Project 1: A Silver Chloride Contact Printing Emulsion
Purpose
Section titled “Purpose”To make a light-sensitive material from two salts and a protein, coat it, print on it, and measure it.
Five grams of silver nitrate go into salted gelatin over five minutes; the emulsion ripens for half an hour; it is coated unwashed on wet watercolour paper; and by the end of the session you have sheets of contact printing paper that nobody else has ever made, together with a step-wedge reading for them and a batch record that says exactly how they came to be that way.
Then you do it again with one thing changed. The second batch is not a repeat: it is the half of the project where a number in a notebook acquires a cause.
Learning objectives
Section titled “Learning objectives”By the end of this session you should be able to:
- read a published emulsion formula, identify who published it and who adapted it, and say which quantities are the original’s and which are somebody’s later choice;
- compute the molar ratio of silver to chloride in a make, express the halide excess as a percentage, and say what that excess is for;
- weigh and dissolve silver nitrate under the Level B controls, and name the three things it must never meet;
- run a controlled three-portion addition into stirred salted gelatin and describe what the emulsion looks like as it forms;
- state why this emulsion is not washed and why it is briefly ripened, and cite the source for each;
- coat, dry, expose, process and read the result, comparing it against the bought emulsion you coated in the previous session;
- design and run a one-variable second batch and interpret the difference.
Prerequisites
Section titled “Prerequisites”Coating your first emulsion — you need the coating stroke in your hands and the step-wedge geometry fixed, and you need the LE-01 sheets to compare against.
Precipitation, nucleation and growth and washing, digestion and sensitisation, which explain what the addition rate and the ripening actually do. This page performs them; it does not re-derive them.
Silver nitrate handling from Part II, in full, including the spill procedure and the ammonia prohibition. That page is the control document for the reagent; this one assumes it rather than repeating it.
Safety classification
Section titled “Safety classification”Level B, and the whole of the reason is the silver nitrate. Nothing else here would reach past Level A: the gelatin is food-grade protein, the potassium chloride is a salt, and the hottest thing in the room is a water bath at about 50 °C. It is the five grams of a corrosive, oxidising, aquatically toxic salt, weighed dry and dissolved in water, that sets the level — and it does so on the substance’s own classification, not on the quantity being large.
What is not a hazard here, and why. There is no ammonia in this make. Most published emulsion formulas of this period are ammoniacal, because ammonia ripens fast at 40 to 45 °C where a neutral make needs 50 °C or far more; the course excludes them all, and the price it pays is working hotter and slower, which is worth knowing rather than wondering about. There is no cadmium and no mercury, both of which appear in period formulas as additives and are excluded at any level. There is no chromium(VI): chrome alum, where a later project uses it, is chromium(III), a different substance with a different classification, and the course’s safety classification page holds the single ruling that separates the two. And there is no strong acid or alkali — the most reactive liquid on the bench after the silver solution is a dilute stop bath.
Hazards
Section titled “Hazards”Silver nitrate, dry and in solution. Corrosive to skin and eyes; an oxidiser, so it is kept away from anything combustible and from every reducing agent in the darkroom, developer included; very toxic to aquatic life, which is why nothing silver-bearing goes to a drain. It stains skin brown, and the stain appears hours later on exposure to light, so you can contaminate yourself thoroughly and not know until the following morning. Weigh it over a tray, in still air, with no fan running.
The ammonia prohibition. Silver nitrate and ammonia must not meet at this level, at any concentration, for the reason Part II’s page sets out in full. Household cleaners containing ammonia do not enter this room during a make.
Hot gelatin and hot glassware. The bath runs at 49 to 52 °C and the near-boiling water used for the ripening stack is hotter than that. Lift with a cloth; pour boiling water away from you; keep the pot’s handle turned in.
Processing chemistry. An alkaline paper developer, a dilute stop and a thiosulfate fixer, as in the previous session.
Metal in contact with silver nitrate. Cheap stainless steel corrodes in silver nitrate solutions, and rust must not reach an emulsion. Use glass, borosilicate or plastic wherever you can, and inspect a stainless whisk before every use.
Darkness. Everything from the melt to the drying box happens under a safelight. Clear the floor first.
Required PPE
Section titled “Required PPE”- Splash goggles, not spectacles, from before the silver nitrate jar is opened until the last rinse water is collected. H314 is the reason and it is sufficient on its own.
- Nitrile gloves, changed if they are splashed, and removed before you touch anything on the dry side.
- An apron or a lab coat, and closed shoes.
- Eyewash within reach and tested, plus the spill provision Part II specifies. This is the first session in the course where the reagent’s own classification makes those two non-negotiable.
- Dedicated utensils, never shared with food preparation and stored apart from it.
