Collotype

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

Collotype is a dichromated colloid process and the chromium policy rules chromium(VI) out at any level anywhere in this course. No procedure appears here.

Collotype and the Woodburytype are two rival answers to one problem: printing a photograph in ink, in continuous tone, without a halftone screen. They are constantly confused, and their identification turns on the same examination — which is why this atlas carries them as neighbouring rows and the course’s glossary treats them together.

The chemistry

The photochemistry is the family’s, and photoglyphic engraving states it: light reduces chromium(VI) to chromium(III) in the presence of an organic colloid, and the reduced chromium binds the colloid so that the exposed layer no longer swells and dissolves. The course has not found a description of that cross-link at the level of which groups are bridged, and does not assert one.

What is particular to collotype is what happens to the gelatin surface, and it is the process’s whole idea: the ink is carried by a reticulated gelatin surface whose own grain does the work of a screen. Where the layer is hardened it takes up less water and accepts more greasy ink; where it is soft it swells with water and rejects the ink; and because the reticulation is fine and irregular, the plate carries a continuous range of ink density without a regular dot pattern anywhere on it.

!What this entry can and cannot state

The corpus behind this course carries collotype as a named member of the bichromate family — Wall’s 1912 and 1924 surveys list it among gum, carbon, photogravure, oil and bromoil and most photographic block and plate making — and it carries the family’s photochemistry in full on the potassium dichromate page.

What it does not carry is a sourced account of how the reticulation is produced and controlled: the gelatin formulation, the substrate preparation, the drying regime that raises the grain, the damping solution, or the press work. Those are real and published somewhere, and they are not in this course’s corpus. The entry therefore states the principle and stops, rather than reconstructing a plate-making procedure from general knowledge — which is what Rule 1 forbids and what would be easiest to do here.

One period detail the corpus does hold, and it is worth having because it says something about the material. Wall’s photomechanical section soaks old collotype plates in a 5 per cent caustic potash solution for four days before the old film is scraped off — the alkali attacking hardened gelatin exactly as it softens an emulsion left too long in an over-alkaline bath. A hardened dichromated gelatin layer is tough, and taking one off again is a four-day job.

Historical workflow

Expose a dichromated gelatin layer on a plate under a negative; wash out the unhardened dichromate; raise and control the reticulation; damp the plate; ink it; print it on a press. The plate is regenerated by the caustic soak above when its working life ends.

Its place in the family’s chronology is with the other photomechanical processes that grew out of Talbot’s 1852 patent and became an industry between then and Wall’s survey of 1912, by which time the bichromate methods covered most photographic block and plate making.

The image

Ink on paper, in continuous tone, with no dots. Under magnification the give-away is the reticulation itself — an irregular, vermiform grain that no screened process produces and no photographic emulsion has.

Its colour is the ink’s, and the process was widely used for book illustration, which is where a reader is most likely to meet one.

Set beside its rival, the difference is where the tone lives. In a Woodburytype a hardened gelatin relief is pressed into soft lead to make a mould, and pigmented gelatin is cast in it, so the ink layer really is thicker in the shadows. In a collotype the ink is carried by the reticulated surface and the grain does the screen’s work. Both descend from the dichromated colloid, and both are ink or pigment on paper rather than silver in a binder.

Permanence

Neither can fade in the ways a silver print fades, because there is no silver in either. The permanence is the ink’s and the paper’s.

The practical consequence for anyone handling one is the one the course’s glossary states for the pair: knowing which family an object belongs to decides which failures to look for, and a pigment- or ink-in-gelatin image will not show any of the silver ones. A conservator who diagnoses a collotype as a faded silver print is looking for a mechanism that is not there.

Hazards

Chromium(VI) in the sensitised layer, ruled out at any level by the chromium policy; EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium.

Caustic potash at 5 per cent, in the plate-regeneration step. Potassium hydroxide’s page records how Wall’s period describes the material — white sticks, poisonous, burning to the skin, extremely deliquescent — and a four-day soak is a standing bath of a strong alkali.

Press work, which is a mechanical hazard rather than a chemical one and is outside anything this course assesses.

Why we study it and do not reproduce it

Because it is half of the proof that continuous tone in ink was solved before the halftone screen made it unnecessary. The Woodburytype is the other half, and the two together are the strongest evidence in this atlas that a technical problem can be solved well and then abandoned for something cheaper.

Because they are common enough in book illustration of the period that a reader will meet one, and because the two are constantly confused with each other and with photogravure.

And because the course cannot teach it and should say why in the right terms. The obstacle is not that the chemistry is exotic — it is the same chemistry as carbon printing and gum bichromate — but that the sensitiser is chromium(VI) and that this course’s corpus does not hold a plate-making account it could publish even if the policy allowed it. Two different kinds of gap, and this entry names both.

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, which follows one photochemical idea from Niépce’s bitumen and Talbot’s dichromated gelatin out into its six descendants — one exposure, six second halves, in the lesson’s phrase.

The section this row belongs to is Printing without a screen: the Woodburytype, collotype and photogravure, and it is the placement this entry has been arguing for: three answers to a single problem, distinguished by how each one holds the ink. The objects: what they look like, and how they are told apart is the companion passage, and it exists because the confusion this entry complains of is the one a reader actually falls into.

And Part XXVI settles a classification question in the open. Chromium(VI) is Level C work under the safety rubric at any concentration, yet the part teaches these processes at Level D. It declares that gap as a choice of the course rather than a reading of the rubric: a domestic reader cannot be assumed to have a fume cupboard, a licensed waste route or supervision, so no procedure goes to anyone. What Level D withholds is a page’s instruction, not a practitioner’s right to work.

The two kinds of gap this entry names are both still real, and the part confirms the second. The sensitiser is chromium(VI), which is a policy question the lesson settles above; and the course’s corpus holds no plate-making account it could publish even if the policy allowed it. Part XXVI does not supply one, and does not pretend to.