Glyoxal

Glyoxal is two aldehyde groups and nothing else — the smallest dialdehyde there can be. That is exactly enough to tie one gelatin chain to another, and it is why the alternative-process community adopted it in place of formalin. It is also a harmonised skin sensitiser and suspected mutagen with no workplace exposure limit anywhere, which is why this course teaches it and does not use it.

In photography

Hardening a gelatin size on paper: the one job with a published dose. Ware’s account is the clearest in the corpus. An unhardened gelatin-sized paper “has a slimy surface texture and the gelatin tends to pass into the wet-processing baths, so a hardener is needed, to cross-link the gelatin molecules rendering it harder and more insoluble, before the paper is coated with sensitizer”. The traditional bath was formaldehyde at 1 to 2 per cent; because formaldehyde “is a noxious and unpleasant substance to handle, especially in a confined space”, Ware proposed glyoxal to the alternative-process community instead, “which is much less offensive due to its lower volatility”, and reports that the suggestion “seems to have been widely and successfully taken up by current practitioners”.

His procedure is specific: papers hardened in a 2 per cent glyoxal bath for about 10 minutes at room temperature, then washed in water for 20 minutes to remove excess hardener, “which may otherwise cause fogging of the sensitizer”. The economical alternative brushes the size on one side only — 3 g of gelatin in 100 cm³ of water with 5 cm³ of 40 per cent glyoxal added directly, enough for ten to twenty sheets of 10 × 8 inch paper.

Hardening an emulsion: the job with no dose at all. The course’s Tier 2 emulsion practitioner uses glyoxal and publishes no quantity for it. Her account of a silvergum base layer that must survive five gum coats and 45-minute soaks says only that “the emulsion was hardened by the addition of glyoxal right before coating”, and gives the price in the same breath: “when emulsion is hardened, the processing chemicals have a hard time penetrating, especially the fixer. Over time, after the print is processed and back in room light, the emulsion in the thick areas will darken.” That is why this course’s default is no hardener in a hand-coated emulsion at all — a hand-coated layer has thick patches by definition.

Why Part V rejected it, on two independent grounds. On evidence: no source at any tier gives a glyoxal dose for an emulsion, and the only quantity statement in the manufacturing literature applies the same molecular proportions as formaldehyde to “these more complicated aldehydes” — which conflicts with the kinetic finding for glyoxal itself, set out below. There is nothing to print. On hazard: the harmonised classification carries a skin-sensitisation and a suspected-mutagenicity statement, and glyoxal appears nowhere in EH40 under any of its names, so there is no exposure limit to work to. The course adopts chrome alum where an in-emulsion hardener is genuinely needed, because chrome alum has published doses and a British exposure limit.

⚗Two aldehyde ends, and a linkage that is not formaldehyde's

ChEBI describes glyoxal as “the dialdehyde that is the smallest possible”, ethane with an oxo group on each carbon. A cross-linker needs two reactive ends and a span between them; glyoxal has the minimum of both.

OHC-CHO + 2 H₂N-R → R-NH-CH(OH)-CH(OH)-NH-R

The course's own sketch of the condensation an aldehyde makes with amino groups on two gelatin chains; the equilibrium and the dehydrated product are not sourced here

Duffin’s manufacturing account calls glyoxal “very active” and reports the kinetic result that distinguishes it: only one glyoxal molecule per two gelatin units, where formaldehyde’s rate law is second order in the hardener. The nature of the link between adjacent chains is therefore “radically different from that formed by formaldehyde”, and that changes hardness, toughness, developer penetrability, and the covering power of the developed image — four properties that a coating bench cares about and that no source in this corpus measures for a hand-coated layer.

The tension is left standing. The same text’s only quantity guidance for the substituted aldehydes is that they are used in the same molecular proportions as formaldehyde, which would put roughly two glyoxal molecules where the kinetics say one is needed. This course does not average two sources that disagree; it prints both and uses neither as a dose.

Properties

C₂H₂O₂, relative molecular mass 58.04, CAS 107-22-2 — the lowest molar mass of any substance in this encyclopaedia except water and the gases. Pure glyoxal is a solid below 15 °C and CAMEO’s description of it is memorable and useless in a darkroom: yellow crystals, a green vapour, a violet flame. The article handled is a 40 per cent aqueous solution, a colourless to pale-yellow liquid with a faint sour odour, and every dose in the literature above is quoted against that solution or against a dilution of it. Ware’s note is explicit that glyoxal, like formalin, “is also available as a 40% aqueous solution”, which is what makes his 2 per cent bath a twenty-fold dilution.

