Woodburytype

☠Level D: chromium(VI), and a lead mould

The relief that starts the process is a dichromated colloid, which the chromium policy rules out at any level; and the mould is made by pressing that relief into soft lead. No procedure appears here.

The Woodburytype and collotype are two rival answers to one problem: printing a photograph in ink, in continuous tone, without a halftone screen. This one answers it by casting rather than printing, and the result is the most literal tone-as-thickness object in this atlas.

The chemistry

The photochemistry is the family’s and is stated in full on photoglyphic engraving and carbon print: light reduces chromium(VI) to chromium(III) in the presence of an organic colloid, the reduced chromium cross-links the colloid, and warm water washes away what light did not harden. 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 the Woodburytype adds is mechanical rather than chemical, and the course’s glossary states it in one sentence: 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.

⚗Why that is a different kind of continuous tone

Three processes in this atlas carry continuous tone in ink or pigment and they do it three different ways.

Carbon print — the relief is the picture. Light hardens the gelatin in proportion, warm water removes the rest, and the pigment that survives is thicker where more light fell.

Woodburytype — the relief is a master. It is pressed into lead, and the resulting intaglio mould is filled with pigmented gelatin, so every copy carries the same thickness variation.

Collotype — there is no thickness variation at all. The ink is carried by a reticulated gelatin surface whose own grain does the screen’s work.

The first is a one-off. The second is an edition of casts. The third is an edition of impressions. All three are the same photochemistry.

Historical workflow

Make a carbon-type relief from a dichromated pigmented gelatin tissue; press it into soft lead under great pressure to make a mould; fill the mould with warm pigmented gelatin; press paper against it; release, trim and mount.

!What this entry can and cannot state

This is the thinnest entry in the atlas, and it says so rather than padding.

What the corpus carries is the principle above, from the course’s own glossary; the family’s photochemistry, in full, from the potassium dichromate entry and the Getty’s carbon atlas; and the family’s reach, from Wall’s surveys of 1912 and 1924, which list the bichromate methods across gum, carbon, photogravure, collotype, oil and bromoil and most photographic block and plate making.

What it does not carry is a sourced account of the practice: the pressures used, the lead alloy, the gelatin formulation, the release agent, the trimming and mounting, the edition sizes, or the dates and names of the process’s development and decline. Those are published somewhere and they are not in this course’s corpus.

So the entry states the mechanism and stops. Reconstructing a plate-making procedure from general knowledge is exactly what Rule 1 forbids, and it would be easy to do here and impossible for a reader to check. A later author with the conservation literature to hand should treat this page as a stub with a correct skeleton rather than as a finished account.

The image

Pigment in gelatin, of genuinely varying thickness, normally trimmed and mounted because the cast layer does not extend to a plate edge in the way an inked impression does.

Its colour is the pigment’s.

And under magnification there are no dots and no reticulation — which is the identification. A screened process shows dots; a collotype shows an irregular vermiform grain; a Woodburytype shows neither, only a smooth layer of varying thickness. That is the examination the glossary says the two rivals turn on, and it separates all three families of ink-and-pigment printing from each other.

Permanence

Pigment in gelatin rather than silver in a binder, so none of the silver failure modes apply: no tarnish, no mirroring, no sulfiding, no yellowed highlights from retained silver.

What remains is the gelatin, the pigment, the mount and the storage climate. A cast gelatin layer is thicker than an emulsion and is not on a rigid support, so its physical vulnerabilities — cracking, delamination from the mount — are its own and are not the same as a print’s. The course has read no conservation account of them and does not describe them further.

Hazards

Chromium(VI) in the relief-making stage, ruled out at any level by the chromium policy.

Lead, in the mould. Lead compounds carry their own classifications and lead appears elsewhere in this course only as a substance to avoid — the lead nitrate page exists so that a reader meeting it in a period toning formula knows what they are looking at, not so that anyone weighs any. A process whose mould is soft lead pressed under great pressure is not one a domestic room can host.

And great mechanical pressure, which is a hazard this course does not assess at all and has no rubric for.

Three hazards, from three different families, in one operation. That is a good part of the reason the process belonged to specialist workshops in its own time as well as in this course’s judgement.

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, and because the two halves are constantly confused with each other.

Because it is common enough in book illustration of the period that a reader will meet one, and identification decides which failures to look for.

And because it is the clearest case in this atlas of an entry the course owes and cannot yet pay in full. The register counts it, this page gives what the corpus supports, and the gap is named rather than filled. That is the same discipline the rest of the atlas applies to a missing threshold or an unnamed reaction product, applied here to an entire practice.

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, a lesson built around the observation that one exposure has six second halves — and this process is one of the six, reached from the same dichromated colloid as carbon printing and diverging only at what is done with the relief.

Two of its sections matter most here. Printing without a screen: the Woodburytype, collotype and photogravure sets this row beside its siblings. And the objects: what they look like, and how they are told apart supplies the identification this entry most needed, because telling the three apart is the practical difficulty a reader meets long before any question of making one.

And the part states plainly where it has departed from its own rubric. Chromium(VI) is Level C material by the safety classification, in any quantity; these pages are Level D. Part XXVI records that as a decision it made rather than a rule it followed, because a course written for a home reader cannot assume a fume cupboard, a waste contractor or a trained second pair of hands. Level D means a page cannot assume the conditions, not that the conditions are unobtainable.

The gap this entry names is narrower than it was, and it has not closed. Part XXVI supplies the chemistry, the placement among the ink processes and the identification, so the entry is no longer the course’s least-paid debt. What no page in the course supplies is a practice, and under the rule above it will not: the process is study-only, and the omission is a decision rather than an absence of evidence.