Experiment: Clearing Time and Fixer Capacity

Purpose

To measure the clearing time of one named film in the three fixers mixed in the previous lab, then to load one of those baths with measured areas of undeveloped film until its clearing time doubles, and to convert the result into a capacity in films per litre that you can apply to your own darkroom and compare with the maker’s published figure.

The hypothesis, in two parts. First, that the ammonium thiosulfate bath will clear the film fastest of the three, by roughly a factor of two to three, matching the published spread between a liquid rapid fixer at 2 to 5 minutes and a sodium acid hardening bath at about 10. Second, that clearing time rises with cumulative silver load along a curve that is nearly flat for most of its length and then turns up sharply, crossing twice the fresh figure somewhere near the maker’s published capacity of 24 films of 135-36 per litre.

The control. A fourth beaker holding the same fresh rapid fixer, at the same volume and in the same water bath, which is never loaded and from which a clearing time is taken at every interval. It is not decoration. It controls for four things at once: evaporation concentrating the bath, temperature drifting across a three-hour session, variation between the scraps of film you are cutting up, and you — because judging “clear” is a skill and you will get better at it during the afternoon, which would otherwise look exactly like a fixer getting faster.

The one variable that changes is the cumulative area of undeveloped film passed through the test bath. Held constant: the film, the fixer and its dilution, the volume, the temperature, the agitation, the beaker, the lighting you judge under, and the person judging.

=What each outcome means, decided before you start

If the loaded bath crosses twice its fresh clearing time near 2.4 films’ worth of area in 100 mL, the hypothesis stands and your bath behaves as its maker says. Scale it: 2.4 films in 100 mL is 24 per litre.

If it crosses much earlier, look first at what you loaded. Undeveloped film sheds every gram of its silver, where a processed negative sheds only the part development did not reduce — so a bath fed undeveloped film is being worked considerably harder per unit area than one processing pictures, and an early crossing may be the correct answer to a harder question. Say which one you measured.

If it crosses much later, suspect the control beaker first. If the control’s clearing time has drifted the same way, the cause is temperature or evaporation and not capacity.

If the acid hardening bath clears faster than the rapid bath, something is wrong with the rapid bath and the specific gravity from the mixing lab will say what: ILFORD publishes 1.070 to 1.080 at 1+4, and a reading near 1.040 is a bath mixed at half strength.

A curve with no corner in it at all is the most likely disappointment, and it means you stopped too soon. The whole point of the shape is that two thirds of the life shows almost nothing.

Learning objectives

Measure a clearing time reproducibly enough to compare two of them; design a control that isolates the operator as a source of drift; correct a rate measurement for temperature and say what the correction rests on; convert a measured crossing point into a capacity per litre and compare it with a published figure; and state, when the two disagree, which of them is a measurement of your darkroom.

Prerequisites

Mixing fixers from scratch, which produced the three baths and the fresh clearing times this session re-measures, and fixer capacity and exhaustion, which states the criterion the session applies. From Part IX, experimental design for the darkroom, whose rules about holding one thing still this page follows. From Part II, measurement and uncertainty.

Safety classification

Level A on the course rubric, and the assessment is short because nothing new comes onto the bench.

What applied. The three baths as mixed in the previous lab, handled by the hundred millilitres in open beakers; no concentrates opened; no solids weighed; nothing heated above a warm water bath at about 20 °C; no mains equipment beyond a timer. The one thing this session has that the mixing lab did not is silver: the loaded bath becomes the most silver-laden liquid this part produces, and the waste section treats it accordingly.

!Why the iodide hypo check is described here and not performed

The classical bench test for a spent fixer is an iodide solution, and this session does not run it, for two independent reasons.

The reagent puts the session at Level B. Potassium iodide carries H317, may cause an allergic skin reaction, and H334, may cause allergy or asthma symptoms if inhaled, along with H372 and H373 for organ damage on repeated exposure. Level A is capped at classifications going no further than irritant, harmful if swallowed or corrosive; a sensitiser is Level B’s first criterion, and sensitisation is not reversible.

The course cannot publish a recipe for it. No document meeting the course’s evidence standard gives a working concentration, a drop count or an interpretation for the iodide hypo check. So the course teaches the principle — below — and prints no numbers, because a plausible recipe assembled from general chemistry would be the course inventing a test and then teaching it back to itself. If you own a bought hypo-check product, its own instructions are the authority and the reagent is its maker’s problem to classify.

