Ambrotype

☠Level D: studied, never performed

An ambrotype is a wet-plate collodion plate, so it inherits that process’s classification whole: ether, nitrocellulose, cadmium salts in the salting mixture, silver nitrate at bath strength, and a fixing step that was frequently cyanide. No procedure appears here.

Nothing about the plate is positive. That is the sentence this entry exists for. The black varnish, cloth or paper turns the pale image silver into highlights and the clear glass into shadow; take the backing away and it is a negative again.

The chemistry

Identical to the wet-plate negative in every reaction — the halide formed inside the collodion film in the silver bath, the physically developed image plated onto the exposed specks by iron(II) out of the solution the plate is still wet with — and different in one setting.

The plate is deliberately made thin. Towler’s 1864 formulas set the two ends of the range and the difference between them is instructive. For ambrotypes and melainotypes he gives three drachms of iron(II) sulfate crystals with four ounces of rain water, three drachms of acetic acid and two of alcohol; for negatives, two drachms of the crystals in thirty-two of water with three drachms each of acetic acid and alcohol. The negative gets less iron because it gets more exposure. Modern practice keeps the same lever from the other side: a concentrated ferrous sulfate developer with a restrainer, diluted 1+3 for tintypes and ambrotypes and developed for twenty to thirty seconds, against 1+5 for glass negatives developed for two minutes after three stops of extra exposure.

⚗Why the picture appears when you put something black behind it

The image silver of a physically developed collodion plate is a pale, scattering deposit lying on the surface of the film. Against a light ground it looks like a weak negative. Against a black ground it has nothing to compete with: the deposit scatters light back to the eye and the bare, clear glass returns almost none, so the deposit reads as highlight and the glass reads as deep shadow.

That is not a chemical change and nothing has been reversed. It is the same plate against a different background, and it makes a unique image out of a negative process. The tintype plays the identical trick with the dark ground built into the support.

Historical workflow

The wet plate’s, run short, and then finished as an object: fixed, washed, varnished, backed with black, and cased.

The fixer is a visible choice. Hardwich’s argument for cyanide is aesthetic rather than practical — hyposulphite contains sulfur and is somewhat unstable, so the image is liable to be superficially darkened, while cyanide is free from that objection and always produces a whiter picture. IPI records the same difference as a process variation with a visible signature: hypo gives a darker, brownish grey image colour, cyanide a lighter, milky tan. The AIC’s ambrotype entry still describes the developed plate as fixed in hypo or potassium cyanide, because both are in the historical record and both are found in surviving objects.

A reader identifying an ambrotype is therefore looking at evidence of a decision somebody made about how white they wanted the highlights, at a cost this course would not now accept.

The image

A pale image against its own black ground, often hand-coloured, cased behind glass like a daguerreotype — which is one reason the two are confused, and one reason the confusion is worth clearing up. A daguerreotype is a mirror and reads positive or negative according to the angle it is held at; an ambrotype is a transparency with something black behind it and does not.

Take the backing off and the polarity flips. Nothing else in this atlas will do that on demand, and it is the cheapest possible demonstration that a photograph’s polarity cannot be read off its appearance.

Detail is the wet plate’s, which is to say very good: a glass support, no fibres, no grain in the gelatin sense.

Permanence

The characteristic loss is mechanical, and it looks exactly like a chemical one. Where the backing has flaked, lifted or been removed, the image silver is intact and its ground has gone. A plate that appears to have faded to nothing may be a plate in perfect chemical condition waiting for a piece of black paper.

That distinction is the single most useful thing a person handling one can know, and it inverts the usual diagnostic instinct. On a silver gelatin print, a pale image means lost silver. Here it may mean lost backing.

Beyond that the object has the wet plate’s own vulnerabilities: a varnish that can crack or be attacked by the wrong solvent, a collodion binder whose long-term behaviour the course will not claim to know, and glass, which breaks.

Hazards

The wet plate’s, in full, and this entry does not restate them: diethyl ether and nitrocellulose, cadmium bromide in the salting mixture, silver nitrate at bath strength, and potassium cyanide where the period fixing is followed.

One additional hazard belongs to the object rather than to the making, and it matters for anyone who owns one. A cased ambrotype is glass, and the backing materials of the period were varnishes whose composition this course has not established. A plate whose backing is flaking is shedding an unidentified material, and the sensible position is to handle it as an unknown rather than to assume it is inert.

Why we study it and do not reproduce it

Because it is the cheapest possible demonstration that a negative and a positive can be the same object seen against two different grounds, and a standing warning against reading a photograph’s polarity off its appearance.

Because it explains why so much of the nineteenth century’s portraiture is unreproducible. The commercial answer to cost was to sell the camera original — the same answer the daguerreotype gave, reached by a completely different route and for a fraction of the price. Two processes, one economic logic, and no negatives left behind either of them.

And because it is a failure mode worth carrying into a collection. The plate that looks faded and is not is the kind of diagnostic trap this course’s troubleshooting atlas exists for, and it belongs in a conservator’s reasoning as much as in a maker’s.

Where the course carries it, and it carries no steps. Part XXVI owns it, in Wet-Plate Collodion: The Process That Made the Nineteenth Century Look Like That, which takes the whole collodion family at Level D and gives no procedure for any of it — the part’s rule, set out in its overview, not a gap in this row.

The section that matters most to this entry is called Three objects out of one chemistry, and it is where the negative, this process and the tintype are separated properly: one sensitised film, three decisions about what to back it with and what to do next. That is this entry’s central claim, taught rather than asserted. The lesson also carries fixing, and the choice that stays visible in the object — the fixer used leaves evidence a century later — and the argument that the varnish is the reason any of these plates survive at all.

And the hazards are counted rather than gestured at: ether, the cadmium salts, and cyanide fixing, which has a lesson of its own explaining why it fixed better in numbers, what it does to a person, and why any acid in the room turns the solid into a gas.

Assignment: Assessing a Historical Process for Hazard closes the part by asking a reader to run the classification themselves, and its identification exercise is where the diagnostic trap this entry describes — the plate that looks faded and is not — belongs in practice.