Project 3: A Bromide Enlarging Emulsion
Purpose
Section titled “Purpose”To wash an emulsion, which is the operation that separates a printing paper from a negative material.
Projects 1 and 2 left everything soluble in the sheet: the potassium nitrate made during precipitation, the halide that was never taken up, and whatever pH the make ended at. Paper absorbs those and carries on working. A bromide emulsion cannot, because the excess halide that ripened it becomes a restrainer the moment ripening stops, and because a material this fast keeps badly with the salts still in it. So this project sets the emulsion to a jelly, shreds it, and washes the salts out — and that costs a second session, an overnight wait and several litres of ice water.
It also carries the part’s most awkward piece of honesty. Neither published formula the course found for this project performs a measured sulfur digestion, and the reason is under Session two.
Learning objectives
Section titled “Learning objectives”By the end of the two sessions you should be able to:
- run a controlled-rate single-jet addition at a stated millilitres per minute, and say what that rate decides;
- set, shred and wash an emulsion, and state the endpoint test you used and whose measurement it is;
- explain why the second gelatin goes in before the wash and not after it;
- convert a published sensitiser concentration expressed in the gelatin into milligrams per mole of silver, and then into a volume of a stock solution you designed;
- read a published commercial formula, scale it, express its hardener as a fraction of its gelatin, and name what you removed from it and why;
- convert a step-wedge count into stops and state what the number does and does not compare.
Prerequisites
Section titled “Prerequisites”Project 2, made and read, and washing, digestion and sensitisation, which owns the mechanism this session performs. The two step wedges from Projects 1 and 2 must exist, or the speed comparison at the end has nothing to compare against.
Two pieces of kit are new and are not in Part II’s list: a potato ricer, and a wide-mouth vacuum flask with a cotton straining bag that fits inside it. Buy them before the session, not during it.
Safety classification
Section titled “Safety classification”Level B, and for the same single reason as the two projects before it: five grams of silver nitrate, weighed dry and handled in solution. Nothing in this make raises the classification.
What is not a hazard here, and why. There is no ammonia in this emulsion, which is why it needs 55 °C and forty-five minutes where an ammoniacal make would need 40 °C and fifteen. There is no cadmium, no mercury, no chromium in any oxidation state, and no acid. The potassium iodide arrives as one millilitre of a 10 per cent solution — a hundred milligrams of a salt sold as a dietary supplement — and the hazard it presents at that quantity is staining, not toxicity. The sodium thiosulfate, if you run the digestion experiment, is the fixer salt you already own: the aggregated notifications report that it does not meet GHS hazard criteria, and the quantity is under a milligram. The one genuinely new physical exposure is two hours at 55 to 60 °C in a domestic water bath, spread over two sessions, which is a scald risk and a timer discipline rather than a chemical one.
Hazards
Section titled “Hazards”Silver nitrate, as established in silver nitrate handling: the harmonised classification is Danger, with GHS03, GHS05 and GHS09 — H272 may intensify fire, H314 causes severe skin burns and eye damage, H400 and H410 very toxic to aquatic life with long lasting effects. Splash goggles, nitrile gloves, apron; weighed in still air; never in a room with ammonia.
Potassium bromide as a hard pellet. The practical hazard is to the emulsion rather than to you: an undissolved fragment reaching the precipitation gives a black slug on the finished sheet. Grind it to a powder in a dedicated mortar and store it airtight.
A 55 °C water bath running for forty-five minutes at a stretch, twice. Use a vessel you can lift with one hand when it is full, keep the bath below the rim of the container, and set a timer rather than watching it.
Several litres of ice water and a cold, wet floor. More people are hurt in a darkroom by slipping than by chemistry. The wash happens on the wet side, over a tray, with the floor dry.
Required PPE
Section titled “Required PPE”Splash goggles, nitrile gloves, apron and closed shoes for the whole of both sessions, with eyewash and spill provision within reach. Gloves matter more in session one than you expect: you will be handling cold wet emulsion by hand through a bag, and silver halide on skin darkens in daylight and takes days to wear off.
Ventilation
Section titled “Ventilation”Ordinary room ventilation. Nothing in this make evolves a vapour: the bath runs at 55 °C, well below the boiling point of anything present, and the only volatile ingredient is five millilitres of ethanol added at the end of the second session, which is a flammability question for the bottle rather than an inhalation one for the room. The control that does matter is still air at the balance, for the same dust reason as the earlier projects.
