlement, at 30 to 50 per cent relative humidity. That protects the paper support rather than the image. Talbot’s own chemical proviso is separate and is about the iodine left in the sheet.
Incompatibilities
Section titled “Incompatibilities”Silver nitrate and gallic acid, on a shelf. In this formula they are mixed on purpose, in small quantities, seconds before use. In a cupboard they are an oxidising silver salt and a reducing polyphenol sharing a space, and a leak from one bottle into the other spoils both. See incompatibilities.
Silver nitrate and any soluble halide, in a vessel. The iodide bath of the previous stage is the obvious danger, and a splash either way ends both solutions in a curd of silver iodide. Separate dishes, separate brushes, separate everything, and this is the reason a calotype worker’s bench had more vessels on it than the process seems to need.
Silver nitrate and tap water. Chloride precipitates silver chloride and converts part of the silver into the wrong halide, in the wrong place, with a different sensitivity. Talbot specifies distilled water for both solutions of this formula and the specification is not decorative.
Silver nitrate and combustible material. Paper, cloth and wood loaded with an oxidiser and then warmed. The historical instance is on this page.
Glacial acetic acid, alkalis and oxidisers. A concentrated carboxylic acid neutralises violently with strong alkali, and Chemical Safety Card 0363 records that it reacts with oxidants. It is also a flammable liquid, which puts it in a different storage class from the aqueous solutions it ends up in.
Gallic acid and iron. Iron gall ink. Not a safety matter; a contamination matter that will stain a solution and a sheet alike, and a reason for glass rather than steel.
Gallo-nitrate and any thiosulphate, in either direction. Contamination upstream kills the sheet; contamination downstream kills the fixer.
And the one this page has to name explicitly: an acidified silver solution and potassium cyanide. Cyanide salts liberate hydrogen cyanide on contact with acid. The period literature puts them together on the same pair of hands. The course does not.
Everything from this formula is a silver-bearing stream, including the water it is washed off into.
Solution A obviously carries silver. So does the mixture. So — and this is the one that gets poured away — does the rinse water: Cundell washes the excess gallo-nitrate off each sheet in several changes of clean water immediately after exciting, and every one of those changes carries dissolved silver nitrate. So do spoiled sheets and offcuts, which are solid silver-bearing waste and not paper recycling.
None of it goes to a drain. All of it is collected into one labelled container under the silver-bearing waste SOP. The reason is not administrative: the harmonised classification of silver nitrate carries H400 and H410, very toxic to aquatic life with long-lasting effects, and a bath that looks like clear water and is legally an aquatic toxicant is exactly the kind that gets emptied down a sink.
The gallic acid fraction on its own would be a minor matter — no aquatic hazard statement is notified for it, and no acute toxicity statement at all — but it is never on its own here. What is bottled is the mixture, and the mixture is a silver stream by design.
What may be discharged is decided where you live, and no page written here can tell you what the rule is there. The disposal caveat explains why the course answers that question with a jurisdiction rather than a number, and the general chemical waste SOP is what to do in the meantime.
Troubleshooting
Section titled “Troubleshooting”The faults below are labelled by where they come from: what Talbot published, what his contemporaries reported in the 1840s and 1850s, and what Ware establishes from the chemistry. Where a reading is the course’s own, it says so.
The mixture goes cloudy or throws a dark precipitate before it reaches the paper. Reported by Talbot, by Hunt in 1844 and by Cundell. The silver has been reduced in the bottle. Two causes, and both are in the sources: too much time, and any light at all. Hunt’s contrast between “almost immediately” in weak diffused light and “many hours” in the dark is the diagnostic. Mix less, mix later, mix darker.
The paper stains or embrowns. Reported by Cundell in 1844. The published strengths are too high for the handling, which is his exact complaint: they “are apt to stain, or embrown the paper” unless skilfully handled. His remedy is dilution to half strength. Course reading, offered as one: free silver nitrate left in the sheet is reduced by the cellulose and sizing, which is Ware’s mechanism for silver-nitrate-only papers, so Cundell’s other habit — washing the excess off at once in several changes of water — attacks the same fault from the other end.
The highlights start to darken and the picture is lost quickly. Reported by Hunt in 1844. Development has run past its end point, and this developer has no natural one. “As this darkening proceeds with rapidity, the picture is soon rendered useless as an original from which copies can be taken.” Watch it; stop it with the fixing liquid.
