KCl gaslight paper emulsion

Two cups, a whisk and a kitchen timer. This is the smallest complete emulsion in the formulary and the only one whose maker has published prints, a step wedge and an assessment of somebody else’s first attempt at it. Everything else in Part V is scaled down from a commercial batch; this was designed at the size it is printed at.

The salted gelatin — the halide, held in the binder

Ingredient	Quantity	Form the source specifies
Potassium chloride	3 g	dissolved first, before the gelatin goes in
Gelatin	25 g	photographic (inert) gelatin, stirred in slowly after the chloride has dissolved, then left to bloom
Water	150 mL, added	Distilled, and stated as 150 g as well as 150 mL, which is the practitioner's habit of weighing rather than measuring. The recipe's own alternative reading is 135 mL "if you are using vodka", which is the trace of the ethanol addition discussed under Mixing.
Covered and bloomed 30 minutes to several hours, then brought up in a water bath preheated to 49 to 52 °C and held until the salted gelatin itself reads 49 to 51 °C. The temperature that matters is the one inside the cup, not the one in the bath.

The silver solution — the silver

Ingredient	Quantity	Form the source specifies
Silver nitrate	5 g	crystalline, weighed dry
Water	25 mL, added	Distilled, stated as 25 g as well as 25 mL.

Mixed in this order — the precipitation, under safelight, on a five-minute timer

1. Start with the whole of The salted gelatin — the receiving vessel, whisked continuously in one direction throughout
2. Then add the whole, in three portions of The silver solution

Start a five-minute timer and whisk the salted gelatin continuously in one direction. Pour in about one third of the silver solution while whisking; whisk one minute; add the second third; whisk one minute; add the remainder and whisk to the end of the five minutes.

Then cover with plastic wrap, stand the stacked cups in a pot, pour in near-boiling water to half the height of the outer cup and put the lid on, for approximately thirty minutes - which is the time it takes to stage the coating. That thermal hold is a ripening, and it is the reason the summary phrase "unwashed and unripened" that this project was designed around is wrong on its second word. The emulsion is never washed.

Purpose

To make a printing paper — a slow, contrasty, blue-blind chloride emulsion coated on watercolour paper and exposed by contact under a negative. It is the first make in the course because it is the shortest: there is no wash, no digestion, no hardener and no dye, so a single session runs from weighing to a coated sheet.

Gaslight paper is the historical name for the class, and it means what it says. These emulsions were slow enough to be handled and printed by gaslight, and Wall places the same composition on the market as the slow gaslight lantern plates of his day.

What the formula is not is a negative emulsion. Its silver concentration, its halide excess and its speed are all a paper’s, and it is used by contact rather than in a camera.

Recommended uses

A first emulsion. Nothing in it needs equipment beyond a water bath that will hold 50 °C, and the tested minimum for that is a nested pair of Pyrex cups standing in a pot of hot water.

Contact printing from a large negative, which is what the source’s own prints are: two named contact prints from 5 × 7 negatives and a whole-plate glass negative printed on the same paper.

A one-variable experiment against Project 2 and Project 3. This emulsion carries 5 g of silver nitrate, and so does the scaled chlorobromide make and the bromide plate make that follow it. Three emulsions at one silver charge is the closest thing this part has to a controlled series, and the comparison is only meaningful because the silver is held constant.

Learning what a coating weight is by laying down far too much of it. At 50 to 55 mL over an 11 × 15 inch sheet this recipe coats roughly three times the commercial rate, and that difference is real, citable and is what makes a hand-coated sheet behave differently from a bought one.

When another formula is preferable

• When you want an enlarging paper, this is not it: a chloride emulsion is far too slow to expose under an enlarger. The bromide paper emulsion is the formula written for that job.
• When you want a warm-tone paper with a longer scale, a mixed halide. Wall’s soft-working chlorobromide is the next step and its silver charge scales onto this one.
• When the emulsion must keep for weeks, none of the Part V formulas are the answer, and this one least of all: it is unwashed, so it carries all its by-product salts, and it has no bacteriocide.
• When you want a formula with a complete published finals set, Wall’s commercial chloride emulsion has one and this does not. It is under Variants, at three hundred times the scale, and it is a washed emulsion of a different composition.

Mixing

Two vessels, one whisk, five minutes, and a temperature taken inside the cup.