Ventilation
Section titled “Ventilation”Ordinary room ventilation, and the control it provides is not against a vapour: nothing in this make boils, fumes or evolves a gas at 50 °C. What air movement is for here is dust while the silver nitrate is being weighed, which is the one moment in the session when the reagent is airborne at all — so the weighing is done in still air with no fan or extractor draught, and the room’s ventilation is used before and after rather than during. That is the opposite of the usual instruction and it is deliberate: a draught over an open balance moves powder, and the exposure route that matters for a soluble silver compound is inhalation of dust.
Materials
Section titled “Materials”| Item | Quantity | Notes |
|---|---|---|
| 90 lb hot-press watercolour paper | 4 sheets, 11 × 15 in | Cotton rag, hot-pressed; the same paper you coated bought emulsion on |
| A tray large enough to soak a sheet | 1 | The paper is coated wet |
| Mylar or acetate sheet, and a squeegee | 1 each | For laying the wet paper down |
| Two nesting heatproof cups or beakers | 1 set | The inner one holds the emulsion, the outer stands in the bath |
| Plastic wrap, masking tape, blotting paper | — | Bed, dams, drying |
| Step wedge, 21-step transmission | 1 | The instrument from the previous session |
| A negative to contact print, or photogram objects | 1 | The same negative you printed last session, if you have it |
| Pencil and the batch-record sheet | — | Opened before anything is weighed |
Chemicals
Section titled “Chemicals”| Chemical | Quantity | Form |
|---|---|---|
| Silver nitrate | 5 g | Crystalline, weighed dry, dissolved in 25 mL distilled water |
| Potassium chloride | 3 g | Crystalline. The course holds no GHS record for it; see the note below |
| Gelatin | 25 g | Photographic (inert) gelatin, added after the chloride has dissolved |
| Distilled water | 175 mL | 150 mL for the salted gelatin, 25 mL for the silver |
| Ethanol | 15 mL | Named in the source’s tool list and not placed in its recipe; see the gaps below |
| Paper developer | 1 L working solution | Development 2.5 to 3 minutes |
| Stop bath | 1 L dilute working solution | 1 minute |
| Fixer | 2 × 1 L | Two baths, 3 minutes in each |
| Water | about 20 L | Washing and rinsing |
Equipment
Section titled “Equipment”The validated coating station: levelled bed, chilled slab, water bath at 49 to 52 °C, drying box, 9 inch puddle pusher, syringes, two thermometers, safelight. Plus a balance reading to 0.1 g or better; a small whisk or a magnetic stirrer; a 5 minute timer; a large pot with a lid for the ripening stack; three processing trays and a fourth for washing; print tongs; a labelled silver-waste container standing open on the wet side before the session begins.
Estimated cost
Section titled “Estimated cost”Cost band ££. Silver nitrate is the item that sets it and 5 g is a small purchase; the gelatin, the potassium chloride and the paper are all modest, and four sheets of good watercolour paper cost more than the silver in some markets. This page quotes no prices; dated figures for the launch market live in the laboratory planner, where the per-batch and per-sheet silver quantities from this page feed the consumables calculator.
For that calculator, the figures are: 5 g of silver nitrate per batch, about 208 g of finished emulsion, and about 50 mL per 11 × 15 inch sheet, so roughly 1.2 g of silver nitrate per sheet and about four sheets per batch.
Estimated consumables cost
Section titled “Estimated consumables cost”One batch is 5 g of silver, 25 g of gelatin, 3 g of potassium chloride and four sheets of watercolour paper — and the page’s own figures say that batch is about 208 g of emulsion, roughly 50 mL a sheet, so about 1.2 g of silver nitrate a sheet. That per-sheet figure is what the consumables calculator needs from this page.
| Consumed | This session | Sourced price | Cost this session |
|---|---|---|---|
| Silver nitrate | 5 g a batch, about 1.2 g a sheet | £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 chloride | 3 g | None. A named price gap: silver-halide salts other than potassium bromide | — |
| Gelatin, photographic inert | 25 g | £17.45 per 100 g, inert photographic gelatin (£0.17 a g) | £4.36 |
| Ethanol, 95 % | 15 mL | None. ethanol carries a cost band and no dated figure |
— |
| Hot-press watercolour paper, 11 × 15 in | 4 sheets a batch | None. A named price gap: hot-press cotton watercolour paper | — |
| Paper developer concentrate | 100 mL, to make 1 L | £10.52–£20.03 per 500 ml to 1 L of concentrate, diluted 1+9 | £2.00–£2.10 |
| Stop bath concentrate | 50 mL, to make 1 L | £10.66–£12.18 per 500 ml of citric acid concentrate, diluted 1+19 | £1.07–£1.22 |
| Rapid fixer concentrate | 400 mL, two baths of 1 L | £21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film | £8.42–£10.39 |
| Distilled water | 175 mL, plus about 20 L of wash water | None. distilled-water carries a cost band and no dated figure |
— |
The priced rows come to £27.84 to £74.53 for one run of this session, at the retail ranges read on 5 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.
