Break/Fix: Emulsion Coating Defects

Twelve coats, each spoiled in a different way, and one habit that gets you from a symptom to a cause without guessing. The habit is worth more than any of the twelve answers, because your own faults will not be these faults.

The situation

A student has coated a batch of eleven sheets and four plates from one melt in a single evening. Six of the sheets are usable. The rest are not, and they are not wrong in the same way: two have a hard line across them a third of the way down, one is covered in small round clear spots, one is grey all over before it was ever exposed, one has an edge that rolled back in the fixer, and one dried with a pattern across it like a watermark. The plates are worse: two have ridges, one has a corner that cleared five minutes after the rest, and one is speckled with black dots that were not there before development.

The station passed its four commissioning tests three weeks ago. Nothing has been rebuilt. The emulsion is the same melt for all fifteen, which is the single most useful fact on the page: anything that differs between sheets from one melt is not the emulsion.

📓These are drawings, and the course says so

The atlas this page feeds is specified to carry only licensed or author-made images, and this course has not yet coated, spoiled and photographed the fault set. So the signatures below are described and drawn from the physics, not reproduced from a real sheet.

That is a real limitation, and it cuts one way: a description can tell you where to look and what shape to expect, and it cannot show you the texture of the real thing. The remedy is the practical at the end — make the faults yourself, keep the sheets, and photograph them. Your own fault set will teach you more than anybody’s illustration, and these entries are written so that your photographs drop straight into them.

The symptoms

Six families, and naming the family is most of the diagnosis. Every one of the twelve faults below belongs to exactly one of them.

A line, a ridge or a streak — something with a direction. Direction almost always means the tool or the stroke, because emulsion left to itself has no preferred direction.

A spot, a hole or a crater — something local and roughly circular. Local means a particle, a bubble or a patch of contamination, and the three are separable.

A gradient — one region thicker or thinner than another, with no edge to it. Gradients mean level, temperature or viscosity.

An edge fault — frilling, lifting, a heavy dark border. Edges are where adhesion and surface tension both do their worst.

A texture — mottle, reticulation, a watermark pattern. Texture is nearly always drying.

A uniform change — everything grey, everything flat, everything weak, with no structure at all. Uniform means the emulsion, the exposure or the processing, and not the coating. This is the family beginners get wrong most often, because it is the one that does not look like a coating fault and frequently is one.

!Two things in the sixth family that are not defects at all

High contrast and low speed. A hand-made chloride paper is contrasty and slow because that is what a chloride emulsion is. Neither is a coating fault, neither is fixed by changing the coating, and both belong in the batch record as properties rather than in this page as problems.

A thin coat mistaken for a poor maximum black. Baker names it directly: it is easy to mistake the effect of too-thin coating for poor maximum black — “low vigor” in the older language — and to go back and change the emulsion when the coating was the problem. Weigh the pot before and after, compute the coating weight, and coat one sheet at twice it before you change anything in the make.

Evidence to collect

Before any diagnosis, four things, and they take ten minutes.

1. Read the sheet in raking light, dry, with a torch held almost parallel to the surface, before it is ever wet. Ridges, stroke lines, dust and craters all show in raking light and several of them disappear once the sheet is processed.

2. Read it again with a loupe, wet from the fixer and dry afterwards. Wall’s instruction about pepper is exact and worth obeying: the specks are sometimes so fine that they cannot be seen by ordinary visual examination, so test plates should always be examined with an eyepiece, particularly in the areas protected from light.

3. Watch the plate clear in the fixer, and time it. Which corner clears last is a thickness map you get for free. A plate that clears evenly was coated evenly, whatever else is wrong with it.

4. Process an unexposed strip alongside. One strip developed and fixed without ever being exposed, and one fixed without development. The difference between them is your base fog, and it separates the whole sixth family from everything else in a single operation.

Then look at the batch as a batch, and ask the question that halves the search: does the fault appear on every sheet, or on some? Every sheet from one melt points at the melt, the bench or the room. Some sheets point at the stroke, the sequence, or the moment.

Hints

In order, from the least to the most specific. Stop as soon as one of them lands.