1. Dissolve 3 g of potassium chloride in 150 mL of distilled water, then stir in 25 g of gelatin slowly. Cover and let it bloom for thirty minutes to several hours.
2. Dissolve 5 g of silver nitrate in 25 mL of distilled water. Under safelight from here on.
3. Bring the salted gelatin up in a water bath preheated to 49 to 52 °C, and hold it until the emulsion itself reads 49 to 51 °C. The thermometer goes in the cup, not the bath.
4. Precipitate on a five-minute timer, whisking continuously in one direction: a third of the silver, a minute; a third, a minute; the last third, and whisk to five minutes.
5. Ripen, about thirty minutes, in the covered stacked cups standing in a pot of near-boiling water with the lid on.
6. Cool to about 40 °C and coat, 50 to 55 mL to an 11 × 15 inch sheet of 90 lb hot-press watercolour paper, wet-coated with a nine-inch puddle pusher. Dry in the dark; store lightproof.

No wash. This is a gaslight paper and gaslight papers are usually unwashed, on Wall’s own division of the two classes; bromide papers are always washed. Everything the precipitation produced — potassium nitrate, and the excess chloride — stays in the coating.

!An ingredient the source names and never places

The recipe’s water line reads “150 ml, or 135 ml if you are using vodka”, and the same author’s tools page states that the first recipe calls for 15 mL of Everclear. The subtraction only makes sense if 15 mL of something alcoholic goes in somewhere. A wetting agent is named in the same list.

Neither the addition point nor the wetting-agent dose is published anywhere the course can read, and her books are named as the place where coating is covered in full. So this entry prints the formula as published, records the omission, and does not invent a step.

What that leaves you with is a choice you must write down. Add nothing, and your make is the recipe as published minus an ingredient the source did not place. Substitute 15 mL of ethanol for 15 mL of the water and record it, and your make is your own labelled variant. The one thing that is not acceptable is adding it and not recording it.

🧮The five numbers this formula is

5 g AgNO₃ ÷ 169.87 g/mol = 0.0294 mol

Silver

3 g KCl ÷ 74.55 g/mol = 0.0402 mol

Chloride

0.0402 ÷ 0.0294 − 1 = 37 mol per cent over the silver

Halide excess

150 + 25 + 3 + 25 + 5 = 208 g

Batch weight

5 g in about 208 mL = 24 g of silver nitrate per litre

Silver concentration

That last figure is the one to hold on to. Baker states that paper emulsions carry fifteen to twenty-five grams of silver nitrate to the litre and negative emulsions forty to fifty; 24 g/L sits at the top of his paper band, which is a useful independent check that a modern adaptation of a 1943 paper formula is still a paper formula.

A thirty-seven per cent excess of chloride is a great deal of unreacted salt to leave in a coating, and that is the point: an unwashed emulsion is stabilised by its own halide excess, and washing it would take away the thing that is keeping it.

Behaviour

It ripens, briefly, and the ripening is thermal. Thirty minutes in a pot of near-boiling water, with no ammonia and no added ripening agent — the excess chloride and the heat are the whole of it. The curriculum summary this project was designed against called the make “unwashed and unripened”; the first word is right and the second is not, and the source’s own procedure is the evidence.

It sets and it can be held. Halved after ripening, refrigerated in a lightproof container, the second half keeps up to a week and is remelted at 50 to 51 °C without stirring. The whole batch can also be refrigerated before ripening. Time is an emulsion variable, and an emulsion held too long fogs.

It coats heavy. Fifty to fifty-five millilitres over an 11 × 15 inch sheet is about 493 mL per square metre, against Baker’s commercial figure of one litre to sixty or eighty square feet — 135 to 180 mL per square metre. Roughly three times the trade’s coating weight, and it is not an error in either source: a hand coat is thicker and less even than a machine coat, and the whole of Part V’s practice follows from that.

It is blind to most of the spectrum, which is why the safelight can be generous: a string of red LED mini-lights or a 25 W yellow bug bulb. A pure chloride emulsion responds to ultraviolet and the edge of the violet and to nothing else.

Thin coats read as weak blacks. Baker’s warning is worth repeating exactly because it sends people chasing the wrong fault: too thin a coating is mistaken for poor maximum black.

Image characteristics

Contrasty, on the maker’s own description, and no number exists. This is the most conspicuous gap in the entry: nobody has published a grade equivalent, a contrast index or a characteristic curve for this paper, and the step wedge on the source page is shown rather than tabulated. It is a figure the course must measure and has not.

Warm tones are easy to reach. Wall attributes the property to the whole chloride and chlorobromide class along with an extremely fine-grained image, more like a stain than a normal silver deposit. Note what he does not attribute it to: his own controlled series across the halide range, developed in one developer, gave the same colour every time, and he concluded that it is the rate of silver deposition — the developer’s business — that decides the colour rather than the composition of the emulsion.