It is miscible with water, which makes it convenient and makes a spill spread: there is no immiscible layer to skim, and a splash on a bench is a thin film of hardener over everything it wetted.

Handling

The page follows the harmonised classification, because one exists. Under Regulation (EC) No 1272/2008 glyoxal carries Warning, GHS07 and GHS08, with H315, H317, H341 — suspected of causing genetic defects — H319 and H332. That is a legal classification, not an aggregate of opinion, and it outranks the notified data for every other candidate hardener in this course. The aggregated ECHA notifications from 1,432 reports agree on all five statements at very high percentages, raise the signal word to Danger, and add three the harmonised entry does not carry — H302 at 30.8 per cent, H335 at 22 per cent, and H334, respiratory sensitisation, at 30.8 per cent. Exactly one report in 1,432 says the substance meets no criteria.

There is no workplace exposure limit anywhere in this course’s sources. EH40 lists nothing under glyoxal, ethanedial or oxalaldehyde, and the NIOSH Pocket Guide has no entry for it either. What NIOSH does have is a note in its appendices: glyoxal is one of nine related low-molecular-weight aldehydes whose carcinogenicity testing “has not been completed”, which “have chemical reactivity and mutagenicity similar to acetaldehyde and malonaldehyde”, and for which “NIOSH recommends that careful consideration should be given to reducing exposures”. EH40’s paragraph 6 says the same thing in the negative: absence from the list does not indicate that a substance is safe.

Why Level C. The course rubric puts at Level C reagents classified as “acutely toxic, carcinogenic, mutagenic, reproductive toxins or sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control”. A harmonised suspected mutagen that is also a harmonised skin sensitiser meets that on either clause. What settles it is the missing number: with no exposure limit in existence, there is no benchmark against which a home darkroom’s ventilation could be shown adequate, and Level B’s controls are written to be demonstrable. The operation that sets the level is diluting the 40 per cent concentrate, which is where both the vapour and the splash are; the 2 per cent bath a practitioner then works in is a milder proposition altogether, but the course does not hand a reader a procedure whose first step it cannot control.

⚠Safety

This course teaches glyoxal as chemistry and gives no home procedure for it. Where a lesson needs a gelatin size hardened, it points to the published practice and to the reason it is not reproduced here; where a lesson needs an emulsion hardened, it uses chrome alum, which has a dose and a limit. If you choose to use glyoxal on your own account, the controls are extraction at the dilution step, goggles and a face shield, gloves changed rather than washed, and Ware’s 20-minute wash — a residue of hardener left in a sheet fogs the sensitiser you coat on it.

Waste

Concentrate and unused bath are kept, labelled and taken to a licensed hazardous waste route; in England and Wales the government’s hazardous waste service finder locates one. Florida Atlantic University’s darkroom guidance says the same thing for photographic hardeners in general — “spent and un-used/concentrated Hardener should be collected and containerized with a Hazardous Waste label” — and although its section is written about formaldehyde, the instruction transfers. The course does not publish a dilution at which glyoxal could be discharged, and the reason is exactly the one that decides the safety level: there is no exposure limit, no discharge standard and no completed carcinogenicity testing in any source held here, so any threshold the course offered would be invented. Check your local regulations, which govern.

History

Glyoxal’s photographic career is a substitution story told twice, in opposite directions. In paper sizing it is the substance that replaced formaldehyde, and the reason given is neither toxicological theory nor regulation but the smell and the confined space: formalin is “noxious and unpleasant to handle”, glyoxal is “much less offensive due to its lower volatility”, and a community adopted it because working with it was tolerable. That is how substitutions actually happen.

In emulsion making it is the substance that was not adopted, and by the same course that excludes formaldehyde. The two exclusions rest on different evidence: formaldehyde went out of Part V because the emulsion literature itself threw it out for fog on keeping, thirty-seven years apart and twice independently, with the hazard record arriving second. Glyoxal went out because nobody published a dose for it in an emulsion and because its harmonised classification has no exposure limit to be measured against. A course that used it anyway would be reasoning from a practitioner’s habit and its own convenience, which is the one thing this course’s first rule forbids.