Dry plate and lantern slide

The dry plate is the point at which photography stops being a chemistry practised by every photographer and becomes a chemistry practised by a manufacturer for them. What it displaced was the wet collodion plate, which had to be coated, exposed and developed before it dried, so that the photographer carried a darkroom everywhere. Part V’s fourth project is written and undoes that separation deliberately.

The chemistry

The emulsion is the bromide paper’s emulsion put on glass instead of paper, so the precipitation, the ripening, the washing and the finals are all that project’s. What is new here is the interface, and it turns out to carry an unresolved disagreement.

Adhesion is the whole subbing question. Gelatin sticks to clean glass by hydrogen bonding and by mechanical keying into a surface that is microscopically rough. Both are defeated by grease, which is why cleaning is the operation that decides whether a plate frills. When the layer swells in the developer it grows in every direction and the glass does not, so the layer is in compression against a rigid surface; if the bond at the edges is weaker than that stress, the edge lifts. That is frilling, and it is a mechanical failure of adhesion rather than a chemical fault in the emulsion.

What a sub does. Chromium(III) cross-links gelatin by forming ionic links between carboxyl groups on different chains, and a chrome alum dip leaves a trace of chromium on the glass that ties the first gelatin it meets to the surface. Duffin notes the condition that comes with it: the action is pH-dependent, effective near the usual coating pH of about 6.0 and much less so higher.

Why plates dry differently from paper. Duffin’s numbers are the mechanism. A coating is about 85 per cent water when it goes down and about 5 per cent when it is dry, because the gelatin holds on to 10 to 12 per cent of its own weight and will not give it up. On paper part of that water leaves downward into the fibres. On glass all of it must leave upward, through a layer that is getting more concentrated and less permeable as it goes.

⁇Does a hand-coated plate need subbing? Two sources, both credible

Baker, Tier 1, 1941, page 140. “Substratuming is done by dipping them into a two per cent solution of chrome alum after the final rinse and racking them without further rinsing. They should be dried in a perfectly dust-free cupboard.”

The tested modern practitioner, Tier 2, from 2013. Clean 1/16 inch glass, no sub at all, coated directly on a moistened plastic-wrap bed. Her plates process and print.

This is less a contradiction than a difference in what is being asked of the plate. Baker is preparing plates for a commercial coating head and for keeping tests measured in weeks; she is coating by hand and processing soon afterwards. Neither is wrong about their own case.

One correction the course makes in the open. Part V’s own manifest asked for a “chrome alum gelatin subbing solution”. No source the course has read gives one. Baker’s dip is plain chrome alum in water with no gelatin in it, and the course publishes what the source gives rather than inventing a compound layer to match a plan.

The test that decides it for your glass and your emulsion: fix a plate and watch the edges. If the layer lifts, rolls back or frills, you need either a sub or a hardening bath. If it does not, you do not.

Historical workflow

Maddox published a gelatine emulsion in 1871 — silver bromide, as Abney puts it, emulsified in a gelatine solution with which plates are coated. What he had was a binder that could be manufactured, stored and sold. Bennett’s process of 1878 is what gave extreme rapidity: keeping the gelatine solution liquid at about 32 °C for six or seven days.

Abney’s Photography with Emulsions of 1885 records the finding that made the material intelligible — silver bromide may be produced in several molecular states, all of which have different degrees of sensitiveness, identical in chemical composition and differing physically. That is the sentence Part V is built on.

The hand-coated version in this course is: choose, cut and clean thin glass; level a bed by watching a puddle of water; spread fifteen millilitres of emulsion on a four-by-five plate; dry; and coat a lantern slide as well.

The image

A negative on glass, and a lantern slide is the same material used as a positive for projection.

A section through a hand-coated plate is mostly a list of absences. Fifteen millilitres on a four-by-five plate is about 1.2 mm thick wet and roughly 80 µm dry — about twice the top of the industrial range at both ends. The glass is 1.5 to 1.6 mm, thin because the plate holders of the period were made for thin glass. And then three things a manufactured plate has and this one does not: no supercoat, so the emulsion surface is the outermost surface; no anti-halation backing, so light that passes through the layer can reflect off the back surface and return, which is the halation Abney explained in 1875; and no hardener in the emulsion, so the wet layer is tender and is handled by the corners.

Working observations worth having. The puddle test is startlingly sensitive — a bed a spirit level calls flat will send a puddle in a definite direction. The emulsion sets faster on glass than on paper, because glass conducts heat away. The first plate is worse than the fourth, so it is worth coating two you intend to throw away. In the fixer an uneven plate clears unevenly, and the last corner to clear is the thickest. And Wall’s instruction about pepper is explicit and worth obeying: test plates should always be examined with an eyepiece, especially in the parts protected from light.

Permanence

Silver mirroring is the characteristic deterioration, and the conservation record is precise about how to tell it from the other one. The AIC describes it as a very common deterioration of dry-plate negatives: a bluish metallic sheen starting from the edges and visible under reflected light. Improperly washed processing chemistry appears instead as yellow or brown stains in transmitted light. Two faults, two lighting geometries, and the pair is a confirming test rather than an impression. The AIC adds a handling consequence: areas with silver mirroring are extremely susceptible to abrasion.

A varnish overcoat protects. IPI records that silver image deterioration by oxidation causes fading, discoloration and mirroring, and that plates protected by a varnish overcoat rarely show signs of image oxidation — which is a statement about the supercoat this hand-coated plate does not have.

Storage is IPI’s: below 18 °C and 30 to 40 per cent relative humidity.

Hazards

Level B, on the emulsion project’s own grounds — silver nitrate and solutions held above 50 °C — plus two that belong to the glass rather than to the chemistry.

Cutting and handling thin glass. The plate is 1.5 to 1.6 mm and is cut to size, so a cut-edge injury is the most likely accident on the page, and it is one no chemical control addresses.

Chrome alum, where a sub is used. It is a chromium(III) compound, and the course’s chromium policy permits chromium(III) as chrome alum at Level B while forbidding chromium(VI) at any level. That distinction is not a technicality: the two oxidation states of one element sit at opposite ends of this course’s safety scheme, and the chrome alum page carries the classification.

Waste is silver-bearing; a chrome alum bath has its own route, and the chromium(III) rinse route is a written procedure.

Where the course teaches it

Part V, as Project 4, and it is written. The register counts dry plate and lantern slide as one row, because the design names them together, and this entry follows it.

The plate is also where several other written parts land. Part VI exposes film in a camera the reader built; Part XIII measures a characteristic curve; Part XIV and Part XV build the sensitometer and densitometer that make the measurement possible. A hand-coated plate is the material those instruments were built to measure, and the fact that it carries no speed number on a box is precisely the point: the number on a box is what the dry plate invented, and Part V is the part that takes it back.