Formaldehyde

There is no argument about whether formaldehyde hardens gelatin. Kodak’s own primer says a 5 per cent solution makes the gelatin of a film completely insoluble in boiling water in less than a minute, which is more than any alum can claim. The argument is entirely about what it does to the person standing over the tray, and that argument was settled decades ago.

In photography

The hardener of last resort, and of hot weather. Kodak’s 1928 primer lists three hardening agents for an acid hardening fixing bath — potassium alum, potassium chrome alum, or formaldehyde at 40 per cent — and then, a chapter earlier, undoes the third of them: formalin is sometimes suggested for hot weather processing, but it should be used only in alkaline or neutral solutions, because in acid solutions it does not harden. An acid hardening fixer built on formaldehyde is therefore working against itself, and the primer’s next sentence gives the other reason it fell out of use: with certain individuals formalin irritates the mucous membranes of the nose and throat, and its use is “very objectionable”.

As a separate bath, it is formidable. Wall’s 1912 dictionary gives the numbers. One part of the commercial 40 per cent formalin in 8 to 12 parts of water, five minutes, and a film on glass or paper will resist boiling water. One part in 20 to 25, ten to fifteen minutes, is enough to prevent frilling. Wall then lists the costs in the same paragraph, which is why the entry is worth reading rather than summarising: the strong solution evolves irritating vapours, sometimes makes the film opalescent, and occasionally produces extreme brittleness “or even a falling into dust”; and after formalin, intensification or reduction of the negative becomes very difficult. Wall’s 1924 formulary keeps a 10 per cent formaldehyde bath among the hot-weather hardening baths, with one advantage over the alum ones — freedom from residual hypo is not so essential.

Sizing paper. Mike Ware’s chrysotype manual records that the usual bath for hardening a gelatin-sized sheet before coating it with sensitiser was, for many years, formaldehyde at about 1 to 2 per cent — the commercial 40 per cent w/w solution diluted twenty times. Unhardened gelatin size is slimy, passes into the processing baths, and takes the image with it. What Ware did about it is the history of this page in one sentence, and it is in the History section below.

Colour processing. Princeton University’s environmental health and safety guidance lists formaldehyde among the prehardeners and stabilisers of colour processes, alongside succinaldehyde, and its recommendation is unambiguous: avoid colour processes using formaldehyde if possible.

⚗The chemistry

The bottle does not contain much formaldehyde. CAMEO states that in solution formaldehyde exists as CH₂(OH)₂ and its polymers HO(CH₂O)ₓH, with x averaging about three:

CH₂O + H₂O → CH₂(OH)₂

What formalin mostly is: methylene glycol

That equilibrium is why the commercial solution carries 6 to 12 per cent methanol, which CAMEO says is added to prevent the precipitation of formaldehyde polymers, and why a methanol-free grade has to be shipped and kept at about 30 °C. The white solid in a cold or neglected bottle is paraformaldehyde, which CAMEO describes as polymers of formula HO(CH₂O)ₓH where x averages about thirty. The hardening reaction draws on the small free-aldehyde fraction and the equilibrium replaces it.

Why it needs an alkaline or neutral bath, this course states as Kodak’s observation rather than as mechanism: the primer records that it does not harden in acid solution, and the course could not verify from a source meeting its standard the molecular reason, which would have to be an account of which groups on the protein react and in what protonation state.

Properties

A gas that behaves like a liquid. Relative molecular mass 30.026; a nearly colourless gas with a pungent, suffocating odour, and an odour threshold CAMEO puts at 1 ppm — well above the 0.1 ppm ceiling NIOSH recommends over fifteen minutes, so smell is a poor warning and not a control. Miscible with water in every source PubChem carries; HSDB puts it at 4.00 × 10⁵ mg per litre at 20 °C.

What the bottle is. Formalin: 37 to 50 per cent formaldehyde by mass, denser than water, with a pungent irritating odour and 6 to 12 per cent methanol. CAMEO’s flammable-solution datasheet gives a flash point varying from 50 to 61 °C and notes that the vapours are heavier than air. Its corrosive formalin datasheet is blunt about the pure material: pure formaldehyde is a gas that is not handled commercially, because it tends to polymerise exothermically and may ignite.

Stability. The polymerisation above is the whole of it. A bottle stored cold, or stored long, throws paraformaldehyde; a bottle stored warm and open loses formaldehyde to the room.

Handling

The aggregated ECHA notifications give Danger, four pictograms — GHS05 corrosive, GHS06 acute toxicity, GHS07 irritant and GHS08 the health hazard — and ten statements. Three of them decide how this substance is handled anywhere.