Materials
Section titled “Materials”The station from the coating station build, plus:
- a potato ricer, stainless or plastic, dedicated to emulsion and never used for food;
- a wide-mouth vacuum flask of about 1 L, and a cotton jelly or straining bag that hangs inside it;
- a second flask or a lidded box of crushed ice, and about 8 to 12 litres of cold water for the six changes;
- a zip-seal bag and a lightproof box that will fit in a refrigerator;
- hot-press watercolour paper, cut to 8 × 10 inch, at least six sheets;
- a 1 mL syringe for the digestion experiment, and a 10 mL syringe for the coating volume.
Chemicals
Section titled “Chemicals”| Chemical | Quantity | Form |
|---|---|---|
| Silver nitrate | 5 g | crystals, weighed dry |
| Potassium bromide | 5 g | ground to a fine powder |
| Potassium iodide | 1 mL of a 10 % w/v solution | 0.1 g of the salt |
| Gelatin | 2 g, then 5 g | photographic, inert |
| Ethanol | 5 mL | drinking-grade, 95 % |
| Distilled water | 45 mL + 25 mL + 45 mL | three separate portions |
| Sodium thiosulfate | 0.3 to 1.8 mL of a 0.05 % w/v working solution | only if you run the digestion experiment |
Equipment
Section titled “Equipment”The validated station, a balance reading to 0.01 g for the thiosulfate stock and to 0.1 g for everything else, a whisk or magnetic stirrer, two timers, the ripening bath, a thermometer that lives in the emulsion, trays, tongs, and the open silver-waste container. Add the ricer, the flask and the bag from Materials, and a gold-mesh coffee filter or a plastic funnel with a fine strainer for the remelt.
Estimated cost
Section titled “Estimated cost”Cost band £££, the highest in the part, and the cost is not the chemistry. Five grams of silver nitrate is the same charge as Projects 1 and 2. What raises the band is the two-session commitment, the ricer, the flask, and the fact that a failed wash costs the whole batch rather than one sheet. This page quotes no prices; dated figures live in the laboratory planner.
Estimated consumables cost
Section titled “Estimated consumables cost”Project 3 costs the same five grams of silver as Projects 1 and 2; what the band above adds is the two-session commitment, the ricer and the flask, all of which are equipment. Per batch, the consumables are the silver, the halides, the gelatin, the alcohol and six sheets of paper.
| 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 |
| Potassium bromide | 5 g | £23.00 per 250 g (£0.09 a g) | £0.46 |
| Potassium iodide | 0.1 g, as 1 mL of 10 % w/v | None. A named price gap: silver-halide salts other than potassium bromide | — |
| Gelatin, photographic inert | 7 g (2 g then 5 g) | £17.45 per 100 g, inert photographic gelatin (£0.17 a g) | £1.22 |
| Ethanol, 95 % | 5 mL | None. ethanol carries a cost band and no dated figure |
— |
| Sodium thiosulfate digestion stock | 0.3 to 1.8 mL of 0.05 % w/v | £14.70 per 1 kg of the raw salt, checked 7 September 2026 | Under a milligram of salt, which rounds to nothing |
| Hot-press watercolour paper, 8 × 10 in | at least 6 sheets | None. A named price gap: hot-press cotton watercolour paper | — |
| Distilled water and 8 to 12 L of cold wash water | about 115 mL distilled, plus the six changes | None. distilled-water carries a cost band and no dated figure |
— |
| Ice for the wash | one lidded box | Household | — |
The priced rows come to £13.67 to £58.13 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: 4 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.
The paper is the row that decides this session’s real cost and it is the one with no dated price: hot-press cotton watercolour paper is a named price gap, and at six sheets a batch it is consumed faster than anything else on the page. The ricer, the vacuum flask, the straining bag and the syringes are equipment.
Waste streams
Section titled “Waste streams”Three, and one of them is much the largest volume in Part V.
The wash water. Six changes, one to two litres each, so eight to twelve litres. It carries potassium nitrate from the precipitation, the unreacted excess potassium bromide and potassium iodide, and dissolved silver as the bromoargentate complex the excess bromide forms. Most of the silver stays in the noodles, which is the whole point of a bag; the fines that escape it do not.
Make residues and failed coats, silver-bearing as before.
Used fixer and first wash from processing, as before.
Alternative route
Section titled “Alternative route”Without silver nitrate, the washing operation can still be learned. Set a plain 5 per cent gelatin solution, press it through the ricer, wash it in six changes and remelt it: you will meet the swelling, the dilution and the handling exactly as they occur in the real thing, and you can weigh the noodles before and after to measure the swelling for yourself. What you lose is the emulsion, not the technique.