Divide the priced subtotal by four for a per-sheet figure and remember that it is a floor: the paper and the potassium chloride, the two rows with no dated price, are consumed sheet for sheet with the silver.
Waste streams
Section titled “Waste streams”Everything silver-bearing is collected, and in this session that is more than the fixer.
The make residues: the emulsion left in the cup, anything spilled, the rinse water from the whisk, the cups, the syringes and the coating rod. All of it carries silver.
The failed coats, wet or dry. A sheet you abandon is a sheet with silver on it.
The fixer and the first wash water, exactly as for a bought paper.
Only the used developer and the used stop are silver-free. Local regulation governs the disposal of all of it, and it differs between authorities even within one country; the arrangement has to be in place before the session, because the moment you have a beaker of silver rinse in your hand is the wrong moment to be deciding.
Alternative route
Section titled “Alternative route”Without silver nitrate. The whole of this project can be run on bought liquid emulsion, at Level A, by treating the coating variables as the experimental ones: coat at three weights, or at three temperatures, or on three substrates, and take the step-wedge readings exactly as below. You lose the make and keep the sensitometry, the coating, the record-keeping and the print.
Without a darkroom. A windowless room at night. This is the most forgiving emulsion in the part — a pure silver chloride material responds to ultraviolet and the violet edge and to nothing else — but it is not insensitive, and the safelight test from the station build is what tells you your room is adequate, not this sentence.
Without an 11 × 15 inch bed. Halve the sheet size and halve the coating volume with it. Coating weight, not volume, is the quantity to hold constant, and the arithmetic is on the previous page.
Preparation
Section titled “Preparation”Reading the formula before you weigh anything
Section titled “Reading the formula before you weigh anything”This is the first covers item for a reason. A formula is a document with a history, and knowing whose history it is changes how you treat every number in it.
What this one is. The recipe is headed as a chloride emulsion by T. Thorne Baker, published in American Photography in March 1943, in a “KCl Gaslight Paper Version” adapted by Denise Ross for The Light Farm in 2012. So there are two authors and two dates in one document, and they own different things: Baker owns the chemistry, Ross owns the scale, the vessels, the timings as she runs them, and the coating method.
What this course has actually read. Ross’s adaptation, in full, on her free tutorial pages. Not Baker’s 1943 article, which survives in this corpus only as a page image that did not yield text. So where this page says “the source”, it means Ross’s adaptation; where it says “Baker’s article”, it is naming an upstream document the course has not read. Ross also states that Baker’s article contains two recipes — a developing-out paper and a printing-out paper — and this is the developing-out one. Anybody citing “Baker 1943” for a formula owes their reader that distinction.
What tier that makes it. Tier 2: a respected specialist practitioner’s own site, not a peer-reviewed publication. What makes it worth choosing over the Tier 1 alternatives is that somebody has demonstrably made it. There are prints on the page, a step wedge, a whole-plate glass negative contact printed on it, and an independent reader’s first batch assessed in an addendum. Wall’s 1929 chloride emulsions are Tier 1 and more complete — they give the wash time and a full finals set — but they are 45-litre commercial batches that nobody in this corpus has made in the modern era, and scaling one by a factor of three hundred would produce an untested variant that the course, not Wall, would own.
The bench, before the safelight goes on
Section titled “The bench, before the safelight goes on”Bench layout for a silver nitrate make, with the waste and quench positions decided first
- Balance on a lipped tray — silver nitrate weighed in still air, before the safelight session, with the jar closed between weighings
- Bath, outer cup, inner cup — thermometer in the salted gelatin, not in the bath
- Silver solution, to the pouring hand — no reach across the body while pouring into a whisked emulsion
- Quench tray — whisk, beaker and splashed gloves go here at once; nothing silver-bearing is carried across the room
- Labelled silver-waste container, on a second tray — open and standing before the session starts, not fetched during it
- Eyewash and spill kit — reachable from the make station without turning round
The sequence, and the two temperatures the source states
Section titled “The sequence, and the two temperatures the source states”The make schedule: what the source states, and what it does not
- Emulsion temperature, plotted only where the source gives one
Show the numbers behind this plot
| Series | Minutes from the start of the session | Emulsion temperature, °C |
|---|---|---|
| Emulsion temperature, plotted only where the source gives one | 0.00 | — |
| Emulsion temperature, plotted only where the source gives one | 41.00 | — |
| Emulsion temperature, plotted only where the source gives one | 42.00 | 50.00 |
| Emulsion temperature, plotted only where the source gives one | 50.00 | 50.00 |
| Emulsion temperature, plotted only where the source gives one | 51.00 | — |
| Emulsion temperature, plotted only where the source gives one | 87.00 | — |
| Emulsion temperature, plotted only where the source gives one | 88.00 | 41.00 |
| Emulsion temperature, plotted only where the source gives one | 112.00 | 41.00 |
Procedure
Section titled “Procedure”Stage 1 — Weigh, in the light (15 minutes)
Section titled “Stage 1 — Weigh, in the light (15 minutes)”Weigh the potassium chloride and the silver nitrate before the room goes dark, in still air, each over a lipped tray, with the jar closed between weighings. Dissolve the 5 g of silver nitrate in 25 mL of distilled water in a small beaker, cap it, label it, and stand it on the make station in the position the plan gives it. Silver nitrate solution is light-sensitive; keep it covered.
