Break/Fix: The Developer That Went Wrong

Every experiment in this part assumed the developer was what the label said. This page is what happens when it is not — and it is deliberately arranged so that you cannot cheat, because the person who broke the three bottles is you, and by the time you open them you will not know which is which.

The situation

Four 250 ml bottles stand on the bench, labelled A, B, C and D in a random order that someone else assigned, or that you assigned yourself weeks ago and sealed in an envelope. One holds sound D-76 stock from the Part VIII lab. The other three were deliberately broken, one way each:

• a mixing fault — the wrong alkali went in;
• a keeping fault — the bottle was left with a large air space and shaken daily until it changed colour;
• a use fault — the bath was worked far past the capacity its maker publishes.

Your job is to say which is which, name the mechanism, and state the one test that confirms it. Then you open the envelope.

📓Making the faults, which is done weeks before the session

None of this can be improvised on the day, and the preparation is part of the teaching. Each bottle is 250 ml, each is labelled with its own formula version code in the private key, and the codes on the outside are A to D in a shuffled order.

The mixing fault. Mix 250 ml of D-76 from Kodak’s published formula — metol 0.50 g, sodium sulfite anhydrous 25.0 g, hydroquinone 1.25 g — but put in 0.50 g of sodium bicarbonate in place of the 0.50 g of borax, dissolving in the order given. It is the error of reaching for the wrong white tub, and it is chosen because the arithmetic below says it should be almost invisible.

The keeping fault. Put 250 ml of sound stock into a 1 litre bottle, so that three quarters of the bottle is air, and either shake it hard for a minute every day or bubble air through it with an aquarium pump for twenty minutes a day — a mains appliance beside a bench of solutions wants a residual-current device and a position from which it cannot fall in. Kodak’s own storage table gives the direction: six months for a full tightly closed bottle against two for a half-filled one, and the 1949 handbook notes that the air space increases every time a large bottle is opened. The course has not measured how long this takes, and the readiness test is the colour and the activity strip rather than the calendar. Expect days rather than hours: a hundred grams of sulfite per litre is a serious preservative and it is doing its job.

The use fault. Kodak’s capacity table gives 16 rolls per US gallon, which is four 135-36 rolls per litre, or one roll per 250 ml, with the footnote that the development time should rise 15 per cent after every four rolls per gallon. So run about four rolls’ worth of fully fogged scrap film through the 250 ml — outdated stock, leaders, the ends of the batch — developed to completion, one piece after another, with no time compensation at all. That is four times the rated capacity, and it puts both things an exhausted bath has into the bottle: less agent, and more of what development gives back.

Before you start

Level A, and it is the mildest practical page in the part: nothing is weighed on the day, and the four baths are handled by the hundred millilitres in open cylinders exactly as in the experiments before it. Metol is a skin sensitiser in every notification that classifies it, and hydroquinone carries serious eye damage, so gloves and eye protection are on from the first cap to the last. HSE’s COSHH essentials sheet P1 sets the ventilation standard for manual film development at more than five air changes an hour with a through draught.

What is not a hazard here, and why. Three of these bottles are broken, and none of them is broken in a way that makes it more dangerous. An oxidised developer is a developer that has lost some of its reducing agent to the air; an exhausted one has lost some of it to silver; a mis-mixed one has a different alkali in it at the two-gram level. The hazard profile of all four is the hazard profile of D-76, and the reason to say so explicitly is that “spoiled chemical” and “hazardous chemical” are different ideas that get confused. The mixing fault is the one worth a second thought and it is still mild: sodium bicarbonate is the least hazardous solid anywhere in this part.

Waste. All four baths, and the rinse that follows them, go into one labelled alkaline developer container per the general waste SOP; the fixer and its first rinse go to the silver stream. The exhausted bottle is the one carrying real silver, and it goes with the developer stream anyway — never combine the two, because one is a concentrated sulfite solution and the other is acidic. Check your local regulations, which govern and which differ between authorities within one country; the disposal page sets out why.

Alternative route

Darkness is needed for one operation only: taking five strips out of the batch tin and giving each its identity nips, about ten minutes with the light off. If the batch was made on ILFORD ORTHO Plus, that ten minutes happens under a deep red safelight instead, and everything else on this page — the colour readings, the pH, the activity strips, the reading and the diagnosis — runs in room light.