1. Direction or no direction? A fault with an axis is the tool. A fault without one is not.
2. Does it appear before processing, in raking light? If yes, it is a coating fault. If it only appears after development, it is emulsion, exposure or processing.
3. Is it on every sheet, or on some? Every, and it is upstream of the stroke. Some, and it is the stroke or the moment.
4. Is it at the edges or in the field? Edges are adhesion and surface tension; the field is the tool, the bed or the air.
5. Does the fault have structure, or is the whole sheet the same? Structure means something local. Uniform means the material.
6. Is it darker or lighter than it should be, and where? A dark fault has extra developed silver — more coating weight, more exposure or more fog. A light fault has less of one of those, or nothing at all, which means the emulsion was not there.

The diagnosis

Twelve entries. Each carries the same five fields, so that they can be lifted into the atlas without rewriting: what you see, the physical or chemical cause, the discriminating test, the fix, and the prevention.

The stroke, and the three marks it leaves when it goes wrong

1. 1The stroke that works — one continuous pass, even speed, the bead held in the angle of the rod
2. 2Stop and restart — a hard line straight across, where a partly set edge met fresh emulsion
3. 3Too fast — the bead breaks and skips: alternating heavy and light bands
4. 4Too cool, or a second pass — ridges parallel to the travel, because the layer is setting as it is laid

Drawn from the mechanism rather than photographed. Every one of the three failures has an axis, and that is the whole of the first diagnostic question: a fault with a direction is the tool, and a fault without one is not.

1. Comets and streaks from a particle

What you see. A short dark or light head with a tail trailing behind it in the direction of the stroke, sometimes several across one sheet, always parallel to each other.

Cause. A solid particle — undissolved gelatin, a grain of halide, a fibre — caught under the rod. The rod rides over it, so the layer behind it is thinner, and the particle ploughs a furrow the length of the stroke.

Discriminating test. Look at the head with a loupe in raking light before processing. A comet has a physical particle at its head; a stroke fault does not. If the particle is a bromide fragment it will also have developed out black.

Fix. Strain the melt through a fine mesh into the coating jug. Discard the sheet: a comet cannot be coated over.

Prevention. Dissolve the halide completely before the gelatin goes in — ground to a powder, and held up to the light to check — and strain every melt. Undissolved potassium bromide is specifically named as the cause of black slugs or sunspots.

2. The stop-and-restart line

What you see. A hard line straight across the sheet, perpendicular to the stroke, with the coating slightly different on each side of it.

Cause. The stroke paused. Gelatin at coating temperature begins to set in seconds, so the bead’s leading edge gelled and the restarted stroke laid fresh emulsion against a partly set edge instead of merging with it. Duffin’s mechanism for setting is the formation of hydrogen bonds between gelatin chains, and it starts as soon as the temperature drops.

Discriminating test. It is straight, it is perpendicular to the stroke, and it is in the same place on every sheet where the stroke hesitated. A gradient has no edge; this has one you can put a fingernail on.

Fix. None on that sheet.

Prevention. One pass, at an even speed, decided before the rod touches the sheet. Do not rework a plate. And coat two sheets you intend to throw away at the start of every session: coating is a motor skill and the first sheet is always the worst.

3. Repellency spots and craters

What you see. Small round areas where the emulsion pulled away, leaving a bare or nearly bare patch with a raised rim.

Cause. Grease or a hydrophobic contaminant lowers the surface energy locally, and the wet layer, which wants to minimise its surface area, retreats from it. Fingerprints, silicone, aerosol overspray and hand cream all do it.

Discriminating test. A crater has a rim — the emulsion piled up where it retreated to — and the bare centre is clean. An air bell leaves a clean circle with no rim. Look in raking light before processing.

Fix. Clean and recoat. On glass, a hot 2 per cent caustic soda clean removes what detergent will not.

Prevention. Cotton gloves on clean glass, nitrile on wet work, and no aerosols in the room. A wetting agent helps and this course cannot tell you how much: the source that supplies Project 1’s formula names a wetting agent in its tools list and gives no dose anywhere. Add one drop to a batch, record it as your own variable, and find your own figure.

4. Bubbles, air bells and pinholes

What you see. Clean round holes, or clean round clear spots after processing, with no rim.