Dissolve the 3 g of potassium chloride in 150 mL of distilled water in the inner cup, stirring until it is completely gone. Undissolved crystals are not a small matter: a grain of undissolved halide reaching the precipitation gives a black speck on the finished paper that no amount of care afterwards removes.
Now sprinkle in the 25 g of gelatin slowly, stirring, and cover the cup. Let it bloom for thirty minutes, or several hours if that suits your evening better. The gelatin must swell before it melts; a dry lump dropped into hot water makes a skin and never fully dissolves.
Stage 2 — Melt (15 minutes, under the safelight)
Section titled “Stage 2 — Melt (15 minutes, under the safelight)”Set the covered cup, standing in its outer cup, into the bath preheated to 49 to 52 °C, and wait until the thermometer in the salted gelatin reads 49 to 51 °C. Not the bath: the gelatin.
While it comes up, warm the silver solution briefly. The reason is a defect with a name. Three sources independently describe pepper — coarse grain distributed through the emulsion that develops out black with no exposure, sometimes too fine to see without a magnifier. Wall names it in 1929 and gives two avoidances, both about the mix: use less water, or add the silver nitrate dry to the chlorized gelatin. Ross gives a third: temper the silver solution warm immediately before precipitation. Three published remedies for one named defect is unusually good evidence that the defect is real; which remedy works for your gelatin is something you will find out.
Stage 3 — The addition (5 minutes, and it is timed)
Section titled “Stage 3 — The addition (5 minutes, and it is timed)”Start the timer, and whisk the salted gelatin continuously, in one direction.
- Pour in about one third of the silver solution while whisking. Whisk for one minute.
- Add the second third. Whisk for one minute.
- Add the remainder, and whisk to the end of the five minutes.
What you are watching for: the clear, faintly yellow salted gelatin turns opalescent and then milky within seconds of the first pour. That milkiness is the silver chloride precipitating, and its appearance is the reaction happening:
The potassium and the nitrate take no part; they stay dissolved, and because this emulsion is never washed, they stay in the coated layer. The rate at which you pour decides how many crystals form and how big they grow — that argument belongs to the precipitation lesson — and the three-portion schedule is the source’s way of making a hand-poured addition repeatable.
Stage 4 — Ripen (about 30 minutes)
Section titled “Stage 4 — Ripen (about 30 minutes)”Cover the cup with plastic wrap. Stand the stacked cups in a pot, pour near-boiling water into the pot to half the height of the outer cup, and put the lid on. Leave it about thirty minutes, which is conveniently the time it takes to stage the coating.
One thing the source does not give is the ripening temperature. It gives a method and a duration. Near-boiling water filling half the height of the outer cup, lidded, for thirty minutes will settle somewhere well above the melt temperature and then fall; where exactly depends on your cups, your pot and your room. Put a thermometer in and record what yours does — that is an original measurement, it costs nothing, and it is the single most useful number you could add to this recipe.
Stage 5 — Cool and coat (40 minutes)
Section titled “Stage 5 — Cool and coat (40 minutes)”Cool the emulsion to about 40 °C, stirring slowly with a plastic spoon or not at all, and coat at 40 to 42 °C. Soak a sheet of the watercolour paper, squeegee it down onto the wet plastic wrap on the levelled bed, draw the coating volume into the syringe, pour it across the top of the sheet and take it down in one pass with the 9 inch rod.
The source’s coating volume is 50 to 55 mL per 11 × 15 inch sheet. Take that as the starting point and weigh the pot before and after, because the same tutorial also says a batch coats ten such sheets, and a 208 mL batch cannot both coat four sheets at 52 mL and ten sheets at anything. The coating lesson sets out that discrepancy in full and does not resolve it; what settles it for your bench is two weighings and a division.
At 52 mL per sheet and about 20 g of silver chloride per litre, an 11 × 15 inch sheet (10.6 dm²) carries about 100 mg of silver halide per square decimetre — two and a half times Baker’s stated ceiling of 25 to 40 mg/dm² for a chloride plate. That is not necessarily wrong. It is a real and citable difference between hand coating and machine coating, and it is worth knowing which side of it you are on.
Dry the sheets in the dark, in the drying box, and store them lightproof.