Without a second session, do not start. This make cannot be compressed: the set is overnight, and an emulsion shredded warm turns to paste in the ricer.
Preparation
Section titled “Preparation”Two candidate formulas, and why the course prints one and reads the other
Section titled “Two candidate formulas, and why the course prints one and reads the other”Trumm’s formula, as published, with one ingredient removed and said so
Section titled “Trumm’s formula, as published, with one ingredient removed and said so”| Trumm, as published | At one hundredth | What the line decides |
|---|---|---|
| A: water 4000 cc, gelatin 350 g, potassium bromide 190 g, potassium iodide 2.5 g | 40 cc, 3.5 g, 1.9 g, 0.025 g | The halide, its excess and the iodide fraction |
| B: distilled water 2000 cc, silver nitrate 250 g | 20 cc, 2.5 g | The silver charge |
| C: water 2000 cc, gelatin 600 g | 20 cc, 6 g | The second gelatin, for body after the wash |
| Emulsify at 49 °C (120 °F), then one hour at 60 °C (140 °F) | unchanged | Ripening rate and grain size |
| Add C at 60 °C, digest a second hour | unchanged | The second heat treatment |
| Wash: about twelve changes of five minutes | unchanged | Salt removal |
| Made up, inclusive of finals, to 16,000 g | 160 g | The coating weight, which is what a made-up weight actually sets |
| Finals: 150 cc of 5 % chrome alum, 1000 cc spirit, |
1.5 cc, 10 cc, none | Hardener, setting aid, bacteriocide |
The scaling is the course’s arithmetic, not Baker’s, and a hundredfold reduction preserves every ratio while preserving none of the addition rate, the heat transfer or the stirring — the same caveat Project 2 carried.
Two sessions, an overnight set between them
Procedure
Section titled “Procedure”Session one — precipitate and set
Section titled “Session one — precipitate and set”Stage 1, weigh in the light (15 minutes). Grind the potassium bromide to a powder. Weigh 5 g of it, 5 g of silver nitrate, 2 g of gelatin and 5 g of gelatin into four labelled containers. Make the 10 per cent potassium iodide solution if you have not got one: 1 g in water to 10 mL, which lasts for years and will do many makes.
Stage 2, the salted gelatin (45 minutes, mostly waiting). Dissolve the 5 g of potassium bromide in 45 mL of distilled water. When it is fully in solution — hold it up to the light and look — sprinkle in the 2 g of gelatin, cover, and bloom for fifteen minutes. Set the covered vessel in the bath at 55 °C and leave it thirty minutes, checking that the bath has come back to 55 °C after the cold vessel went in. Then stir in the 1 mL of 10 per cent potassium iodide.
Bloom the second gelatin at the same time: 5 g in 25 mL of water, covered, in the bath.
Stage 3, temper the silver (5 minutes). Dissolve 5 g of silver nitrate in 45 mL of distilled water, and stand that solution in a small, very hot water bath immediately before you use it. The source gives the reason and it is worth having: warming the silver solution cuts down pepper, the coarse grain that develops out black without ever having been exposed. Wall, seventy years earlier, names the same defect and gives two different avoidances — less water in the mix, or the silver nitrate added dry to the salted gelatin. Three published remedies for one named fault, none of them agreeing, is a good sign that you are looking at something real.
Stage 4, the addition (10 minutes, timed). Under the safelight, with the salted gelatin at 55 °C and stirring steadily and consistently, add the silver solution at one teaspoon — 5 mL — per minute until it is gone, and keep stirring to the end of ten minutes.
The rate is the point. Ten minutes for 45 mL is slow enough that most of the silver arriving finds an existing crystal to grow on rather than making a new one, which is what precipitation, nucleation and growth means by growth rather than nucleation. Change nothing else in this make and halve the addition time, and you will have made a different emulsion.
Stage 5, hold (10 minutes). Leave the emulsion in the bath, not stirring. This is physical ripening at its simplest: the excess bromide lifts silver off the smallest crystals and puts it on the larger ones.
Stage 6, second gelatin and set (10 minutes). Remove the stirrer. Add the second gelatin — the 5 g in 25 mL — and stir it in thoroughly. Pour the emulsion into a zip-seal bag, seal it with as little air as you can, put it in the lightproof box, and refrigerate it for a few hours to a few days.
