Project 2: A Chlorobromide Paper Emulsion

Purpose

To move one variable and watch what it does.

Project 1’s emulsion was pure silver chloride. This one carries the same five grams of silver nitrate and puts six mol per cent of its halide in as bromide instead. That match is not luck: the published formula is a commercial gaslight paper whose batch carried 500 g of silver nitrate, and one hundredth of that is exactly Project 1’s charge — which is what turns a comparison into an experiment.

The project also corrects something. It was designed around the idea that halide ratio sets the image colour of a warm-tone paper. The best Tier 1 evidence in this corpus says otherwise, in the words of the man who ran the experiment, and the page is built around what he found.

Learning objectives

By the end of this session you should be able to:

• scale a commercial published formula to student scale, say what scaling does not preserve, and label the result as a variant the maker owns;
• compute a halide ratio and a halide excess in mol per cent from a formula’s masses;
• add a hardener at a dose expressed as a fraction of the gelatin, and explain why that is the form the dose has to take;
• state, with its source, what halide ratio does and does not control;
• design and read a comparison in which the developer rather than the emulsion is the variable;
• carry a disagreement between two Tier 1 sources without resolving it, and say what you would measure to resolve it yourself.

Prerequisites

Project 1 — made, coated, read and recorded. This project is only worth doing if the KCl-01 step wedge exists to compare against.

Safety classification

Level B, for the same reason as Project 1 — five grams of silver nitrate — plus one addition, chrome alum in the finals.

What is not a hazard here, and why. The chrome alum is chromium(III), not the chromium(VI) of dichromate, and that difference is the whole argument. Chrome alum’s notified classification is signal word Warning with GHS07 — H315, H319, H335 — and no sensitisation and no carcinogenicity statement notified at all. The British exposure limit for chromium(III) compounds is 0.5 mg/m³ as chromium; for chromium(VI) it is 0.01 mg/m³ with the carcinogen and sensitiser notations. Fifty times, plus two notations, separates two substances that share a word. The course’s safety classification page holds that ruling once and this page does not restate it in its own words.

The quantity is small: 30 mg per batch, delivered from a stock solution, so the powder is weighed once and never again. Everything absent from Project 1 is still absent — no ammonia, no cadmium, no mercury, no chromium(VI), no strong acid.

Hazards

Silver nitrate, exactly as in Project 1: corrosive, oxidising, very toxic to aquatic life, and never in the same room as ammonia. Splash goggles, nitrile gloves, apron, eyewash and spill provision, and the weighing done in still air. Part II’s silver nitrate handling is the control document.

Chrome alum powder, once, while the stock is made. The hazard is dust: weigh over a tray in still air, gloves and eye protection while the jar is open, and make the whole packet up into a 5 per cent stock in one operation so it never has to be opened again. Label the stock with the substance, the strength and the date.

Citric acid, 0.5 g of a mild solid organic acid. Ethanol, 2.5 mL, flammable in the bottle rather than in the emulsion; keep the bottle away from the bath. Hot gelatin at 50 °C for well over an hour, longer than any previous session: set a timer and do not leave the room.

Required PPE

Splash goggles, nitrile gloves, apron and closed shoes throughout, with eyewash and spill provision within reach, as Project 1 established. Add eye protection and gloves specifically for the one occasion the chrome alum jar is open, and a disposable dust mask if you are making the stock from a bulk packet — a precaution against a fine powder rather than compliance with a limit, since the exposure limit is an airborne concentration and you are not measuring one.

Ventilation

Ordinary room ventilation, as before, and again the point of concern is dust rather than vapour: nothing here evolves a gas at 50 °C. The two weighings — silver nitrate, and chrome alum if you are making stock — happen in still air, with no fan or extractor draught running, and the room is aired before and after rather than during.

Materials

As Project 1, with the addition of a labelled bottle for the 5 per cent chrome alum stock, a 1 mL syringe for dispensing it, and a second negative print’s worth of paper. You will want at least four coated sheets: two for step wedges and two for the developer comparison.

Chemicals

Quantities are the one-hundredth scaling of Wall’s published formula. The right-hand column gives his original figures so that the scaling is visible rather than hidden.