The shadows are not opaque enough to print from. Reported by Hunt in 1844. The opposite fault: stopped too soon, or under-exposed. Talbot’s remedy for the second is to repeat the wash of gallo-nitrate and warm again, which is the same operation as reviving a faded negative.
The lights sink and become opaque. Reported by Cundell in 1844. The sheet was allowed to dry before the gallo-nitrate was washed off. His remedy is to retard the drying by wetting the back of the paper, or to bring the image out over the vapour from hot water or a horizontal jet of steam. Course reading, offered as one: Talbot’s experiment 3 points at the same physics from the other side, since he finds the fastest darkening at the moment the sheet becomes nearly dry, which is also the moment the solution left in it is most concentrated. Talbot does not give that explanation and neither does Cundell.
Uneven development, with the edges of the wetted area darker than the middle. Reported by Talbot, experiment 3. A concentration and drying effect at the boundary of the wet patch. Cundell’s answer is to flood a glass slab with the solution and bring the paper down onto it, keeping the sheet moving and never letting one part dry before the rest.
The negative is very dark, fogged, and reads only by transmitted light. Reported by Ware, of Greenlaw’s large negatives. Physical development depositing excess silver in the shadow values, compressing the density range. It is the characteristic over-run of this developer rather than of any one step, and it is the strongest argument for the dilutions Cundell proposed.
The excited sheet fogs before it can be exposed. Reported by Talbot, Hunt, Cundell and Ware, with four different keeping figures. Free silver in the sheet plus the gallic acid in the sheet plus time. The remedies the period found were, in order: wash the excess off at once (Cundell); dilute the solutions (Cundell); and leave the gallic acid out of the exciting solution altogether (Adamson, and after 1846 almost everybody).
A finished negative fades after a few printings. Reported by Talbot. Sunlight acting with residual iodine. His own remedy was another wash of gallo-nitrate and a warming, which redeposits silver and sometimes reveals detail never seen before — the only repair in this book that a modern developer cannot perform.
The negative prints very slowly. Reported by Hunt in 1844 and by Talbot’s 1843 patent. The yellow silver iodide of the sheet underneath, absorbing the blue and ultraviolet the printing paper needs. The period cure is the hot hyposulphite bath of the second patent; Ware records that many photographers did not bother.
Experiments
Section titled “Experiments”None of these makes a calotype, because this page is not a procedure and the course does not teach the process. Each tests a claim the page makes, using chemistry the course already teaches at Level B: the plain salting solution and salted paper sensitiser, with gallic acid and acetic acid from the shelf. Read the silver nitrate handling SOP first, and note that every one of these produces silver-bearing waste.
Hunt’s 1844 experiment, which is the cheapest and settles the most. Hypothesis: a gallic acid and silver nitrate mixture is decomposed by light and keeps in the dark. Control: a stoppered tube in a lightproof box. Variable: illumination. Mix equal volumes of a 5 per cent silver nitrate solution and a cold saturated gallic acid solution in four identical tubes. Put one in the dark, one on a north-facing windowsill, one in direct sun, and one in the dark with acetic acid added to 7 per cent v/v. Record the time to first visible cloudiness in each. Hunt predicts “almost immediately” for the lit tubes and “many hours” for the dark one; Ware and Wall between them predict that the acidified dark tube outlasts the plain dark one. If it does not, this page’s account of what the acetic acid is for is wrong.
The physical development experiment, which is the one worth doing. Hypothesis: a developer carrying silver nitrate deposits silver from solution onto an existing image, and a developer without it cannot. Control: a printed-out salted paper strip, fixed and washed, treated with cold saturated gallic acid alone. Variable: the presence of silver nitrate in the treating solution. Make two identical, deliberately weak salted paper prints of a step wedge. Fix, wash and dry both. Treat one with gallic acid alone and the other with a freshly mixed gallo-nitrate at Cundell’s half strength, both for the same time in the same light, then wash and dry. Read both step wedges against the originals with a densitometer, or against a grey scale by eye. The gallic-acid strip should be essentially unchanged; the gallo-nitrate strip should gain density in the darker steps and much less in the light ones. That difference is physical development, and it is Talbot’s revival of a faded negative reproduced with the course’s own chemistry.
How much of the sensitivity the gallic acid actually buys. Hypothesis: gallic acid in the sensitising solution raises the speed of the sheet and shortens its life, and both effects come together. Control: half a batch of salted paper sensitised in the ordinary way. Variable: gallic acid added to the sensitiser at 0.5 per cent w/v. Sensitise, dry in the dark, and split each half again: expose one pair immediately under a step wedge, and the other pair after twenty-four hours in a drawer. Four strips, one variable, two time points. Ware’s account predicts that the gallic acid strip is faster fresh and worse after a day, which is the trade the whole profession made in 1846.
