Gelatin

Every other page in this encyclopaedia opens with a formula and a molar mass. This one cannot, and the reason is the most useful thing to know about the substance: gelatin is not a compound. The U.S. Food and Drug Administration’s substance registry classes it as a mixture, and the entries underneath it are called things like GELATIN TYPE B BOVINE (150 BLOOM) — an animal, a curing process and a mechanical test, because that is what identifies a gelatin. It is bought to a specification, and two gelatins meeting different specifications behave differently in an emulsion.

In photography

It is what keeps the crystals apart. Kodak’s 1928 primer describes making an emulsion as soaking gelatin in water until it is swollen, dissolving it with heat, adding a soluble bromide, and running in a silver nitrate solution slowly. The precipitate that forms is silver bromide, and the primer is explicit about what the gelatin is for: were there no gelatin in the solution, the silver compound would still form, but it would settle to the bottom, “and an emulsion would not be formed”. The gelatin holds a crowd of microscopic crystals in suspension, and when the coated support cools, the whole sets to a jelly with the crystals fixed where they were.

It made the dry plate possible. The Image Permanence Institute’s account is a list of consequences rather than a description: gelatin binder made possible photographic emulsions, exposure times in the camera of a second or less, and ready-to-use light-sensitive plates. Because the halide was already suspended in the coating, nothing had to be precipitated at the moment of use, and because gelatin is hygroscopic the plate could be exposed in the field and developed weeks later. The wet collodion photographer’s portable darkroom went with it.

It is a chemical participant, not a bag. Two findings make this the important sentence on the page. First, halogen: Mike Ware writes that gelatin may be the important halogen acceptor in development emulsions, where it surrounds every silver halide crystal and the halogen released by latent-image formation is minute — while noting that at print-out levels of exposure gelatin is not an effective scavenger, so the sizing on a salted-paper sheet is doing a different job. Second, sulfur: Ware records Sheppard’s 1925 paper in the Photographic Journal, from research at Kodak in the 1920s, showing that traces of sulfur compounds in gelatin — allyl isothiocyanate, which is mustard oil, and allyl thiourea — sensitise silver halide emulsions. For fifty years before that, emulsion makers knew that some batches of gelatin gave faster plates than others and could not say why.

It colours the image. Reilly records that gelatin lends a characteristic reddish colour to salted paper prints, and that Talbot’s first salted papers showed it — not because he added gelatin, but because the paper-maker had already sized the sheet with it. Ware describes the same effect in gold printing, where gelatin acts as a protecting agent for colloidal gold and favours red images, and is “quite inimical” to platinum printing. The binder is part of the picture’s colour.

It is also the layer that gets scratched. The Getty Conservation Institute’s atlas notes that early silver gelatin printing-out papers were not protected by a supercoat of hardened gelatin, so handled prints show surface scratches in the gelatin under a stereomicroscope. Hardened gelatin over soft gelatin is the answer, and everything on the hardener pages exists for that reason.

⚗The chemistry

Kodak’s 1928 primer gives the behaviour that makes gelatin work, and it is worth reading as physics rather than as description.

In cold water it does not dissolve; it swells — “as if, instead of the gelatin dissolving in the water, the water dissolves in the gelatin”. There is no definite solubility: add more gelatin and the solution simply gets thicker. Heat melts it, cooling sets it, and a jelly can be melted and reset many times — but each cycle changes it, permanently leaving it thinner than before, and if the process is continued long enough the solution will refuse to set at all. Ware puts the working limit at 40 to 45 °C for a sizing solution, above which the gelatin is denatured; Reilly gives the melting point of a swollen mass as above 32 °C.

Dried, it becomes a horny, glassy, slightly brittle layer that will take up water and swell again at once. That reversible swelling is what lets a processing solution reach a crystal buried in the coating, and it is also the emulsion’s weakness: a small quantity of acid or alkali increases the swelling considerably, so the alkaline developer and the acid fixer both swell it, especially when warm, and a swollen layer can lift from its support or reticulate. Hardening is the control, and Kodak states that gelatin emulsions have a hardener added before they are coated.

Properties

Identity. CAS 9000-70-8, EC 232-554-6, no molecular formula, no molar mass. The registry’s component entries — porcine and bovine, type A and type B, 150 and 160 and 195 Bloom, and a marine non-gelling grade — are the identity that matters in practice.

Manufacture. The Image Permanence Institute describes gelatin as a protein product manufactured by the partial hydrolysis of collagen from the connective tissues and skin of animals. Reilly gives the process: cooking skins, tendons and bones in a pH-controlled vat of water, with very pure grades obtainable if the temperature and pH of the cooking liquor are carefully controlled. Ware adds the distinctions a buyer meets — skin or bone (bone gelatin being called ossein), and cured with acid or with lime, with a marginal preference for acid-cured in gold printing.

Gel behaviour. As in the callout above: swells cold, dissolves hot, sets on cooling, degrades with repeated melting, denatures above about 45 °C. Ware’s sizing procedure is the practical form — swell about 30 g in 600 cm³ of water at room temperature for half an hour, make up to a litre, warm to 40 to 45 °C with stirring, and everything dissolves within half an hour.