Chemical	This make	Wall’s published quantity
Potassium bromide	0.25 g	25 g
Sodium chloride	2.0 g	200 g
Citric acid	0.5 g	50 g
Gelatin	6.25 g	625 g
Ethanol	2.5 mL	250 ccm alcohol
Water, solution A	50 mL	5000 ccm
Silver nitrate	5 g	500 g
Water, solution B	16.6 mL	1660 ccm
Chrome alum, in the finals	0.03 g, as 0.6 mL of a 5 % stock	3 g
Made up to	110 g	11,000 g

!This is a scaled variant, and the course owns it rather than Wall

Every Tier 1 emulsion formula in this corpus is a commercial batch — Wall’s run from 8 to 45 litres, Baker’s from 1 to 16 kilograms — and there is no Tier 1 domestic-scale formula anywhere in it. So anything published at student scale from a Tier 1 source is a scaled variant the course owns, and Rule 6 requires it to be labelled as one.

Scaling preserves every ratio: halide to silver, gelatin to silver, salt to salt, hardener to gelatin. Those are the numbers that define the emulsion, and dividing them all by a hundred changes none of them.

Scaling does not preserve the addition rate per unit volume, the heat transfer or the stirring. A 45-litre crock stirred by a paddle and a 60 mL cup whisked by hand are not the same reactor, and addition rate is the variable that decides how many crystals form. Wall gives the temperature, the digestion and the finals for these papers but not an addition rate, so this page carries over Project 1’s schedule — that carry-over is this course’s decision, not Wall’s.

One caveat travels with every formula from this book, from the publisher of the scan the course holds: Wall’s formulas assume active gelatin, sulfur sensitisation had not been invented when he wrote, and he omits information that makes his formulas hard to reproduce. Modern photographic gelatin is inert by design. Expect a slower emulsion than Wall expected, and record what you get.

🧮The four numbers that describe this emulsion

Halide ratio. 0.25 g of potassium bromide ÷ 119.00 = 0.00210 mol; 2.0 g of sodium chloride ÷ 58.44 = 0.03422 mol; total halide 0.03632 mol, of which chloride is 94 mol per cent and bromide 6. This is a chloride paper with a bromide addition, not a balanced chlorobromide, and the page says so rather than letting the name imply otherwise.

Halide excess. Silver: 5 g ÷ 169.87 = 0.02943 mol, so the excess is 0.03632 ÷ 0.02943 − 1 = 23 mol per cent, against Project 1’s 37. Less ripening agent, and a longer, hotter digestion to use it.

Gelatin per mole of silver. 6.25 ÷ 0.02943 = 212 g per mole, against Project 1’s 849 — the second-largest difference between the two makes after the halide.

Silver concentration. 5 g in 110 g is 45 g of silver nitrate per litre. Baker states that paper emulsions carry fifteen to twenty-five grams per litre and negative emulsions forty to fifty, so Wall’s made-up gaslight paper lands in Baker’s negative band. Two Tier 1 sources, no resolution offered here; what it means for you is practical, and it is in the coating stage.

Equipment

As Project 1: the validated station, a balance to 0.1 g, whisk or stirrer, timers, ripening pot, trays, tongs and the open silver-waste container. Add a 1 mL syringe for the chrome alum stock and a second processing tray if you intend to run the two developer conditions side by side.

Estimated cost

Cost band ££, and almost identical to Project 1: the same 5 g of silver nitrate, a little less gelatin, and three small quantities of common salts. Chrome alum is bought once and lasts for years at 30 mg a batch. This page quotes no prices; dated figures live in the laboratory planner.

Estimated consumables cost

Almost exactly Project 1’s bill: the same five grams of silver, a little less gelatin, and three small quantities of common salts. The chrome alum is the only new substance and 30 mg a batch means one purchase lasts for years.

Consumed	This session	Sourced price	Cost this session
Silver nitrate	5 g	£59.95–£112.90 per one jar: 25 g at the lower figure, 10 g at the higher (£2.40–£11.29 a g)	£11.99–£56.45
Sodium chloride	2.0 g	None. sodium-chloride carries a cost band and no dated figure	—
Potassium bromide	0.25 g	£23.00 per 250 g (£0.09 a g)	£0.02
Citric acid	0.5 g	£10.00 per 250 g of the monohydrate	£0.02
Gelatin, photographic inert	6.25 g	£17.45 per 100 g, inert photographic gelatin (£0.17 a g)	£1.09
Ethanol, 95 %	2.5 mL	None. ethanol carries a cost band and no dated figure	—
Chrome alum	0.03 g, as 0.6 mL of a 5 % stock	None. A named price gap: chrome alum, potassium alum and sodium hydroxide	—
Hot-press watercolour paper	at least 4 sheets	None. A named price gap: hot-press cotton watercolour paper	—
Disposable FFP3 respirator	1, for the weighing	None. A named price gap: an FFP3 disposable respirator	—

The priced rows come to £13.12 to £57.58 for one run of this session, at the retail ranges read on 5 and 7 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 5 of the 9 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

Five of the nine rows have no dated price and two of them — the paper and the respirator — are consumed every session rather than bought once. Treat the priced subtotal as the chemistry only.

