Gallic acid

The latent image was not discovered; it was developed. Gallic acid is the substance Talbot washed over an exposed sheet in 1840 to bring out an invisible impression, and it is the reason a calotype exposure takes seconds where a photogenic drawing took an hour.

In photography

Talbot’s gallo-nitrate of silver. His 1841 account gives the recipe exactly. Solution A is 100 grains of crystallised silver nitrate in two ounces of distilled water with one sixth of its volume of strong acetic acid. Solution B is “a saturated solution of crystallized gallic acid in cold distilled water. The quantity dissolved is very small.” Mixed in equal volumes they make what Talbot names the gallo-nitrate of silver, and he warns to mix only a little at a time “because the mixture does not keep long without spoiling”. Washed over iodised paper by candlelight, left half a minute, dipped in water and blotted, that 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”. A second’s exposure on a dull winter day suffices — and leaves nothing to see. The impression “is latent and invisible”, and is made visible by washing the sheet once more with the same gallo-nitrate.

One solution doing two jobs. That is the calotype’s economy and it is worth stating plainly: the same liquid sensitises and develops. Talbot offers one substitution and rejects it in the same breath — “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”. Hunt’s 1854 manual repeats the method and records the variants that grew round it: io-gallic paper, iodised paper simply washed with gallic acid and kept in a portfolio until silver nitrate makes it sensitive; and a paper washed with twenty-six parts of saturated gallic acid solution to one part of the usual silver nitrate solution, which “can then be dried without fear of spoiling”.

Its afterlife is short. Wall’s 1912 dictionary already writes it in the past tense: gallic acid “was used for developing in the collodion and waxed-paper processes, and has been also suggested for developing gelatino-chloride paper”. By then the developing agents of the gelatine era — hydroquinone, metol, amidol, glycin — had displaced it entirely, and gallic acid’s survival in the trade was as the raw material for one of them.

⚗Physical development, and the molecule pyro is made from

A developer that contains silver is a different kind of developer. Wall’s 1912 definition of chemical development is drawn against precisely this case: in chemical development “we have no silver nitrate slowly depositing silver on the film, but the sensitive silver salt itself is reduced”. Talbot’s gallo-nitrate does contain silver nitrate, so by Wall’s own distinction the calotype is the other kind — silver arriving from solution and building on the exposed specks. The course reads Wall’s definition against Talbot’s recipe and reports the reading as a reading; neither author says the word “physical” of the calotype, and no source held here measures how much of a calotype’s density comes from solution silver.

Pyro is gallic acid minus one carbon dioxide. Wall records that pyrogallic acid “is prepared by sublimation from gallic acid”, and gives its formula as C₆H₆O₃ = 126 against gallic acid’s HC₇H₅O₅ = 170.

C₇H₆O₅ → C₆H₆O₃ + CO₂

Decarboxylation: 170.12 less 44.01 leaves 126.11, which is the arithmetic behind Wall's sentence

The two are the same trihydroxybenzene ring with and without a carboxyl group, and that one group is the difference between the developer of 1841 and the developer that dominated the 1880s.

Properties

White hygroscopic crystals, or a white to cream powder; 3,4,5-trihydroxybenzoic acid, C₇H₆O₅, relative molecular mass 170.12, CAS 149-91-7. Wall’s 1912 formula, HC₇H₅O₅ = 170, is the same molecule and the same mass — one of the few places in this encyclopaedia where a period formula needs no correction.

Two traps at the balance. The first is the hydrate: the article usually sold is the monohydrate, CAS 5995-86-8 at 188.13, which is 10.6 per cent heavier than the anhydrous acid this record describes. The second is that the substance is hygroscopic, so even a correctly labelled anhydrous jar left open gains water. Where a historical formula asks for a saturated cold solution, as Talbot’s does, neither matters; where a modern formula asks for a mass, both do.

Sparingly soluble cold, freely soluble hot, and this is the property the calotype exploits. One gram dissolves in 87 mL of cold water and in 3 mL of boiling — about 1.1 per cent against 33 per cent. Talbot’s “the quantity dissolved is very small” is that first figure observed in 1841 and it agrees with the modern one.

