Hand-coated silver chloride contact paper

This is the first emulsion in the course that a reader makes rather than buys, and its design is a teaching decision as much as a chemical one: it is not washed, so it can be made, coated and printed in one session, and everything the make left behind stays in the sheet. The Part V project is the written page that owns it, with the quantities and the record sheet.

The chemistry

One precipitation, in a viscous medium. Silver ions meet chloride ions and silver chloride comes out of solution, because its solubility product is small — 1.6 × 10⁻¹⁰ at 25 °C. In pure water that would give a coarse, settling precipitate. In gelatin it does not, and the reason is peptisation: the gelatin adsorbs onto the growing crystals, restrains their growth and keeps them dispersed.

The excess chloride does three jobs at once, and they are worth separating because they act at different times. It drives the precipitation to completion by the common-ion effect, leaving little dissolved silver. It provides the ripening agent for the half hour of heat, because a chloride ion can lift a silver ion off a crystal surface as a soluble complex, and that complex is how material moves from the small crystals to the large ones. And it leaves the crystals carrying adsorbed chloride rather than adsorbed silver, which is the state a printing emulsion wants.

⚗Why the paper is slow, and stays slow

Silver chloride is the least sensitive of the three halides and its intrinsic response reaches only into the ultraviolet and the violet edge. Nothing in this make adds sensitivity: there is no sulfur digestion, no gold, no dye. That is exactly why it can be handled under a generous safelight and printed by contact under a domestic lamp, and exactly why it will never be an enlarging paper.

The excess halide is also why it cannot be sensitised without being washed first. Duffin is explicit that chemical sensitisation is retarded by the presence of excess halide, which is the chemical reason a bromide emulsion must be washed and a gaslight paper need not be. Wall states the same division from the practical side in 1929: bromide papers are always washed emulsions, the gaslight papers usually unwashed.

Historical workflow

The course’s own make is T. Thorne Baker’s chloride emulsion of March 1943, in Denise Ross’s potassium chloride gaslight-paper adaptation for The Light Farm, run at about one hundredth of the manufacturing scale: salted gelatin, five grams of silver nitrate added in three portions, half an hour’s ripening, and coating while warm onto a sheet of watercolour paper that has been soaked and squeegeed down wet.

The historical workflow it stands in for is the gaslight paper trade. Wall’s 1929 manual sets out commercial emulsion making at the scale it was then done — chloride and chlorobromide emulsions in forty-five-litre batches, with wash times and full finals sets — and the distinction that matters here is the one about washing, because it is a distinction between two kinds of product rather than two levels of care.

The image

Pale cream, not white, in the dry sheet, with the paper texture showing through. That is the first consequence of coating a paper with nothing under the emulsion.

Contrasty, with a shorter exposure scale than a normal enlarging paper.

Neutral to slightly warm in a normal paper developer, and the colour is set by the developer and the rate of silver deposition rather than by the emulsion — the point Project 2 makes at length with Wall’s own controlled series behind it.

🔬A section through the sheet, and two absences

For a sheet coated at fifty-two millilitres over an eleven by fifteen inch sheet, the emulsion carries about 100 mg of silver halide per square decimetre — about two and a half times Baker’s stated ceiling of 25 to 40 mg/dm² for a chloride plate. It also contains, permanently, everything the make left behind: the potassium nitrate formed in the precipitation and the 37 mol per cent excess of potassium chloride.

Then two absences that matter as much as the layers present. No supercoat, so the emulsion surface is the outermost surface and is what gets scratched. No baryta layer, where a commercial paper carries barium sulfate in gelatin between base and emulsion — which is one reason a hand-coated sheet’s maximum black and its paper white both differ from a bought paper’s.

Permanence

The unwashed salts are the honest cost of the design, and this is the entry that has to say so. A commercial chloride paper is washed; this one is not, so potassium nitrate and a large excess of potassium chloride remain in the layer for the life of the sheet. The course has read no study of what that does to a print over decades, and does not claim either that it is harmless or that it is fatal. What can be said is that it is a known departure from manufacturing practice, made deliberately, for a stated teaching reason.

Beyond that, the print’s permanence is a silver gelatin print’s permanence, with two structural differences. There is no supercoat, so the emulsion is exposed to abrasion. And there is no baryta, so there is no barrier between the image layer and the paper — which means the fibre’s own chemistry, its sizing and anything the sheet was buffered with are in contact with the silver.

The fixing and washing arguments of Parts XI and XII apply unchanged and are where the numbers live.

Hazards

Level B, and the rubric puts it there on two counts rather than one.

Silver nitrate, named explicitly under Level B, weighed as a solid and handled in solution.

Heating and holding solutions above 50 °C, which is emulsion making’s own criterion: the make runs warm and the ripening is half an hour of held heat.

Add to those the ordinary controls — nitrile gloves, eye protection, dust control when weighing, and a dedicated set of vessels that never meet food. The waste is silver-bearing throughout and belongs to recovery; the Part V project page carries the waste streams and the disposal considerations in full.

Where the course teaches it

Part V, Photographic Emulsions, as Project 1 — and it is written, which makes this one of the few entries in this atlas that points at a finished lab rather than at a plan. The page gives the preparation, the procedure, the expected observations, the data to record, the analysis and the troubleshooting, and it asks for the make to be repeated with one deliberate change so that the batch record shows a cause and an effect.

Its neighbours in the same part are the other three rows of this atlas’s hand-coating cluster: the chlorobromide paper, which adds 6 mol per cent of bromide; the bromide enlarging emulsion, which is the first washed emulsion; and the dry plate and lantern slide, which puts an emulsion on glass. The assignment that closes the part asks for one finished print on an emulsion the reader made, presented beside the same image on bought paper.