The route without a pH meter. Three of the four diagnostic steps do not need one, and the fourth is the step whose arithmetic says the electrode cannot decide it anyway. Indicator papers covering pH 7 to 10 will tell you that no bottle has moved by a unit, which is all this fault set actually requires. Record the range you could resolve, and say in the report that the alkali branch of the tree was decided on the record rather than on a reading.

The symptoms

Start with what the negatives actually show, because a diagnosis that starts from a hypothesis finds it.

Develop five wedge strips from the Part IX exposure batch: one in each of the four bottles at D-76 stock, 8 minutes at 20 °C — ILFORD’s published time for this film at EI 125 — and one shared reference at the standard condition of 1+1 for 11 minutes. Same agitation script as every other strip in the part. Water rinse, fix for twice the clearing time, wash, dry, read blind.

The reference is diluted from the part’s own stock bottle and not from bottle A, and that is not a detail. You do not yet know which of the four is sound, so a reference made from one of them would be a reference of unknown condition — which is the one thing a reference may never be.

The session, and where the 150 minutes goes. Fifteen minutes to open the laboratory and bring the bench to 20 °C; twenty-five for the non-destructive evidence below; thirty for the four activity strips; forty-five for the five wedge strips in two batches; fifteen to wash and hang everything together. Then the strips dry, unattended and not counted, and the last twenty minutes are the blind reading, the evidence table and the reveal. The overview budgets three strips for this session; the design spends five, because the fourth is the sound control and the fifth is the shared reference, and neither is optional.

ILFORD’s own description of what a sound negative looks like is the benchmark: a full range of tones, with some parts almost clear like the rebates and others so dense you can only just read print through them.

What you see on the strip	What it is called	What it tells you on its own
Every step lighter than the control’s, and the top steps never reach a real black	Thin	Less silver was reduced. Says nothing yet about why
The steps run together — fewer separable steps between first density and maximum	Flat	Contrast is down. Usually travels with thin, and the pair is the signature of under-development
Density on the masked patch, which saw no light at all	Chemical fog	The developer reduced unexposed halide. Points up the alkali ladder, not down it
Density on the masked patch that is lower than the control’s	Self-restraint	Something in the bath is holding development back at the toe
A milky or violet cast over the whole strip, wet or dry	Under-fixing	Not a developer fault at all. Rule it out first
A general yellow or brown stain in the gelatin	Staining	Oxidised developer carried into the film, or an exhausted fixer
Streaks running downward from the dense steps	Bromide drag	An agitation fault, not a chemistry fault

✕The step wedge is immune to the commonest wrong answer

When a real roll comes back thin, the first suspect is always exposure, and on a pictorial negative you usually cannot tell. On a wedge strip you can, and this is the reason the part uses them.

A wedge strip carries the whole exposure range in one frame, three log units of it. Under-exposure cannot make it thin; it can only slide the pattern along the wedge, leaving the density at the clearest step where it was. Under-development lowers the density at every step including the clearest, and squeezes the separable range.

So: if the clearest step is as dark as the control’s clearest step, the developer is doing its job and the fault is elsewhere. If it is not, you are looking at a developer fault and this page is the right one. That single comparison is worth more than any amount of squinting at the middle of the strip.

Evidence to collect

Take all of this before you form a hypothesis, and record it against the bottle letters rather than against your suspicions.

1. The record, which is evidence and is usually skipped. For each bottle: its version code in the key if you can read it without spoiling the blinding, the mixing date, the volume, the headspace, and the capacity log — how much film has gone through it, in rolls or square inches, and whether any time compensation was applied. Kodak’s list of the causes of an out-of-control process begins with improper mixing, improper storage and keeping, and contamination, and every one of those leaves its trace in a notebook rather than in the bottle.

2. Colour, against a white card, in one light. Fresh D-76 is water-clear to faint straw. Write a colour and an intensity for each bottle — water clear, faint straw, clear yellow, amber, brown — and never “darker than the last one”.