Cause. Air whipped into the melt during stirring, or trapped under the layer as it was poured. A bubble sitting at the surface breaks and leaves a hole; one trapped underneath holds the emulsion off the support and leaves a clear disc.

Discriminating test. No rim, and a size distribution rather than a single size. If they are all the same size and evenly spread, suspect the support rather than the melt.

Fix. Let the melt stand covered in the bath for ten minutes before coating and the bubbles will rise and break. Chase the ones on the sheet out to the edge during the stroke, where they can burst against the dam bar.

Prevention. Stir, do not whisk, once the emulsion is made; pour down the inside of the vessel rather than from a height; and do not shake a bottle of emulsion to remix it.

5. Uneven thickness: the gradient

What you see. One region consistently heavier than another, with no edge between them — a heavier bottom, a heavier corner, or a wedge.

Cause. Three, and they are separable. The bed is not level, so the wet layer flows before it sets. The emulsion is too warm, so it stays fluid long enough to flow. Or the viscosity is wrong — too little gelatin, and the layer is thin and will not hold its thickness.

Discriminating test. The fixing tray. A plate clears last where it is thickest, so the pattern of clearing is a thickness map. Then ask whether the direction is the same on every sheet — which means the bed — or different on each, which means temperature or the stroke.

Fix. Re-run the puddle test on the bed. Coat at the top of your temperature window rather than above it. For a thin, streaky coat specifically, the published remedy is to thicken the melt by adding 2 to 3 g of gelatin at a time and re-testing.

Prevention. Level with a water puddle, not a spirit level; keep the thermometer in the emulsion, not in the bath; and record the coating temperature in the batch record every time.

6. Frilling and lift-off at the edges

What you see. The layer lifts, rolls back or detaches at the edges during processing — sometimes bubbling first, then frilling, then coming away in one piece.

Cause. Adhesion failure. Wet gelatin swells in every direction and the support does not, so the layer is in compression against a rigid surface; where the bond is weaker than that stress, the edge goes first because it has a free face to move into. Warm baths, long soaks and vigorous handling all make it worse.

Discriminating test. Did the plate clear evenly? If yes, thickness is ruled out and the fault is the interface: contamination, no sub, or a bath too warm.

Fix, and this one is sourced with a case history. A working practitioner records developing a chloride paper in Selectol Soft after years of never having an emulsion lift: it bubbled, then frilled at the edges, then lifted clean off — and a hardening fixer prevented it completely. Her rule follows from that: if your emulsion lifts, switch developers or use a hardening protocol. She gives the same remedy for plates, as a hardening fixer in the second fixing tray.

Prevention. Clean the support properly; keep every bath within a couple of degrees of the others; handle wet sheets by the corners and never squeegee them; and reach for an acid hardening fixer — Kodak’s F-5, or the F-53 stock made up as F-54a — before you reach for a hardener in the emulsion.

⚗Why the fix is a hardening fixer and not a hardener in the emulsion

The course’s default is no hardener in the emulsion, and the reasoning is the same on both sides of the trade.

Unhardened emulsion tones more readily, spots more readily, and — the point that decides it for beginners — lets processing chemistry penetrate the thick, uneven patches that hand coating inevitably produces. Hardened emulsion resists penetration, especially by the fixer, so a thick patch fixes incompletely and darkens later, in room light, after the print is finished and put away. Duffin says the same thing from the manufacturer’s side: excessive hardening of the layers interferes with developer penetration.

So the layer is hardened in the bath, where the dose is a soak time you can stop, rather than in the emulsion, where it is a decision you made an hour before you knew you needed it. If you do put chrome alum in an emulsion, the two things that go wrong are both in the same sentence of the literature: it must go in slowly, into moving emulsion, because a local excess coagulates the batch, and it works only near a coating pH of about 6.0.

7. Mottle and drying marks

What you see. A soft irregular texture across the sheet, like a watermark, visible in reflected light and often only after drying.

Cause. Uneven drying. The layer goes down at about 85 per cent water and finishes at about 5 per cent, and gelatin holds on to 10 to 12 per cent of its own weight and will not release it. Where a part of the sheet dries faster — a draught, a warm spot, contact with the rack — the gelatin sets to a different density and the difference is permanent.