Waste streams

As Project 1 — make residues, failed coats, fixer and first wash water, all silver-bearing and all collected — with one addition. This make puts chromium(III) into the coating-station rinse water and, because the hardener is in the emulsion, into the processing and wash water as well.

That is a new constituent and it is stated rather than absorbed; the detail is under Disposal considerations below. The quantity is 30 mg per batch, and the option that puts no chromium anywhere at all is the course’s default of no hardener — which is what Project 1 did.

Alternative route

Without silver nitrate, run the developer comparison alone on the bought-emulsion sheets from the practice run. It is the half of this project that carries the correction, and it needs no make.

Without chrome alum, omit it. You will be making Wall’s formula minus its hardener, which is a labelled variant and a perfectly good emulsion; the consequence is a tender wet layer, handled by the corners, with an acid hardening fixer available if it frills.

Preparation

Read Project 1’s provenance section again, then note the differences in this document’s status. Wall’s book is Tier 1 and public domain, and the formula is given, in his words, as used commercially — but nobody in this corpus has made it in the modern era, where Project 1’s recipe had prints, a step wedge and an independent reader’s batch behind it. You are trading testing evidence for provenance and for a complete finals set, and it is worth knowing which way that trade runs before you weigh anything.

Make the chrome alum stock in daylight, before the session: 5 g of chrome alum in water to 100 mL, in a labelled bottle. This make needs 0.6 mL of it.

The make schedule, with the seventy-five-minute digestion marked

This make

Dissolve salts, sprinkle gelatin, bloom 30 minMelt and hold at 50 °C, 15 minAddition, 5 minDigest 75 min at 50 °CFinals, 5 minCool and coat, 20 min

Project 1, for comparison

Bloom 30 minMelt to 49–51 °C, 15 minAddition, 5 minRipen about 30 minCool and coat, 20 min

030 min60 min90 min120 min150 min

Blocks are drawn to a common scale of one unit to five minutes, so the widths are proportional to the times named. Both makes carry five grams of silver nitrate; the two visible schedule differences are the length of the hot hold — seventy-five minutes against about thirty — and the finals step, which Project 1 has not got. The halide ratio, which is the change this project is actually about, does not appear on a timeline at all.

Procedure

Stage 1 — Solution A (45 minutes, mostly waiting)

Dissolve the 0.25 g of potassium bromide, 2.0 g of sodium chloride and 0.5 g of citric acid in 50 mL of water, completely; grind the bromide first if it came as hard pellets, because an undissolved grain reaching the precipitation gives a black speck on the paper.

Sprinkle in the 6.25 g of gelatin, stir, cover and bloom for thirty minutes, then add the 2.5 mL of ethanol. Wall places his alcohol in solution A — worth noticing, because Project 1’s source named an ethanol addition and never said where it went. Bring solution A to 50 °C, thermometer in the emulsion.

Stage 2 — Solution B and the addition (10 minutes)

Dissolve the 5 g of silver nitrate in 16.6 mL of water. That is a strong solution — about 300 g per litre — and it will need a moment and a gentle warm to go fully into solution. Keep it covered.

Then run Project 1’s addition schedule, whisking continuously in one direction: one third, one minute, one third, one minute, the remainder, whisk to the end of five minutes. This schedule is the course’s carry-over and not Wall’s instruction, for the reason the scaling note gives.

Two precipitations happen at once, and they do not happen equally:

AgNO₃ + NaCl → AgCl(s) + NaNO₃

The bulk of the make: 94 mol per cent of the halide

AgNO₃ + KBr → AgBr(s) + KNO₃

And the 6 mol per cent that changes its character

Silver bromide’s solubility product is about three hundred times smaller than silver chloride’s — 5.0 × 10⁻¹³ against 1.6 × 10⁻¹⁰ — so bromide is taken up preferentially and can displace chloride that has already precipitated:

AgCl(s) + Br⁻ → AgBr(s) + Cl⁻

Why a little bromide does not stay evenly spread

Stage 3 — Digest, seventy-five minutes at 50 °C

Hold the emulsion at 50 °C for seventy-five minutes, Wall’s own figure. It is Project 1’s ripening run two and a half times as long on a two-thirds smaller excess.