What this session uses instead is clearing time, which is free, and pH and specific gravity, which ILFORD publishes for its own fixer so that a reading can be checked against something.

What is not a hazard here, and why. No solid is weighed, so there is no dust; the acid was bought dilute and was diluted again into a bath, so what is in the beaker is milder than the stop bath next door; the water bath is at room temperature and there is no heat source. The three baths’ hazards are their ingredients’, which the previous lab assessed, and none of them becomes more dangerous by dissolving silver — it becomes more valuable and more of a disposal problem, which is a different thing and is easy to confuse with a safety problem.

Hazards

What	Why	Control
Working-strength fixers in open beakers	Splash and skin contact; the aggregated classifications of the thiosulfates are absent on a thin evidence base rather than benign	Gloves, eye protection, an apron; beakers in a tray so a knock is contained
The acid hardening bath meeting a spilled sulfite or a stop bath	The pair on the incompatibility matrix; acid and thiosulfate together give sulfur dioxide	Nothing acid on this bench except the fixer itself; the stop bath stays in its bottle
Silver-laden fixer at the end of the session	Not a health hazard at these concentrations, but a waste stream that must not go to a drain and is worth recovering	The silver-bearing container, open on the bench, before you start
Cutting film scrap	A craft knife and a metal rule	A cutting mat, a straight edge you can hold, and the blade away from the hand
Three hours of repetitive timing	Fatigue is the commonest source of drift in a long session	The control beaker, which measures exactly this

Required PPE

Nitrile gloves at the thickness HSE’s COSHH essentials sheet P1 names — single-use, about 0.2 mm — changed when splashed. Eye protection from the first pour to the last. An apron or overall, because dried fixer is a white bloom that marks whatever the sleeve touches next. The glove page records that the permeation guide the course read carries no entry for either thiosulfate, so the glove is splash protection with no breakthrough time behind it.

Ventilation

General ventilation at HSE’s standard for manual film development, more than five air changes an hour with a through draught. Nothing in this session is intended to produce a vapour: the baths are at room temperature, the acid bath is buffered, and the sulfurisation reaction the previous lab demonstrated is exactly what the mixing order and the sulfite exist to prevent. The ventilation is here because a three-hour session with four open beakers of thiosulfate at the bench is a long exposure to a small thing.

Materials

Undeveloped film, and a good deal of it — this is the cost of the session and it should be planned before the day. Anything that has never been developed carries its full silver charge: outdated stock, the ends of bulk rolls, leaders, unwanted exposed rolls, expired sheet film. It does not matter whether it was exposed, because exposure does not remove silver; only development does. Handle it in room light, because you are going to dissolve the image anyway.

You need roughly 1,400 cm² for the standard 100 mL run, plus about 300 cm² of test strips. Kodak’s area table converts: a black-and-white 135-36 roll is 550 cm², a 120 roll is 500 cm², and a 5 × 4 inch sheet is 129 cm². So the run is about two and a half 135-36 rolls’ worth, or eleven sheets of 5 × 4, or any mixture you can measure.

Also: a cutting mat, a steel rule, a craft knife, and a sheet of squared paper to measure areas against.

Chemicals

Chemical	Quantity	Form
Ammonium thiosulfate fixer	200 mL of working strength at 1+4	100 mL for the test bath, 100 mL for the control, both from the litre mixed in the previous lab
Sodium thiosulfate plain bath	100 mL of the 25 % w/v bath	For the fresh-clearing-time comparison only
The acid hardening bath	100 mL	Thiosulfate, sulfite, acetic acid and potassium alum, as mixed previously; for the comparison only
Potassium iodide	none	The hypo check is described and not performed; see the safety classification

Equipment

Four 250 mL beakers, tall rather than wide. One tray large enough to stand all four in as a water bath. A thermometer that will stay in the control beaker for three hours. A timer with a second hand or a stopwatch — Part II’s timer is the one to use, and the same one for every reading. A 100 mL graduated cylinder. Tweezers or tongs. A sheet of white card and a lamp, fixed in one position for the whole session. A pH meter and a hydrometer if you have them. The notebook, ruled up before you start.

Estimated cost

Band £ for the chemistry and rather more for the film. The three baths were made in the previous lab and this session uses a fifth of a litre of them. The real cost is the 1,700 cm² of undeveloped film, which the planner does not carry as an item because it is a by-product rather than a purchase: it is the leaders, the ends and the outdated stock that accumulate. If you have none, halve the whole experiment — 50 mL of bath and 700 cm² of film — and say on the report that you did, because the crossing point scales with volume and the numbers stay comparable.