Discriminating test. It appears on drying, not on coating: photograph or note the sheet in raking light while wet, and again dry. If it was not there wet, it is drying.

Fix. None on that sheet.

Prevention. Dry with the box shut, at an even temperature, with airflow that does not blow directly on the sheets — the published arrangement is a drying space near 24 °C with fabric taped over the air vent so that air moves and dust does not.

And one cause that is not yours. A practitioner records a lot of a named paper whose sizing recipe had been changed by its maker, which produced blistering during processing and took weeks and half a dozen emulsion batches to diagnose, and a later lot of a different paper that gave mottled fading late in the wash. Keep the paper brand and lot in the batch record. A change you did not make is still a change, and without the lot number it is invisible.

8. Dust

What you see. Specks, hairs and fibres embedded in the layer, appearing as small clear or dark points.

Cause. The room. Nothing else.

Discriminating test. The plain gelatin test coat from the station’s commissioning: coat a sheet with gelatin alone, dry it in the box, and count what you find in a marked 100 mm square under raking light. That count is your room’s baseline, and comparing today’s silver coat against it tells you whether the specks came from the emulsion or the air.

Fix. Spotting, later, on the finished print.

Prevention. Baker is blunt about the stakes and they are worse than cosmetic: dust and bacteria are the plate maker’s enemies, mould spores travel on dust, and a speck on gelatin-coated glass may deposit a spore that finds an ideal medium — especially where plates dry slowly in a warm humid atmosphere. Shut the box, damp-wipe rather than sweep, and do not dry slowly.

9. Black specks that were not there before development

What you see. Fine black dots scattered through the image and, crucially, through the unexposed areas too. Sometimes invisible to the naked eye.

Cause. Two, and they are different faults with the same appearance. Pepper is a characteristic coarse grain distributed through the emulsion and reduced to the metallic state without any exposure to light. Slugs are undissolved halide that developed out.

Discriminating test. The loupe, on an area that was never exposed. And ask whether every sheet from the melt has them — which points at the melt — or only some, which points at the coating.

Fix. None on that sheet.

Prevention, and there are three published ones for pepper alone, which is a good sign that the fault is real and the mechanism is not settled. Two are from 1929: reduce the quantity of water during the mix, or add the silver nitrate dry to the salted gelatin. The third is modern practice: warm the silver solution immediately before precipitation. For slugs, grind the halide to a powder and dissolve it completely.

10. A layer that will not set

What you see. The coated sheet is still fluid after twenty minutes on the chilled slab, or sets and then softens again.

Cause. Not enough gelatin for the volume, or too much water. Noodle washing dilutes an emulsion — concentration can fall to nearly half — and a coating solution needs its gelatin not to drop appreciably below 5 per cent.

Discriminating test. Measure the finished volume and compute the gelatin concentration. Do not assume the recipe’s figure: the wash changed it.

Fix. Remelt and add gelatin, in 2 to 3 g steps, testing a small coat between each addition.

Prevention. Put the second gelatin in before the emulsion is set and shredded, which is what every recipe in this part instructs and is exactly why. And measure the finished volume rather than inheriting it from the recipe.

11. Fog: everything grey before it was exposed

What you see. Base density where nothing was exposed. In its mild form it does not look like fog at all: it looks like a general loss of contrast, which is how the safelight literature warns you it presents.

Cause. Four, and the discriminating tests separate them cleanly.

Candidate	The test that isolates it
Safelight	The four-step fog test at 0, 1, 2 and 4 minutes, run on your own emulsion and not on a commercial paper
Over-ripening, or a make run too warm	Fog rises throughout digestion while speed peaks and falls; compare an unexposed strip from this batch against one from a shorter ripening
Keeping	Develop an unexposed strip from a sheet coated today and one coated three weeks ago, side by side
Contamination	Was the same vessel used for fixer or for a developer? Rust from cheap stainless in contact with silver nitrate is specifically warned against

Fix. The bromide restrainer, at the published dose, in the finals of the next batch.

