Lab: Coating Your First Emulsion

Purpose

To learn to coat, using an emulsion somebody else guaranteed.

Making an emulsion and coating it are two different skills, and a beginner who attempts both at once has no way to read the result. A streaky, mottled, wedge-shaped sheet that will not reach a black could be a bad make or a bad coat, and the batch record will not tell you which. So this session buys the emulsion. The formula is fixed, the material is known to work, and the only variable left in the room is your hands — which means that every fault you produce today is a fault you have diagnosed.

It also builds the two instruments the rest of Part V is judged by: a step-wedge test run at a fixed lamp, a fixed distance and a fixed time, and a batch record with a formula version code. From Project 1 onward, every emulsion you make is compared against the sheets you coat today and against each other through those two things. Keep the sheets.

Learning objectives

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

• explain why coating skill is separated from making skill here, and what a student who cannot obtain silver nitrate keeps by taking this route for the whole part;
• melt a liquid emulsion at the maker’s stated temperature without whipping air into it, and say what a bubble costs on the dried sheet;
• coat paper with a rod and with a brush, and glass by pour and tilt, and describe what each method does to the evenness of the layer;
• measure the volume you laid down and convert it to a coating weight in milligrams of silver halide per square decimetre;
• dry and, where the maker specifies it, harden the coating, and state what hardening costs as well as what it buys;
• read a step-wedge print for threshold, for maximum black and for exposure scale, and convert a step count into stops;
• open a batch record with a formula version code, and keep it well enough that a coat made in six months can be compared with this one.

Prerequisites

Build an emulsion coating station. The bed must be levelled, the bath logged, the drying box tested and the safelight passed. This session uses all four results.

The latent image made visible, for the safelight test method, the develop-out threshold and the vocabulary this page uses to describe a print.

Coating, drying and hardening, which explains the coating methods, the temperature window, coating weight and the hardening ruling. This page performs what that lesson argues.

Safety classification

Level A. A bought liquid emulsion is a silver halide suspended in gelatin, supplied as a ready-to-use liquid; the processing chemistry is the paper developer, stop bath and fixer already used in Part IV; and the only energy source is a water bath at coating temperature. Every control here is one the safety classification puts at Level A.

What is not a hazard here, and why. There is no silver nitrate on this page. Silver in a liquid emulsion is already precipitated as silver halide and locked in gelatin, which is a completely different proposition from the soluble, corrosive, oxidising salt Project 1 will ask you to weigh: it is not corrosive, it does not stain your hands brown in daylight, and it does not need goggles and a spill kit in the way the nitrate does. That difference is the reason this page is Level A and Project 1 is Level B, and it is worth understanding rather than memorising — the hazard belongs to the chemical form and the quantity, not to the word “silver”.

Two things are still true and are not softened by that. The product has its own safety data sheet, because formulations differ and some carry preservatives; read it before the bottle is opened, and take your controls from it rather than from this page. And the fixer and the first wash water are silver-bearing, exactly as they are for a bought paper, so they are collected.

Hazards

The emulsion itself. Read the supplier’s safety data sheet for the product you actually buy. This course has not obtained a current data sheet for any liquid emulsion, so it states no classification for one; what it can say is that the general precautions for a gelatin suspension of a silver halide are gloves, no skin contact with a warm emulsion, and no food or drink at the bench. If the product ships a separate hardener, that hardener has its own sheet and its own classification, and it is the item on the bench most likely to need a control the emulsion does not.

Hot water and hot glassware. The bath runs at the maker’s melting temperature, typically somewhere in the range this part works in, and a jar lifted from it holds the heat. Lift with a cloth. For a scald, the first-aid sequence is in Part II’s laboratory safety and PPE.

Processing chemistry. Paper developer is alkaline, the stop is acidic, and the fixer is a thiosulfate solution: the same three trays as Part IV, at the same dilutions, with the same tongs and the same rule that a tong that has been in the fixer does not go back into the developer.

Glass. Plates have edges. Dress them with a diamond pad before they are coated, not after, and handle a coated plate by the edges with clean hands or cotton gloves.

Darkness. The commonest injury in a darkroom is not chemical. Clear the floor before the lights go out, know where the trays are without looking, and leave the route to the door empty.

Required PPE

• Nitrile gloves whenever a solution is poured or a tray is handled, and while the emulsion bottle is open.
• Eye protection from before the first cap comes off until the last tray is emptied.
• An apron or an old shirt, because emulsion that lands on cotton fixes there and stains black in daylight a day later.
• Dedicated utensils, kept apart from anything used to prepare food, as Part II establishes.
• Cotton gloves or clean dry hands for handling dried, coated sheets; fingerprints on an emulsion surface print.