The second gelatin goes in now, before the set, and not after the wash. Duffin gives the reason: the noodles swell as the salts leave, and the emulsion concentration commonly falls to almost half its starting value by the time the halide is low enough to digest — while a coatable emulsion needs the gelatin not to drop appreciably below 5 per cent. Add the gelatin after the wash and you are dissolving it into cold wet noodles, which does not work.
Session two — wash, ripen, finish and coat
Section titled “Session two — wash, ripen, finish and coat”The wash rig, and the four things it has to do at once
- Ricer — the set emulsion pressed through into ice water; noodles no bigger than a pea
- Straining bag — holds the noodles, passes the water; tie the neck above the rim
- Vacuum flask — keeps the change cold for its full three minutes without more ice
- Waste tray — every change is collected: nitrate, bromide, iodide and dissolved silver
Stage 7, wash (20 minutes). Under the safelight, with the ice water ready and the bag in the flask: press the cold rubbery emulsion through the ricer straight into the ice water. Gather and tie the bag. Dunk it up and down for three minutes, squeeze it gently, lift it out, throw the water into the waste tray, refill with fresh ice water, and repeat. Six changes in all, about eighteen to twenty minutes. Then squeeze the bag firmly in the ricer until little or no water comes off, pat it dry, and scrape the noodles into a 400 to 500 mL container.
Stage 8, ripen (45 minutes). Stir 5 mL of ethanol into the drained noodles and set the container in the bath at 55 °C for forty-five minutes without stirring, or pull it when the emulsion itself reaches 55 °C. If you are using a kitchen-grade gelatin rather than an inert photographic one, the source drops the bath to 50 °C and pulls at 50 °C — an active gelatin brings its own sulfur and needs less heat before fog arrives.
Stage 9, the finals (5 minutes). The source’s finals for this emulsion are the five millilitres of ethanol already added, and nothing else. The variant she offers is 2 drops of a 10 per cent potassium bromide solution, described as added insurance against excessive base fog; that works out at about 340 mg of potassium bromide per mole of silver, and Baker’s published finals for a different emulsion carry about 170 mg of ammonium bromide per mole, so the two agree in order of magnitude. Add it if your last batch fogged, and record that you did.
No hardener. That is this course’s default and its tested source’s practice, and the coating lesson holds the ruling. The consequence is that the wet layer is tender: handle it by the corners, never squeegee it, and fix in an acid hardening fixer if it frills.
Stage 10, cool and coat (30 minutes). Set the container in cool water and stir very slowly with a plastic spoon down to coating temperature — the low-to-mid 30s to the lower 40s °C. Strain through the mesh filter into the coating jug.
Stage 11, expose and process. Under the safelight, expose one sheet with the step wedge at your fixed lamp, distance and time. This emulsion is faster than the two before it, so start by doubling the lamp distance rather than by guessing at a time: distance is the one exposure variable you can change without touching the lamp or the timer, and a lamp behaving as a point source at that distance quarters its illuminance when the distance doubles, which is two stops. Process to Project 1’s sequence and handle the wet sheets by the corners.
Expected observations
Section titled “Expected observations”- The salted gelatin is clear and colourless; the emulsion after the addition is a pale, opaque yellow. Held against the safelight, an undyed bromide emulsion transmits pale yellow, and the iodide deepens it. A pure chloride emulsion transmits white. That ladder is a maker’s check: it tells you what you have made before you expose anything.
- The set emulsion is rubbery, not stiff, and presses through the ricer like cold marzipan. If it crumbles, it is too cold; if it smears, too warm.
- The wash water of the first change is the cloudiest. By the sixth it should look like water.
- The noodles get bigger. They take up water as the salts leave, which is the swelling that dilutes the emulsion, and you can weigh it: put the drained noodles on the balance before the first change and after the sixth.
- The emulsion is visibly thinner at coating temperature than Project 2’s, because it has been diluted by the wash rather than concentrated by a made-up weight.
What is happening chemically
Section titled “What is happening chemically”The excess bromide is doing two opposite jobs, and the wash is what separates them. During ripening it is the solvent: it lifts silver off small crystals as a soluble complex and lets it redeposit on large ones.
Once ripening stops, the same excess halide becomes a restrainer. 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 division from the practical side in 1929: bromide papers are always washed emulsions, the gaslight papers usually unwashed.
The iodide is not a fourth of a per cent of anything. At 2 mol per cent of the silver it sits mostly in the crystal as a solid solution, distorting the lattice and deepening the electron traps that make a latent image. That is the mechanism Part IV’s latent image page owns, and it is the reason a negative emulsion carries iodide and a printing paper does not.