Very slow. Contact printing exposures of twenty seconds and two and a half minutes appear on the source page, under her lamp, at her distance, with a Yupo sheet as a diffuser. They are a starting point for a first test strip and nothing more.

Maximum black depends on the coating weight, which on a hand coat varies across the sheet. A patchy maximum black is usually a patchy coat.

The mechanism

One reaction, run slowly on purpose.

AgNO₃ + KCl → AgCl + KNO₃

The precipitation, in the gelatin

Silver nitrate and potassium chloride are both soluble; silver chloride is not, so it comes out of solution the instant the two meet. Everything that makes this a photographic emulsion rather than a precipitate is in how it is allowed to happen.

The gelatin is there first, and that is the whole trick. Silver chloride precipitated into water is a curd. Precipitated into a warm gelatin solution it is held as microscopic crystals, each one wrapped in adsorbed protein that stops it meeting its neighbours. The binder is a protective colloid before it is ever a coating medium.

The addition is slow because crystal size follows addition rate. Silver poured in at once makes a great many nuclei at a huge instantaneous supersaturation; silver added in three portions over five minutes, into a well-stirred vessel, makes fewer and grows them. Slow addition and a stirred vessel are the only controls this formula has over grain, and they are the ones the timer enforces.

The excess chloride does three jobs. It drives the precipitation to completion, so no free silver nitrate is left to fog the paper. It keeps the crystals in a chloride-rich condition that resists reduction. And it survives into the coating, because the emulsion is never washed.

The ripening is Ostwald ripening, at 55 °C or so, without ammonia. Small crystals dissolve and redeposit on larger ones, so the size distribution moves upward and the emulsion gains a little speed. Half an hour of it is a short ripening by the standards of the literature, and that is why this paper is slow.

Function of every ingredient

Silver nitrate, 5 g, in 25 mL of distilled water. The silver, the whole of the light sensitivity and about nine-tenths of the cost. Five grams is 0.0294 mol, and every mole of it that meets a chloride ion becomes silver chloride. At this batch size it gives 24 g of silver nitrate per litre of finished emulsion, which is a paper’s concentration and not a negative’s. More silver at the same chloride would exhaust the halide excess and leave free silver nitrate in the coating, which fogs and stains; less thins the coating weight and, past a point, gives the weak maximum black Baker warns is mistaken for something else. Distilled water is a specification: tap water carries chloride of its own and, more to the point, carries whatever else it carries into a bath where a trace of the wrong ion is a defect. Warming the silver solution before it goes in is a tested precaution against pepper in this class of emulsion, and it is the same avoidance Wall reaches for by a different route.

Potassium chloride, 3 g, dissolved before anything else. The halide, and it is in 37 mol per cent excess over the silver, which is not slack but the formula’s stabiliser. Dissolving it first is not arbitrary either: gelatin added to water first would thicken the solution before there was anything to dissolve the salt into, and undissolved halide during a precipitation shows up as black specks in the finished print. More chloride raises the excess and slows the emulsion further; less risks running out of halide before the silver does. It is hygroscopic, so a caked jar has taken up water that will be weighed as salt, and in an emulsion a halide error is a real error. There is no bromide in this formula at all — that is what makes it a chloride paper and what the next two entries change.

Gelatin, 25 g, stirred in slowly after the chloride has dissolved and bloomed before it is melted. Three jobs in one ingredient. It is the protective colloid during precipitation, keeping the crystals separate as they form. It is the binder that holds them on the paper afterwards. And its own composition matters photographically: a photographic (inert) gelatin carries one to two parts per million of active sulfur against about a hundred in an active one, which is why an inert gelatin gives a slower and cleaner emulsion and an active one can bring speed and fog together. More gelatin thickens the melt, and the tested remedy for a coat that is too thin is to add 2 to 3 g at a time and try again; less thins it towards a coating that will not cover. The blooming is not optional — dry gelatin dropped into warm water lumps, and a lump is a coating defect that arrives an hour later.

Water, 150 mL in the salted gelatin and 25 mL with the silver, both distilled. Not a filler. The 150 mL sets the concentration at which the precipitation happens, which is one of the two variables that decide crystal size, and Wall’s own remedy for pepper in a chloride emulsion is to reduce the quantity of water in the mix. It is also the figure the source qualifies with “or 135 ml if you are using vodka”, which is where the unplaced ethanol addition shows through.

