Tannic acid

Two very different things are called tannin in a photographic text. One is a preservative that let a collodion plate be exposed a week after it was coated. The other is a hardener that turns gelatin insoluble. They are the same jar, and the jar does not contain one substance.

In photography

Major Russell’s dry plate, and why a preservative was worth having. A wet collodion plate must be coated, sensitised, exposed and developed while it is still wet — an hour’s work at most, all of it in the field, with a tent and a bath of silver nitrate. Towler’s 1864 manual gives the tannin process as one of the answers. After the plate is collodionised, sensitised and thoroughly washed, it is flowed with a preservative solution of tannin, drained, and dried. “When perfectly dry, the plates will keep in the dark for a long time.”

The dose is published and it is a range with a reason. Towler: “This solution may vary in strength from ten to thirty grains of tannin to one ounce of water, depending upon the light and the nature of the collodion”, and his worked formula is 15 grains of tannin to 1 ounce of distilled water — about 3.4 per cent by weight in water. It is to be dissolved and filtered through paper before use, and only then given four or five minims of alcohol to the ounce, “but always after filtration”. The plate is flowed two or three times with a small quantity to displace the wash water, then with fresh solution, drained, and dried away from light. And the range is a control, not a tolerance: “When the contrasts of the landscape are very marked, and the light brilliant, a less quantity of tannin may be used; the greater the quantity of tannin, the greater the density of the shades.”

What it gives and what it forbids. Towler records an image whose colour “is rich and warm; its tone is very agreeable”, and adds one prohibition with a mechanical reason: the developed plate is fixed in hyposulphite of soda “but not of the cyanide, because it is apt to loosen the film”.

The second use is on gelatin. Wall’s 1912 entry for gelatine lists the substances that act on it, and puts tannin in the same clause as one of this course’s adopted hardeners: “Chrome alum and tannin render it insoluble, but capable of absorbing water.” That is the whole of what this course can source for tannin as a gelatin hardener — a statement of the effect, with no dose, no temperature and no comparison. Wall’s warning in the same entry is the more useful half for a coating bench: gelatin in water “soon putrefies”, and alum, alcohol, carbolic, salicylic and boracic acids, thymol, formalin and zinc salts act as antiseptics.

⚗Tanning is a reaction with protein, and that is all the course will claim

The word is older than the chemistry. Tanning a hide, hardening a gelatin layer and precipitating a protein from solution are the same operation: a polyphenol with many hydroxyl groups binds across protein chains and stops them dissolving. Wall’s sentence — insoluble, but still able to absorb water — describes a cross-linked network rather than a coating, which is what makes it a hardener and not a varnish.

One in-emulsion dose is published, and it comes with its own warning. Wall’s 1929 emulsion manual records Wratten’s ratio as 1 of tannin to 286 of gelatine — about 0.35 per cent of the gelatin weight, which puts it in the same band as the 0.5 to 2 per cent the manufacturing literature gives for chrome alum — and adds that it “is very effective; but it must be used with caution, as it tends to give a yellow stain during development, if used in too large a quantity”. That stain is why the course adopts chrome alum where an in-emulsion hardener is genuinely needed, and keeps tannin as history.

Not sourced here. No document in this course’s corpus gives the number of linkages, the groups on the protein involved, or a comparison of tannin with the metal-salt hardeners on a common scale of hardness. The page states the effect and the one published ratio, and stops.

Properties

A light yellow to tan powder, faintly smelling, freely wetted by water. And then the difficulty: tannic acid is not a compound. PubChem’s CID 16129778 carries CAS 1401-55-4 and depicts one idealised molecule, a glucose core esterified ten times over with galloyl and digalloyl groups — C₇₆H₅₂O₄₆, relative molecular mass 1701.2. ChEBI’s description of that structure is three words: “Chinese gallotannin is a tannin.” The ECHA notifications attached to the substance are filed under a different idealisation again, a pentagalloyl glucose, EC 226-562-9. What is in a jar is a mixture of gallotannins whose degree of galloylation depends on the plant it came from.

