The Becquerel variant

☠Level D, and classified here for the first time in the course

The register that plans this atlas classifies the Becquerel process Level D in the design’s own words, and this entry is where that classification is first stated in the course. It is worth being precise about why, because the obvious reading is wrong: the variant removes the mercury and it does not remove the reason for the level.

No procedure appears here. No quantity, no temperature, no time, no apparatus, no sequence.

The Becquerel variant is named on the Part I daguerreotype page and not described there; that page ends by saying that whether and how the course would ever classify it is Part XXVI’s decision. This entry records the classification the register made and the little the corpus establishes, and marks the rest as open.

The chemistry

The AIC’s conservation record gives the operation in one sentence: a variant dating from 1840 in which the exposed plate is developed by red light instead of by mercury vapour, needing no mercury and no bromine or chlorine sensitising, with the plates around ten times slower in consequence.

What the course can say about the mechanism it says by placing the observation in a family it has already established. Eder records that Draper in 1842, Lerebours in 1846 and Claudet in 1847 all found the same red-light action on the latent image of the iodised daguerreotype plate that Herschel had found on visible print-out silver, and that the work was later extended. So the effect is not isolated: red light acting on an already-exposed silver iodide surface does something the same light will not do to an unexposed one.

!What the course does not state

The course has read no account of the mechanism that meets its own source standard, and does not supply one. In particular it does not assert what the image substance of a Becquerel plate is, how it differs from the silver–mercury amalgam of the mercury process, or why the plate should be about ten times slower. The speed ratio and the absence of mercury are the AIC’s; everything beyond them is open.

That gap is not decorative. A page that cannot say what the image is made of cannot argue about its permanence either, and this one does not try.

Historical workflow

Sensitise over iodine; expose in the camera; develop under red light rather than over mercury; fix. The first and last of those are the mercury daguerreotype’s own operations, unchanged, and that entry carries them.

The variant sits in the same two years as the accelerations that made portraiture possible on the main process — Goddard’s bromine of December 1840, Kratochwila’s independent result of September 1840, Claudet’s iodo-chloride of May 1841, and Petzval’s lens of 1840. A variant that gives up halogen acceleration and runs an order of magnitude slower is arriving exactly as the trade is moving the other way, which is most of the explanation for why it stayed a variant.

The image

The object is a daguerreotype plate, so the properties that follow from the plate rather than from the development are unchanged: it is a unique image, laterally reversed unless the camera carried a mirror or prism, and ambivalent — reading positive or negative according to the angle it is held at, because the picture is a contrast between scattering and reflection rather than between densities.

Whether a Becquerel plate looks different from a mercury-developed one, and how a conservator would tell them apart, this course has not established. That is a real question with a real answer somewhere in the conservation literature, and the honest thing to do with it here is to name it as outstanding rather than to guess.

Permanence

Inherited rather than argued. The plate is silver on copper with a surface image and no binder, so the mercury daguerreotype’s permanence discussion applies to the object: chemically stable once the iodine is removed, mechanically fragile, and dependent on a sealed case.

What the course cannot say is whether the image substance produced by red-light development is as stable as an amalgam, or whether gilding does the same thing for it. No source read here addresses either question.

Hazards

The mercury is gone and the iodine is not, and that is the whole classification argument.

Sensitising a plate means iodine vapour in a closed box and then an open one. HSE’s EH40 gives iodine a short-term exposure limit of 0.1 ppm, and a limit exists because the substance is judged to need control. The absence of the mercury step removes the worst hazard of the parent process and leaves a vapour-phase halogen operation, carried out beside photographic materials, with no engineered extraction that a domestic room can assume.

The course’s Level D policy turns on one question — can a reader assemble the controls that make this reasonable? — and the answer here is the same as for the parent process, reached by a shorter argument.

Why we study it and do not reproduce it

Three reasons, and the first is the interesting one.

It is the clearest case in the course of a variant that is safer and still not safe enough. The temptation with a Level D process is to look for the substitution that rescues it, and the Becquerel variant is exactly the substitution one would look for: same picture, no mercury. The course’s Level D policy forbids “a substitution that makes it safer” as a way of getting a procedure onto a Level D page, and this entry is what that rule looks like when applied to a real and genuinely milder variant. A partial removal of a hazard is not a control.

It is evidence about the latent image. That red light can bring out an exposure on iodised silver, and that three independent workers found the same effect between 1842 and 1847 on the plate and Herschel had found it on print-out silver, is a fact Part IV’s account of the latent image has to accommodate.

It is honestly incomplete, and the atlas says so. Of the fields this atlas asks of a process, this entry can fill the chemistry in outline, the workflow in outline, the speed, the hazard and the classification. It cannot fill the image colour, the tonal scale, the identification signature or the permanence. Part XXVI owns the mercury daguerreotype and is where those gaps should be closed, from the conservation literature, when it is written.