Thymol

Gelatin is meat. Left wet and warm it does what meat does, and a hand-made emulsion or a pot of size spends hours at exactly the temperature that suits the bacteria best. Thymol is one of the answers the photographic literature gives, and it has been giving it since 1885.

In photography

What goes wrong without it. Wall’s 1912 entry for gelatine is the clearest statement of the problem: gelatine “will keep indefinitely in the dry state, but in the presence of water it soon putrefies, turning first acid and then alkaline; and at this stage ammoniacal vapours are given off.” An emulsion whose pH is drifting from acid to alkaline is an emulsion whose behaviour is drifting too, and ammonia in a silver halide system is not a passive contaminant.

The list of answers, twice, twenty-seven years apart. Abney in 1885: “Alum, alcohol, carbolic acid, salicylic acid, glycerine, fuchsin, hydrate of chloral, thymol, and salts of zinc act as antiseptics, preventing the decomposition of gelatine, even in small quantities.” Wall in 1912: “Alum, alcohol, carbolic, salicylic, and boracic acids, thymol, formalin, and the salts of zinc act as antiseptics.” The two lists overlap in six substances, and thymol is in both.

And a dose, which is unusual for this family of additives. Abney gives one directly. If a dissolved gelatine emulsion is to be kept, “to 10 ounces of emulsion add 10 grains of salicylic acid dissolved in 1 drachm of alcohol, or replace the salicylic acid by the same weight of thymol oil, as a protection of the gelatine against change caused by a long warming, putrefaction, &c.” Ten grains in ten fluid ounces is 0.65 g in about 284 mL — roughly 0.23 per cent, or 2.3 g per litre of emulsion — and it is carried in a single fluid drachm, about 3.5 mL, of alcohol.

The same trick in a conservation workshop. Reilly’s manual for albumen and salted paper prints gives a wheat starch paste for mounting and finishes it with “0.4 ml of a saturated solution of thymol in methyl alcohol. Thymol is a fungicide and does not appear to affect the adhesive properties of the paste.” A fraction of a millilitre of an alcoholic saturated solution, in a paste that will then be refrigerated for up to a week. Ninety-five years after Abney, the same substance, the same solvent, and the same reason.

⚗Why it always arrives dissolved in alcohol

Thymol’s water solubility is about 0.9 to 1 gram per litre. HSDB’s own comparison makes the point without any arithmetic: one gram dissolves in about 1000 mL of water and in 1 mL of alcohol. A thousandfold difference in solvent power is why every published photographic use of thymol in this course’s corpus adds it as an alcoholic solution and never as crystals stirred into water — Abney’s drachm of alcohol, Reilly’s saturated solution in methyl alcohol.

Check Abney’s dose against the limit and the reason becomes concrete. His 2.3 g per litre is more than twice what water alone will hold; the alcohol is not a convenience, it is what makes the dose possible at all. That the thymol then stays in solution in a mostly aqueous emulsion is a fact the sources report by using the method, and none of them explains it. The course does not supply the explanation — whether the gelatin itself solubilises the phenol, or the dose sits near a limit that the small alcohol fraction raises just enough, is not established by anything held here.

What it is. A phenol, C₁₀H₁₄O — ChEBI calls it a natural monoterpene derivative of cymene — with pK_a1 10.62 at 20 °C in the IUPAC compilation, assessed approximate. Above about pH 11 it is the phenolate and freely soluble; in any photographic bath it is the neutral molecule.

Properties

White crystals with a strong aromatic odour, melting well below the boiling point of water; 5-methyl-2-(propan-2-yl)phenol, C₁₀H₁₄O, relative molecular mass 150.22, CAS 89-83-8. The descriptions in the record disagree pleasantly about the smell — “herbaceous, warm” in the JECFA specification, “like thyme” in HSDB, “like phenol” in a supplier’s sheet — and the disagreement is worth noticing, because a substance that smells of a herb is a substance people underestimate.