And the sensitisation that is not happening. With an inert gelatin at 1 to 2 ppm of active sulfur, this emulsion is close to the chemical floor. Duffin’s account of what a proper sulfur digestion does — a few thousand silver sulfide atoms in a handful of specks on a crystal of a billion ions, multiplying speed several-fold — is on the washing and sensitisation page, and the gap between what that page describes and what your emulsion will measure is the thing this project’s experiment is for.
Data to record
Section titled “Data to record”Project 2’s fields, with these added: the addition rate in mL per minute and the measured total time; the bath temperature every ten minutes through both the hold and the ripening; the noodle mass drained, before and after washing; the pH of each wash change, if you measured it; the number of changes and their duration; the finished emulsion volume, measured rather than assumed, and the silver halide concentration computed from it; the coating volume and computed coating weight for paper and for glass separately; and the number of remelts each portion of the batch has had.
Version code the batch BrI-01a. No source in this course’s corpus quantifies how many times an
emulsion may be remelted before it suffers, so the count is recorded rather than limited, and after a
few batches your own record will tell you more than the literature does.
Analysis
Section titled “Analysis”1. Speed against Projects 1 and 2, in stops
Section titled “1. Speed against Projects 1 and 2, in stops”Lay the three step-wedge strips side by side and read each one the same way: the first step that is distinguishable from base plus fog, the first step at maximum black, and the count between them.
A 21-step transmission wedge has a density increment of 0.15 per step, which its maker gives as half a stop. So a threshold four steps further along than Project 1’s is two stops faster.
2. The one-variable experiment, and the sulfur digestion inside it
Section titled “2. The one-variable experiment, and the sulfur digestion inside it”Three levers are available, and each of them is a real published question rather than an invented one.
(a) Digestion time, holding everything else. Split the drained noodles in two, ripen one half for forty-five minutes and the other for ninety, coat both at the same weight, and read the step wedges. Duffin’s curves say speed rises to a maximum and then falls, and that fog rises the whole way — so the useful stopping point is where fog starts to move, not where speed peaks.
(b) Iodide percentage. Make a second batch with 2 mL of the 10 per cent potassium iodide instead of 1 mL, taking it from 2.0 to 4.0 mol per cent of the silver, still inside Duffin’s 1 to 8 per cent range. One variable, one number moved, and a direct test of a published claim.
(c) The sulfur digestion, which is the one the project was originally designed around and which belongs here rather than in the procedure.
What a digestion experiment is looking for: two curves, and the point where you stop
- Relative log speed — drawn from a verbal description, not measured
- Fog density — drawn, not measured
Show the numbers behind this plot
| Series | Digestion time at 55 °C (minutes) | Relative log speed, and fog density — shape only |
|---|---|---|
| Relative log speed — drawn from a verbal description, not measured | 0.00 | 0.02 |
| Relative log speed — drawn from a verbal description, not measured | 20.00 | 0.12 |
| Relative log speed — drawn from a verbal description, not measured | 40.00 | 0.30 |
| Relative log speed — drawn from a verbal description, not measured | 60.00 | 0.45 |
| Relative log speed — drawn from a verbal description, not measured | 90.00 | 0.58 |
| Relative log speed — drawn from a verbal description, not measured | 120.00 | 0.60 |
| Relative log speed — drawn from a verbal description, not measured | 150.00 | 0.55 |
| Relative log speed — drawn from a verbal description, not measured | 180.00 | 0.47 |
| Fog density — drawn, not measured | 0.00 | 0.02 |
| Fog density — drawn, not measured | 20.00 | 0.03 |
| Fog density — drawn, not measured | 40.00 | 0.03 |
| Fog density — drawn, not measured | 60.00 | 0.04 |
| Fog density — drawn, not measured | 90.00 | 0.09 |
| Fog density — drawn, not measured | 120.00 | 0.17 |
| Fog density — drawn, not measured | 150.00 | 0.30 |
| Fog density — drawn, not measured | 180.00 | 0.50 |
3. Using the paper
Section titled “3. Using the paper”Contact print at a short exposure. This sheet is much faster than Projects 1 and 2, so start at a quarter of the time that worked for them and bracket downwards. A glass-and-board sandwich under a weight is the arrangement; Part XVI builds the sprung frame.
Reserve two sheets, unexposed and labelled, in the lightproof box for Part VI, where a paper negative exposed in the camera is one of the routes the pinhole work uses. Write the version code and the coating weight on the back in pencil before you put them away; in three weeks you will not remember which batch they came from.