The carcinogen classification, stated from the data. H351, suspected of causing cancer, is notified by 64.1 per cent of reports; H350, may cause cancer, by 19 per cent, a minority placing it one category higher. H341, suspected of causing genetic defects, also stands at 19 per cent. PubChem separately carries the California Office of Environmental Health Hazard Assessment’s statement that formaldehyde gas can cause cancer according to an independent committee of scientific and health experts, and ChEBI lists carcinogenic agent among its roles. HSE’s EH40 attaches the Carc notation to formaldehyde, and its paragraphs on carcinogens state the consequence in regulation: where a substance carries H350, COSHH requires exposure to be reduced to as low as is reasonably practicable, which is a different duty from staying under a number.

It is a sensitiser. H317, may cause an allergic skin reaction, is the most widely notified statement of the ten, at 90.5 per cent. Florida Atlantic University’s guidance puts the practical meaning plainly: the more a person is exposed, the smaller the dose it then takes to have an effect.

The exposure limits are small. NIOSH sets its recommended limit as Ca — the Pocket Guide’s carcinogen designation — with a time-weighted average of 0.016 ppm and a ceiling of 0.1 ppm over fifteen minutes, and gives the immediately-dangerous-to-life-or-health value as Ca [20 ppm]. HSE’s EH40 lists 2 ppm, 2.5 mg m⁻³ for both the eight-hour and the fifteen-minute reference periods. NIOSH’s symptom list is irritation of the eyes, nose, throat and respiratory system, lacrimation, cough and wheezing, with target organs given as eyes and respiratory system and nasal cancer named in brackets.

Why Level C. The course rubric does not require an argument here: its Level C criteria name “chromium(VI) compounds, formaldehyde and similar substances in any quantity”, and the controls it attaches are a fume cupboard or equivalent engineered extraction, a written procedure and spill plan, a second competent person present, and waste collected and disposed of through a licensed route. Everything above supports that placement independently — an acute-toxicity pictogram, a corrosivity statement, a 90.5 per cent sensitisation notification and a carcinogenicity classification would each on its own put the substance past Level B’s ceiling.

⚠Safety

The course teaches formaldehyde as chemistry and history and does not give a home darkroom procedure for it. Where it is used at all it is used under engineered extraction with the containers kept closed, with splash goggles and a face shield, gloves selected against permeation data, and eyewash and drench facilities within reach. Keep it away from hydrochloric acid and from any acid chloride source: both NIOSH and CAMEO record that the reaction produces bis(chloromethyl) ether. Keep it away from strong oxidisers, with which CAMEO records violent reactions, and from phenols, where the polymerisation can develop sudden destructive pressure. Once a person is sensitised, avoiding further exposure is the only remedy that remains.

Waste

A used hardener or prehardener bath is an aldehyde solution carrying an acutely toxic, corrosive and sensitising substance with a carcinogenicity classification, and it is not drain load. Collect it in its own closed, labelled container; keep it separate from acid wastes, because of the chloride reaction above, and from oxidisers; and route it through a licensed hazardous waste service rather than assuming a household collection point will take it. Florida Atlantic University’s photographic chemicals guidance treats hardeners as a stream its environmental health and safety office collects, which is the institutional form of the same rule. In England and Wales the government publishes a finder for local hazardous waste services. Check your local regulations; they govern.

History

Formalin entered photography as a general-purpose stiffener and antiseptic. Wall’s 1912 dictionary files it under the aldehyde of methylic alcohol, notes that it was sometimes sold as Antipyr, lists it among the antiseptics that prevent putrefactive change in gelatinous mixtures, and reports the period claim that one drop to a quart will preserve milk. Its photographic use is cross-referenced from two directions in the same book — Films, Hardening and Hardening of Gelatine Films both say only “See Formalin”.

Kodak’s primers mark the turn. The 1924 edition gives the hardening power and the restriction to alkaline or neutral solutions; the 1928 edition adds the sentence about irritation of the nose and throat and the judgement that its use is very objectionable. A manufacturer describing one of its own listed hardening agents that way in 1928 is the beginning of the end of the substance in the darkroom, and it was the alums, not formaldehyde, that went into the standard fixing baths.

The last chapter is recent and is a straight substitution. Ware records that after many years of the formaldehyde bath for gelatin-sized paper he suggested to the alternative-process community a hardening agent “much less offensive due to its lower volatility” — glyoxal, at 2 per cent for about ten minutes followed by a twenty-minute wash — and reports that it was widely and successfully taken up, with glutaraldehyde also proposed. The reason given is volatility rather than potency: formaldehyde’s problem was never that it failed to harden gelatin, but that a substantial part of every bath left the tray and went into the room.