What the acid is worth, measured as working time. Hypothesis: the acetic acid extends the usable life of the mixture rather than changing what it eventually does. Control: gallo-nitrate mixed with no added acid. Variable: acetic acid at 0, 3.5, 7 and 14 per cent v/v. Mix four small quantities, and every minute brush a stripe of each across a strip of the same salted paper and note when a stripe first fails to develop a visible image on an identically pre-exposed area. Cundell’s “a few minutes” is the figure to test against. This is also the experiment that tells you whether the acid slows development itself, which the sources assert and none of them quantifies.
Saturation as a variable nobody controlled. Hypothesis: the strength of solution B varies with room temperature enough to matter. Control: none needed; this is a measurement. Saturate gallic acid in distilled water at 5, 15 and 25 °C, filter each, evaporate a measured volume of each to dryness and weigh the residue. Wall’s figures predict about 1 per cent cold and 33 per cent boiling, so the spread across an unheated room in winter and a warm one in summer is worth knowing. The only quantity this formula’s second solution has is that number, and in the years since 1841 no source held by this course appears to have published it for a darkroom at a stated temperature.
Sources for this page
14 cited · checked 2026-09-06
- 01An Account of some recent Improvements in Photography, in Abstracts of the Papers Printed in the Philosophical Transactions of the Royal Society of London, volume 4William Henry Fox Talbot, 1841§ Pages 312 to 315, which are the whole of the published account. In particular: the liquid prepared by dissolving 100 grains of crystallized nitrate of silver in two ounces of distilled water and adding to it one-sixth of its volume of strong acetic acid, called A; the saturated solution of crystallized gallic acid in cold distilled water, called B, of which "the quantity dissolved is very small"; the instruction to mix A and B in equal volumes "but only mix a small quantity of them at a time, because the mixture does not keep long without spoiling", and the naming of that mixture the gallo-nitrate of silver; the washing of the iodised sheet on the marked side by candlelight, the half minute's rest, the dip into water, the blotting and the cautious drying at a distance from the fire; the option of using the paper moist; the three months in a press against the recommendation to use it within a few hours; the tincture of galls diluted with water offered as a substitute for B and judged not altogether so satisfactory; the sensitivity "which transcends a hundred times or more that of any kind of photographic paper hitherto described" and the one second of dull winter daylight that leaves a latent and invisible impression; development by washing once more with the gallo-nitrate and warming gently before the fire, the exposed part darkening in a few seconds, the weaker impression brought out by repeating the wash and the stronger needing no heat and appearing in a minute or two; the artist watching the picture develop and stopping it with the fixing liquid; the revival of a faded negative by another wash of gallo-nitrate and a warming, with details appearing that had never been seen; and experiments 1 to 4, including the finding that the lesser quantity of gallo-nitrate darkens the paper more than the greater and that the fastest darkening is at the moment the sheet becomes nearly dryarchive.org/download/jstor-110751/110751_djvu.txttier 1, primary2026-09-06
- 02Researches on Light: an examination of all the phenomena connected with the chemical and molecular changes produced by the influence of the solar rays, embracing all the known photographic processes, and new discoveries in the art, 1st editionRobert Hunt, 1844§ Section 75, which reprints Talbot's two solutions and the gallo-nitrate word for word three years after the Royal Society reading, adding that the excited paper "has been found to keep for two or three months" but "is often rendered useless by spontaneous change, in the dark"; section 76, on the discovery of gallic acid's effect on silver iodide as the most valuable of Talbot's contributions; section 77, on the yellow tint of the negative interrupting the rays active in producing chemical change; section 78, the period account of what the gallic acid is doing — that it separates the metallic oxides very readily from the powerful acids, that a mixture of gallic acid and nitrate of silver precipitates almost immediately in weak diffused light while the same solution "will often remain clear for many hours in the dark", that the formation of "the gallate of the oxide of silver" proceeds over the light-struck parts with an energy equal to the intensity of the light that acted on them, that warmth accelerates it, and that some experience is required to check the action at the proper time because the lights begin to darken if it runs on; and section 79, Hunt's own comparison of gallic acid development across silver nitrate, chloride, bromide, tartrate, oxalate, phosphate, carbonate, benzoate, cyanate and ferrocyanate papersarchive.org/stream/b2930488x/b2930488x_djvu.txttier 1, primary2026-09-06