Estimated consumables cost

Band £ for the chemistry and rather more for the film — except that the film is not bought. The 1,700 cm² of undeveloped film this session dissolves is leaders, ends and outdated stock, which is why the planner carries it as no item at all: it is a by-product of the rest of the course rather than a purchase.

Consumed	This session	Sourced price	Cost this session
Undeveloped film, any age or format	about 1,700 cm²: two and a half 135-36 rolls, or eleven 5 × 4 sheets	A by-product: leaders, ends and outdated stock	—
Ammonium thiosulfate fixer	200 mL of working strength at 1+4	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£0.84–£1.04
Sodium thiosulfate plain bath, 25 % w/v	100 mL, from the previous lab	£14.70 per 1 kg of the raw salt, checked 7 September 2026; this 100 mL is drawn from the litre costed in the previous lab	—
The acid hardening bath	100 mL, from the previous lab	None. A named price gap: chrome alum, potassium alum and sodium hydroxide	—

The priced rows come to £0.84 to £1.04 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: 1 of the 4 rows carries 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.

If you have no scrap film, halve the whole experiment — 50 mL of bath and 700 cm² of film — and say on the report that you did, because the crossing point scales with volume and the numbers stay comparable. The two bath rows are drawn from the litre mixed in the previous lab, which is where their substances are costed — the hypo now with a dated figure there and the alum still without one, for the same reasons each carries or lacks one here.

Waste streams

This is the session where the fixers become waste, and it is the first silver-bearing stream Part XI produces. The test bath at the end of the afternoon holds every gram of silver that came off 1,400 cm² of undeveloped film, and it is the most concentrated silver solution in this part of the course.

• The loaded test bath, the control bath and both comparison baths go to the silver-bearing container, which stands open on the bench beside the sink before the session starts, not in a cupboard.
• The first rinse off every beaker, every pair of tongs and every gloved hand goes to the same container. It carries most of what the rinse removes.
• The cleared film scraps, which are now clear plastic with a gelatin layer and almost no silver in it, are solids. ILFORD’s guidance for domestic users in the United Kingdom treats small quantities of scrap film as normal household waste; the silver that was on them is in the bottle.

Preparation

1. Rule the notebook up before the day. Columns: reading number, clock time, cumulative area loaded in cm², cumulative film-equivalents, bath temperature, test-bath clearing time, control-bath clearing time, ratio to fresh, and a notes column. A table drawn at the bench under time pressure is a table with a column missing.
2. Cut the loading film into measured pieces and record the area of each. Squares of a stated size are easiest; strips of 35 mm film can be measured by length, at 3.5 cm wide. Keep them in a labelled envelope in the order you will use them.
3. Cut the test strips, one per reading, all the same size, from the same film. About twenty of 15 cm² each. They are consumed by the measurement and — this matters — they load the bath too, so their area goes in the running total.
4. Stand all four beakers in the tray, fill the tray with water at the temperature you intend to work at, and leave the whole assembly for half an hour to come to one temperature before the first reading.
5. Fix the lamp and the white card. Every judgement of “clear” must be made in the same light, from the same angle, against the same background.

The bench, and why every beaker stands in the same tray

1. 1The water bath — one tray, all four beakers, half an hour to equilibrate before the first reading
2. 2Test bath and control, adjacent — same fixer, same volume, same water; only one of them ever gets loaded
3. 3The viewing station — fixed lamp, white card, one angle. Every "clear" is judged here or none of them are comparable
4. 4The silver-bearing container — open, beside the sink, before the session starts — not in a cupboard

Plan view, not to scale. The one thing the drawing is trying to make obvious is the adjacency of beakers 3 and 4: the control only works if it experiences the same afternoon as the test bath.

Procedure

Phase one: three fresh baths, one film

1. Pour 100 mL of each bath into its beaker and let the tray bring them to one temperature. Record the temperature.
2. Take a test strip. Put one drop of the bath you are about to measure on its emulsion side and leave it until a clear spot appears — ILFORD says 30 to 60 seconds. That spot is your reference for what “clear” looks like on this film, and it removes the argument.
3. Immerse the strip, start the timer, and agitate exactly as you will agitate every time — one gentle inversion of the tongs every fifteen seconds is a workable rule and the point is that it is the same rule every time.
4. Stop the timer when the rest of the strip matches the reference spot, judged at the viewing station. Record.
5. Repeat twice more in the same bath, on fresh strips. Three readings, because two cannot tell you your own scatter.
6. Do the same for the other two baths. That is nine readings and it is the whole of phase one.