Prevention. Re-run the safelight test whenever the material changes — an orthochromatic emulsion needs a different filter class from a chloride paper, and most emulsions retain some sensitivity to the colours their recommended filter transmits, so exposure under it is minimised rather than treated as free. The one published acceptance number the course can offer is Baker’s: fog on a trial coating on glass should not exceed 0.02 density.

12. Incomplete fixing that shows up weeks later

What you see. Nothing, at the time. Then, weeks after the print was finished and put away, the thickest areas of the layer darken in room light.

Cause. Fixer never reached the bottom of a thick patch, so undeveloped silver halide stayed in the layer and slowly printed out. Hardening makes it worse, which is the trap: a hardened thick patch is exactly the case in which the fixer cannot penetrate.

Discriminating test. The fixing time, against the thickness. And a clearing test: put an offcut in fresh fixer and time how long it takes to go clear, then fix for at least twice that clearing time.

Fix. Refix and rewash while the print is still recent; after that, the damage is done.

Prevention. Coat evenly; fix for twice the clearing time; use a two-bath fix; and do not harden the emulsion when the real problem is that the coat is thick and uneven. The hardening fixer is a remedy for frilling, and it is applied in the bath, where the time is under your control.

The fix

One line each, so the table can be used at the bench.

Fault	Confirming test	Fix	Prevention
Comet	A particle at the head, in raking light	Strain the melt; discard the sheet	Dissolve halide fully; strain every melt
Stop-and-restart line	Straight, perpendicular to the stroke, at the pause	None on that sheet	One pass, even speed, two sacrificial sheets first
Crater	Has a rim; centre bare and clean	Clean and recoat	Cotton gloves on clean glass, no aerosols, a recorded drop of wetting agent
Air bell	Clean circle, no rim, mixed sizes	Stand the melt ten minutes; chase bubbles out	Stir rather than whisk; pour down the vessel wall
Gradient	Fixing tray: last corner to clear is thickest	Re-level; coat cooler; thicken with 2–3 g gelatin	Puddle test, thermometer in the emulsion, temperature recorded
Frilling	Cleared evenly, so thickness is ruled out	Hardening fixer; switch developer	Clean support, matched bath temperatures, handle by corners
Mottle	Absent wet, present dry	None on that sheet	Even drying near 24 °C, box shut, no direct draught
Dust	Compare against the plain gelatin baseline count	Spot the print	Box shut, damp wipe, do not dry slowly
Pepper and slugs	Loupe, on unexposed areas	None on that sheet	Warm the silver; less water in the mix; grind the halide
Will not set	Compute gelatin per finished volume	Remelt, add gelatin in 2–3 g steps	Second gelatin before the set; measure the finished volume
Fog	Four-step safelight test on your own material	Bromide restrainer in the next finals	Re-test whenever the material changes
Late darkening	Clearing time against thickness	Refix and rewash promptly	Fix twice the clearing time; two-bath fix; do not harden a thick coat

Alternative route

The diagnostic work on this page needs no darkroom: the twelve entries, the six families and the hint sequence can be worked through on somebody else’s sheets, or on the faults already in your own batch records from Projects 1 to 5.

Only the optional practical needs one, and it can be done entirely on a bought liquid emulsion at Level A, with no silver nitrate handling at all.

Optional practical: make one on purpose, then coat it out

Reproduce one fault deliberately with the commercial emulsion from the practice run, then correct it and coat the same sheet size again. Four are safe, quick and unmistakable:

• The stop-and-restart line. Stop the rod half way down and count to five.
• The crater. Touch the paper with a bare finger before coating.
• The gradient. Prop one edge of the bed with a coin.
• The air bell. Whisk the melt for ten seconds before pouring.

Photograph both sheets — the fault and the correction — in raking light, side by side, with a scale in frame. That pair is your first atlas entry, and it is worth more than the drawing above because it is your material, your room and your hand.

⚠Level A, and why

Nothing on this page introduces a new substance. The optional practical uses a bought liquid emulsion and the same working-strength developer, stop and fixer as every other practical page in this part, with the same controls: nitrile gloves, eye protection, one pair of tongs per tray never swapped, and ordinary room ventilation rather than extraction, because none of the three baths generates a vapour that would need it.