3. pH at 20 °C, meter calibrated at 7 and 10 per the SOP, with the electrode’s stated accuracy written beside every reading. ILFORD publish 8.60 to 8.70 for ID-11 stock, which is the closest published figure to D-76 the course holds; there is no published pH for D-76 itself, so the comparison that matters is bottle against bottle, not bottle against a book.

4. Specific gravity, if you have a hydrometer. ILFORD publish 1.090 for ID-11 stock. The course has no published figure for D-76 and offers none, so this reading is a comparison between your four bottles and nothing more — but a bottle that has been topped up with water, which is a fifth kind of fault this page does not stage, would show here and nowhere else.

5. The fogged-strip activity test, which is the one test that measures what you actually care about. Part VIII’s aerial oxidation experiment sets the method and it is used unchanged: take a scrap of the same film, expose it fully to room light, dip it in the bath for exactly 60 seconds at 20 °C with gentle agitation, lift, rinse 30 seconds, fix 2 minutes, wash and dry. One strip per bottle, all four in the same ten minutes, read side by side.

⚗Where the 60-second activity test comes from, and what it is not

No manufacturer publishes this test for developers. What ILFORD publish is its mirror image for fixers: drop film on a scrap, time how long it takes to clear, and discard the bath when the used clearing time exceeds twice the fresh one. The developer version reverses the reaction — time to blacken instead of time to clear — and the course states plainly that the “twice the fresh figure” threshold is borrowed reasoning and not a published rule for developers.

What makes it a measurement rather than an impression is the fresh control in the row beside it. A 60-second strip from bottle A is the zero; the others are read against it. Without that strip the test returns an opinion.

6. Clearing behaviour, to rule the fixer out. Time the clearing of a film scrap in your working fixer and compare it with fresh. If it exceeds twice the fresh time, ILFORD’s rule says the bath is finished — and a milky strip is then a fixing fault that has nothing to do with any of these four bottles. Rule it out on paper before you spend a diagnosis on it.

7. Base plus fog on every strip, read against a piece of clear, fixed, unexposed film from the same batch, and ranked. You cannot put a number on it without a densitometer. The ranking is what separates two of the three faults.

Hints

Read these in order, and stop as soon as one of them opens the problem.

Hint 1. Three of the four strips are thinner and flatter than the fourth. That tells you which bottle is sound and nothing else. Every fault on this page is a species of under-development, and Kodak says so in one line: underdevelopment gives a decrease in density and contrast in the control strip and a loss of density, contrast and shadow detail in the negative.

Hint 2. Two readings separate faults that look alike on the strip: the colour of the solution and the fog floor of the negative. Look at both before you look at anything else.

Hint 3. One of the three faults should have raised the pH, one should have lowered it very slightly, and one should have left it alone. Work out which is which from the chemistry before you look at your readings — and then notice that only one of those three predictions is larger than your electrode’s stated accuracy.

Hint 4. A bath that has developed a great deal of film has something in it that a bath which has merely oxidised does not. ILFORD name it in one sentence: each film processed releases halides and other by-products into the developer that act as a restrainer on the development of subsequent films. A restrainer has a specific, published signature, and it is not “less of everything”.

Hint 5. One of these faults is smaller than the instruments you have. When the effect you are looking for is inside the uncertainty of every test you can run, the diagnosis does not come from the bench. It comes from the record, and the page said in the first callout that the record is part of the evidence.

The diagnosis

From a thin negative to a named cause, and the test that confirms each one

1. 1Is the clearest step as dark as the control’s? — if yes, the developer is not the fault — look at exposure, fixing and agitation
2. 2Colour of the solution — amber or brown convicts oxidation; clear convicts nothing, because the sulfonates are colourless
3. 3Fog floor against clear fixed base — lower than the control means a restrainer is present, and only one of these faults supplies one
4. 4pH against the control bottle — clearly higher means too much alkali; within the electrode’s accuracy convicts nobody
5. 5The mixing record — where the fault that is smaller than the instrument is finally decided

The tree is drawn to be walked downwards in this order and no other. Colour comes before pH because it is free, instant and non-destructive; pH comes last among the solution tests because it is the one whose uncertainty is closest to the size of the effect.