Phase two: loading one bath until it doubles

7. The test bath is the rapid fixer. Take its fresh clearing time as the mean of its three phase-one readings, and write it at the head of the page as t₀. Everything after this is a ratio to t₀.
8. Load. Put a measured piece of undeveloped film into the test bath, agitate as before, and leave it until it has cleared and then some — twice its own clearing time is the right amount, because you want its silver in the bath and not still in the gelatin. Remove it, let it drain back into the beaker for five seconds, and put it in the rinse.
9. Add its area to the running total. Add the area of every test strip too.
10. Measure, both baths, after every 150 cm² or so of loading — about every 0.27 of a film-equivalent. Test bath and control bath, same procedure, same lamp, same card. Record the temperature every time.
11. Repeat 8 to 10. Expect the first two thirds of the afternoon to look like nothing is happening. It is not: the curve is flat there and that is the finding.
12. Stop when the test bath’s clearing time exceeds 2 × t₀, and take two more readings past it if you have film left, because the shape after the crossing is worth having.
13. If you have a pH meter and a hydrometer, read both baths at the start, at the crossing, and at the end.

One loading cycle, repeated until the curve turns

1. 1LoadOne measured piece of undeveloped film into the test bath, fixed for twice its own clearing time so that its silver is really in the bath.
2. 2CountAdd its area to the running total — and the area of every test strip you have used, because those load the bath too.
3. 3Measure bothA fresh test strip in the test bath, then one in the control bath. Same lamp, same card, same agitation.
4. 4RecordTime, area, temperature, both clearing times, and the ratio to t₀ computed on the spot so that you can see the corner arriving.
5. 5Stop at 2 × t₀Then take two more readings past it.

Expected observations

Phase one should separate the three baths clearly. ILFORD gives 2 to 5 minutes as the fixing time for general-purpose film in its rapid fixer, and fixing time is twice clearing time, so a clearing time of about one to two and a half minutes is the expectation. Kodak Limited’s 1949 handbook expects F-5 to fix films properly in 10 minutes, implying a clearing time near 5. The plain hypo bath has no published figure at all in the corpus, and where it falls is genuinely your measurement.

The scatter within a bath will surprise you the first time. Three readings on identical strips in the same bath will not agree to the second. That is not sloppiness: it is the real precision of a human judging an optical endpoint, and knowing its size is what lets you say whether a later change is real.

The curve will be flat and then it will not be. Two thirds of the loading will move the clearing time by less than a fifth, which for a two-minute clearing time is under twenty-five seconds and is inside the scatter you just measured. Then it will move a great deal in one or two loadings.

The control should stay flat throughout. If it does not, the ratio between the two is still meaningful and the absolute numbers are not, which is exactly what a control is for.

What is happening chemically

Two processes, and they compound.

Thiosulfate is being consumed and locked up. Two thiosulfate ions leave circulation with every silver ion that enters solution, permanently. Part III’s scissors argument is the picture: the free thiosulfate still available falls as twice the silver dissolved, while the free thiosulfate the equilibrium requires to hold that silver rises in proportion to it, and where the two lines meet the bath stops. The crossing sits at about 91 per cent of the stoichiometric limit, which is why the flat part of your curve is so flat and the turn so abrupt.

Halide is accumulating, and it is a product.

AgBr + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + Br⁻

Every ion of bromide released pushes back on the reaction that released it

Bromide and the complex rise together, so the numerator of the equilibrium expression grows as the square of the work done while the denominator shrinks. That is the mathematical shape of the corner you are measuring. And if your loading film is a high-iodide emulsion, a small quantity of released iodide does disproportionate damage, because silver iodide’s solubility product is three orders of magnitude below silver bromide’s and iodide is therefore the ion best placed to take silver back out of a complex.

What the doubling rule is really measuring. Not the point at which the bath cannot dissolve halide — that is much further on — but the point at which it can no longer complete stage two in a reasonable time. A bath whose clearing time has doubled is a bath in which the mono complex is forming freely and the fresh thiosulfate needed to convert it is in short supply, and that is the state that leaves the invisible sparingly-soluble compound in the film.