Stir occasionally and keep the vessel covered. Watch the level: seventy-five minutes of uncovered evaporation would concentrate the emulsion and falsify every number you computed.

Stage 4 — The finals (5 minutes)

Wall’s finals for this formula are two things: chrome alum, and water to the made-up weight.

Add the 0.6 mL of 5 per cent chrome alum stock slowly, into moving emulsion. Duffin gives the failure mode precisely: too rapid an addition can coagulate the emulsion if there is a temporary excessive local concentration of the hardener. Then make up to 110 g total with water, weighing rather than measuring by volume — Wall specifies a weight, and so does Baker for his own paper emulsion.

⚗Why the hardener dose is expressed against the gelatin, and why the citric acid matters to it

Wall states the principle in one line on page 152: the quantity of chrome alum used depends on the total quantity of the gelatine and not on the bulk of the emulsion. That is why a dose survives scaling and a bulk quantity does not.

Check it: 3 g of chrome alum against 625 g of gelatine is 0.48 per cent of the gelatin weight. Duffin gives the general rule as 0.5 to 2 per cent, introduced carefully into the molten emulsion immediately before coating; Wall’s own rule of thumb elsewhere works out at 0.33 to 0.40 per cent, and Baker’s published finals for a different emulsion at 0.625. A manufacturing text and two working formulas, inside the same band, arrived at independently.

One condition is attached, and this formula happens to satisfy it. Chrome alum’s action is pH-dependent: effective near the usual coating pH of about 6.0 and far less effective higher. Solution A carries 0.5 g of citric acid, so this emulsion is acid. Wall gives no reason for the citric acid at all; Duffin’s pH dependence means an alkaline emulsion would not harden at this dose, and yours is not alkaline.

What nobody has established: no source at any tier gives a hardener dose tested at domestic scale. Every figure is either a fraction of the gelatin, which scales by arithmetic, or a batch quantity, which does not. This is a design rule with a test, not a tested recipe, and the test is whether your sheet frills.

Wall adds that all paper emulsions should have saponin or quillaia tincture added as a surfactant. He gives no dose on this page, referring the reader to another; this course has not read that page. So the page names the ingredient and does not supply a quantity. Coat without it, or add one drop of a wetting agent and record it as your own variable.

Stage 5 — Coat, and hold the coating weight rather than the volume

Cool to 40 to 42 °C and coat as in Project 1 — but not at the same volume per sheet, and this is where the two makes stop being interchangeable.

This emulsion carries about 39 g of silver halide per litre against Project 1’s 20: the same silver in half the bulk. Coating at Project 1’s 52 mL per 11 × 15 inch sheet would lay down about 190 mg per square decimetre, nearly twice Project 1’s coat. To hold the coating weight at Project 1’s figure of about 100 mg/dm², coat at about 27 mL per sheet — this course’s arithmetic from the coating-weight equation, not Wall’s. It also makes the batch go the same distance: about four sheets.

Stage 6 — Expose, process, and set up the developer comparison

Expose one sheet under the step wedge at the fixed lamp, distance and time. Then print your negative twice, on two sheets from the same batch, changing only the developer condition — see the analysis below. Process to Project 1’s sequence: develop, dilute stop, two-bath fix, wash. Handle the wet sheets by the corners; a light hardening is not a licence to squeegee.

Expected observations

• The emulsion is whiter, more opaque and thicker at coating temperature than Project 1’s, because there is more silver halide in less liquid.
• By transmitted light 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. That ladder is a maker’s check: it tells you what you have made before you expose anything. It is also the safelight warning. 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 you add. 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.
• The step wedge should show a longer scale than Project 1’s, which is to say lower contrast — but see the contested block, because two Tier 1 sources are recorded as disagreeing about which way this goes.
• Speed against Project 1 is unknown. No source at any tier publishes a speed figure for either emulsion; your step counts are the only comparison that exists.

What is happening chemically

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, and this is the page where you feel it.

And the part the project was designed to get wrong.