No silver nitrate is weighed or handled, which is the difference between Level A here and Level B on the project pages. Used fixer and first wash are silver-bearing and are collected into the labelled container Part II established; local regulation governs the rest and differs between authorities even within one country.

What to record

Two things, and the second is the one that compounds.

The corrected batch record. Do not erase a wrong entry. Strike it through, write the right one beside it, and add a sentence saying which field was wrong and how you know. A record that shows its own corrections is the only kind that teaches anything, and a season of them is a map of your own characteristic mistakes.

An atlas entry, per fault. Each of the twelve above is written in the form the diagnostic atlas uses — one defect, one set of causes, one confirming test, one corrective action, one prevention — so an entry drafted from this page looks like this:

---
title: Stop-and-restart line in a hand-coated emulsion
pageType: reference
refKind: defect
defect:
  id: stop-and-restart-line
  name: Stop-and-restart line
  aka: [stroke line, restart ridge]
  appearsOn: [print, plate]
  stage: [coating]
  likelyCauses:
    - The coating stroke paused, letting the bead's leading edge begin to gel
    - A second pass made over a layer that had already started to set
---

## What you see
A hard, straight line across the sheet, perpendicular to the stroke, with the
coating slightly different on each side of it and an edge you can feel.

## Likely causes
The stroke stopped and restarted; or the plate was reworked after setting began.

## The chemistry and physics
Gelatin sets by forming hydrogen bonds between chains as the temperature falls,
and it begins within seconds at coating temperature. Fresh emulsion laid against
a partly gelled edge does not merge with it.

## Diagnostic questions
Is it straight, and perpendicular to the stroke? Is it in the same place on every
sheet where the stroke hesitated? Was it visible in raking light before processing?

## Corrective action
None on that sheet.

## Prevention
One continuous pass at an even speed, decided before the rod touches the sheet;
never rework a plate; coat two sacrificial sheets at the start of each session.

Write one for every fault you produce, whether or not it is on this page’s list, and keep the sheet with it. Part XXVIII builds the general diagnostic method on exactly this foundation, and it will want your twelve rather than these.

📓Three faults this page does not cover, and where they go

So that the boundaries are clear. Uneven development, flow marks and streaking in the tray are processing faults and belong to Part IX and Part XXVIII, not to the coating. Fixing and washing faults — staining, residual thiosulfate, fading — belong to Part XI and Part XII, with the single exception of entry 12 above, which is here because the coating is what caused it. And exposure faults belong to the part that owns the exposure.

If your sheet is fogged and streaked, you have two faults, and the commonest way to get a diagnosis wrong is to look for one cause of two independent symptoms.

Check your understanding

Question 1. Fifteen sheets and plates come from one melt in one evening. Six are fine, and the faults on the others differ. What does that single fact establish before you look at any of them?

• That the emulsion is at fault, since it is common to all of them
• That anything differing between sheets is not the emulsion, because the emulsion was the same for all fifteen — so the search narrows immediately to the stroke, the moment, the support or the room
• That the safelight is at fault
• Nothing useful

Show the answer and why

Answer: That anything differing between sheets is not the emulsion, because the emulsion was the same for all fifteen — so the search narrows immediately to the stroke, the moment, the support or the room

This is the most valuable question on the page and it costs nothing to ask. A single melt is a perfect control: whatever it did, it did equally to every sheet. So a fault that appears on some sheets and not others cannot be the melt, and a fault that appears on all of them cannot be distinguished from the melt by this test alone and needs a second one. The corollary is worth building into a working habit: coat every batch on more than one sheet, and record which sheet was coated in which order, because a fault that appears on sheets four to six and not one to three is telling you the emulsion was cooling as you worked.

Question 2. A round bare patch and a round clear spot look similar on a processed sheet. How do you tell a repellency crater from an air bell?