A triprotic acid whose measurements disagree. The IUPAC compilation carries several determinations and they do not settle the first constant: pK_a1 is given as 3.13 at 20 °C in 0.1 mol/L potassium nitrate and as 4.27 at 25 °C in 0.1 mol/L potassium chloride, with a temperature series from that second study running 4.46, 4.49, 4.53 and 4.56 at 30, 40, 50 and 60 °C. That is a spread of more than a whole log unit between two approximate determinations in different background electrolytes, and this course quotes both and reconciles neither. The second constant is better behaved — 8.84 at 20 °C, 8.68 and 8.78 at 25 °C — and the third, 12.4, is assessed uncertain. The carboxyl group ionises in any darkroom solution; the phenolic hydroxyls do not, until the pH passes 8.

Handling

The aggregated ECHA notifications give Danger with the corrosion and exclamation-mark pictograms, and they are the only classification in the record: no harmonised CLP entry exists. The majority statements are ordinary irritation — H319 at 75.3 per cent, H315 at 75.1, H335 at 71.9 — and 5.4 per cent of the 405 reports say the substance meets no criteria at all. The signal word is Danger only because 19.3 per cent of notifiers give H318, causes serious eye damage, one category above the majority’s H319. The course carries both, because a one-in-five report of eye damage is not a rounding error when the substance is a powder that will be tipped into water.

There is no workplace exposure limit in EH40 or in the NIOSH Pocket Guide under any name searched, and EH40’s paragraph 6 says that absence from the list does not indicate a substance is safe.

Why Level B. On classification alone this would sit in Level A of the course rubric, which admits substances “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. What takes it past that is Level B’s other named criterion, fine powders that must not be inhaled: gallic acid is bought as a light hygroscopic powder, three quarters of notifiers give a respiratory irritation statement, and Princeton University’s darkroom guidance asks for extraction or an enclosure whenever powdered developers are mixed. The operation that sets the level is weighing the powder. Working from a made-up saturated solution, as Talbot’s method does after the first day, the same substance is Level A handling — and the calotype’s own level is set not by the gallic acid but by the silver nitrate it is mixed with.

⚠Safety

Goggles rather than glasses while the jar is open, nitrile gloves, and the powder weighed over a tray in still air with extraction or an enclosure. Keep it away from silver nitrate on the shelf; they meet in the dish, deliberately, and nowhere else. And keep it out of iron and steel vessels: gallic acid and iron(III) make the blue-black of iron gall ink, which will stain a solution and a print alike.

Waste

Small volumes, bottled separately. Nothing in the classification says otherwise — there is no aquatic statement and no acute toxicity statement, and ChEBI records gallic acid as a plant polyphenol with roles as an antioxidant and an astringent. What the course bottles is the mixture: a calotype developer is a silver-bearing solution by design, which is exactly the case Kodak’s J-300 rule about developers carrying negligible silver does not cover. Treat spent gallo-nitrate as a silver stream, label it, and take it to a licensed household hazardous waste route — in England and Wales through the government’s hazardous waste service finder, which is the route ILFORD gives domestic users. Check your local regulations; they decide.

History

Gallic acid reached photography from the tannery and the ink-pot. Wall’s 1912 entry records it as “obtained by fermentation from powdered galls” — the growths a wasp induces on an oak, which had supplied the iron gall inks of European manuscript culture for a thousand years before Talbot washed one over a sheet of iodised paper. The chemistry Talbot needed had been sitting in the stationer’s shop.

What he did with it changed the exposure. A photogenic drawing had to be printed out, the image forming visibly under prolonged light; the gallo-nitrate made a brief and invisible exposure sufficient, because the developer supplies the density that the light no longer has to. Talbot’s own demonstration of the difference is the one this page opened with — half a sheet covered, the other half given one second of dull winter daylight, and a strong impression whose “existence would not be suspected by any one who was not forewarned of it by previous experiments”.