Solubility governs every use. About 1 g/L in water; 1 g per mL of alcohol; 1 g in 0.7 mL of chloroform, 1.5 mL of ether and 1.7 mL of olive oil. Also soluble in fixed alkali hydroxide, as a phenol should be, which is a fact with a consequence: an alkaline processing bath will pull thymol out of a gelatin layer that was preserved with it.

Handling

The page follows the harmonised classification. Under Regulation (EC) No 1272/2008 thymol carries Danger, three pictograms, and three statements: H302, harmful if swallowed; H314, causes severe skin burns and eye damage; and H411, toxic to aquatic life with long lasting effects. The aggregated ECHA notifications, from 2,007 company reports, agree at above 99 per cent on all three. A corrosive that is also a food flavouring is an uncomfortable combination to hold in the head, and the resolution is the ordinary one — the classification is for the pure crystalline substance, and a mouthwash is a dilution by three or four orders of magnitude.

Two additions are worth a reader’s attention. HSDB and a 10.5 per cent minority of notifiers add H318, subsumed in H314 in practice. And Japan’s NITE-CMC revised its entry in 2024 to add H317, may cause an allergic skin reaction — a statement no other scheme carries, from the most recent assessment in the record, and therefore one to treat as a live minority view rather than a rejected one.

There is no workplace exposure limit. EH40 lists nothing for thymol, and the NIOSH Pocket Guide has no entry either; thymol appears in the Guide only inside another substance’s incompatibilities note, as a 0.01 per cent stabiliser added against decomposition by light. EH40’s paragraph 6 states that absence from the list does not indicate that a substance is safe.

Why Level B. Level A of the course rubric admits substances classified “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, and the qualifying clause is what decides this. Two things put the jar past Level A: it is a corrosive solid that sublimes, so both the crystals and the vapour in a closed container are corrosive; and Level A assumes waste that goes for silver recovery or down a drain, which a near-unanimous H411 is not. Level B names “toxic or corrosive reagents” and fine powders explicitly. The operation that sets the level is making the alcoholic stock — weighing corrosive crystals and dissolving them in a flammable solvent. Dosing that stock afterwards, a drachm or a fraction of a millilitre at a time, is Level A handling, and Reilly’s paste and Abney’s emulsion are both at that end.

⚠Safety

Goggles and gloves while the jar is open. Weigh over a tray with extraction or an enclosure, and cap the jar promptly — thymol sublimes, and the smell in a cupboard is the substance escaping. Label the alcoholic stock as flammable as well as corrosive. And do not let the pleasant smell set the handling: the harmonised statement is severe skin burns and eye damage, and the classification does not care what a substance smells like.

Waste

Bottled, never poured. The harmonised classification carries H411 and 99.5 per cent of 2,007 company reports agree; thymol is a biocide, which is a statement about what it does to small organisms rather than an incidental property. Crystals, alcoholic stock and any unused preserved solution go to a licensed household hazardous waste route, labelled with the substance and the date — in England and Wales through the government’s hazardous waste service finder, which is the route ILFORD gives domestic users for photographic chemicals. The milligrams that leave a treated emulsion in a processing bath are a separate, far more dilute stream and travel with that bath. Check your local regulations; they decide.

History

Thymol reached photography as one of a shortlist of antiseptics that a Victorian chemist could buy at a pharmacy, and it has outlasted most of the shortlist. Abney’s 1885 list contains fuchsin and chloral hydrate; Wall’s 1912 list contains carbolic acid and formalin. Of the substances on both lists, boric acid and thymol are the two this course would still put in a reader’s hand — carbolic acid is phenol itself, formalin is excluded from Part V on the emulsion literature’s own evidence, and chloral hydrate is a controlled drug.

What kept it is the combination that makes the entry worth writing: it works at a fraction of a per cent, it is compatible with gelatin, and it dissolves a thousand times better in the alcohol that a coating bench already has on it than in the water it is going into. Reilly’s conservation manual records the practical test that matters for any additive to a working solution — “Thymol is a fungicide and does not appear to affect the adhesive properties of the paste” — which is the question every preservative has to answer and the reason most of the 1885 list did not survive.