The enlargement is deferred. No enlarger has been built or characterised at this point in the course, so the first enlargement onto your own bromide paper is a named return-visit task in Part XIX, and it carries this batch record and this contact-tested speed with it.
Troubleshooting
Section titled “Troubleshooting”| What you see | Likely cause | What to do |
|---|---|---|
| Black specks in unexposed areas, visible with a loupe | Pepper, or an undissolved bromide fragment | Three published avoidances: warm the silver solution before the addition; reduce the water in the mix; or add the silver nitrate dry. Grind the bromide finer and dissolve it completely |
| The emulsion will not press through the ricer, or comes out as paste | Too warm, or too little gelatin for the volume | Refrigerate it longer. Paste cannot be washed: set it again rather than pushing on |
| The wash water is still cloudy at the sixth change | Noodles too large, or the water not cold enough | Fines are escaping the bag, or the noodles are melting. Re-ricer to a smaller size next time, and use more ice |
| The remelted emulsion is watery and coats thin | The swelling, working as Duffin describes | Expected, not a fault. Measure your finished volume and recompute the coating volume from it rather than reusing 15 mL |
| High fog: grey where nothing was exposed | Over-ripening, an active gelatin at 55 °C, or a safelight that was adequate for a chloride paper and is not for this | Drop the ripening to 50 °C; re-run the safelight fog test on this material; add the 2 drops of 10 per cent potassium bromide to the finals |
| Very low contrast and low maximum black | Too thin a coat, before you suspect the emulsion | Baker’s warning exactly. Weigh the pot before and after coating, compute the weight, and coat one sheet at twice it before changing anything in the make |
| The layer lifts or frills in the fixer | No hardener, which is the default, plus a thick coat | Fix in an acid hardening fixer. Do not add a hardener to the emulsion to solve a processing problem |
Clean-up
Section titled “Clean-up”Everything that touched emulsion is quenched in the tray of clean water on the wet side and that tray goes into the silver-waste container. The ricer and the bag need particular attention: gelatin left in the perforations sets hard and comes out only with a long hot soak, and a bag with dried emulsion in it sheds specks into the next wash. Wash both in hot water and detergent immediately, then dry them.
Wash gloves before removing them. Return the silver nitrate to its store, and label and date the thiosulfate stock if you made one.
Storage
Section titled “Storage”The washed noodles, drained and patted dry, keep in a lightproof container in the refrigerator for several days. The instruction from the tested source is emphatic and worth obeying: never freeze them. Duffin’s general figures agree in shape — below 5 °C, a stable finished emulsion keeps for several months, undigested material usually only a few days.
Coated sheets, dry, flat, interleaved, in a lightproof box, out of warm damp storage, with the version code and coating weight pencilled on the back.
What nobody has published: the keeping life or the keeping fog rate of a hand-coated bromide paper. Baker gives the industrial test — ten days at 41 °C (105 °F) below 65 per cent relative humidity, with incubated plates showing no more than 0.02 extra fog against controls — and no figure exists for anything made by hand. Keep two sheets back and print one a month later against a fresh one. That is a keeping test nobody has run for this material.
Disposal considerations
Section titled “Disposal considerations”Two streams and one of them is large.
Eight to twelve litres of wash water, carrying potassium nitrate, unreacted potassium bromide and iodide, and dissolved silver as the bromoargentate complex. That is much the biggest aqueous volume in Part V and it is collected, not drained: silver compounds carry H400 and H410, very toxic to aquatic life with long lasting effects, and the collecting arrangement is Part II’s chemical waste and silver waste.
Emulsion residues, failed coats, used fixer and first wash, as in every project here.
No source in this course’s corpus characterises the silver content of emulsion wash water, which means the course cannot tell you how much is in your bucket, and says so rather than estimating. What it can say is that the classification method in the launch market covers wastes from photographic processes explicitly, that a household producing them is not exempt from classification, and that local regulation governs what may then be done with the collected waste — and differs between authorities even within one country.
Questions
Section titled “Questions”- Recompute the halide excess for this make if the potassium bromide is raised from 5 g to 7 g, everything else held. Say what you would expect that to do to grain size and to ripening rate, and which published statement you are relying on.
- Trumm’s paper carries 645 g of gelatin per mole of silver and this make carries 238. Both are emulsions on the same element. Explain what the gelatin load is doing differently in each, and predict which of the two dries faster on the same paper.
- Convert 150 ppm of thiosulfate in the gelatin into milligrams per mole of silver, then into millilitres of the 0.05 per cent working solution for a batch carrying 8 g of silver nitrate. State the one assumption in the first conversion and the one ambiguity in the second.