Data to record

• Film type and format used for loading and for the test strips, with the batch if you have it.
• Bath, dilution, volume, date mixed, and the fresh clearing time from the mixing lab for comparison.
• For every reading: number, clock time, cumulative loaded area in cm², cumulative film-equivalents, temperature, test-bath clearing time, control-bath clearing time, and the ratio to t₀.
• pH and specific gravity of both baths at the start, at the crossing and at the end, if you have the instruments.
• The scatter of the three phase-one readings in each bath, as a range.
• Anything that went wrong, with the reading number it affected.

Analysis

1. Convert area to films. Kodak’s area table gives a black-and-white 135-36 roll as 592 square feet per thousand, which is 550 cm².

films = total area in cm² ÷ 550

Film-equivalents from measured area

2. Correct for temperature, and say what the correction rests on. Kodak’s 1928 primer gives one measured pair: a film clearing in 95 seconds at 18 °C clears in about 60 at 29 °C. Fit an exponential through those two points and the rate constant is

ln(95 ÷ 60) ÷ (29 − 18) = 0.042 per °C

A two-point fit to Kodak's pair

so a clearing time measured at temperature T can be brought to a 20 °C reference by multiplying by e^{0.042 × (T − 20)}. Two points define an exponential exactly and test it not at all, and the course has no third point to check it with, so this is a working correction and not a measurement. Use it, state that you used it, and print the uncorrected numbers as well.

3. Find the crossing. Plot the ratio to t₀ against cumulative film-equivalents and read where it passes 2. Interpolate between your last two readings rather than taking the first reading over 2, because the curve is steep there and your reading interval is coarse.

4. Scale to a litre and compare. Multiply by ten if you ran 100 mL, or twenty if you ran 50. Set the answer beside ILFORD’s 24 films per litre at 1+4 — or Foma’s 17 per litre at 1+5, if that is the fixer you used, which is a different product at a different dilution and not a discrepancy.

5. Say which question you answered. You loaded undeveloped film, which sheds all of its silver. A darkroom processing pictures loads its fixer with only the fraction development did not reduce. So your figure is a lower bound on the capacity the same bath would show in normal use, and the honest form of the result is: this bath took N films of undeveloped 135-36 equivalent per litre before its clearing time doubled, measured under the criterion stated in the capacity lesson. That is the course’s own criterion and the sentence should say so.

6. What you cannot compute. Silver, in grams. It would take the film’s silver coating weight, and no published coating weight for any black-and-white camera film appears in any source the course holds — Kodak’s J-210 tabulates colour and radiographic families only. So the result stays in area and in films, which are the units you actually work in, and the bath’s silver concentration remains something Part XII measures rather than something this page infers.

The temperature correction, from Kodak's two points

• Factor = e^(0.042 × (T − 20))

Show the numbers behind this plot

A single rising exponential curve of correction factor against bath temperature, used to bring a clearing time measured at any temperature back to a twenty degree Celsius reference. The curve passes through one at exactly twenty degrees, falls to 0.88 at seventeen degrees and 0.92 at eighteen, and rises to 1.09 at twenty-two degrees and 1.15 at twenty-three and a half. The curve is computed from the only measured pair the course holds, from Kodak's 1928 primer, which gives ninety-five seconds to clear at eighteen degrees and about sixty seconds at twenty-nine, implying a rate constant of 0.042 per degree. Two markers on the curve show what the correction is worth in practice: a two degree drift across a session is a nine per cent change in clearing time, which is comparable with the entire change over the first two thirds of an exhaustion curve, and is therefore capable of hiding or of faking the result the experiment is looking for. A note records that two points define an exponential exactly and test it not at all, so the curve is a working correction rather than a measurement, and that the primer separately warns against letting an acid bath rise above twenty-one degrees Celsius because it is then apt to precipitate sulphur.