Stage 6 — Hold and split, if you want two batches from one make
Section titled “Stage 6 — Hold and split, if you want two batches from one make”The source gives two ways to hold the emulsion, and either lets a second batch be coated on another day. The batch may be halved after ripening, with the second half refrigerated up to a week in a lightproof container and remelted at 50 to 51 °C without stirring. Or the whole batch may be refrigerated before ripening, remelted at 50 °C, and then cooled to 40 °C to coat. Note that these are two different experiments: the first holds ripening constant and the second does not.
Stage 7 — Expose, process, wash (35 minutes)
Section titled “Stage 7 — Expose, process, wash (35 minutes)”Under the safelight, expose one sheet with the step wedge on it at the fixed lamp, fixed distance and fixed time from the previous session. Expose a second with your negative in contact, in a glass-and-board sandwich under a weight; this part has no printing frame, which Part XVI builds.
Process to the source’s sequence: develop 2.5 to 3 minutes; dilute stop, 1 minute; two-bath fix, 3 minutes in each; wash. Ross’s own step wedge was developed in Photographers’ Formulary BW65 at what she writes as 1:1:4, which on the usual reading of that notation for a two-part paper developer is one part of each stock solution to four parts of water — this course reads it that way and says so, because the source does not spell it out. Handle the wet sheets by the corners: this emulsion has no hardener in it and the wet layer is tender.
Expected observations
Section titled “Expected observations”- The salted gelatin is clear before the silver goes in, and milky within seconds afterwards. Nothing about the change is gradual.
- The emulsion thickens as it cools, quite suddenly, over about two degrees.
- A cool emulsion coats thin and streaky; the addendum to the source treats a thin coat as a gelatin problem and adds 2 to 3 g of gelatin at a time to correct it.
- The dry sheet is pale cream, not white, and shows the paper texture through it.
- The step-wedge print is contrasty. The source describes this paper as contrasty and it should show a shorter exposure scale than a normal enlarging paper.
- The image colour is neutral to slightly warm in a normal paper developer. What sets it is the developer and the rate of silver deposition rather than the emulsion — the point Project 2 makes at length.
- A first batch usually has faults in the coating rather than in the emulsion. The independent reader whose batch the source assesses got rich blacks and clean whites on his first attempt, with the faults in how it went onto the paper.
What is happening chemically
Section titled “What is happening chemically”One precipitation, in a viscous medium. Silver ions meet chloride ions and silver chloride comes out of solution, because the solubility product of silver chloride is small — 1.6 × 10⁻¹⁰ at 25 °C. In pure water that would give a coarse, settling precipitate. In gelatin it does not, and the reason is peptisation: the gelatin adsorbs onto the growing crystals, restrains their growth and keeps them dispersed. The gelatin lesson is where that argument lives.
The excess chloride does three jobs at once. It drives the precipitation to completion by the common-ion effect, leaving little dissolved silver. It provides the ripening agent for the half hour of heat, because a chloride ion can pick a silver ion off a crystal surface as a soluble complex:
and that complex is how material moves from the small crystals to the large ones during ripening. And it leaves the crystals carrying adsorbed chloride rather than adsorbed silver, which is the state a printing emulsion wants.
Why the paper is slow, and stays slow. Silver chloride is the least sensitive of the three halides and its intrinsic response reaches only into the ultraviolet and the violet edge. Nothing in this make adds sensitivity: there is no sulfur digestion, no gold, no dye. That is exactly why it can be handled under a generous safelight and printed by contact under a domestic lamp, and exactly why it will never be an enlarging paper.
A sheet of your chloride paper in section, with this batch's own numbers
The finals, and what this formula does not have
Section titled “The finals, and what this formula does not have”The curriculum promised “finals for a chloride paper: bromide restrainer, wetting agent, hardener if used”. Three of those are not in this formula, and saying so is more useful than quietly supplying them.
No bromide restrainer. There is none in the recipe. A little bromide would be a restrainer, suppressing fog, and Project 5 uses one with a published dose; this make has nothing of the kind. If your paper fogs, that is a real experiment waiting rather than a gap you should fill by guessing a quantity.
No hardener. None, and none is needed. This part’s default is no hardener in the emulsion at all, for the reasons the coating lesson argues in full — an unhardened layer tones and spots more readily, and processing chemistry penetrates the thick patches hand coating produces. If a sheet frills or lifts in the developer, the remedy is a bath: an acid hardening fixer, which the same source reports prevented the problem completely when one of her emulsions lifted in a soft-working developer.
A wetting agent and an ethanol addition that the source names but never places. This is the honest gap and it is worth stating precisely. The tools list says the first recipe calls for 15 mL of ethanol, and the recipe’s water line reads “150 mL, or 135 mL if you are using vodka” — which only makes sense if 15 mL of something alcoholic goes in somewhere. Neither the addition point nor the wetting-agent dose is given anywhere on the free site. The books are named as the place where coating is covered in full and this course has not read them.