- 03A Manual of Photography, 4th editionRobert Hunt, 1854§ History of Photography, section II, The Calotype, which reprints Talbot's specification with the two solutions; section III, Improvements in Calotype, for the nine claims of the second patent, including the hot hyposulphite bath, the warm iron behind the paper in the camera, io-gallic paper, and the twenty-six parts of saturated gallic acid to one of the usual silver solution; Pictures on Porcelain Tablets, for the third patent's albumenised porcelain sensitised with gallo-nitrate and for the alcoholic solution of gallo-nitrate of silver washed over a warm steel plate; and Practice of Photography, Mr Cundell's calotype process, reprinted from the Philosophical Magazine of May 1844 — the nitrate of silver at fifty grains to the ounce of distilled water with one-sixth part of its volume of glacial acetic acid, the saturated gallic acid, the judgement that "for many purposes these solutions are unnecessarily strong, and, unless skilfully handled, they are apt to stain or embrown the paper", the recommendation that they "may with advantage be diluted to half the strength", the gallic acid solution keeping "not more than a few days", the mixed gallo-nitrate that "speedily changes, and will not keep for more than a few minutes", the mixing in equal volumes by a graduated drachm tube, the five or ten seconds of contact at the exciting stage, the twenty-four hours' keeping of the excited sheet, the bringing-out with radiant heat from an iron held within an inch or two, and the warning that if the paper dries before the gallo-nitrate is washed off "the lights sink and become opaque"archive.org/details/manualofphotogra00huntrichtier 1, primary2026-09-06
- 04Argyronomicon: Silver Photographs on Paper — Chemical History of their Invention, Deterioration, and ConservationMike Ware, 2019§ 7.8.2 Exciting, for the readings that put numbers on Talbot's grains and ounces — the exciting solution containing 4.9 per cent w/v (0.288 M) silver nitrate, 7 per cent v/v acetic acid and 0.5 per cent w/v gallic acid, made by mixing equal volumes of an aceto-nitrate of silver stock of 9.8 per cent w/v (0.577 M) silver nitrate and 14 per cent v/v strong acetic acid with a saturated gallic acid solution of approximately 1 per cent w/v; the statement that the silver iodide already in the paper is made sensitive by the excess silver ions, that the gallic acid increased the sensitivity further but was not essential, that the acetic acid inhibited spontaneous decomposition, and that even so the excited paper "remained critically unstable" and had to be exposed within an hour or two before it fogged. 7.8.3 and 7.8.4 for the exposure and for development being physical rather than chemical, for the deposition of more silver upon the image to build density, and for the revival of under-exposed or faded negatives. 7.8.5 and 7.8.6 for the fixing, the larger image silver particles that follow from physical development, and Talbot's testing in notebook Q of an exciting solution without gallic acid and a developer of gallic acid alone, neither of which he published. 6.2 for the discovery in the days following 20 September 1840 and the hundredfold gain; 6.7 for patent No. 8,842 of 8 February 1841 and No. 9,753 of 1 June 1843; the chronology table for 23 September 1840. 10.5 for Cundell's published objection that Talbot's solutions were "unnecessarily strong", for the further dilution of ten to forty times proposed in the Philosophical Magazine of August 1846, for John Adamson's advice that stability was greatly increased by excluding the gallic acid, and for the consequent adoption after 1846 of aceto-nitrate of silver alone as the exciting solution. 11.1 for Blanquart-Evrard's claims; 11.2 for Guillot-Saguez's 7.1 per cent aceto-nitrate with no gallic acid; 13.1 for Flacheron's developer of saturated gallic acid alone; 14 for Greenlaw's 1 per cent gallic acid with 0.2 per cent aceto-nitrate and for the base fog attributed to physical development; 15 and Table 1 for the two categories of exciting and the two means of development. 23.1 to 23.6 for the chemical model: the Ag+ + e– = Ag half-reaction that has no accompanying oxidation in pure silver nitrate, the role of halogen acceptors, the adsorption of impurity ions that makes a silver halide "sensitized" or "fixed", and the disproportionation equilibria that are extensive for chlorine, slight for bromine and negligible for iodine until free silver ions drive them, with the consequence that fixed silver iodide "is completely insensitive even to direct sunlight"mikeware.co.uk/downloads/Argyronomicon.pdftier 2, specialist2026-09-06