Ethanol is named by the source and is not in the formula above. The tools page says the recipe calls for 15 mL of it; the recipe never says where it goes. In this class of emulsion alcohol is a setting and coating aid rather than a chemical participant, and Wall’s own commercial chloride finals carry a great deal of it. None of that is a licence to place it: the course prints what the source prints and states the omission.

Interactions

With light, from step 2 onwards. Everything after the silver is weighed happens under safelight, and the safelight can be generous because the emulsion is blind beyond the violet: red LED mini-lights or a 25 W yellow bug bulb are the tested options. Run a safelight test on your own lamp rather than trusting the description.

With metal. A cheap stainless whisk corrodes in silver nitrate, and rust must not go near an emulsion. Plastic or good stainless, dedicated to the job.

With temperature, in both directions. Below about 38 °C the melt begins to set as it is spread and the coat drags; much above 45 °C it runs thin and floods to the edges. The coating window is narrow and the thermometer is a coating instrument.

With the paper. The sheet is coated wet, laid down on a moistened bed, and 90 lb hot-press watercolour paper is the tested stock. A sized paper takes the emulsion differently from an unsized one, and this is one of the few variables in the whole make that costs nothing to test.

With the developer, which is where the image colour is actually decided. Wall’s controlled series is the evidence: change the developer and the colour changes; change the halide ratio and it does not.

With time. An emulsion held too long fogs, and the published holds are up to a week refrigerated after ripening.

Variants

Wall’s commercial chloride emulsion, 1929, is the Tier 1 formula this entry was chosen over. Ammonium chloride 275 g, hydrochloric acid 12.5 ccm, gelatine 1225 g and water 7500 ccm; the gelatine soaked, the acid and salt added, raised to 50 °C; 500 g of silver nitrate in 1200 ccm of water added in a fine stream at room temperature with vigorous stirring; digested thirty minutes at 44 °C with continual stirring; set, and washed about three hours. Remelted at 44 °C, its finals per 45,000 ccm are gelatine 900 g, alcohol 2500 ccm, basic chrome alum solution 100 ccm and hydrochloric acid 5 ccm. It is complete in a way the recipe above is not — it has a wash, a digestion time and a full finals set — and it is a forty-five litre washed emulsion that nobody in this corpus has made with modern inert gelatin. Scaling it is not a translation: ratios survive a division by three hundred and addition rate, heat transfer and stirring do not.

Wall’s higher-contrast variant of the same formula substitutes citric acid 225 g and cupric chloride 7.5 g. The course does not carry it: a copper(II) salt adds a hazard and a waste stream to a Level B project for a contrast effect reachable by other means.

A dry-silver variant, also Wall’s, adds the 500 g of silver nitrate dry to the chlorized gelatine at 44 °C. It is one of his two named avoidances of the pepper defect, and it is a genuinely different operation rather than a variation in quantity.

No course variant is offered, and the reason is that the course has already made one silently and said so. The published recipe minus its unplaced ethanol is what this page prints. Anything you add to that — the ethanol, a wetting agent, a hardener — makes your make a variant that you own, and the batch record is where it is owned.

Safety

Level B, set by silver nitrate and by nothing else on the bench. Its classification is Danger, with the oxidiser, corrosive, health-hazard and environmental pictograms; the detail is on its own page and is not restated here.

What that means at this scale: eye protection that is splash goggles rather than glasses, nitrile gloves, an apron, and a spill plan before the jar is opened. Five grams of silver nitrate will stain skin black over the following day and the stain wears off; a splash in an eye is a different matter.

Do not repeat the source’s own framing of the hazard. She writes that silver nitrate is no more hazardous than many common household cleaning chemicals. That is her assessment and not a classification, and this course gives the GHS statements instead.

Potassium chloride is Level A and is unclassified on a broad but shallow evidence base — 825 of 875 reports say it does not meet GHS criteria, on information from under six per cent of companies. The safety card that leaves its own classification box empty still records that the substance is irritating to eyes and respiratory tract, so the dust discipline that applies to every powder applies here too.

Gelatin is Level A, and the hazard associated with it is bacteriological rather than chemical: this is a warm nutrient broth, and the formula has no bacteriocide.

The only heat hazard is a 50 °C water bath, and the only other physical hazard is a pot of near-boiling water carried across a darkroom in the dark. Stage it in the light.

No ammonia, no cadmium, no mercury and no chromium of any oxidation state appears in this make.

Storage

The emulsion: refrigerated in a lightproof container, halved after ripening, up to a week; remelted at 50 to 51 °C without stirring. Never frozen.

