Gum arabic

Gum arabic is the second of this encyclopaedia’s two great natural binders, and it is the one that carries no silver. Where gelatin holds a light-sensitive crystal, gum arabic holds a pigment — an ordinary artist’s colour with no photographic properties at all — and lets light decide where the pigment stays. That is a different way of making a picture, and it is why this page belongs beside the silver ones rather than inside them.

In photography

Gum bichromate is its principal use, and the gum does the holding while the dichromate does the chemistry. Mike Ware’s summary is the clearest the course has read: an aqueous solution of gum arabic is used as a viscous binder for the chosen artists’ pigments, mixed with a soluble dichromate before it is brushed onto the paper; on exposure through a negative the chromium(VI) is photochemically reduced to chromium(III), which partially insolubilises the gum by cross-linking its macromolecular structure, trapping a proportional amount of pigment in the hardened gum where the light fell; and the excess pigmented gum in the lighter tones is dissolved away by washing in water. The result is negative-working, and was once called photo-aquatint.

Two things follow from that description. The image substance is the pigment, so a gum print’s colour, permanence and grain are the pigment’s, not silver’s. And the light-sensitive component is the dichromate — which is why the course studies this process and does not perform it, on the ruling recorded in the chromium policy: chromium(VI) is never used, at any level, anywhere in this course.

It appears in the iron processes too, as a coating rather than as a binder. The Getty Conservation Institute’s atlas describes Pellet’s positive cyanotype, patented in 1877 and used into the 1940s, in which paper is coated with a mixture of gum arabic, ammonium ferric citrate and ferric chloride; light reduces the iron(III), and the developing solution of potassium ferrocyanide then forms Prussian blue only where unreduced iron(III) remains. The atlas calls the process much more difficult and delicate than the ordinary cyanotype. Ware records the attempt that preceded it: Herschel, working towards a positive-working cyanotype, added gum arabic to his potassium ferrocyanide developer in an attempt to inhibit the spreading of the image substance — and Ware’s comment is that had he added it to the sensitiser instead, as Pellet later did, he might have met with greater success.

And it turns up repeatedly as a size, a subbing and a protective coat. The atlas for platinotype lists starch, agar-agar, gum arabic and gelatin among the sizing materials described in the photographic literature. Ware’s list of the additives put into salted papers as protecting agents runs milk, isinglass, rice water, honey, gum arabic, sugar candy and egg white. The AIC’s collodion page records an aqueous pre-coat of gum arabic, dextrin, albumen or gelatin applied before varnishing a collodion negative, so that the alcohol in the varnish does not dissolve the collodion binder. Its daguerreotype page lists gum arabic among the materials used to coat plates.

⚗What cross-linking a polysaccharide does, and what the course does not claim

The mechanism the sources support is a short one. Chromium(VI) is reduced by light in the presence of the organic colloid; the chromium(III) produced cross-links the gum’s macromolecules; a cross-linked network no longer dissolves in water; and pigment trapped inside it stays where it is. That is Ware’s account and the course states it in his terms.

What the course does not do is describe the cross-link. No source read here says which groups on the gum the chromium(III) binds to, in what ratio, or whether the gum itself is oxidised in the process. Gum arabic is a plant exudate rather than a compound — the FDA’s registry classes it as structurally diverse and names the part simply as resin — which is the same problem gelatin and bitumen present, and the reason this page carries no formula.

One consequence is worth carrying into a conservation context: the Getty’s atlas notes that the analytical signal of gum arabic is very weak and lies in a spectral region prone to interference, so even a laboratory has difficulty proving it is there. A material that is hard to detect is a material whose presence in a historical object is often inferred rather than measured.

Properties

A water-soluble exudate that dissolves to an unusually concentrated solution without becoming unworkably viscous, which is exactly what a pigment vehicle needs and what distinguishes it from the other natural binders. It has no melting point, no molar mass and no molecular formula; it is bought to a specification — species, grade, spray-dried or in tears — in the same way a photographic gelatin is bought by Bloom number.

Its behaviour in solution has the two properties every colloid page in this encyclopaedia repeats. It is a growth medium: a standing aqueous solution ferments, which is why historical formulas add a preservative. And it is a binder that changes irreversibly when it is cross-linked, so a hardened gum layer cannot be recovered, redissolved or reworked.

Ware supplies one more observation that belongs under properties rather than under history. Both Pizzighelli’s platinum sensitiser variants and Jarman’s 1907 water-development paper add gum arabic to a ferric oxalate sensitiser — Ware’s judgement is that it was there “presumably to ‘improve’ the surface qualities of the print”, that these are undesirable additional variables, and that a formulation with as many as eight ingredients makes it very improbable that the proportions were ever experimentally optimised. The course records that as a caution about reading period formulas: an ingredient in a historical recipe is not evidence that the ingredient does anything.

Handling

There is no GHS classification and no exposure limit, and neither absence is reassurance. The front matter above sets out what the course looked for and did not find, and quotes EH40’s own caution. What remains is straightforward and unglamorous: a fine organic powder that should not be raised as a dust when it is weighed, a solution that spoils, and a supplier’s safety data sheet that describes a product — which may contain a preservative — rather than the exudate.

Why Level A. Against the course rubric, gum arabic 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; there is no mains-voltage work; and the waste from the gum itself is dilute rinse water.

⚠The one operation that takes this out of Level A

Adding a dichromate. Everything on this page about gum arabic as a material is Level A, and the moment a soluble dichromate goes into the solution the assessment belongs entirely to the dichromate: potassium dichromate’s notified classification runs to twelve statements including H340, H350 and H360, and HSE’s EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium with the Carc and Sen notations. The course’s ruling is not a level at all but a prohibition — chromium(VI) is never used, anywhere, and gum bichromate is taught as chemistry and history in Part XXVI with no procedure. That ruling lives on the chromium policy page and is cited, not restated, here.

Nothing about the gum requires it. A dichromated gum layer is Level D because of one ingredient, and the binder that carries the pigment is the same food-grade material either way. That is the clearest case in this encyclopaedia of the rule that a level is assigned to a procedure and not to a substance.

Waste

Dilute gum in rinse water is organic load. Everything else about the waste belongs to what the gum was carrying. A pigmented, dichromated washing-off bath is a chromium(VI) waste and goes by the route on the chromium policy page; a Pellet coating’s rinse carries iron salts and ferrocyanide and follows the cyanotype route; a size solution carrying a preservative carries the preservative’s classification. Bottle each stream separately and label it with what it contains and the date, as ILFORD instructs domestic users to do. Check your local regulations; they govern.

History

Gum arabic reached photography already established as something else. The registry codes on its FDA record show what that something else is even now: a food additive with an international number, two United States food regulations and a GRAS notification. Photography borrowed a material that was already being made, graded and shipped at industrial scale for other buyers, which is why no photographic supplier ever had to develop it.

Its career here is accordingly unlike any other substance in this encyclopaedia. It did not arrive, dominate and retreat; it arrived as a studio material, acquired a light-sensitive partner in the middle of the nineteenth century — Ware calls the result a “‘painterly’ but photographic method” — and is still in use by printmakers today. What the course withholds is not the gum. It is the partner.