Ventilation

Ordinary room ventilation — an openable window, or a working extractor — is the control, and the reason is not vapour. None of the four liquids on this bench boils, fumes or generates a mist at the temperatures used, and extraction is not among the controls on this page for that reason. What air movement is for is that this is a two-and-a-half-hour session in a small dark room with a heat source running, and a person who stops concentrating in the middle of a timed sequence makes the kind of mistake that costs the session.

If the drying box shares the room, give the room more air rather than the box less: sheets dry by giving water to the air, and a sealed room reaches a humidity at which nothing dries.

Materials

Item	Quantity	Notes
Hot-press watercolour paper, 90 lb or heavier	6 sheets	The paper the projects use; buy the same paper now so today’s coats are comparable with them
Glass plates, 1.5–1.6 mm, edges dressed	2	4 × 5 inch is the size Part VI’s camera work assumes
A commercial photographic paper, graded or variable contrast	4 sheets	The comparison print, and the safelight re-test
A negative to print by contact	1	Any negative from Part IV or VI; a strong, simple one
Objects for a photogram	a handful	If you have no negative
Step wedge, 21-step transmission	1	No substitute is described here, and a hand-cut one is not equivalent: the session reads relative speed off a calibrated transmission scale. Part XIII owns the wedge and its calibration
Plastic wrap, masking tape, blotting paper	—	Bed surface, dam bars, drying
Pencil, and a printed batch-record sheet	—	The record starts today

Chemicals

Chemical	Quantity	Form
Liquid photographic emulsion	150–250 mL	As supplied. Sold ready to melt; the maker’s own leaflet is the authority for its temperature, coating rate, drying and safelight
Hardener supplied with the product	as the maker states	Only if one is supplied. Its dose is the maker’s, not this course’s
Paper developer concentrate	100 mL	Diluted to the maker’s working strength, typically making 1 L; the same product Part IV used
Stop bath concentrate	50 mL	Diluted to make 1 L. A citric acid stop bath, or dilute acetic acid
Fixer concentrate	200 mL	Diluted to the maker’s paper strength to make 1 L. An ammonium thiosulfate rapid fixer, or a sodium thiosulfate fixer
Acid hardening fixer, if the coat frills	1 L	A bought acid hardening fixer at the maker’s dilution. The hardening agent in the published reference formulas is potassium alum
Gelatin	10 g	For the rehearsal coat only
Water	about 15 L	Dilution, washing, the bath

!The product is not named here, and that is a decision rather than an omission

Liquid emulsions are sold by several makers, and the three families a student in the launch market is most likely to meet are the FOMA, Rollei and Liquid Light lines. This course has not obtained a current technical leaflet or safety data sheet for any of them, so it prints no melting temperature, no coating rate, no drying time, no hardener composition and no safelight recommendation for a product. Printing one from memory or from a retailer’s description is exactly what Rule 1 forbids.

What you do instead takes five minutes and is a better habit anyway. Before the session, read the leaflet that came with your bottle and copy these seven figures onto the first page of the batch record:

1. melting temperature, and whether the maker gives a range or a single figure;
2. working temperature for coating, if it differs from the melting temperature;
3. coating rate, in millilitres per square metre or per sheet of a stated size;
4. safelight — filter class or colour, wattage and distance;
5. drying conditions and time;
6. hardener: supplied or not, dose, and when it goes in;
7. shelf life and storage, including whether the bottle may be remelted and how often.

Those seven are the specification of your material. Every one of them is a number this page would otherwise have had to invent, and every one of them is a number you will need again when you write your own formula in Project 1 and discover that you must decide it yourself.

Equipment

The coating station, validated: levelled bed, chilled slab, water bath, drying box, rod, brushes, syringes, two thermometers, safelight. Three trays and a fourth for washing; two graduates; print tongs; a lamp on a stand that can be fixed at a measured distance, with a way to switch it for a measured time; a tape measure; a sheet of clean glass heavy enough to hold a negative in contact; a timer; a torch for raking-light inspection; and a soft pencil.

Estimated cost

Cost band ££, and the bottle of emulsion is most of it. One bottle coats far more than this session needs, which is the point — the surplus is what the Further experiments below spend, and what the alternative route uses for the rest of the part. Everything else is either already in the station or is paper. This page quotes no prices; dated figures for the launch market live in the laboratory planner.

Estimated consumables cost

The bottle of bought emulsion is most of the band above, and one bottle coats far more than this session needs — the surplus is what Further experiments and the alternative route through the rest of Part V run on. The row below therefore prices what this session pours, and the bottle is a purchase that lasts.