Bottle C, the oxidised stock: convicted by colour, confirmed by activity

What you see. An amber or brown solution. A strip that is thin and flat with a fog floor about the same as the control’s. An activity strip visibly lighter than the fresh one at 60 seconds. A pH within the electrode’s accuracy of the control’s, because oxidation consumes the agent and not the alkali.

The mechanism. Oxygen takes electrons from the developing agent, and the agent that has given them to oxygen cannot give them to silver. The bottle was three quarters air, and every shake put a fresh interface between the two. Kodak’s troubleshooting chart has an entry for exactly this — developer or replenisher too old or oxidized — against a contrast index and a speed both trending down, which is “thin and flat” in process-control language.

The confirming test. The activity strip beside the fresh one, because it isolates the property that matters. And there is a caution attached that is more important than the test:

✕Colour convicts, but clear does not acquit

Kodak’s 1928 primer gives two fates for the quinone that hydroquinone becomes, and reconciles neither. On one route the sulfite reduces it back to hydroquinone. On the other it becomes hydroquinone mono- and disodium sulfonates, which are colourless and are not hydroquinone — and the same book warns in so many words that a colourless developer is no indication of undiminished power.

So a brown bottle is proof of oxidation, and a clear bottle is proof of nothing. That asymmetry is why the activity test exists, and why “it still looks fine” is not a statement about a developer.

Bottle D, the exhausted bath: convicted by the fog floor, confirmed by the log

What you see. A solution that may be slightly yellowed but need not be. A strip that is thin and flat and has a lower fog floor than the control’s, with its threshold further down the wedge. The activity strip is lighter than fresh.

The mechanism, and the discriminator. An exhausted bath has lost agent to silver rather than to air, and that is only half of it. ILFORD state the other half: each film processed releases halides and other by-products into the developer that act as a restrainer on the development of subsequent films. So an exhausted bath is a weakened developer with a restrainer in it, and a restrainer has the signature the activity series has already put in front of you: fog down first, threshold down after.

That is the discriminator. Oxidation removes agent and adds nothing. Exhaustion removes agent and adds bromide. Both give thin and flat; only one gives a cleaner unexposed patch than the control.

The confirming test. Two of them, and they cost almost nothing.

• The capacity log against the published figure. Kodak’s table gives four 135-36 rolls per litre, so 250 ml is rated for one roll, with a 15 per cent time increase after every four rolls per gallon. Four rolls’ worth through 250 ml is four times the rating with no compensation applied. ILFORD’s parallel table says the same thing in the other direction: 10 per cent more time for each successive film in a litre, to a maximum of ten films at plus 90 per cent.
• A second strip at twice the time. Hurter and Driffield established that a restrainer retards rather than stops, and that the image will appear in full force if sufficient time be allowed. A bath held back mainly by bromide should recover a great deal at 16 minutes. A bath that has simply lost its agent should recover less. That is a prediction with a direction, and it is worth one strip.

Bottle B, the wrong alkali: the fault the bench cannot convict

What you see. A water-clear solution. A pH a fraction below the control’s. An activity strip barely distinguishable from fresh. A wedge strip that is slightly thinner and flatter than the control — and by “slightly” the page means a step, or less than one.

And that is the lesson. Here is the arithmetic, worked before the session so that the reading has something to be measured against.

D-76 carries 100 g of sodium sulfite per litre, which is 0.79 mol/L, and metol arrives as a hemisulfate whose acid converts about 0.012 mol/L of that to bisulfite at the moment of mixing. The borax contributes 0.0105 mol/L of borate and the same of boric acid — a pool a sixtieth the size of the sulfite pool.

pH = 7.19 + log([SO₃²⁻] / [HSO₃⁻])

Predicted pH of D-76, and of D-76 with the borax replaced by bicarbonate

Conserve protons across the couples and the answer comes out at 8.97 with the borax and 8.88 with 0.5 g/250 ml of sodium bicarbonate in its place — a difference of 0.09 pH units. Sodium bicarbonate sits between its own two pK_a values, 6.35 and 10.33, at this pH; it is neither donating nor accepting protons in any quantity, so it is very nearly a spectator.