Hardening changes it and does not slow washing. Reilly states the ladder plainly: alum gives gels that are harder and less permeable; potassium chrome alum gives gels that are completely insoluble; formaldehyde also hardens. Kodak corrects a common belief about the consequence — hardened material does not wash more slowly than unhardened. Hardening contracts the network of the sponge without contracting the sponge, so diffusion is unchanged, unless the gelatin has been dried after hardening: a negative thoroughly hardened and then dried down will not expand much when soaked again, and diffusion through it is then difficult.

Long-term behaviour. The Image Permanence Institute’s account of what goes wrong is a humidity story. High relative humidity promotes mould growth in the gelatin layer, which solubilises the binder and destroys the image, and makes the layer stick to whatever it touches; low humidity shrinks it and worsens lifting and flaking from the support; and rapid swings do the most damage of all. Its storage recommendation is below 18 °C at 30 to 40 per cent relative humidity.

Handling

There is no GHS classification to quote, and the page says so rather than filling the gap. The course’s own query of PubChem returns only substance depositions for gelatin and no compound record, so the aggregated ECHA notifications this encyclopaedia normally reports do not exist for it in that form; the FDA registry entry carries identifiers and no hazard classification; and HSE’s EH40 does not list it, while stating that absence from its list does not indicate a substance is without risk. Nothing in that paragraph is evidence of an absence of hazard. It is an absence of evidence, and the correct response is to read the supplier’s safety data sheet for the grade in hand, which describes a product that frequently contains a preservative.

Three things are worth controlling in practice. A fine organic powder should not be raised as a dust when it is weighed. A warm solution at 40 to 45 °C is a scald risk in the ordinary way and nothing more. And a protein solution spoils: Ware’s few days of refrigerated life for a sizing solution, and Wall’s period list of antiseptics for gelatinous mixtures, both describe the same problem, and a bottle that smells wrong is a bottle to discard rather than to use.

Why Level A. Against the course rubric, gelatin meets Level A on every criterion: there is no classification approaching the rubric’s ceiling of irritant, harmful if swallowed or corrosive; nothing is heated above 50 °C, since the material’s own denaturation limit sits below that; there is no mains-voltage work; and the waste is dilute rinse water. Two adjacent operations are not Level A, and the distinction matters. Making an emulsion is not: it involves silver nitrate and holding solutions warm, which the rubric puts at Level B. Hardening a coating or a size with an aldehyde is not: formaldehyde and glutaraldehyde are Level C substances, and adding them to a gelatin bath makes that bath a Level C operation. The level belongs to the procedure, not to the most innocuous thing in it.

⚠Safety

Gloves and eye protection on the ordinary darkroom footing, unhurried weighing of the powder, and a warm-water bath rather than direct heat. Read the safety data sheet for the grade you have bought before opening it, because the sheet describes the product and this page describes the protein. The consequential safety decisions around gelatin are not about gelatin at all: they are about what is suspended in it and what is being used to harden it, and both of those live on other pages.

Waste

Dilute gelatin in rinse water is oxygen-demanding organic load and nothing more exotic; a spoiled sizing solution is decomposing protein. But a gelatin bath in a darkroom is rarely only gelatin. A discarded coating solution carrying silver halide belongs with the silver stream and should go to recovery. A spent hardening bath belongs to its hardener: alum-bearing solutions travel with the fixer, and aldehyde baths need the licensed hazardous waste route described on their own pages. A size solution carrying a preservative carries the preservative’s classification with it.

Bottle each stream separately and label it with what it contains and the date, as ILFORD instructs domestic users to do for photographic wastes. Check your local regulations; they govern.

History

Gelatin’s photographic career begins with a publication and takes seven years to become an industry. The Image Permanence Institute dates the first publication on a gelatin silver bromide emulsion to Richard Leach Maddox in 1871, describes the method — a silver bromide precipitate formed in a warm gelatin solution, fine enough to stay in suspension — and then follows the improvements that made it practical. John Burgess was selling bromide emulsion by 1873, with inconsistent results. Richard Kennett washed the emulsion after precipitation to remove soluble by-products that spoiled the plates, and by 1874 was selling both ready-made plates and a dried emulsion he called pellicle, to be dissolved in warm water and poured onto glass by the photographer. Only in 1878, when Charles Bennett found the effect of ripening the emulsion on its light sensitivity, did dry plates become widely used — and in the same year Wratten & Wainwright introduced the first commercially successful plates in London, followed by John Carbutt’s Keystone Dry Plate Works in 1879 and George Eastman’s Eastman Dry Plate Company in 1880. Machine coating through the 1880s built the modern photographic industry on this one protein.

The hardening story runs alongside it from the beginning. The Institute records that the glass support of a dry plate was given a substratum — a thin layer of gelatin hardened with a chrome alum solution — so that the emulsion had something to hold on to, and that after development the binder itself was hardened with a potassium or chrome alum solution before fixing. The Getty’s X-ray fluorescence still finds that chromium in printing-out papers a century later.

Gelatin outlasted the plate. The Institute records dry plates being displaced by nitrate film from 1890 and almost entirely by the end of the 1920s, while continuing in spectroscopic and astronomical work, which needed a perfectly flat and dimensionally stable support. The binder simply moved to the new support and stayed there, which is why “silver gelatin” is still the name of the medium.