!Halide ratio does not set image colour. The man whose formula you just made proved it.

Project 2 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, magnesium ribbon, a Nernst lamp and incandescent gas; and developed every one of them 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. It does not itself set the colour.

That is why the comparison below runs one developer at two strengths rather than two products: dilution moves the rate of deposition directly.

⁇What more bromide does to contrast: two Tier 1 sources, recorded as disagreeing, and an honest account of what this course can see

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

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.”

What this course can and cannot say. Side by side, in the wording available here, the two appear to point the same way — more bromide, longer scale, softer contrast — and the recorded disagreement may turn on the period phrase long scale, which in Baker’s trade usage means a soft material rather than a contrasty one. This course has not re-read either original page with a second reader, so it neither overturns the research note nor asserts an agreement it has not verified.

What it does instead is hand you the measurement. You have a pure-chloride and a 6 per cent bromide emulsion on the same silver charge, and a step wedge that reads exposure scale directly. Count the steps; a shorter scale is higher contrast. One evening settles for your materials what two manuals are recorded as disputing.

What halide ratio is and is not responsible for

• Speed against bromide content — drawn from verbal statements, not measured

Show the numbers behind this plot

A plot with bromide content of the halide running from zero to one hundred mol per cent along the horizontal axis and relative log speed, in arbitrary units, up the vertical. A single rising curve runs from bottom left to upper right, steep at first and flattening towards the bromide-rich end, drawn to render in a shape what Wall and Duffin state in words: the more bromide an emulsion contains in comparison with the chloride, the faster it is as a rule, and chlorobromide emulsions are of greater speed as the bromide content increases. Two markers sit on the curve near its left-hand end. The first, at zero per cent bromide, is labelled Project 1, pure silver chloride. The second, at six per cent, is labelled Project 2, and it is only a short distance along the axis, which is the point of the drawing: the change these two projects make is small on the scale of what the ratio can do, and the whole of commercial gaslight-paper practice lives in that same crowded left-hand corner. A shaded region spanning the left-hand fifth of the axis is labelled paper emulsions and a second region across the right-hand half is labelled negative and enlarging emulsions. A note beneath the plot records that no point on this curve is measured: the shape is a rendering of two verbal statements from Tier 1 sources, no source in this course's corpus publishes a speed figure for any hand-coated emulsion, and the two markers show where the projects sit on the axis rather than any speed that has been read from them.

Series	Bromide as a percentage of the total halide (mol %)	Relative log speed — shape only
Speed against bromide content — drawn from verbal statements, not measured	0.00	0.00
Speed against bromide content — drawn from verbal statements, not measured	2.00	0.18
Speed against bromide content — drawn from verbal statements, not measured	6.00	0.42
Speed against bromide content — drawn from verbal statements, not measured	12.00	0.70
Speed against bromide content — drawn from verbal statements, not measured	20.00	0.95
Speed against bromide content — drawn from verbal statements, not measured	35.00	1.28
Speed against bromide content — drawn from verbal statements, not measured	50.00	1.50
Speed against bromide content — drawn from verbal statements, not measured	70.00	1.72
Speed against bromide content — drawn from verbal statements, not measured	85.00	1.83
Speed against bromide content — drawn from verbal statements, not measured	100.00	1.90

Not measured. The curve renders in a shape what two Tier 1 sources state in words about speed; no source in this corpus gives a speed figure for a hand-coated emulsion, so there are no points to plot. What the ratio does not set, on Wall's own controlled evidence, is the colour of the image. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Data to record

Project 1’s batch-record fields, with these added: the halide ratio in mol per cent, the halide excess, the gelatin per mole of silver, the digestion temperature measured every fifteen minutes for the seventy-five minutes, the chrome alum dose as a percentage of the gelatin weight, the made-up weight, the coating volume per sheet and the computed coating weight, and, for each print, the developer condition — dilution, temperature and time — recorded as carefully as the emulsion was.

Version code the batch CB-01a, and the second halide ratio, if you make one, CB-02a. Not CB-01b: a different formula is a different batch, and a different coating of the same melt is the letter.

Analysis

1. Speed and scale against Project 1

Lay the KCl-01 and CB-01 step-wedge strips side by side and read both: threshold step, first maximum-black step, and the count between them. Each step is half a stop.