So the page publishes the omission. Add nothing, and your make is the recipe as published minus an ingredient the source did not place; add 15 mL of ethanol in place of 15 mL of the water and record it, and your make is your own labelled variant. The one thing not to do is to add it and not write it down.
Data to record
Section titled “Data to record”| Field | Example |
|---|---|
| Version code | KCl-01a |
| Date, session, room temperature | 2026-09-05, evening, 19 °C |
| Silver nitrate: mass, supplier, lot | 5.02 g |
| Potassium chloride: mass | 3.01 g |
| Gelatin: mass, type, Bloom, lot | 25.0 g, photographic |
| Water: salted gelatin, silver solution | 150 mL, 25 mL |
| Ethanol: added or not, volume, where | none — recipe gap, see the page |
| Derived: mol Ag, mol Cl, halide excess | 0.0294, 0.0402, 37 % |
| Derived: g AgNO₃ per litre, g AgCl per litre | 24, 20 |
| Bloom: duration | 45 min |
| Melt: bath temperature, emulsion temperature, time to reach | 51 °C, 50 °C, 14 min |
| Addition: schedule, stirring, total time | thirds at 0, 1, 2 min; whisked one direction; 5 min |
| Ripening: method, duration, measured temperature | stacked cups, 30 min, peaked 58 °C, fell to 49 °C |
| Coating: temperature, volume per sheet, sheets coated | 41 °C, 52 mL, 4 |
| Coating weight computed | about 100 mg/dm² |
| Drying: box temperature and humidity, time | 22 °C, 55 %, 16 h |
| Step wedge: lamp, distance, time; threshold step; maximum-black step; scale in steps | as the previous session |
| Development, stop, fix, wash | 2.5 min; 1 min; 3 + 3 min; 20 min |
| Faults, in raking light and after processing | one stroke line; three specks |
| Silver waste collected | 900 mL |
| The one change for batch b | — |
Analysis
Section titled “Analysis”1. Speed and contrast against the bought emulsion
Section titled “1. Speed and contrast against the bought emulsion”Put the KCl-01a step-wedge strip beside the LE-01a strip from the previous session and read both the same way: threshold step, first maximum-black step, and the count between them. At 0.15 in density per step, each step is half a stop.
Expect your paper to be slower and more contrasty than the bought emulsion, and expect the difference to be large. Then note what this course cannot tell you: no source at any tier publishes a speed or contrast figure for a homemade emulsion, so there is no number to check yourself against. The reading you take is the reading; write it down and it becomes the baseline that Projects 2 and 3 are measured against.
2. Image colour, and a warning about attributing it
Section titled “2. Image colour, and a warning about attributing it”Look at the colour of the deposit in the deep tones and in the light ones, and describe it in words alongside a print made on the same negative on bought paper. Then resist the obvious conclusion. Project 2 goes into this properly, and the short version is that a controlled series across the whole halide range, developed in one developer, produced the same colour every time. What sets image colour is the rate of silver deposition — a developer question — not the composition of the emulsion. If your paper is warm, the interesting question is what your developer and your development time were doing.
3. The controlled variation
Section titled “3. The controlled variation”Make a second batch changing exactly one thing, and choose it from the list below rather than from enthusiasm. Everything else — masses, temperatures, addition schedule, coating volume, exposure, developer, time — is held.
| Variable | Change | What the sources let you predict |
|---|---|---|
| Chloride excess | 2 g or 4 g of potassium chloride instead of 3 g | Excess halide is the ripening agent in a neutral make, so more of it should ripen further in the same half hour. No source in this corpus quantifies chloride excess for this emulsion, so the direction is reasoned and the magnitude is yours |
| Gelatin | 20 g or 30 g instead of 25 g | The source’s own addendum treats gelatin as the thin-coat remedy, adding 2 to 3 g at a time. Expect a thicker coat and a change in coating temperature behaviour |
| Ripening time | 15 minutes, or 60 | Physical ripening changes the size distribution. This is the variable with the least published guidance and the most to teach |
| Bromide | replace 0.1 g of the potassium chloride with potassium bromide | A restrainer and a halide-ratio change at once — which makes it two variables, not one. It is really a small Project 2 |
| Coating weight | half or double the volume | Not an emulsion variable at all, which is exactly why it is worth running: it separates coating faults from make faults |
Coat, dry, expose and process the second batch identically, read its step wedge, and write one paragraph linking the change to the difference. If there is no difference, say so; a null result you can defend is worth more than a difference you cannot.
