Photoglyphic engraving

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

Every process in this cluster rests on a chromium(VI) compound, and the course’s chromium policy is absolute: chromium(VI) is never used, at any level, anywhere in this course. No procedure appears here.

Photoglyphic engraving is where the dichromated colloid enters photography as a patented process, and its importance is out of all proportion to the number of prints made by it. Talbot’s patent of 29 October 1852 covers the sensitivity of dichromate mixed with gelatin or gum — and the whole chromium(VI) family, carbon printing, gum bichromate, photogravure, collotype and the Woodburytype, descends from that one observation.

The chemistry

Light does not act on the dichromate alone. The 1911 Encyclopaedia Britannica states the constraint plainly: a dichromate solution absorbs only the rays that are effective in altering it, its spectral action beginning in the ultraviolet and reaching no further than the blue-violet, and the change is only possible in the presence of organic matter of some kind, such as gelatin or albumen.

The Getty Conservation Institute calls what happens the photochemical reduction of the dichromate salt: chromium(VI) takes electrons from the colloid around it and falls to chromium(III), and the reduced chromium binds the colloid so that the exposed layer no longer swells and dissolves.

!What the course states about the cross-link, and what it does not

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 colloid’s macromolecules; a cross-linked network no longer dissolves in water.

The course has not found in its sources a description of that cross-link at the level of which groups are bridged, and does not assert one. The gum arabic page says the same and gives the reason: no source read there states which groups on the gum the chromium(III) binds to, in what ratio, or whether the gum itself is oxidised in the process.

This is heliography’s family rather than silver’s: light hardens a coating rather than reducing a metal salt, and development is a solvent removing what light did not act on. Ponton’s own 1839 observation makes the point in the plainest terms available, in his words as Hunt quotes them: “To fix it, all that is required is careful immersion in water, when it will be found that those portions of the salt which have not been acted on by the light are readily dissolved out, while those which have been exposed to the light are completely fixed on the paper.”

Historical workflow

Talbot’s object was the same as Niépce’s: a printing plate, not a picture. A metal plate carries a dichromated colloid resist; light through a negative hardens it in proportion; the unhardened colloid washes away; and the bared metal is etched so that it holds ink by tone. The persistent difficulty is the one Niépce gave up on in 1827 — the acid either bites or it does not, and a continuous-tone image has no way of instructing it — which is why the half-tone problem is a nineteenth-century industry in itself.

⁇Who found the light-sensitivity of chromates?

Eder’s own chapter heading credits the chromate processes to Ponton in 1839, and then immediately complicates it.

Vauquelin had found in 1798 that chromic acid forms a carmine-red silver salt that darkens in light — but that is a silver compound, so the reaction belongs to silver photochemistry.

Suckow, in 1832, found that chromate salts mixed with organic substances are light-sensitive without any silver. Eder: “He must be recognized as the first discoverer of this light reaction.”

Ponton, in 1839, made the first photographic application of it to printing on paper — and Eder adds that Ponton’s own explanation of the chemistry “was quite incorrect”, and that he missed the far more important sensitivity of dichromate mixed with gelatin or gum, which is what Talbot patented in 1852.

Becquerel repeated Ponton’s work in 1840 and identified the paper’s internal size as the active ingredient — which is the same finding as the Britannica’s rule, arrived at experimentally.

What the course states. Suckow, 1832, for the effect; Ponton, 1839, for the photographic application; Talbot, 1852, for dichromated gelatin. Where a source says simply “Ponton discovered it”, read that as shorthand for the middle claim.

The image

There is no photograph here, and that is the point. What photoglyphic engraving produces is a printing plate, and what a reader sees is an ink impression pulled from it. Its colour is the ink’s, its surface is paper, and it can be produced in an edition.

The tonal question is the whole difficulty. A silver print carries continuous tone natively; a press carries ink or no ink. Everything the dichromate family did over the following fifty years — carbon tissue, the Woodburytype’s relief cast, collotype’s reticulated grain, photogravure’s aquatint — is a different answer to that one question, and Wall’s 1912 “Bichromate Methods” shows the reach by then: gum, carbon, photogravure, collotype, oil and bromoil, and most photographic block and plate making besides.

Permanence

The finished object is ink on paper. It cannot tarnish, sulfide or mirror, and it cannot fade in any of the ways a silver print fades; its permanence is the ink’s and the paper’s.

That is the family’s strongest argument and it is worth stating once here rather than five times over. Take silver out of the image entirely and every silver failure mode goes with it, which is a stronger claim than any toner can make.

Hazards

Chromium(VI). Potassium dichromate’s notified classification runs to twelve hazard statements, and the Level D policy names dichromates and chromic acid as one of the families it covers. EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium.

And the etching. A photogravure or half-tone plate is bitten in an iron(III) chloride bath, which the anhydrous salt’s page records at 33 to 46 per cent — a solution that is not a neutral salt but a strong acid.

The course’s position on the whole family is not that the controls are difficult but that chromium(VI) is not used at all, which is a policy rather than a risk assessment and is stated as one.

Why we study it and do not reproduce it

Because it is the root of a family this atlas carries six rows of, and because the family is where continuous tone in ink was solved.

Because the course teaches the chemistry of light-hardened colloids elsewhere and needs the historical anchor. The dichromated colloid is one of the two great families of photochemistry, and its patent date and its dispute belong somewhere findable.

And because the retreat is instructive. As potassium dichromate’s page puts it, the retreat was not a chemical discovery: nothing better at hardening gelatin arrived. What arrived was an understanding of what chromium(VI) does to lungs and skin, and the exposure limits that followed. Carbon and gum survived in equipped hands, photomechanical printing moved into plants built for it, and the amateur darkroom dropped both the chromium intensifier and the dichromate tray cleaner for substitutes that do nearly as much.

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