The silver charge is identical, which is what makes the comparison meaningful. But record what else moved: gelatin per mole of silver differs by a factor of four, halide excess is 23 per cent against 37, digestion 75 minutes against 30, and this emulsion is hardened and acid where the other was neither. The strict one-variable experiment is the second batch below; the between-project comparison gives you a silver-matched pair and a list of what to control next.

2. The developer comparison, and what it is actually testing

Print the same negative twice on sheets from the same batch, changing only the developer condition:

• Condition A — the developer at working strength, developed to completion.
• Condition B — the same developer diluted, typically 1+1 or 1+3 with water, developed for proportionally longer to reach a similar maximum black.

That is a rate of deposition experiment, the variable Wall’s series identified. A weaker developer deposits silver more slowly, builds finer particles and — on his evidence and Chapman Jones’s — should give the warmer image. For the second arm Wall names, add extra potassium bromide to the diluted developer as a restrainer; a soft-working paper developer of this period already carries about 4 g per litre, so doubling it is a defensible start and is your labelled variant, not a published formula.

Why not a named neutral developer and a named warm-tone one? Because the course has not chosen that pair — paper developers are Part XVIII’s decision. One developer at two strengths is also the better experiment: it changes one thing.

The comparison grid, with its caption corrected

1. 1Across a row: developer condition — working strength, diluted, diluted plus bromide — the rate of silver deposition falls left to right
2. 2Down a column: halide ratio — pure chloride above, 6 mol % bromide below — speed and the shape of the scale change; the colour does not
3. 3What you are looking for — a colour difference along the rows and none worth naming down the columns

Drawn as a prediction, not as a result: the shading shows what Wall's controlled series leads you to expect, and the six prints you make are what will confirm or refute it on your materials.

3. The optional second halide ratio, one variable only

If you want the strict experiment, make CB-02a from the same formula with only the two halide masses changed, holding the total halide moles constant so the excess does not move as well. Doubling the bromide to 0.5 g and reducing the sodium chloride to 1.88 g holds the total halide at 0.0363 mol — 0.00420 plus 0.03212 — while taking bromide from 6 to about 12 mol per cent. Everything else — gelatin, citric acid, alcohol, water, silver, temperature, addition schedule, digestion, finals, coating weight, exposure, developer — is held.

That is one variable, and its step wedge against CB-01a’s is the cleanest evidence this project can produce.

4. What it teaches about the papers you can buy, and about toning

Commercial warm-tone papers are chlorobromide emulsions, and this project explains the shape of that fact. A chloride-rich composition gives a slow, fine-grained material whose development is easy to hold back; the maker then reaches the warm image by controlling the rate at which silver is laid down. The emulsion makes warmth available; the developer takes it — which is why a warm-tone paper still prints cold in the wrong developer.

It also sets up a distinction to carry into the toning parts. Everything here changes colour by changing the size and form of the metallic silver. Toning changes colour by changing what the image is made of. Two mechanisms, two vocabularies, kept apart.

Troubleshooting

What you see	Likely cause	What to do
The emulsion coagulates or goes lumpy as the finals go in	Chrome alum added too fast, giving a local excess	Duffin’s named failure mode. Add the stock slowly into moving emulsion, and dilute it further if your batch is small
Black specks throughout, unexposed areas included	Undissolved potassium bromide, or pepper	Grind the bromide to a powder and dissolve it fully; the three published pepper remedies are in Project 1
The coat is far heavier than Project 1’s at the same volume	It is: this emulsion has twice the silver halide per litre, and the level may also have dropped through evaporation	Coat at about 27 mL per sheet, not 52. Hold weight, not volume, and keep the vessel covered during the digestion
The sheet still frills in the developer	The hardener is at the bottom of Duffin’s band and the layer is thick	Fix in an acid hardening fixer. Do not raise the chrome alum dose above 2 per cent of the gelatin without recording it as a variant
Both prints in the developer comparison look identical	The dilution did not change the deposition rate enough, or both were developed to completion in the same way	Increase the dilution and extend the time further; the variable is rate, and a print taken to completion twice can look the same by two different routes
The step wedge shows no difference from Project 1 at all	Possible, and interesting	Check that the coating weights match. A six per cent bromide change is a small move along the ratio axis

Clean-up

As Project 1: quench everything in the tray of clean water on the wet side as you finish with it, and empty that tray into the silver-waste container. The plastic wrap off the bed carries emulsion and goes with the silver-bearing solid waste. Rinse the 1 mL chrome alum syringe separately and keep it with the stock bottle, so it is never the syringe that measures emulsion. Wash gloves before removing them, and return both the silver nitrate and the chrome alum to their own storage.