Consumed	This session	Sourced price	Cost this session
Liquid photographic emulsion	150–250 mL of a 250 g bottle	£49.85 per 250 g bottle (£0.20 a g)	£39.88
Hot-press watercolour paper, 90 lb or heavier	6 sheets	None. A named price gap: hot-press cotton watercolour paper	—
Glass plates, 1.5–1.6 mm	2, at 4 × 5 in	None. A named price gap: glass plates, 1.5 to 1.6 mm, cut to 4 × 5 in	—
Commercial photographic paper for the comparison	4 sheets	£16.06–£44.71 per 25 to 100 sheets, 5 x 7 in, variable contrast RC (£0.45–£0.64 a sheet)	£1.79–£2.57
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
Fixer concentrate	200 mL, to make 1 L	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£4.21–£5.20
Gelatin	10 g, rehearsal coat only	£17.45 per 100 g, inert photographic gelatin (£0.17 a g)	£1.74
Plastic wrap, masking tape, blotting paper	one bed and a few sheets	None. tape-and-adhesives carries a cost band and no dated figure	—
Water	about 15 L	Metered supply; the planner prices no water	—

The priced rows come to £50.69 to £52.71 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: 3 of the 10 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 emulsion row is priced per gram of a 250 g bottle, which is the only figure the price file carries for it; if your bottle is a different size, scale it. The hardener supplied with the product, where one is supplied, is the maker’s and carries no separate cost.

Waste streams

The fixer and the first wash water are silver-bearing and are collected, into the labelled container Part II’s chemical waste and silver waste sets up. So is any emulsion left in a tray, a failed coat scraped off a plate, and the water used to rinse the coating rod and the beakers, because all of it carries silver halide.

Used developer and used stop carry no silver. Dried, fixed and washed test sheets are ordinary paper. Local regulation governs the disposal of every one of these streams, and it differs between authorities; find out what yours accepts before the session rather than after it.

Alternative route

No silver nitrate, now or later. Everything in Part V except the five makes can be done on bought emulsion, and this page is the entry to that route. You keep the coating, the drying, the step-wedge sensitometry, the defect work, the plates for Part VI and the hand-coated portfolio print; you lose the makes and the batch-to-batch chemistry. Where a later project asks you to change a variable in the emulsion, change one in the coating instead — weight, temperature, substrate, method — and the experimental structure survives intact.

No plate glass. Coat the paper work only, and take the plates in Part VI on a bought material.

No controllable lamp for the step wedge. A window with a fixed aperture and a fixed time works, provided the light is steady and you record the conditions; the test is comparative and does not need an absolute source. It does need the same source every time, which is why an overcast sky is better than a sunny one.

Preparation

1. Read the maker’s leaflet and fill in the seven figures above. Nothing else in this session can be planned until they exist.
2. Cut the paper to a size your trays take, and cut two or three strips of the same paper for the test coats. Do this in daylight, before anything is dark.
3. Dress the plate edges with the diamond pad and wash the plates: detergent, hot water, a final rinse in distilled water, and rack them to drain in the drying box. Do not touch the faces.
4. Rehearse the stroke with plain gelatin. Melt 10 g in 200 mL of water and coat one strip of paper and one plate, in the light, with the same rod, at the same temperature, on the levelled bed. This is free, it is the only chance you get to watch what you are doing, and it is where you discover that the pour is heavier than you expected.
5. Set the trays out in the order developer, stop, fixer, wash, and mix all three to the maker’s working strength. Bring them to 20 °C.
6. Fix the lamp at a measured distance above the bed where you will make the step-wedge exposure, and write the distance down.
7. Run the safelight check once more with a sheet of the commercial paper, because the room is now full of apparatus that was not there when you tested it.

📓Open the batch record before you open the bottle

The record starts now and it opens with a formula version code. Use the scheme the rest of the part uses: an identifier for the emulsion, a two-digit batch number, and a letter for the coating variant — so LE-01a is the bought liquid emulsion, batch 1, coating variant a. When you coat the same bottle a second time at a different temperature, that is LE-01b, not a note in the margin.

The point of a version code is that it survives being wrong. Six months from now the only thing that will make a sheet in a box interpretable is a code on its back that leads to a page in a notebook.

Procedure

The coating run, in the order it must happen

1. 1Meltin the bath at the maker's temperature; never on direct heat; do not stir, swirl
2. 2Temperthermometer in the emulsion, not the bath; let the reading settle before pouring
3. 3Pour and spreadmeasured volume from the syringe; one stroke of the rod, no second pass
4. 4Setonto the chilled slab within seconds, before the layer can creep
5. 5Dryin the box, door shut, slowly; three hours before a sheet is moved
6. 6Hardenonly if the maker supplies one and says to; the default is no hardener
7. 7Storelightproof, flat, labelled with the version code

Stage 1 — Melt (20 minutes)

Under the safelight, stand the closed bottle in the bath and bring it to the maker’s melting temperature. Two rules matter more than the number.

Never melt on direct heat. A gelatin emulsion scorched against a hot surface is finished: the gelatin degrades, the layer will not set properly, and the damage is invisible until the sheet dries. The bath exists to make direct heat impossible.