🧮Why a teaching electrode cannot see this, and what that means

A sealed teaching electrode is specified at ±0.2 pH units. The effect is 0.09. The instrument’s own uncertainty is more than twice the size of the thing it is being asked to find, so a pH reading cannot convict this bottle and cannot acquit it either.

The same arithmetic explains why the negative is only slightly wrong. The bath’s activity is set by its pH, the pH is set by the sulfite–bisulfite couple, and two grams of anything per litre cannot outvote 100 grams of sulfite. The borax in D-76 is not what makes D-76 alkaline. It is a fine trim on a pH the sulfite has already decided, which is exactly why D-23 — the same sulfite, the same agent, no alkali at all — is a working developer.

Two consequences worth carrying out of this page. First, the rival hypothesis “they left the borax out entirely” predicts almost the same result, so no test on the solution can separate the two: only the mixing record can. Second, if you want to break a D-76 by getting the alkali wrong, you have to get it wrong by a factor of ten or swap it for a stronger base. Two grams of sodium carbonate in place of two of borax computes to about pH 9.4, half a unit up, which the same electrode would see easily and which the activity series has already shown you the consequences of: more contrast, and more fog.

The fix

Bottle C, oxidised: discard it. There is no restoring an agent that has given its electrons to oxygen. What is fixable is the practice. Kodak’s storage table is a design specification, not a warning: six months full, two months half-filled. Decant into smaller bottles as you use a batch so that each one is full; the 1949 handbook’s advice is to leave only a small air space, and to remember that the space grows every time a large bottle is opened. Where you can, buy or mix the volume you will actually use.

Bottle D, exhausted: discard it, and start counting. A capacity log is four columns — date, what went through, running total, time compensation applied — and it is the difference between using a developer to its published capacity and using it past it. Kodak’s route for a working bath is 15 per cent more time after every four rolls per gallon; ILFORD’s is 10 per cent per film in a litre to a maximum of ten. Neither is a licence to run a bath forever, and both stop.

Bottle B, mis-mixed: discard it, and change the labelling rather than the chemistry. The fix here is not chemical at all, because the failure was a hand reaching for a tub. Label every solid with its full name and its formula, keep the alkalis apart on the shelf, and weigh with the formula in front of you. The labelling SOP exists for this and this page does not restate it.

And the general fix, which is the point of the page. Every one of these three bottles would have been caught in a minute by a process-control strip: one strip from the standard batch, developed at the standard condition, before the session that matters. The whole of Part IX runs a shared reference strip for exactly that reason, and this page is what a session looks like when nobody did.

📓What the Visual Diagnostic Atlas will carry, and what it does not yet

The troubleshooting atlas is the course’s reference of named defects, organised by what you see: appearance, likely causes in order of how often they turn out to be the answer, the chemistry that produces the appearance, the questions that separate one cause from another, the fix and the prevention.

Its entries for the defects on this page have not been written yet. Three of them are owed by this part’s work — thin negative, flat negative and chemical fog — and each will carry the discriminator this page has just used: the clearest-step comparison that separates a developer fault from an exposure fault, the fog-floor comparison that separates exhaustion from oxidation, and the arithmetic that says the alkali fault is smaller than the instrument. Writing that down here is not a promissory note; it is the specification a later author works to, and it comes from strips rather than from a plan.

What to record

The blind sheet, filled in before the envelope is opened. Four rows, one per bottle letter, with your diagnosis, your confidence in a word, and the single piece of evidence you would cite for it. Writing the evidence down before the reveal is the whole exercise; a diagnosis reconstructed afterwards always looks better than it was.

The evidence table. One row per bottle: colour and intensity with the light it was read in; pH with the electrode’s stated accuracy; specific gravity if taken; the 60-second activity strip’s rank; the capacity and headspace from the log; the mixing date.

The strip table. One row per wedge strip: threshold step, scale length, the density of the clearest step against the control’s, the fog floor ranked against clear fixed base, and any stain or streak with its position.

The reveal. The key, opened and pasted in, with a line for each bottle saying whether you were right, which piece of evidence carried the decision, and — for the one you got wrong or could not call — what test would have decided it.

The capacity log you are starting today, for every developer bottle you own: date, film area or roll count, running total against the published capacity, and the compensation applied.