- Your washed emulsion measures 195 mL rather than the 150 to 180 the source implies. Compute the new silver halide concentration and the coating volume that holds 100 mg/dm² on an 8 × 10 inch sheet, and say what the larger volume tells you about the wash.
- Trumm’s finals contain 7.5 g of chrome alum against 950 g of gelatin. Express that as a percentage of the gelatin and as a percentage of the made-up emulsion, and explain which of those two numbers survives a hundredfold scaling and why.
- This project’s step wedge shows a threshold five steps further along than Project 1’s. State the speed difference in stops, then list three things other than the emulsion that could have produced part of that difference.
Further experiments
Section titled “Further experiments”The coating-weight series. Coat four sheets from one melt at 5, 10, 15 and 25 mL per 8 × 10 inch sheet, compute the weight for each, expose them all identically and read maximum black and threshold. That is the experiment that separates the emulsion from the coating, and it is the one worth doing before any other, because until you have run it every result you get is a result about both.
Trumm at one hundredth. Make the scaled formula in the table above, phenol omitted, and put its step wedge beside this one. You will be comparing a 45 per cent halide excess against 9.5, 238 g of gelatin per mole against 645, and no hardener against 0.79 per cent of the gelatin — four variables, so it is a comparison rather than an experiment, but it is the only way to see what a purpose-designed paper emulsion does that a negative emulsion on paper does not.
The wash you can weigh. Weigh the drained noodles before the first change and after the sixth, and compute the water taken up as a percentage. Duffin says the emulsion concentration commonly falls to almost half. Nobody has published that measurement at domestic scale, and it takes two weighings.
- A bromide emulsion is washed, and the reason is chemical rather than traditional: excess halide ripens the crystals and then restrains the sensitisation that follows, so it has to come out.
- The make is 0.0294 mol of silver, 45 mol per cent halide excess, 2.0 mol per cent iodide on the silver, 238 g of gelatin per mole, added at 5 mL a minute for ten minutes at 55 °C.
- Noodle washing is set, shred, six three-minute changes of ice water, drain. It dilutes as it washes, which is why the second gelatin goes in before the set.
- The endpoint the course can offer is one practitioner’s pH curve from 10 to 7.8, presented as hers.
- There is no measured sulfur digestion in this procedure, because no published home formula performs one. Duffin’s 5 to 30 mg per mole is a published range, the stock that delivers it is the course’s own design, and both belong in the experiment rather than the recipe.
- Trumm’s bromide paper is the formula you read and scale, phenol omitted and said so; its chrome alum works out at 0.79 per cent of the gelatin, inside Duffin’s band, and its commercial coating weight at 23 to 31 mg/dm², three to four times lighter than a hand coat.
- Every speed number in this project is yours. No source at any tier publishes one.
Check your understanding
Sources for this page
9 cited · checked 2026-09-04
- 01The Light Farm: silver gelatin emulsion making for the artistDenise Ross§ Tutorial Workshops, Plain Silver BrI Dry Plate Emulsion — The Recipe, headed as an adaptation of Kodak Publication No. AJ-12, 1969 Revision: the 55 °C bath, the salted gelatin of 45 mL water, 5 g potassium bromide and 2 g gelatin, the 1 mL of 10 per cent potassium iodide, the second gelatin of 5 g in 25 mL, the silver solution of 5 g in 45 mL warmed immediately before use to cut down pepper, the addition at 5 mL per minute over ten minutes, the ten-minute unstirred hold, the refrigerated set, the potato-ricer wash in six three-minute changes of ice water, the 45-minute ripening at 55 °C with 5 mL of ethanol, the coating window in the low-to-mid 30s to lower 40s °C, and the 15 mL per 4 × 5 inch plate at ten to twelve plates per batch. Dry Plate recipes page for the pH endpoint measured from 10 down to 7.8 on two trials, and for the several-days refrigerated keeping of washed noodles with the instruction never to freeze. Odds and Ends for the transmitted-colour ladder of undyed emulsionsthelightfarm.comtier 2, specialist2026-09-04