• By size: craters are larger
• By the rim. A crater has emulsion piled up around its edge, because the wet layer retreated from a low-energy contaminant; an air bell leaves a clean circle with no rim. Read it in raking light before processing
• By counting them
• They cannot be told apart

Show the answer and why

Answer: By the rim. A crater has emulsion piled up around its edge, because the wet layer retreated from a low-energy contaminant; an air bell leaves a clean circle with no rim. Read it in raking light before processing

The mechanisms are different and the mechanisms leave different marks. A crater is a wetting failure: grease lowers the surface energy locally and the layer, minimising its own surface, pulls back and heaps at the boundary — so there is a rim, and the bare centre is clean. A bubble is a physical inclusion: it simply holds the emulsion off the support, and when it goes there is a hole with nothing piled around it. Note the second discriminator in the entry, which is worth having when raking light is ambiguous: bubbles come in a range of sizes because the whisking that made them was not uniform, while a set of identically sized, evenly spread spots points at the support rather than the melt.

Question 3. An emulsion frills and lifts in one developer and not in another. What is the sourced remedy, and what does the course specifically not recommend?

• Add chrome alum to the emulsion; the course does not recommend changing developer
• A hardening fixer, which a practitioner records as preventing it completely, or switching developers; the course specifically does not recommend adding a hardener to the emulsion, because a hardened thick patch fixes incompletely and darkens later in room light
• Reduce the coating weight; the course does not recommend hardening at all
• Nothing can be done

Show the answer and why

Answer: A hardening fixer, which a practitioner records as preventing it completely, or switching developers; the course specifically does not recommend adding a hardener to the emulsion, because a hardened thick patch fixes incompletely and darkens later in room light

The case history is specific: a practitioner who had never had a paper emulsion lift in years of making them had one bubble, frill and lift clean off in Selectol Soft, and a hardening fixer prevented it completely. Her rule follows — if your emulsion lifts, switch developers or use a hardening protocol — and she gives the same remedy for plates. The reason for hardening in the bath rather than in the emulsion is where the two halves of the trade meet: hardening resists penetration, especially by the fixer, so exactly the thick uneven patches that hand coating produces are the ones that fix incompletely and print out later. A soak time can be stopped; a hardener you put in an hour ago cannot.

Question 4. A print is flat and grey with a weak maximum black. Which family does that belong to, and what should you rule out first?

• A coating fault; check the level of the bed
• The uniform family, which means the emulsion, the exposure or the processing — but rule out too thin a coat first, because it is easy to mistake thin coating for poor maximum black and change the emulsion when the coating was the problem
• A drying fault
• Fog, which is the only cause of low contrast

Show the answer and why

Answer: The uniform family, which means the emulsion, the exposure or the processing — but rule out too thin a coat first, because it is easy to mistake thin coating for poor maximum black and change the emulsion when the coating was the problem

Uniform means no structure, and no structure rules out the tool, the particle and the bubble. But the family boundary is exactly where the commonest mistake in emulsion making lives: coating weight sets maximum density, so a thin coat produces the same symptom as a weak emulsion and invites you to change the wrong thing. Weigh the pot before and after coating, compute the weight, and coat one sheet at twice it before touching the formula. Fog is a real candidate in the same family and its own test is separate — an unexposed strip developed and fixed against one fixed without development — and note the way mild safelight fog actually presents, which is not visible greyness but a general loss of contrast.

Question 5. Why does the page insist that the safelight fog test be run on your own emulsion rather than on a commercial paper?

• Because commercial papers are more sensitive
• Because the test certifies the geometry for the material it was run with, and your emulsions are not that material — a bromide emulsion reaches further into the blue than a chloride paper, and an orthochromatic one needs a different filter class entirely
• Because commercial paper is expensive
• It does not matter which material is used

Show the answer and why

Answer: Because the test certifies the geometry for the material it was run with, and your emulsions are not that material — a bromide emulsion reaches further into the blue than a chloride paper, and an orthochromatic one needs a different filter class entirely

A safelight test is a statement about one lamp, one distance, one time and one material, and only the first three of those carry over. The five emulsions in this part span three sensitivity classes: a pure chloride paper sees ultraviolet and the violet edge, a bromide with iodide reaches through violet into blue, and the dyed orthochromatic emulsion sees green as well and needs a deep red filter of a different class. On top of that the manufacturers are explicit that most emulsions retain some sensitivity to the colours their recommended filter transmits, so safelight exposure is minimised rather than treated as free. The practical form is a rule: whenever the material changes, the test is run again.

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