Storage

The chrome alum stock, labelled with the substance, the strength and the date, in a closed bottle. It is made once and used 0.6 mL at a time.

Coated sheets, dry, flat, interleaved with clean paper, in a lightproof box, pencilled on the back with the version code, out of warm damp storage.

One keeping question this course cannot answer. No source in the corpus quantifies the keeping life of an unwashed chlorobromide paper; Wall notes only that bromide emulsions are sometimes poured into ice-cold pans after digestion so they set quickly and the ripening stops. Everything soluble that went into this make is still in the sheet. Date your sheets, keep two aside, and print one in a month against a fresh one — a keeping test nobody has published for this material.

Disposal considerations

Two constituents leave this session, not one. Silver — as dissolved nitrate in rinse water, as silver halide in residues and failed coats, and as the silver-thiosulfate complex in used fixer — all collected, none to a drain, because silver nitrate carries H400 and H410, very toxic to aquatic life with long lasting effects. And chromium(III), 30 mg per batch, in the coating rinse water and, because the hardener is in the layer, in the processing and wash water too.

The waste classification method in the launch market treats compounds of chromium as transition-metal compounds where they are classified as hazardous, and chrome alum is classified — so the assessment is not automatically trivial, and this course states the chemistry and names the method rather than computing a threshold. Local regulation governs what may then be done with the collected waste, and it differs between authorities even within one country. Part II’s chemical waste and silver waste owns the collecting arrangement.

Questions

1. Recompute the halide ratio and the halide excess for a make in which the potassium bromide is doubled to 0.5 g and the sodium chloride held at 2.0 g. Say why that is a two-variable change, and what to do instead.
2. Wall’s made-up emulsion carries 45 g of silver nitrate per litre; Baker says paper emulsions carry 15 to 25 and negative emulsions 40 to 50. Compute the volume per sheet that holds your coating weight at Project 1’s figure.
3. Explain, using Wall’s controlled series, why this project runs one developer at two strengths rather than two different developers. What would a two-product comparison confuse?
4. The chrome alum dose is 3 g against 625 g of gelatine. Express it as a percentage of the gelatin weight, compare it with Duffin’s published band, and explain why a dose stated this way survives a hundredfold scaling when a dose in grams per batch would not.
5. Your CB-01 step wedge shows a scale two steps longer than KCl-01’s. State what that means for contrast, list four differences between the two makes that could also have produced it, and describe the measurement that would settle the Baker–Duffin question on your own materials.

Further experiments

The halide ladder. Four batches at 0, 6, 12 and 25 mol per cent bromide, total halide moles held constant, everything else held, coated at matched coating weight and read on the step wedge. Four points on the axis of the plot above, which currently has no measured points on it at all.

Wall’s series, at your scale. Develop strips from each of those four batches in one developer and look at the image colour. If Wall was right, the colour will be the same across all four and will move only when the developer moves. A hundred-year-old result nobody in this corpus has reproduced.

The digestion question. Split one melt after the addition; digest one half 75 minutes and the other 30, Project 1’s time, and coat both at the same weight. That isolates the digestion from the halide change and tells you which of the two schedule differences your step wedges have been reading.

Recap

Wall’s soft-working gaslight paper at one hundredth scale: 94 mol per cent chloride, 6 mol per cent bromide, 23 mol per cent halide excess, 212 g of gelatin per mole of silver, digested seventy-five minutes at 50 °C because there is no ammonia to do it faster, hardened with chrome alum at 0.48 per cent of the gelatin weight, made up to 110 g, and coated at about 27 mL per sheet to hold Project 1’s coating weight. The scaling is the course’s, and so is the addition schedule.

And the correction that matters more than the formula: halide ratio sets speed and the shape of the scale, and decides how easily a warm tone can be reached. It does not set image colour. The rate of silver deposition does — the developer’s business — and Wall proved it across the whole halide range in one developer, and wrote it down.

Check your understanding

Question 1. Wall made emulsions from pure chloride through 5, 10 and 20 per cent bromide to pure bromide and developed them all in one metol-hydroquinone developer. What did he find, and what follows?