Troubleshooting
Section titled “Troubleshooting”| What you see | Likely cause | What to do |
|---|---|---|
| Black specks scattered through the print, unexposed areas included | Pepper, or undissolved halide grains | Three published remedies for pepper: less water in the mix, dry silver nitrate added to the chlorized gelatin, or the silver solution tempered warm before the addition. For undissolved halide, dissolve the chloride completely before the gelatin goes in |
| The coat is thin, streaky and will not cover | Emulsion too cool, or too little gelatin | Check the thermometer is in the emulsion. The source’s remedy for a persistently thin coat is 2 to 3 g more gelatin at a time |
| Irregular islands of low density with a matt sheen, black spots on the back | Emulsion too warm; it soaked into the paper | Drop the coating temperature; look across the wet sheet under the safelight before coating the next |
| One corner clears far later in the fixer | The bed was not level | Re-level with the water puddle; this is the acceptance test the station build named |
| The emulsion lifts or frills in the developer | Unhardened gelatin and that developer | Switch developers, or fix in an acid hardening fixer, which the source reports prevented it completely |
| Overall grey in unexposed areas | Fog: safelight, keeping, or an over-ripened batch | Re-run the safelight test first, because it is the cheapest to eliminate. Then compare a freshly coated sheet against one that has been stored |
| The whole batch is grey before coating | Light reached it during the make, or the ripening ran far too hot | The stacked-cup method has no thermostat. Measure the ripening temperature next time — this is the reason to |
| The print is contrasty but will not reach a black | Coating weight, not the emulsion | Weigh the pot before and after; compute mg/dm²; coat one sheet at double the volume before touching the formula |
| Brown stains appearing on your hands the next morning | Silver nitrate on skin, developing in daylight | Not harmful in itself, but it is evidence a control failed. Review gloves and the quench routine |
Clean-up
Section titled “Clean-up”Quench the whisk, the beakers, the cups, the syringes and the rod in the tray of clean water on the wet side, immediately, and empty that tray into the silver-waste container. Wipe the bench with damp paper and collect the paper. Peel the plastic wrap off the bed and bin it — it has emulsion on it, so it goes with the silver-bearing solid waste rather than into recycling.
Wash gloves before removing them, and wash hands afterwards. Close the silver nitrate jar and return it to its locked or separated storage, away from every combustible and every reducing agent.
Storage
Section titled “Storage”Unmade silver nitrate: in its amber jar, dry, closed, separated from combustibles and reducing agents, as Part II specifies.
Held emulsion: refrigerated, lightproof, up to a week for the split-batch route; remelted at 50 to 51 °C without stirring. The source warns that time is itself an emulsion variable and that an emulsion held too long will fog.
Coated sheets: dry, flat, interleaved, in a lightproof box, each sheet pencilled on the back with its version code. Keep them out of warm damp storage.
Disposal considerations
Section titled “Disposal considerations”Silver leaves this session as dissolved silver nitrate in rinse water, as silver chloride in emulsion residues and failed coats, and as the silver-thiosulfate complex in used fixer. All three are collected; none goes to a drain. The classification reason is on the label: silver nitrate carries H400 and H410, very toxic to aquatic life with long lasting effects, and a sewage works is a biological process.
The chemistry is general and the instruction is local. Regulation governs what may then be done with the collected waste, and it differs between authorities even within one country — some accept small quantities of bottled, labelled chemical waste at household recycling centres, some do not, and none of them wants streams mixed together. Part II’s chemical waste and silver waste is the page that owns this; find out what your authority accepts before the session.
Questions
Section titled “Questions”- Recompute the halide excess for a make in which the potassium chloride is reduced to 2.2 g, the silver held at 5 g. Express it in mol per cent, and say what you would expect the reduced excess to do to the half-hour ripening and why.
- This page says the emulsion is unwashed but ripened, and corrects the manifest on the second word. Give the evidence for each half of that statement and name the source of each.
- Baker gives 15 to 25 g of silver nitrate per litre for a paper emulsion and 40 to 50 for a negative emulsion. Where does this make sit, and what would you have to change to turn it into a negative emulsion by that measure alone? Say why that change would not actually give you a negative material.
- The source’s recipe implies 15 mL of ethanol and never says where it goes. Set out the two defensible things a maker can do about that, and say what each one obliges them to write in the batch record.
- Your first sheet shows black specks in areas that received no exposure at all. Name the defect, give the three published avoidances, and describe an experiment that would tell you which of them your gelatin needs.
- You coat at 52 mL per 11 × 15 inch sheet and compute about 100 mg of silver halide per square decimetre, against Baker’s ceiling of 40 for a chloride plate. Give two reasons why that might be the right thing to do anyway, and one measurement that would tell you whether it is.
Further experiments
Section titled “Further experiments”Measure the ripening temperature and publish it. Put a thermometer in the emulsion through the plastic wrap, read it every five minutes for the thirty minutes, and plot it. The source gives a method and a duration; nobody has given the curve. Yours would be the only one.
Ripening series, four batches. Fifteen, thirty, sixty and ninety minutes, everything else held, four step wedges. This is the cleanest experiment in the whole project and it addresses the variable with the least published guidance.
Split one melt and coat at two weights. Half the batch at 25 mL per sheet and half at 50, dried together, exposed together. The difference between the two step wedges is coating weight alone, and it is the calibration that lets you interpret every later make.
