Gum bichromate

☠Level D: chromium(VI), never used at any level

The gum is a Level A material and the pigments are artists’ colours. The light-sensitive component is the one substance this course will not use at any level, and the chromium policy settles it. What the course withholds here is not the gum; it is the partner. No procedure appears on this page.

Gum bichromate is the process in which the print is most obviously made rather than taken, which is why Pictorialism adopted it and why it is still practised. It is also the most useful corrective available to a reader trained on silver: nothing about it can be diagnosed with silver reasoning.

The chemistry

Mike Ware’s summary is the clearest the course has read, and it is worth having whole. 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 directly.

The image substance is the pigment, so a gum print’s colour, permanence and grain are the pigment’s, not silver’s.

The light-sensitive component is the dichromate, which is why the course studies this process and does not perform it.

!What the course does not claim about the cross-link

The mechanism the sources support is short: 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 its own page carries no formula.

What more or less of the dichromate does is published and it runs opposite to intuition. Wall’s account of gum printing has a mixture rich in colour and poor in dichromate tending to hardness and deep shadows, while doubling the dichromate gives greater sensitiveness, a flatter picture and the fainter gradations. More sensitiser therefore buys speed and costs contrast, which is the reverse of what a silver printer expects from more of anything.

Historical workflow

Size the paper; mix pigment, gum and dichromate; brush it on; dry; expose under a negative; develop in plain water, sometimes helped along with a brush; dry; and, if the tonal range demands it, do the whole thing again in register.

Multiple coats are the method, not a repair. One coat carries a short scale, so gum is usually printed in several coats in register, and each coat can carry a different pigment. And the printer’s hand during development is part of the method rather than a lapse — a brush, a hose or a jet moves pigment locally in a way no silver process allows.

Ware’s judgement of formulations with eight ingredients is worth carrying: the process attracts elaboration that its chemistry does not require.

The image

Whatever colour the printer chose, in one or several layers, with brush marks and coating character visible as part of the picture.

Short-scaled in a single coat, long-scaled only by accumulation.

And diagnosable only in its own terms. For a reader trained on silver this is the most useful thing gum has to offer: what fails is a coating, a registration or a development, never an exposure that failed to be amplified. There is no latent image here, no development in the silver sense, no fixing, and no residual halide. A fault that looks like fog is a coating fault; a fault that looks like a speed problem is a dichromate ratio.

Permanence

The pigment’s, and therefore as good as the pigment is. Nothing in a finished gum print can tarnish, sulfide or mirror, and its failure modes are the paper’s, the sizing’s and the gum’s rather than a metal’s.

What the course cannot tell a reader is how much chromium remains in a finished print and what it does there over decades. That is a real conservation question — chromium(III) is the cross-linker and is part of the hardened layer by definition — and no source read here quantifies it.

And the material is hard to detect. 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, which is worth knowing before trusting any attribution.

Hazards

Chromium(VI), and it is the whole of the assessment. Both the potassium and the ammonium dichromate appear in gum formulations, and both carry notified classifications severe enough that the Level D policy names the family. EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium.

The chromium policy settles the course’s position without a risk assessment: chromium(VI) is never used, at any level, anywhere. Chromium(III), as chrome alum, is permitted at Level B, and the distinction between the two oxidation states of one element is the sharpest in this course’s safety scheme.

Gum arabic itself is a Level A material — a food additive with an international number and two United States food regulations — and the pigments are artists’ colours, whose own hazards belong to their own labels. The hazard is entirely in the sensitiser.

Why we study it and do not reproduce it

Because it is the clearest corrective to silver thinking in the course. A reader who has spent twelve parts on latent images, amplification, fixing and residual thiosulfate needs one process where none of those words apply, and this is it.

Because it is still practised, by printmakers rather than by photographers, which makes it a live process the course excludes rather than a historical one it merely records.

And because the exclusion is honest about what it costs. Gum arabic did not arrive, dominate and retreat like most substances in this encyclopaedia; 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 in use today. The course withholds the partner and says so.

Part XXVI owns “Light-Hardened Colloids: Bitumen, Dichromate and the Pigment Processes”, and is not yet written.