Do not stir. Swirl. Every bubble you beat into a warm emulsion is a hole in the finished coat, and gelatin at coating viscosity holds bubbles for a long time. Swirl the vessel gently, let it stand two or three minutes, and look across the surface in raking light before you pour. If foam has formed, wait: it will thin from the middle outwards, and the last bubbles to go are at the wall.

Now move the thermometer into the emulsion and let it settle. The bath’s temperature is not the emulsion’s temperature, and the gap is largest in the first ten minutes, which is precisely when impatience says pour.

Stage 2 — Coat paper with the rod (25 minutes)

Wet the bed glass, lay the plastic wrap, wet the wrap. Soak a sheet of the watercolour paper in a tray of water, lift it, squeegee it down onto the wrap and work the air out from the middle to the edges. The paper is coated wet: that is the tested method for this paper, and it is why the sheet stays flat instead of buckling while the emulsion is on it.

Draw the measured volume into the syringe. Pour it in a line across the top of the sheet, set the rod behind the pool, and draw it down the sheet in one steady pass, letting the surplus run off the far edge onto the glass. Do not stop, do not go back, do not re-pour. A stroke that stops leaves a line, because the layer has begun to set where the rod paused; a second pass drags a skin that has already formed.

Coat three sheets, changing exactly one thing between them and writing down which:

• sheet a: the maker’s coating rate, at the maker’s temperature — the reference;
• sheet b: about half that volume;
• sheet c: about twice that volume.

That is a coating-weight series, and it is the most useful three sheets you will coat all year.

Stage 3 — Coat paper with a brush (10 minutes)

Take a fourth sheet and coat it with the hake brush: flood it, then work in one direction, then once lightly across. Then a fifth with the foam brush and no attempt to hide the marks. Both will be less even than the rod, and one of them may be more beautiful. Brush coating is not the poor relation of rod coating; it is a different intention, and the visible edge is the signature people buy hand-coated prints for.

Stage 4 — Coat glass by pour and tilt (15 minutes)

Arrange the two plates on the wet wrap, with taped card dam bars around them, leaving a small gap between plate and bar. Pour the measured volume onto the centre of the first plate, tilt it to each of the four corners in turn, and drain the surplus off one corner — or, better for evenness, spread the pool with the back of a round-bowled spoon and chase bubbles out to the edges.

Do not rework a plate. The layer sets fast on glass, which is cold and does not absorb, and a second pass leaves a mark that no amount of care afterwards will remove. Move the plate to the chilled slab as soon as it is covered, and leave it alone.

✕Two temperature faults, and what each one looks like on the dried sheet

Too warm. The emulsion behaves, in the practitioner’s phrase, like cooking oil poured into a very hot pan: it spreads too readily, sinks into the paper, and dries to irregular islands of low density with a matt sheen surrounded by deep black. On thin paper you will find black spots on the back of the sheet where it soaked through. The saving grace is that it is visible under the safelight while the sheet is wet, so look before you coat the next one.

Too cool. A thick, viscous, uneven coat with a stubborn curl that will not flatten under contact printing glass, which costs you sharpness wherever the negative loses contact. If the emulsion is hardened as well, thick patches fix incompletely and darken later in room light.

Both are read from the thermometer in the emulsion. Neither is read from the bath.

Stage 5 — Set, dry and harden (drying is overnight)

Slide each coated sheet onto the chilled slab for a minute, then hang or lay it in the drying box, door shut. Give the sheets three hours before you move them and, if the room is cool and damp, overnight. Moving a sheet before the layer has stopped being able to flow leaves a flow mark that is permanent.

Hardening happens only if the maker supplies a hardener and says to use it, at the maker’s dose and at the maker’s point of addition. This part’s default, which the coating lesson argues in full, is no hardener in the emulsion at all: an unhardened layer tones and spots more readily, and — the point that decides it for a beginner — processing chemistry still penetrates the thick, uneven patches that hand coating inevitably produces, where a hardened thick patch fixes incompletely and bronzes months later. The cost is that the wet layer is tender: handle it by the corners, never squeegee it, do not process it warm, and do not give it long soaks.

If a coat frills at the edges, bubbles or lifts during processing, the remedy is a bath rather than an additive: an acid hardening fixer. The published reference formulas are Kodak’s F-5 and the F-53 stock used in F-54, and the hardening agent in both is potassium alum — which is the one candidate in this whole area with no GHS hazard classification at all, forty-three of forty-three notifiers reporting that it does not meet the criteria. Mixing F-5 from raw chemicals involves glacial acetic acid and belongs to Part XI at that part’s own classification; for today, a bought acid hardening fixer at the maker’s dilution does the same job. Kodak’s own warning travels with it: prolonged immersion at high temperature is harmful.

Stage 6 — The step-wedge test (25 minutes)

This is the instrument, and it is worth setting up carefully once.