The coated paper: dried in the dark and stored lightproof. No keeping figure is published for hand-coated sheets by any source in this corpus, and the only related figure anywhere is Baker’s industrial oven test — ten days at 105 °F, with no more than 0.02 extra fog density against controls — which is a specification for a factory and not a shelf life for a drawer.

The dry chemicals: silver nitrate dark, dry and labelled; potassium chloride dry and closed, because a caked jar weighs its own absorbed water.

Date everything. Time is a variable of this emulsion in a way it is not for a bath: the source’s own warning is that an emulsion held too long will fog, and there is no number attached to “too long”.

Incompatibilities

Silver nitrate with organic matter, dust, skin and cloth, all of which reduce it and all of which stain. See incompatibilities.

Silver nitrate with chloride, which is the reaction the formula wants — and the reason a stray splash in a cup of tap water clouds instantly, and the reason nothing that has held a halide solution is used to measure the silver.

Rust and bare steel, which put reduced silver and iron where neither belongs.

Any bath or vessel that has held fixer, whose thiosulfate dissolves silver halide. One set of glassware for the make.

Strong oxidisers with the chloride, and concentrated sulfuric acid with any chloride, which liberates hydrogen chloride.

Bacteria and mould, which are an incompatibility in the honest sense on a formula with no bacteriocide: small batches, refrigeration and use are the only defences the course offers.

Waste

Silver-bearing from the first drop. Everything that has touched the silver — the measuring vessel, the whisk rinse, the tail of the coating that ran off the sheet, the first wash of the processed print — goes into the silver stream.

The processing waste is the larger part. Two fixing baths of a print carrying this much silver halide is where most of the silver actually leaves, and a fixer is silver-bearing by design.

Bottle it, label it, and follow the disposal ruling and the general chemical waste SOP. The wash water also carries potassium nitrate and the unreacted chloride, neither of which is the reason the water is collected.

Nothing here carries chromium, because this formula has no hardener. That is one of the quiet advantages of the course’s default.

Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Black specks, or “sunspots”, in the print. Undissolved halide during the precipitation, or rust. Make sure the potassium chloride has fully dissolved before the gelatin goes in, and keep steel out of the cup.

A general grey fog with no exposure. Several candidates and they are separable: the emulsion was held too long, the safelight is not as safe as it looks, or the ripening overran. Run a safelight test first, because it is the only one of the three you can rule out in an hour.

Pepper — a fine coarse grain reduced to metal without exposure. Wall’s own defect, and he says test plates should be examined with an eyepiece, particularly in the areas protected from light, because it can be too fine to see. His two avoidances are less water in the mix, or adding the silver dry; the modern tested precaution is to warm the silver solution before it goes in.

The coat is thin and streaky. Emulsion too cool, too little of it, or a rod without plugged ends. The tested remedy for a persistently thin coat is more gelatin, 2 to 3 g at a time.

Weak maximum black. Before blaming the developer or the exposure, measure the coating weight. Baker’s warning is that too thin a coating is mistaken for poor maximum black, and on a hand coat it usually is.

The sheet frilled or lifted in the developer. The remedy the tested source reaches for is a hardening fixer in the second tray — F-5 or an equivalent — rather than a hardener in the emulsion.

A thick patch has darkened weeks after processing. That is a fixing failure, not a keeping failure: the thick area did not fix all the way through. Fix longer, coat thinner, and do not harden the emulsion.

Experiments

Measure the contrast nobody has published. Expose a 21-step transmission wedge on this paper, develop it in a named developer at a named dilution and time, and read it. The single largest gap in this entry is that its contrast exists only as an adjective.

Hold the silver, change the halide. This make, the chlorobromide paper at one hundredth, and the bromide plate make all carry 5 g of silver nitrate. Coat all three at the same computed coating weight and print the same negative on each. That is the halide-ratio experiment done properly, and it is only possible because the silver is constant.

Ripen for nothing, for thirty minutes and for ninety. Three batches, everything else held, each read on a step wedge. Half an hour is what it takes to stage the coating rather than a measured optimum, and the speed you are being given by it has never been quantified.

Coat at the trade’s weight. Compute the volume that gives Baker’s 135 to 180 mL per square metre for your sheet size — roughly a third of the recipe’s 52 mL — and coat a sheet at it. Then decide for yourself whether three times the coating weight is a fault of hand coating or a property of it.

Place the missing ethanol, and record what happens. Two batches: one as published, one with 15 mL of ethanol replacing 15 mL of the water in the salted gelatin. Coat both at the same volume and compare the evenness of the coat and the drying time. You will not have recovered what the source meant; you will have a labelled variant of your own with evidence attached, which is the honest form of filling a gap.