• The bromide-rich emulsions gave colder images, proving that ratio sets colour
• Every emulsion gave the same coloured image, so the rate of silver deposition, not the composition of the emulsion, decides colour
• The images differed only under one of the three light sources
• The experiment was inconclusive and he recommended further work

Show the answer and why

Answer: Every emulsion gave the same coloured image, so the rate of silver deposition, not the composition of the emulsion, decides colour

His words are that in every case the same coloured image was obtained, and that warmer tones came only when he weakened the developer and raised its bromide — and then they came on all the plates equally. That is the definition of a controlled result: the thing he changed between plates did nothing, and the thing he changed between developers did everything. Chapman Jones supplies the mechanism from the other side, showing image colour depends on silver particle size, which slow deposition makes fine.

Question 2. This make carries the same 5 g of silver nitrate as Project 1 but is made up to 110 g rather than 208 g. What follows for coating?

• Nothing: the same volume per sheet gives the same result
• It has about twice the silver halide per litre, so coating at Project 1's volume would lay down about twice the coating weight; about 27 mL per sheet holds the weight constant
• It must be diluted to 208 g before coating
• It should be coated hotter to compensate

Show the answer and why

Answer: It has about twice the silver halide per litre, so coating at Project 1's volume would lay down about twice the coating weight; about 27 mL per sheet holds the weight constant

Coating weight, not coating volume, is the quantity that has to be held constant for two sheets to be comparable — it is what decides maximum density and, through it, most of what a print looks like. Same silver in half the bulk is twice the concentration, so half the volume per unit area. Diluting the emulsion to match Project 1's bulk would also work arithmetically, but it would depart from the published formula and change the gelatin concentration at coating temperature as well.

Question 3. Why is the chrome alum dose given as a percentage of the gelatin weight rather than as grams per batch?

• Because gelatin is the most expensive ingredient
• Because the hardener acts by cross-linking gelatin chains, so what matters is how much gelatin there is, not how much liquid; a gelatin-relative dose therefore survives scaling
• Because chrome alum does not dissolve in water
• Because the batch weight is not known until the finals are added

Show the answer and why

Answer: Because the hardener acts by cross-linking gelatin chains, so what matters is how much gelatin there is, not how much liquid; a gelatin-relative dose therefore survives scaling

Wall states the principle directly: the quantity used depends on the total quantity of the gelatine and not on the bulk of the emulsion. It follows from the mechanism — chromium(III) forms ionic links between carboxyl groups on different gelatin chains, so the reagent is being matched to its substrate. It is also the reason a hundredfold scaling of this formula is arithmetic rather than guesswork, where scaling an addition rate or a heat-transfer rate is not.

Question 4. Silver bromide has a solubility product of 5.0 × 10⁻¹³ and silver chloride 1.6 × 10⁻¹⁰. What does that difference do inside a chlorobromide make?

• Nothing; both precipitate together in proportion to the salts present
• Bromide is taken up preferentially and can displace chloride that has already precipitated, so the bromide is not evenly distributed through the crystals
• The chloride precipitates first because it is more abundant
• It makes the emulsion faster to fix

Show the answer and why

Answer: Bromide is taken up preferentially and can displace chloride that has already precipitated, so the bromide is not evenly distributed through the crystals

A factor of about three hundred in solubility product is a large thermodynamic preference, and it means silver bromide forms wherever bromide ion is available and can replace silver chloride that has already come down. The consequence is structural: a chlorobromide emulsion is not a blend of two emulsions, and the bromide concentrates where it precipitated and at crystal surfaces — which is part of why a few mol per cent changes the material's behaviour more than its bulk composition suggests.

Question 5. The course records Baker and Duffin as disagreeing about which direction more bromide moves contrast. What does this page do?

• Adopts Duffin, as the later and more technical source
• Adopts Baker, as the practical manual
• Prints both statements, says that on the wording available they appear to point the same way, declines to overturn or confirm the research note without a second reading, and hands the reader a measurement that settles it for their own materials
• Omits the question as unresolvable

Show the answer and why

Answer: Prints both statements, says that on the wording available they appear to point the same way, declines to overturn or confirm the research note without a second reading, and hands the reader a measurement that settles it for their own materials

Where two Tier 1 sources are in dispute, the course records both and picks neither silently — and it also does not manufacture an agreement it has not verified. What makes this more than a shrug is that the reader is holding the instrument that answers it: two emulsions on the same silver charge and a step wedge that reads exposure scale directly. A dated step count in your own notebook outranks an unresolved note in somebody else's, for your materials.

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