Lay a dry coated sheet on the bed under the fixed lamp, emulsion up. Put the step wedge on it, emulsion to emulsion, and weight it flat under clean glass. Expose for a measured time at the measured distance. Develop for a fixed time — two minutes is a reasonable starting point for a hand-coated paper — stop, fix in two baths, and wash.

Fixed lamp, fixed distance, fixed time. All three go in the record, and none of them changes for the rest of the part. The wedge measures materials against each other, not against an absolute standard, so its whole value lies in nothing else moving.

Reading a step-wedge print: the three things to find and the arithmetic that follows

1. 1Base white — the unexposed, developed and fixed paper; everything is read against this
2. 2Threshold step — the first step distinguishable from base white at normal viewing distance
3. 3First maximum-black step — the first step indistinguishable from its neighbour to the right
4. 4Exposure scale — steps between 2 and 3; multiply by 0.15 for log exposure, halve for stops

Drawn as an arrangement of the reading, not as a measured print. The step increment of 0.15 is the wedge maker's published figure; every other number on your strip is yours.

Stage 7 — Print the negative, twice (20 minutes)

Contact print your negative onto one of the coated sheets and onto a sheet of the commercial paper, processing both identically. Do the exposures by test strip rather than by guess; the coated sheet’s speed is unknown and is very likely to be far from the bought paper’s.

Two prints of one negative, side by side, is the comparison the whole session exists to make. Look at the maximum black, at the separation in the light tones, at the surface, at the colour of the image and at the paper white showing through it. Write down four differences before you form an opinion about any of them.

Expected observations

• The wet coat looks alarming. A freshly coated sheet is translucent, uneven-looking and much paler than the print it will make. Judge nothing until it is dry.
• The half-volume sheet dries fastest and looks best, and then fails to reach a black.
• The double-volume sheet curls as it dries, and may still be tacky the next morning.
• The rod-coated sheets have a faint border where the surplus ran off, and a narrow uncoated margin where the rod’s plugs rode.
• The plates set almost at once on the chilled slab and are visibly more even than the paper, because glass is flat and paper is not.
• The step-wedge strip from the reference sheet shows a run of distinguishable tones somewhere in the middle of the wedge, with base white at one end and a black plateau at the other. If it shows only white, the exposure was too short; only black, too long. Re-expose rather than re-coating.
• The commercial print reaches a deeper black than the hand-coated one, in most first attempts. That difference has two candidate causes, coating weight and the absence of a baryta layer, and the coating-weight series is what separates them.

What is happening chemically

Very little, and that is the point of coating a bought emulsion: the chemistry was done in a factory, and what happens on your bench is mostly physics.

Melting and setting are not a reaction. Gelatin in the sol state is a mass of random coils; on cooling they take up regular patterns held by hydrogen bonds, which is why a set layer melts again when it is warmed. Duffin reports Janus’s evidence that the guanidino groups of arginine dominate the process: destroy them and the gelatin no longer gels at all. A hydrogen bond carries less energy than a covalent one, so a moderate temperature rise breaks it — and that single fact explains why coating temperature is critical, why the chilled slab works, and why a hardener, which makes covalent or ionic links instead, changes the layer permanently.

Drying is a collapse. About 85 per cent of the freshly coated layer is water. Drying takes it to about 5 per cent, because gelatin holds on to 10 to 12 per cent of its own weight of water and will not give it up. A layer 300 micrometres thick when wet may finish at 20. Everything about the dried material — how much silver halide sits under a square millimetre, how far a developer has to diffuse, how the surface takes light — follows from that collapse.

Exposure and development are Part IV’s chemistry, unchanged, and the one place the coating touches them is thickness. Fixing has to reach the bottom of the layer and dissolve the silver halide there, and a thick layer is a longer diffusion path:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

What the fixer must reach, at the bottom of the layer as well as the top

That is why a heavy hand-coated sheet needs a longer fix than a machine-made paper, why a hardened thick patch is the classic incomplete-fixing failure, and why two-bath fixing is the sensible default for a material whose thickness you are still learning to control.

🧮Turning millilitres into a coating weight you can compare

Coating weight is the number this part turns on, and it is arithmetic rather than judgement. The formula below is the coating lesson’s, restated here because this is the page where you stand at a bench with a syringe in your hand.

Measure the volume you laid down — draw it from the syringe, and if you want to be exact, weigh the vessel before and after. Take the silver halide concentration from the maker’s leaflet if it gives one; if it does not, you can still compare your own sheets with each other, because the concentration is the same in all of them.