- 05The Pencil of NatureWilliam Henry Fox Talbot, 1844§ Brief Historical Sketch of the Invention of the Art, for Talbot's own retrospective account of the calotype and of the sensitiveness of the paper, and for the proviso that a negative will yield an almost unlimited number of copies "provided that every portion of iodine has been removed from the picture before the copies are made"gutenberg.org/cache/epub/33447/pg33447.txttier 1, primary2026-09-06
- 06The Dictionary of Photography and Reference Book for Amateur and Professional Photographers, 9th editionE. J. Wall, edited by F. J. Mortimer, 1912§ Development and Developers, for the definition of physical development against chemical development — the wet plate covered with adherent silver nitrate solution, the nascent silver deposited on the light-affected places, the statement that "at present there is no proof that the sensitive salt is itself reduced", and the remark that the precipitation of silver from a reducing agent and silver nitrate is "delayed by the presence of organic acid such as acetic or citric"; and Gallic Acid, for the formula HC7H5O5 = 170, the solubility of 1 per cent in cold water and 33 per cent in boiling, the preparation by fermentation from powdered galls, and the use in the collodion and waxed-paper processesarchive.org/details/dictionaryofphot1912walltier 1, primary2026-09-06
- 07Photographic Negatives: Nature and Evolution of Processes, 2nd editionMaria Fernanda Valverde, Advanced Residency Program in Photograph Conservation, 2005§ Calotype Negative — Process, steps 2 to 4: the sensitising of the iodised sheet, the exposure of one to ten minutes made while the paper was still moist, and the development with the same solution used to sensitise itrit.edu/ipi/sites/rit.edu.ipi/files/documents/negatives_poster_booklet.pdftier 1, primary2026-09-06
- 08Photogenic Drawings, Salted Paper Prints, and Calotype Prints, in the Photographic Materials Group section of the AIC Conservation WikiAmerican Institute for Conservation, Photographic Materials Group (Luisa Casella, Amanda Maloney, Stephanie Watkins)§ Identification Characteristics, for the image layer of silver deposited directly in the paper support and the range of image colour across photogenic drawings, salted paper prints and calotypes; and Conservation, Housing and Storage Considerationsconservation-wiki.com/wiki/Photogenic_Drawings,_Salted_Paper_Prints,_and_Calotype_Printstier 1, primary2026-09-06
- 09PubChem compound summary: Silver Nitrate (CID 24470)National Center for Biotechnology Information§ GHS classification — the harmonised entry under Regulation (EC) No 1272/2008, and the notified additionspubchem.ncbi.nlm.nih.gov/compound/24470tier 1, primary2026-09-06
- 10PubChem compound summary: Gallic acid (CID 370)National Center for Biotechnology Information§ GHS classification, aggregated from the ECHA C&L Inventory; solubility; physical descriptionpubchem.ncbi.nlm.nih.gov/compound/370tier 1, primary2026-09-06
- 11PubChem compound summary: Acetic Acid (CID 176)National Center for Biotechnology Information§ GHS classification, aggregated from the ECHA C&L Inventory; physical description; solubilitypubchem.ncbi.nlm.nih.gov/compound/176tier 1, primary2026-09-06
- 12International Chemical Safety Card 0363: Acetic acidPrepared by an international group of experts on behalf of the International Labour Organization and the World Health Organization, with the financial assistance of the European Commission, 2010§ Physical properties; chemical dangers; effects of short-term exposure; storageinchem.org/documents/icsc/icsc/eics0363.htmtier 1, primary2026-09-06
- 13NIOSH Pocket Guide to Chemical Hazards (DHHS (NIOSH) Publication No. 2005-149)National Institute for Occupational Safety and Health, 2007§ Entry for acetic acid (npgd0002) — specific gravity 1.05, and the note that the pure compound is a solid below 62 °F (16.7 °C), which is where the freezing point and the molarity arithmetic on this page come fromcdc.gov/niosh/npgtier 1, primary2026-09-06
- 14PubChem compound summary: Potassium Cyanide (CID 9032)National Center for Biotechnology Information§ GHS classification — H300, H310 and H330, fatal if swallowed, in contact with skin or if inhaled; and the reactivity with acids. Cited only for the period stain-removal instruction this page refusespubchem.ncbi.nlm.nih.gov/compound/9032tier 1, primary2026-09-06
Formulas, hazard statements, historical dates and process descriptions on this page were checked against the sources above on the date shown. Safety data changes: obtain the current safety data sheet for the product you actually buy before you open it.