Try the printing-out arm. The source states that Baker’s 1943 article contained two recipes, a developing-out paper and a printing-out one, and this project follows the first. Expose a coated sheet in strong daylight for a long time without developing it, and see whether an image appears. That would be evidence about what a chloride emulsion at this silver concentration can do, and the course has read only one of the two recipes.
Five grams of silver nitrate, three grams of potassium chloride and twenty-five grams of gelatin, in about 208 mL: a 37 mol per cent chloride excess, 24 g of silver nitrate per litre, which is the top of Baker’s own published range for a paper emulsion. Added in three portions over five minutes into whisked, melted salted gelatin at 49 to 51 °C. Ripened for about half an hour in a hot-water stack — ripened, not merely rested — and not washed, which is what a gaslight paper is. Coated at 40 to 42 °C on wet watercolour paper at a weight two to three times commercial practice, dried in the dark, developed for two and a half minutes and fixed in two baths.
Three things this formula does not contain, all of them stated rather than supplied: a bromide restrainer, a hardener, and a placed ethanol and wetting-agent addition. Two things this page corrected: the make is ripened, and image colour is not read off the halide.
And one number that is now yours rather than anybody else’s: the step count of a paper you made.
Check your understanding
Sources for this page
8 cited · checked 2026-09-04
- 01The Light Farm: silver gelatin emulsion making for the artistDenise Ross§ Tutorial Workshops: KCl Gaslight Paper — Background, and KCl Gaslight Paper — The Recipe, headed "A homemade CHLORIDE EMULSION, by T. Thorne Baker for American Photography, March 1943. KCl Gaslight Paper Version, adapted by Denise Ross for The Light Farm, 2012": the quantities, the 49 to 51 °C melt, the five-minute three-portion addition with continuous whisking, the thirty-minute ripening in a hot-water stack, the 40 to 42 °C coating window, 50 to 55 mL per 11 × 15 inch sheet of wet 90 lb hot-press watercolour paper, the processing sequence, the hold-and-split instruction, the 30 January 2013 addendum on thin coats and gelatin additions, and the safelight. Also Getting Started (Tools and Materials) for the ethanol, the puddle pusher and the whisk-corrosion warningthelightfarm.comtier 2, specialist2026-09-04
- 02Photographic Emulsions: their preparation and coating on glass, celluloid and paper, experimentally and on the large scaleE. J. Wall, 1929§ Pages 92 to 94, the slow chloride gaslight emulsions and their finals set of gelatine, alcohol, basic chrome alum solution and hydrochloric acid per 45,000 ccm; page 92, the "pepper" defect and its two avoidances; pages 101 and 113, the division between always-washed bromide papers and usually-unwashed gaslight paperskeyesphoto.com/wp-content/uploads/2018/09/Photographic-Emulsions-by-E-J-Wall-1929.pdftier 1, primary2026-09-04
- 03Photographic Emulsion TechniqueT. Thorne Baker, 1941§ Page 166, the silver concentrations of negative and paper emulsions — forty to fifty grams of silver nitrate per litre for negatives and only fifteen to twenty-five for papers — and one litre of emulsion coating sixty to eighty square feet; page 165, fog on a trial coating on glass not exceeding 0.02 density and the warning that too-thin coating is mistaken for a poor maximum blackarchive.org/stream/photographicemul00bake/photographicemul00bake_djvu.txttier 1, primary2026-09-04
- 04Photographic Emulsion Chemistry (The Focal Library)G. F. Duffin, 1966§ Chapter IV: physical ripening by excess halide, the neutral against ammonia distinction and the higher temperatures a neutral make needs; the effect of excess halide on grain growththelightfarm.com/BookImages/Duffin.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 and GHS09, with H272, H314, H400 and H410 — and the wider ECHA C&L aggregationpubchem.ncbi.nlm.nih.gov/compound/24470tier 1, primary2026-09-04
- 06EH40/2005 Workplace exposure limits, containing the list of workplace exposure limits for use with COSHHHealth and Safety Executive, 2005§ Table 1: silver, soluble compounds as Ag, 0.01 mg/m3 long-term; metallic silver 0.1 mg/m3; and the statement that absence from the list does not indicate that a substance is safehse.gov.uk/pubns/priced/eh40.pdftier 1, primary2026-09-04
- 07Chemistry 2e, Appendix J: Solubility ProductsPaul Flowers, Klaus Theopold, Richard Langley and William R. Robinson, for OpenStax§ Appendix J: solubility products at 25 °C for silver chloride and silver bromideopenstax.org/books/chemistry-2e/pages/j-solubility-productstier 1, primary2026-09-04
- 08Transmission Step WedgesStouffer Industries, doing business as Stouffer Graphic Arts§ The T2115 21-step transmission guide: 21 steps, density increment 0.15, maximum density 3.05stouffer.net/TransPage.htmtier 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.