Hand-coated chlorobromide paper

Six mol per cent of the halide is bromide instead of chloride, and everything else about the make is Project 1. That is a deliberately small change in a deliberately controlled comparison, and the thing it was designed to demonstrate turned out to be wrong — which is why this entry is worth reading even by somebody who will never coat a sheet. The Part V project is written and owns the quantities.

The chemistry

Two precipitations and one competition. The two silver halides are not interchangeable. Silver bromide’s far smaller solubility product means bromide is taken up first and can replace chloride already laid down, so it ends up concentrated where it precipitated and at crystal surfaces rather than spread evenly through the lattice. Six mol per cent of bromide therefore does more to the material than six per cent of anything ought to, and a chlorobromide emulsion is not a blend of two emulsions.

The digestion is Project 1’s ripening run harder. Excess halide lifts silver off the smallest crystals as a soluble complex and redeposits it on the larger ones; the size distribution shifts up and the crystal count falls. This make has a smaller excess — 23 mol per cent against 37 — and answers with two and a half times the time. That trade is forced by the absence of ammonia: with it, 40 to 45 °C is often enough; without it, as Duffin puts it, much higher temperatures are needed. The course’s exclusion of ammonia is therefore also a decision to work hotter and longer.

!Halide ratio does not set image colour, and the man whose formula this is proved it

The project was designed around the idea that halide ratio and developer together set the image colour of a warm-tone paper. The first half of that is wrong, and the evidence against it is in the same book as the formula.

Wall ran the controlled series. He made emulsions from pure chloride, through 5, 10 and 20 per cent bromide, to pure bromide; exposed them under a test plate of densities 1 to 3.01 — an exposure range of 1 to 1024 — to three different light sources; and developed every one in one metol-hydroquinone developer. His conclusion, in his words: “In every case the same colored image was obtained from the lightest to the deepest deposit, which conclusively proved that the length of exposure with a constant developer has nothing to do with the color of the image. In further experiments with a weaker developer and increased bromide content, warmer tones were obtained, but they were the same on all plates, thus proving that it is the rate of development, or in other words, the rate of deposition of the silver, which is the determining factor of the color of the image, and not the composition of the emulsion.”

He cites Chapman Jones alongside, who showed image colour depends on the size of the silver particles — the same finding from the other side: slow deposition builds fine particles, and fine particles look warm.

So what does halide ratio do? Wall answers in the next breath: “The more bromide an emulsion contains in comparison to the chloride, the faster it is, as a rule, the longer the scale of gradation and the less easy to obtain warm tones.” Ratio sets speed and the shape of the scale, and decides how easily a warm tone can be reached — because a chloride-rich emulsion develops slowly, and slow development is what makes warmth available.

Historical workflow

The gaslight and warm-tone paper trade, at manufacturing scale, is Wall’s 1929 subject: chloride and chlorobromide emulsions in forty-five-litre batches, with wash times and full finals sets. What the course reproduces is the chemistry rather than the scale, and it reproduces it unwashed, which puts this sheet in the gaslight-paper class rather than the enlarging-paper class.

The image

Whiter, more opaque and thicker at coating temperature than the chloride emulsion, because there is more silver halide in less liquid.

By transmitted light the colour of the emulsion is a maker’s check, and it is one of the most useful habits in the part. An undyed chloride emulsion looks white, a bromide one pale yellow and an iodobromide one deeper yellow; at 6 mol per cent bromide expect very nearly white, with at most a hint of cream. It tells you what you have made before you expose anything.

That ladder is also the safelight warning, and the course states its own limit here. No source establishes a safelight margin for a mixed-halide emulsion, and bromide reaches further into the blue than chloride does, so the margin narrows with every mol per cent added. Six per cent is a small move and this emulsion is still very largely a chloride paper — but the honest instruction is to re-run the fog test on a coated sheet of your own emulsion rather than to assume that Project 1’s verdict transfers.

Speed is unknown. No source at any tier publishes a speed figure for either emulsion, and a step count against the chloride sheet is the only comparison that exists.

⁇What more bromide does to contrast: two Tier 1 sources, recorded as disagreeing

The course’s research pass recorded Baker 1941 and Duffin 1966 as disagreeing about which direction more bromide moves contrast, and ruled that the page carry the disagreement rather than pick a side.

Baker, page 95: if the bromide is much in excess of the chloride, “the tone will be colder but the gradation will usually have a longer scale; an excess of the chloride will tend towards higher contrast and a shorter scale, but will more readily yield color on suitable development.”

Duffin, page 74: “chlorobromide emulsions are of greater speed and softer in contrast as the bromide content increases.”

Read carefully, the two may not be describing the same variable over the same range — but the course does not reconcile them by assuming that, and the project’s own step wedge is the measurement that would settle it for one particular make.

Permanence

The chloride paper’s, unchanged in kind: an unwashed emulsion, so the potassium nitrate of the precipitation and the 23 mol per cent halide excess stay in the sheet permanently; no supercoat, so the emulsion is the outermost surface; no baryta, so there is no barrier between the image and the paper.

One addition specific to this make. Bromide sits at crystal surfaces rather than spread through the lattice, so a chlorobromide emulsion’s surface chemistry is not a weighted average of its two halides. Whether that matters to a finished print’s stability is not something the course has a source for, and it is not asserted.

Hazards

Level B, for the same two reasons as the chloride project: silver nitrate, named explicitly in the rubric under Level B, and holding solutions above 50 °C — here for two and a half times as long as Project 1, which is the practical cost of leaving ammonia out.

Nitrile gloves, eye protection, dust control when weighing, dedicated vessels, and a silver-bearing waste stream that belongs to recovery. The Part V project page carries the hazards, the PPE, the ventilation, the waste streams and the disposal considerations in full.

Where the course teaches it

Part V, as Project 2, between the chloride paper and the first washed emulsion. It is written.

Its role in the part is not really the paper. It is the controlled series: the project runs one developer at two strengths rather than two different products, because dilution moves the rate of deposition directly, and the rate of deposition is what Wall’s evidence says sets the colour. A reader who takes only one thing from this entry should take that — a warm-tone paper is a paper that develops slowly, and the halide ratio’s job is to make slow development available rather than to supply the warmth itself.