- 02Photographic Emulsion TechniqueT. Thorne Baker, 1941§ Chapter IX, Bromide and Chloride Papers, pages 166 to 168: Trumm's bromide paper emulsion with solutions A, B and C, the funnel and barometer-tubing jet, the half-hour swell, emulsification at 120 °F (49 °C), the hour at 140 °F (60 °C), the second gelatin and second hour, setting in a cold crock in ice water, the wash in about twelve changes of five minutes, the make-up to 16,000 g inclusive of finals, the finals of 150 cc of 5 per cent chrome alum, 1000 cc of spirit and 10 g of phenol, the rice-starch matt option and the saponin or quillaia note; page 166 for negative emulsions at forty to fifty grams of silver nitrate to the litre against paper emulsions at fifteen to twenty-five, and for one litre of emulsion coating sixty to eighty square feet; page 165 for the 0.02 fog-density limit on a trial coating and the warning that too-thin coating is mistaken for poor maximum blackarchive.org/stream/photographicemul00bake/photographicemul00bake_djvu.txttier 1, primary2026-09-04
- 03Photographic Emulsion Chemistry (The Focal Library)G. F. Duffin, 1966§ Page 94, Quantities of Sensitizer: the 1 to 2 parts per million of active sulphur in inert gelatins against 100 in active ones, Bekunov's 25 to 150 parts per million of thiosulphate in the gelatin for an iodobromide emulsion of 3.1 molar per cent iodide, and the assumption of 200 g of gelatin per gram mole of silver; page 95, the speed-against-digestion-time curves at four doses, the finding that larger quantities give a lower maximum speed, the statement that only about 10 per cent of the sensitizer has broken down at the speed maximum, and the warning that very small quantities probably bring on fog before useful sensitivity; page 74 for the 1.0 to 8.0 per cent iodide range in a bromide emulsion; pages 75 to 81 for noodle washing, its dilution of the emulsion and the pAg endpoint; page 158 for chrome alum at 0.5 to 2 per cent of the gelatin weightthelightfarm.com/BookImages/Duffin.pdftier 1, primary2026-09-04
- 04Photographic Emulsions: their preparation and coating on glass, celluloid and paper, experimentally and on the large scaleE. J. Wall, 1929§ Page 101: bromide papers are always washed emulsions where the gaslight papers are usually unwashed, and the practice of pouring bromide emulsions into ice-cold pans immediately after digestion to set quickly and stop the ripening; page 92 for pepper, the coarse grain reduced to metallic silver without exposure that may be too fine to see without an eyepiece, and its avoidanceskeyesphoto.com/wp-content/uploads/2018/09/Photographic-Emulsions-by-E-J-Wall-1929.pdftier 1, primary2026-09-04
- 05PubChem compound summary: Silver Nitrate (CID 24470)National Center for Biotechnology Information§ GHS Classification: the harmonised entry under Regulation (EC) No 1272/2008 — Danger, GHS03, GHS05, GHS09, with H272, H314, H400 and H410pubchem.ncbi.nlm.nih.gov/compound/24470tier 1, primary2026-09-04
- 06PubChem compound summary: Sodium Thiosulfate Pentahydrate (CID 61475)National Center for Biotechnology Information§ GHS Classification and Names and Identifiers: the ECHA C&L result reported as not meeting GHS hazard criteria, and the molecular weight of the pentahydratepubchem.ncbi.nlm.nih.gov/compound/61475tier 1, primary2026-09-04
- 07PubChem compound summary: Sulfuric acid, chromium(3+) potassium salt (2:1:1) (CID 61489)National Center for Biotechnology Information§ GHS Classification, ECHA C&L Inventory EC 233-401-6: signal word Warning, GHS07, H315, H319 and H335, with no sensitisation and no carcinogenicity statement notifiedpubchem.ncbi.nlm.nih.gov/compound/61489tier 1, primary2026-09-04
- 08Transmission Step WedgesStouffer Industries, doing business as Stouffer Graphic Arts§ Transmission step wedge specification table: the T2115 with 21 steps at a density increment of 0.15, which the maker gives as half a stop per stepstouffer.net/TransPage.htmtier 1, primary2026-09-04
- 09Waste Classification: Guidance on the classification and assessment of waste, Technical Guidance WM3 (1st edition, version 1.2.GB)Environment Agency, Natural Resources Wales and the Scottish Environment Protection Agency§ List of Waste chapter 09, wastes from the photographic industry, and the statement in Step 1 that nearly all household, commercial and industrial wastes need to be classifiedassets.publishing.service.gov.uk/media/6152d0b78fa8f5610b9c222b/Waste_classification_technical_guidance_WM3.pdftier 1, primary2026-09-04
Formulas, hazard statements, historical dates and process descriptions on this page were checked against the sources above on the date shown. Safety data changes: obtain the current safety data sheet for the product you actually buy before you open it.