An 11 × 15 inch sheet is 165 in². One square inch is 6.4516 cm², so the sheet is 1,064 cm², which is 10.6 square decimetres.

coating weight (mg/dm²) = volume laid down (mL) × silver halide concentration (g/L) × 100 ÷ area (dm²)

Coating weight

The volume is in millilitres, the concentration in grams of silver halide per litre, the area in square decimetres, and the factor of 100 turns grams into milligrams and litres into millilitres at once. Baker’s target for one class of material gives you something to aim at: chloride and chlorobromide plates should carry not more than 25 to 40 mg of silver halide per square decimetre, which is 2.5 to 4.0 g per square metre. His warning is the more useful half, because it names the mistake a first-time maker actually makes: it is easy to mistake the effect of too-thin coating for a poor maximum black, and to go back and change the emulsion when the coating was the problem.

Data to record

One batch-record entry per coated sheet, written as you go.

Field	Example
Version code	LE-01a
Date, session, room temperature	2026-09-04, evening, 19 °C
Emulsion: maker, product, batch or expiry	as printed on the bottle
The maker’s seven figures	melting temp, coating temp, rate, safelight, drying, hardener, storage
Melt: bath temperature, emulsion temperature, time to reach it	46 °C bath, 42 °C emulsion, 18 min
Substrate	90 lb hot-press watercolour, wet-coated
Coating method and tool	rod, 9 in, ends plugged
Volume laid down, and sheet area	52 mL over 10.6 dm²
Computed coating weight	mg/dm², with the concentration used
Emulsion temperature at the moment of coating	41.5 °C
Setting: slab temperature, time on the slab	6 °C, 60 s
Drying: box temperature and humidity, time	22 °C, 55 %, 14 h
Hardener: supplied, dose, point of addition, or none	none
Step-wedge exposure: lamp, distance, time	40 W tungsten, 900 mm, 30 s
Development: developer, dilution, temperature, time	working strength, 20 °C, 2 min
Fix: product, dilution, baths, time in each	1+9 equivalent, two baths, 2 min each
Threshold step, maximum-black step, exposure scale in steps	6, 15, 9
Faults seen, in raking light and after processing	two bubbles, one stroke line at the top
One change for the next sheet	drop 3 °C and re-coat

Analysis

1. What the coating-weight series says

Put sheets a, b and c side by side under the same light. The half-volume sheet will very likely reach a weaker maximum black and look “thin”; the double-volume sheet may reach a good black and show longer fixing, curl, or a mottled surface. Somewhere in that series is the weight your material and your hand want, and it is a number, not an impression: compute mg/dm² for all three.

Then compare against the commercial print. If your reference sheet is short of the bought paper’s black and the double-volume sheet is not, the difference was coating weight. If the double-volume sheet is still short, look elsewhere — the absence of a baryta layer under a hand-coated sheet means the light that gets through the emulsion is scattered by paper fibres instead of being returned by a white ground, and a hand-coated sheet’s white and its black both differ from a manufactured paper’s for that reason.

2. Speed and contrast from the step count

Both figures come out of the same strip, and both are relative.

Speed. Find the threshold step — the first one distinguishable from base white. On a 21-step wedge at 0.15 density per step, each step is half a stop. If one material’s threshold falls three steps further towards the dense end of the wedge than another’s, everything else held constant, that material is one and a half stops faster.

Contrast. Count the steps from the threshold to the first step that reaches maximum black. That run is the material’s exposure scale. Multiply by 0.15 for the log exposure range; a shorter scale is a higher contrast, because the material uses up its whole range of tones over less exposure. A contact paper with an eight-step scale and an enlarging paper with a fourteen-step scale are doing genuinely different jobs.

!This number is yours and there is no published one to check it against

Not one of the sources this course has read gives a speed or a contrast figure for a hand-coated emulsion — not a step count, not an ISO(P) figure, not a characteristic curve. The practitioner whose recipes this part uses gives exposures under her own lamp, which are not transferable to yours.

So every speed and contrast statement in Part V is the course’s own measurement, or yours, and it is labelled as such wherever it appears. That is not a weakness of the evidence; it is the reason the step wedge is set up at a fixed lamp and a fixed distance on this page rather than on a later one. Your readings are the standard the rest of the part is measured against, which means today’s carelessness is next month’s confusion.

3. The comparison print, judged rather than scored

Write four sentences: one about maximum black, one about the separation in the light tones, one about image colour, and one about surface. Then a fifth sentence saying which of the four differences you would most want to close, and what you would change to close it. That fifth sentence is the beginning of the design thinking Projects 1 to 5 formalise.