Questions

1. A strip is thin and flat, and its clearest step is exactly as dark as the control’s clearest step. What have you just ruled out, and what are the three things now worth checking?
2. Bottle C is water-clear and its activity strip is plainly weaker than fresh. Can you rule out oxidation? Give the reason in terms of what happens to quinone in a sulfite solution.
3. Two bottles both give thin, flat strips. One has a fog floor lower than the control’s and the other has a fog floor about the same. Name each fault and give the sentence from a manufacturer’s literature that justifies the discrimination.
4. Your electrode is specified at ±0.2 pH units and the effect you are looking for is 0.09. Write the sentence you would put in the report about bottle B’s pH reading.
5. Somebody proposes that bottle B was mixed with no alkali at all rather than with bicarbonate. Design the test that would separate those two hypotheses, or explain why no test on the solution can.
6. You have a litre of D-76 stock and you develop six 135-36 rolls in it over a month, applying no time compensation. Using Kodak’s published capacity, say how far past the rating you are and what you would expect the sixth roll to look like against the first.

Further work

Stage the fault this page declined to stage. Mix 250 ml of D-76 with 0.50 g of sodium carbonate in place of the borax and put a fifth strip through it at the same 8 minutes. The arithmetic predicts about pH 9.4 against the control’s 8.97, and the activity series predicts what half a unit does. It is the same mixing error with a different tub, and it is the version you can actually see.

Break the fixer instead. Every fault on this page is a developer fault, and a good half of the negatives that come back wrong are not. Run a clearing-time series on a fixer used past ILFORD’s twice-the-fresh-time rule, and add the milky strip to your evidence table so that next time it is ruled out in thirty seconds rather than diagnosed for an hour.

Run the reveal on somebody else’s bottles. The blinding on this page is imperfect, because you made the faults and you remember roughly what you did. Two students who prepare four bottles each and swap them get a much harder and much more honest exercise, and the preparation is identical.

Check your understanding

Question 1. Why does replacing D-76’s 2 g/L of borax with 2 g/L of sodium bicarbonate change the developer so little?

• Because bicarbonate and borate have almost the same pKa
• Because the pH is set by the sulfite–bisulfite couple at 0.79 mol/L, and a 0.024 mol/L addition sitting between its own two pKa values cannot outvote it — the computed shift is about 0.09 units
• Because bicarbonate decomposes in the bottle
• Because hydroquinone buffers the solution

Show the answer and why

Answer: Because the pH is set by the sulfite–bisulfite couple at 0.79 mol/L, and a 0.024 mol/L addition sitting between its own two pKa values cannot outvote it — the computed shift is about 0.09 units

A hundred grams of sodium sulfite per litre is 0.79 mol/L, and metol’s hemisulfate converts about 0.012 mol/L of it to bisulfite at mixing time, so D-76 arrives as a buffer before any alkali is added. Bicarbonate at 2 g/L is 0.024 mol/L and sits between pKa1 6.35 and pKa2 10.33, where it neither donates nor accepts protons in quantity. The arithmetic gives 8.97 with borax and 8.88 without it. That is also the reason D-23 — the same sulfite, the same agent, no alkali at all — is a working developer.

Question 2. A student reports that bottle C cannot be oxidised because the solution is still clear. What is wrong with the inference?

• Nothing; a clear developer is a fresh developer
• Kodak’s primer gives two fates for quinone in a sulfite solution, one of which produces colourless hydroquinone sulfonates, and warns that a colourless developer is no indication of undiminished power — so colour convicts but clear does not acquit
• Oxidised developers are always clear
• Colour can only be judged wet

Show the answer and why

Answer: Kodak’s primer gives two fates for quinone in a sulfite solution, one of which produces colourless hydroquinone sulfonates, and warns that a colourless developer is no indication of undiminished power — so colour convicts but clear does not acquit

The two routes are given in the same book and reconciled nowhere. On one, sulfite reduces the quinone back to hydroquinone and the reservoir is topped up; on the other, the quinone becomes mono- and disodium sulfonates that are colourless and are not hydroquinone, so the reservoir drains while the bottle stays clear. The asymmetry is why the diagnostic tree puts colour early — it is free and instant when it fires — and why a negative colour reading sends you on to the activity strip rather than closing the case.