The clearest measure of that is historical. Wall’s 1912 dictionary gives tannic acid as “C27H13O17 = 618” and adds, correctly and usefully, that “this is not a true acid, but a glucoside obtained from galls”. The modern record gives 1701.2 for the same article. Nothing about the substance changed between 1912 and now; what changed is which idealised structure the reference work chose. A formula for tannic acid is a convention, not a composition, and this page prints PubChem’s because a validator needs one, not because a jar contains it. The consequence for practice is small but real: tannin baths in the literature are given as a mass in a volume, never as a molarity, and there is no sound way to convert them.

Solubility is extreme and imprecisely known. HSDB has one gram dissolving in 0.35 mL of water; Wall has ten parts in eight. Those differ by a factor of 2.3 and the course states the disagreement rather than choosing. Either way Towler’s 15 grains in an ounce is nowhere near saturation, which is why his instruction is to weigh.

Handling

The classification is the mildest in this family. The aggregated ECHA notifications give Warning with the exclamation-mark pictogram and a single hazard statement, H319, causes serious eye irritation, from the one notification that carries codes; 17.6 per cent of all reports say the substance meets no GHS hazard criteria. There is no harmonised entry, no national scheme in the record, and no workplace exposure limit — EH40 and the NIOSH Pocket Guide list neither tannic acid nor tannin, and EH40’s paragraph 6 states that absence from the list does not indicate that a substance is safe. With 34 reports behind the whole classification this is a thin evidence base, and the thinness is structural as well as commercial: notifiers are classifying a defined molecule, and suppliers are selling a plant extract.

Why Level A. Level A of the course rubric admits substances “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled” with gloves, eye protection, ventilation and dedicated utensils, and a 3 per cent aqueous solution of an eye irritant is exactly that. No respiratory statement is notified, so the Level B criterion for fine powders is not triggered by the classification, though the powder is still weighed over a tray. The operation that would take a page using tannic acid out of Level A involves no tannic acid at all: in the tannin process the level is set by the collodion — ether, alcohol and nitrocellulose — and by the silver nitrate bath, and those are what any page teaching it must be classified on.

⚠Safety

Goggles while the jar is open and while the solution is poured; that is the only classified hazard. Weigh over a tray. Keep it out of iron and steel: tannin and iron(III) make iron gall ink, and a rusty dish will blacken the bath and mark the plate. Filter the solution through paper before use, as Towler directs, and add the alcohol afterwards — a filter will strip alcohol-borne fines out of a solution that has already been wetted.

Waste

Dilute, organic and readily biodegradable, with no aquatic or acute-toxicity statement notified. The course still separates it, for the ordinary reason: in the tannin process the tannin bath is preceded and followed by silver-bearing washes, and mixing an organic load into a silver-recovery stream helps nobody. Keep the tannin solution to itself, bottle the silver-bearing rinses as any silver process requires, and follow ILFORD’s advice to domestic users in the United Kingdom — bottle each waste chemical separately, label it, and leave it in the chemical cupboard at a Household Waste and Recycling Centre. Check your local regulations; they decide.

History

Tannin entered photography as one answer among many to the wet plate’s tyranny. Towler’s contents page lists it beside the honey process, the resin process, Sutton’s and Keene’s rapid dry processes and Dr Hill Norris’s, and he places the tannin process among the four that “come out most prominently” in the practice of 1864, alongside wet and dry collodion and albumen. The honey variant is described as “one which seems to possess considerable advantages in sensitiveness” — the dry plate’s standing complaint, since every preservative bought keeping time at the cost of speed.

What ended the argument was the gelatine emulsion, which needed no preservative because it was already dry. Wall’s 1912 entry records the outcome without sentiment: tannic acid’s “photographic use being limited almost, if not entirely to the old collodion process”. It survives in the course as history, as the second half of a sentence about hardening gelatin, and as the reason the jar next to it is labelled gallic acid — both came out of the same oak gall.