Troubleshooting

What you see	Likely cause	What to do
Round clear spots in the dried coat	Bubbles beaten in during melting, or trapped under the rod	Swirl, never stir; let the melt stand; look across the surface in raking light before pouring
A hard line straight across the sheet	The rod stopped and restarted	One pass. If you cannot do it in one, the sheet is too big for the rod or the emulsion is too cool
A dark border down both long sides	Emulsion escaped past the rod’s end plugs	Rebuild the plugs so they stand proud of the rod and sit outside the sheet width
Irregular islands of low density with a matt sheen	Emulsion too warm; it soaked into the paper	Drop the coating temperature; check the thermometer is in the emulsion
Black specks on the back of the sheet	Emulsion soaked through	Same cause; also consider heavier paper
Thick, ridged, uneven coat with a stubborn curl	Emulsion too cool and too viscous	Raise the temperature a degree at a time; do not add water to thin it
One corner clears far later in the fixer	The bed was not level, so the layer is wedge-shaped	Re-level with the water puddle; the fixer is the acceptance test
The layer frills at the edges or lifts in the developer	Unhardened gelatin, softened by that developer	Switch developers, or use an acid hardening fixer; the practitioner who found this reports the hardening fixer prevented it completely
The print bronzes or darkens weeks later in the thick patches	Incomplete fixing in a hardened, thick layer	Fix longer, in two baths, and reconsider hardening a coat this heavy at all
The strip is white everywhere, or black everywhere	Step-wedge exposure wrong, not the coat	Change the time, not the sheet. Bracket by factors of four until tones appear
Specks and hairs across every sheet	Dust in the drying box, or on the bed	Compare against the station’s baseline speck count; if it has risen, the box has

Clean-up

Rinse the rod, the syringes, the beakers and the brushes in warm water before the gelatin sets, and collect that rinse water — it carries silver halide. Peel the plastic wrap off the bed and bin it. Wipe the bed and slab with a damp cloth. Wash the trays, empty last, and hang the tongs separately so that the fixer tong cannot be picked up for the developer next time.

Storage

The bottle. Cap it tightly and store it as the maker directs, in the dark and cool. Record how many times you have remelted it: repeated melting is not free, and the leaflet’s shelf life assumes a reasonable number of cycles.

The coated sheets. Dry, flat, interleaved with clean paper, in a lightproof box or a double black bag, each sheet pencilled on the back with its version code. Keep them out of a warm damp cupboard; gelatin at high relative humidity is a growing medium, and a coated sheet is more vulnerable than a processed one.

Keep them. The whole point of coating a known material today is to have it to compare against. When Project 1’s first sheet comes out of the fixer, you want the LE-01a sheet beside it, not a memory of it.

Disposal considerations

Silver leaves this session in three places: the fixer, the first wash water, and any emulsion or coating that never became a print. All three are collected, because the silver in them is the reason they cannot be treated as ordinary drain waste — a used fixer accumulates dissolved silver as it works, which is also why it eventually stops fixing.

Used developer and used stop bath carry no silver. Processed, washed test sheets are paper.

Local regulation governs all of it, and it differs between authorities even within one country: some accept small quantities of bottled, labelled chemical waste at household recycling centres and some do not, and none of them will thank you for mixing streams together. The chemistry is general; the instruction is local. Find out what yours is before the session.

Questions

1. This session coats a bought emulsion rather than one you made. State the experimental reason for that ordering, in terms of how many variables a beginner can interpret at once, and say what would be lost by reversing it.
2. Your bath reads 46 °C and the emulsion in the jar reads 41 °C, still rising. You coat anyway. Predict what the first sheet and the third sheet will look like, and say which of the two temperature faults in this page you have committed.
3. Sheet a took 52 mL over an 11 × 15 inch sheet. Convert that to millilitres per square metre, and then, assuming an emulsion carrying 20 g of silver halide per litre, to milligrams per square decimetre. Compare with Baker’s 25 to 40 mg/dm² and say what you would do next.
4. Two strips are exposed identically under the same wedge. Strip A’s threshold is at step 5 and its first maximum-black step at step 17. Strip B’s threshold is at step 8 and maximum black at step 16. Which material is faster, by how many stops, and which has the higher contrast? Show the arithmetic.
5. A coat lifts off the paper in the developer. Give the remedy this page recommends, and explain why the course’s default is nevertheless not to harden the emulsion.
6. Your hand-coated print will not reach the black the commercial print reaches, even at double coating weight. Name two explanations that are consistent with that observation, and describe an experiment that would separate them.

Further experiments

A temperature series. Coat four sheets from one melt at four temperatures across the maker’s window and a couple of degrees either side, holding volume constant. Dry them together, expose them under the wedge together, and read the exposure scale of each. You are looking for whether coating temperature moves the sensitometry or only the appearance.

Substrate against everything else. Coat the same emulsion at the same weight on hot-press watercolour paper, on a smooth cartridge paper, and on glass. The step wedges will not agree, and the disagreement is the substrate — mostly what it does to the light that gets through the layer.

The dry-down question. Read the maximum black of a wet print and again when it is bone dry, then again after a week. Paper prints dry down, and how much a hand-coated sheet dries down is a number nobody has published for a material like yours.