Question 3. Two thin, flat strips. One shows a fog floor lower than the control’s. Which fault is it, and what is the published basis?

• Oxidised stock, because oxidation products are restrainers
• The exhausted bath, because ILFORD state that each film processed releases halides and other by-products that act as a restrainer on the development of subsequent films — so exhaustion removes agent and adds a restrainer, while oxidation removes agent and adds nothing
• The mis-mixed bottle, because bicarbonate restrains development
• Neither; a fog floor cannot be read without a densitometer

Show the answer and why

Answer: The exhausted bath, because ILFORD state that each film processed releases halides and other by-products that act as a restrainer on the development of subsequent films — so exhaustion removes agent and adds a restrainer, while oxidation removes agent and adds nothing

This is the one discrimination on the page that a by-eye reading makes cleanly, and it works because the two faults differ in kind rather than in degree. A restrainer’s signature was established in the activity series: fog falls first and the threshold follows. So a bath that is weak and cleaner than fresh has bromide in it, which means film went through it. The fog floor is ranked rather than measured, which is enough, because the comparison is against the control strip developed in the same session.

Question 4. What does the clearest step of a wedge strip tell you that no part of a pictorial negative can?

• The film speed
• Whether the developer worked at all: the wedge carries the whole exposure range in one frame, so under-exposure can only slide the pattern along it, while under-development lowers the density at every step including the clearest
• The temperature the bath was at
• Whether the fixer was exhausted

Show the answer and why

Answer: Whether the developer worked at all: the wedge carries the whole exposure range in one frame, so under-exposure can only slide the pattern along it, while under-development lowers the density at every step including the clearest

On a real roll, "thin" is ambiguous between too little light and too little development, and that ambiguity is where most darkroom arguments start. A step wedge removes it by construction, because every strip carries three log units of exposure and the clearest step is guaranteed to be at the top of the film’s range. If that step is as dark as the control’s, the developer reached maximum density and the fault is elsewhere — exposure, fixing, agitation or drying.

Question 5. Kodak rates D-76 at 16 rolls per gallon. You put four rolls’ worth of fogged scrap through 250 ml with no time compensation. Where does that leave the bath?

• Just inside the rating
• At four times the rating, since 16 per gallon is 4 per litre and therefore 1 per 250 ml, and with none of the 15 per cent time increases the footnote calls for after every four rolls per gallon
• At half the rating
• The rating does not apply to fogged film

Show the answer and why

Answer: At four times the rating, since 16 per gallon is 4 per litre and therefore 1 per 250 ml, and with none of the 15 per cent time increases the footnote calls for after every four rolls per gallon

The unit conversion is the whole of it: 16 rolls per US gallon is 4 per litre, so a 250 ml bottle is rated for one 135-36 roll. Fogged scrap is if anything a harder test than pictorial film, because every crystal develops, so four rolls’ worth is a thorough exhaustion. The footnote matters as much as the headline figure — the rating assumes a rising development time, so a bath run to capacity at constant time was already under-developing before it reached the number.

Question 6. Which fault on this page cannot be convicted by any test on the solution, and what follows for practice?

• The oxidised bottle, because oxidation leaves no trace
• The mis-mixed bottle, because its predicted pH shift of 0.09 units is well inside a teaching electrode’s ±0.2 accuracy and its rival hypothesis predicts the same result — so the mixing record is the only evidence, which is why the record is kept
• The exhausted bottle, because bromide cannot be measured at home
• None; every fault has a solution test

Show the answer and why

Answer: The mis-mixed bottle, because its predicted pH shift of 0.09 units is well inside a teaching electrode’s ±0.2 accuracy and its rival hypothesis predicts the same result — so the mixing record is the only evidence, which is why the record is kept

It is worth being blunt about: not every fault has a signature you can find, and a diagnostic method that assumes otherwise will manufacture a confident wrong answer. Here the effect is 0.09 pH units against an instrument specified at ±0.2, and "borax replaced by bicarbonate" and "borax left out altogether" predict almost the same number, so even a better electrode would not separate them. That is the argument for the notebook, and it is a stronger argument than any exhortation to be tidy.

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