Photogravure

☠Level D: chromium(VI) and a strong acid etch

The mature process rests on a dichromated colloid resist and on an iron(III) chloride etching bath at 33 to 46 per cent, which is not a neutral salt solution but a strong acid. The chromium policy settles the first and the Level D policy the second. No procedure appears here.

Photogravure is the end of the line of work Niépce began, since his object was never a picture but a plate a printer could ink. It is also the answer to a question silver photography never solved: how to put a photograph on the same page as text, in an edition, without the picture being a mosaic of dots.

The chemistry

A resist is exposed and developed, and the metal beneath it is etched so that it holds ink by tone.

The resist is a dichromated colloid in the mature process and bitumen in the earliest one, and the difference between them is chemistry rather than principle. Both belong to the family in which light hardens a coating rather than reducing a metal salt; both are developed with a solvent that removes what light did not act on; and in both the surviving coating’s job is to keep acid off the metal underneath it.

⚗The rule a beginner gets backwards

Wall’s 1924 formulary gives a ladder of iron(III) chloride etching baths at 43, 40, 38, 36 and 33 degrees Baumé — corresponding to 46, 41, 39, 36 and 33 per cent — with 6 per cent of freshly precipitated iron(III) hydroxide added to each and allowed to settle. The rule that governs their use is: “The weaker the solution the stronger the etching.”

The course reproduces Wall’s rule and does not claim to have verified the explanation usually given for it, which concerns how fast each strength of bath penetrates the swollen gelatin resist rather than how fast it attacks bare copper. The half-tone bath is separate and simpler: a 35 per cent solution of the anhydrous salt, a first bite of thirty seconds, and three and a half to four minutes in total after stopping out.

Why an intaglio plate can carry continuous tone at all is worth stating, because it is what separates gravure from every screened process. The ink sits in recesses bitten into the metal; a deeper recess holds more ink; and a deeper recess is where the resist was thinnest, which is where least light fell. The tonal information survives as depth rather than as dot area. Because the ink layer is genuinely continuous, a gravure sits with the pigment processes for permanence rather than with the silver ones.

Historical workflow

The oldest form is Niépce’s, and it did not go away. Look at the British patent abridgments for photography for 1877 to 1883, fifty years after his death, and the class is full of specifications that are recognisably his process: a metal plate coated with bitumen or asphaltum dissolved in benzene or turpentine, exposed under a negative in a printing frame, the unaffected coating cleaned off with a solvent, and the bared metal then etched with acid or built up in a galvanic bath. Change the resin, change the solvent, keep the logic, and you have photolithography and photogravure — and, a century later, the manufacture of every integrated circuit ever made.

The mature form substitutes the colloid. Talbot’s patent of 29 October 1852 covers the sensitivity of dichromate mixed with gelatin or gum, and Wall’s 1912 “Bichromate Methods” shows how far it had gone by then: gum, carbon, photogravure, collotype, oil and bromoil, and most photographic block and plate making besides.

The half-tone that this atlas does not have an entry for is the third answer, and it is the one that won commercially: break the image into dots with a screen, and the acid’s all-or-nothing bite stops mattering. Photogravure is the answer that refused to do that.

The image

Ink on paper, in continuous tone, in an edition. Its colour is the ink’s and its surface is the paper’s.

And it is the form in which many nineteenth-century photographs actually reached their audience, which is the reason this entry matters to a reader who will never make one: a reader looking at a reproduction may be looking at the original object.

The tonal argument is the whole aesthetic case. A gravure carries the negative’s tones as ink depth rather than as dot area, so the transitions are continuous and the blacks are ink rather than an average of dots and paper.

Permanence

A pigment process, by the reckoning that matters. Because the ink layer is genuinely continuous and carries no silver, a gravure cannot fail in any of the ways a silver print fails, and its permanence is the ink’s and the paper’s.

That places it with the carbon print and the Woodburytype rather than with anything in the silver half of this atlas, and it is why identification matters: a pigment- or ink-in-paper image will show none of the silver failures, so a conservator’s first job is to establish which family the object belongs to.

Hazards

Chromium(VI) in the resist, which the chromium policy rules out at any level anywhere in this course.

Iron(III) chloride at 33 to 46 per cent. Its own encyclopaedia entry records the property that matters: the anhydrous salt’s solution is not a neutral salt but a strong acid, and CAMEO groups it with the strong non-oxidising acids. A bath of it deep enough to etch a plate is a large volume of that.

Organic solvents, where the bitumen resist is used — benzene or turpentine in the patent specifications, and the Level D policy names the dangerous solvents.

And a metal-bearing waste stream carrying dissolved copper and iron, which is not a domestic disposal question.

Why we study it and do not reproduce it

Because it is where Niépce’s line ends up. Heliography states the principle — light changes a coating’s solubility, and a solubility difference can protect metal from acid — and this is that principle industrialised. The most useful sentence in the whole chain is on the heliography page: Niépce did not solve the problem he set out to solve; he solved a neighbouring one, and the neighbouring one turned out to matter more.

Because it answers the question the half-tone screen answered differently, and comparing the two answers is the clearest way to see what a screen costs.

And because a reader will meet one. Photogravure is not a curiosity at the edge of the subject; it is how a great deal of nineteenth-century photography was published, and identifying it is part of reading the period’s images correctly.

Where the course carries it, and it carries no steps. Part XXVI owns the light-hardened colloids, in Light-Hardened Colloids: Bitumen, Dichromate and the Pigment Processes, and it is the page that closes Niépce’s line for this course. Bitumen: the first member, and one honest gap carries the beginning and names what is not known about it; the section headed 1832, 1839, 1852, 1855 separates four claims that usually get collapsed into a single origin story, which is the sort of correction this entry’s history depends on somebody having made.

This row is placed in Printing without a screen: the Woodburytype, collotype and photogravure, and the comparison this entry exists to make — what a half-tone screen costs, against what these three do instead — is worked out in the half-tone problem, and three answers to it rather than asserted.

And the lesson closes Niépce’s line for the course. Heliography states the principle; Part XXVI’s Bitumen: the first member, and one honest gap carries the first member and names what is not known about it, and its section on 1832, 1839, 1852 and 1855 separates four claims that usually get collapsed into one — which is the kind of correction this entry’s history section depends on somebody having made.

And the part is candid about a classification it has bent. The safety rubric assigns chromium(VI) to Level C, specialist or supervised laboratory, in any amount; Part XXVI teaches these processes at Level D and says the discrepancy is its own decision. The ground is that a home reader cannot be assumed to have extraction, a chemical waste contract, or somebody trained in the hazard beside them — so the course gives the procedure to nobody rather than to the wrong person.