Coat one sheet badly on purpose. Bubbles, a stopped stroke, a tilted bed, a rewetted plate. Dry it, print it, and put it in the front of the notebook labelled with what you did. This part’s break/fix session on emulsion coating defects asks you to diagnose faults later; the fastest way to learn a fault is to have manufactured it once deliberately.

Recap

You bought the chemistry and supplied the hands, which is the only way to find out which of the two is responsible when something goes wrong. Melting is done in a bath and never on direct heat; the thermometer lives in the emulsion; the stroke is one pass; the slab stops the layer moving; the box dries it slowly and cleanly; and hardening is a decision with costs on both sides rather than a routine.

Two instruments came out of the session. The step wedge, exposed at a fixed lamp, a fixed distance and a fixed time, converts a step count into stops at half a stop per step and gives you speed and contrast as relative numbers. The batch record, opened under a formula version code, is what makes a sheet found in a box next spring mean anything at all. Every project from here on is measured with those two things and against the sheets you coated today.

Check your understanding

Question 1. Why is the emulsion for this session bought rather than made?

• Because home-made emulsions are unreliable
• Because coating and making are separate skills, and attempting both at once leaves a failed sheet with two possible causes and no way to choose between them
• Because a commercial emulsion is the only one that will coat evenly
• Because the safety level would otherwise be higher

Show the answer and why

Answer: Because coating and making are separate skills, and attempting both at once leaves a failed sheet with two possible causes and no way to choose between them

The reasoning is experimental rather than practical. A hand-coated sheet that will not reach a black could be a bad make or a bad coat; with a bought emulsion the make is held constant, so every fault you produce is attributable to technique and therefore diagnosable. The safety level does drop as a side effect, but that is a consequence of the choice and not the reason for it.

Question 2. A step wedge has 21 steps at a density increment of 0.15. Your first coat reaches threshold at step 4; a second coat, exposed identically, reaches threshold at step 10. What can you say?

• The second coat is three stops faster
• The second coat is six stops faster
• The second coat is three stops slower
• Nothing, because thresholds cannot be compared

Show the answer and why

Answer: The second coat is three stops faster

Higher step numbers on the wedge pass less light, so a material that still records a tone through step 10 needed less light than one that stopped at step 4: it is faster. Six steps at 0.15 in density is 0.90 in log exposure, and since 0.30 in log exposure is one stop, that is three stops. The arithmetic is worth doing rather than remembering, because a different wedge has a different increment.

Question 3. Why does a heavy hand-coated layer need a longer fixing time than a machine-made paper?

• Because hand-coated emulsions contain more silver iodide
• Because the fixer has to diffuse to the bottom of a thicker layer and the reaction products have to diffuse back out
• Because the paper base absorbs fixer
• Because hand coating uses a different silver halide

Show the answer and why

Answer: Because the fixer has to diffuse to the bottom of a thicker layer and the reaction products have to diffuse back out

Fixing is a diffusion-limited process: thiosulfate has to reach every crystal and the soluble silver complex has to leave. Hand coating commonly lays down two to three times the commercial coating weight, so the path is longer in both directions. This is also the mechanism behind the classic late failure — a thick, hardened patch that was never fully fixed, discolouring in room light weeks later.

Question 4. The course's default for these emulsions is no hardener at all. What is the main reason given, and what does it cost?

• Hardeners are too hazardous to use at home; it costs nothing
• Unhardened layers dry faster; it costs image colour
• Processing chemistry still penetrates the thick uneven patches hand coating produces, and unhardened emulsion tones and spots more readily; the cost is a tender wet layer that must be handled by the corners and never squeegeed
• Hardeners react with the developer; it costs contrast

Show the answer and why

Answer: Processing chemistry still penetrates the thick uneven patches hand coating produces, and unhardened emulsion tones and spots more readily; the cost is a tender wet layer that must be handled by the corners and never squeegeed

The argument is about what hand coating actually produces. Thick, uneven patches are the beginner's normal condition, and a hardened thick patch is exactly the case that fixes incompletely and bronzes later; an unhardened one processes through. The trade is real and stated rather than hidden: the wet layer is fragile, and the remedy when it frills is a hardening bath at the fixing stage rather than an additive in the emulsion.

Question 5. Why does this page insist that the step-wedge lamp, distance and time never change again for the rest of the part?

• Because the wedge is only calibrated for one exposure
• Because no published speed figure exists for any hand-coated emulsion, so every comparison in Part V is relative and depends on nothing else moving
• Because changing the lamp would fog the safelight test
• Because the developer time depends on the lamp

Show the answer and why

Answer: Because no published speed figure exists for any hand-coated emulsion, so every comparison in Part V is relative and depends on nothing else moving

Not one source this course has read publishes a speed or contrast figure for a hand-coated emulsion. That leaves comparison as the only available method, and a comparison is worth exactly as much as the constancy of everything you did not intend to change. Fixing the lamp, the distance and the time is what converts a step count into evidence.

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