Series	Bath temperature, °C	Multiply the measured clearing time by
Factor = e^(0.042 × (T − 20))	16.00	0.85
Factor = e^(0.042 × (T − 20))	17.00	0.88
Factor = e^(0.042 × (T − 20))	18.00	0.92
Factor = e^(0.042 × (T − 20))	19.00	0.96
Factor = e^(0.042 × (T − 20))	20.00	1.00
Factor = e^(0.042 × (T − 20))	21.00	1.04
Factor = e^(0.042 × (T − 20))	22.00	1.09
Factor = e^(0.042 × (T − 20))	23.00	1.13
Factor = e^(0.042 × (T − 20))	24.00	1.18

Computed from a two-point fit to Kodak's 1928 pair, not measured. The reason it earns a place on the page is the size of the effect: two degrees is worth more than the first two thirds of an exhaustion curve. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Troubleshooting

What you see	Most likely cause	What to do
The three phase-one readings differ by more than 20 per cent	Endpoint judgement, not the bath	Use the reference spot rather than your memory of “clear”, and fix the lamp and card before continuing
Both baths drift the same way	Temperature or evaporation	Read the tray thermometer; top up the tray; apply the correction and say so
The control drifts and the test bath does not	Something is wrong with the control — a splash of the test bath, or the wrong beaker	Discard the reading, note it, and mark the control beaker unmistakably
The curve rises steadily from the first reading	The bath was not fresh, or was mixed weak	Check the specific gravity against the maker’s published figure; a bath at 1.040 where 1.070 to 1.080 is published was mixed at about half strength
Clearing time falls during the session	You are getting better at judging the endpoint, which the control will confirm	Report the ratio rather than the absolute time, which is what the control is for
The acid hardening bath goes cloudy during the session	Sulfurisation, or aluminium sulfite	Pale yellow and slow-settling is sulfur; white and gelatinous is aluminium sulfite. Either way the bath is finished and the reading is void
You run out of film before the crossing	The commonest outcome	Report the curve you have and the area at which you stopped, and say the crossing was not reached. An incomplete curve honestly labelled is a result

Clean-up

Every bath into the silver-bearing container, through a funnel, followed by the first rinse of every beaker. Second and later rinses to the general stream. The cleared film scraps drained, dried and bagged as solids. The thermometer, the tongs and the cylinder rinsed and dried. Update the running-volume line on the waste container’s label and add the date, per the labelling procedure.

Storage

Nothing from this session is stored. The four baths are spent — even the control, which has fixed twenty test strips — and they go to waste together. The only thing that survives the afternoon is the notebook, and the fresh clearing time you wrote at the head of it, which is the number the break/fix session will ask for.

Disposal considerations

The chemistry. The loaded bath carries dissolved silver as the bis(thiosulfato)argentate complex, along with accumulated bromide, unspent thiosulfate and the products of thiosulfate’s slow decomposition. Silver is the constituent that makes the stream both valuable and unwelcome in water; thiosulfate is oxygen-demanding. The complex is stable in the bottle and the silver stays in solution, so a full container is a concentrated silver solution rather than a sludge, and it is worth what a refiner will pay for it. Do not acidify it — that is the sulfur dioxide reaction — and do not mix it with a sulfide, which would precipitate silver sulfide and release hydrogen sulfide.

General practice. ILFORD’s guidance for domestic users in the United Kingdom is to bottle used chemistry separately, label it, and take it to a household waste and recycling centre; small quantities of scrap film and paper it treats as normal household waste. The disposal page sets out how the course reads the classification, and the silver-bearing waste procedure is the one to follow at the bench.

None of that chemistry changes with where you are, and the rules for the bottle do. Check your local regulations; they govern, they differ between authorities within one country, and they are the reason the disposal page computes no threshold and this page issues no instruction.

Questions

1. Your crossing came at 1,100 cm² in 100 mL. What capacity is that per litre, in 135-36 equivalents, and how does it compare with the figure your fixer’s maker publishes?
2. You measured at 22.5 °C throughout. What is the correction factor to 20 °C, and would applying it move your crossing point?
3. The control’s clearing time fell by 12 per cent over the afternoon while the test bath’s rose by 90 per cent. What do you report, and what would you have concluded from the test bath alone?
4. Why do the test strips have to be counted in the running total, and roughly what fraction of your loading were they?
5. A reader objects that loading with undeveloped film is unrealistic. Answer the objection in a way that makes your result more useful rather than less.

Further experiments

Run the same curve on the acid hardening bath. It is a sodium bath and a slower one, so it should start with a longer t₀; the question worth asking is whether its curve has the same shape. If the corner arrives at the same silver load in a bath that fixes half as fast, the corner is stoichiometric and the rate is a separate matter — which is what the theory says and is worth having measured.

Split the loading between two baths and run the two-bath rotation. Load bath one to its crossing, promote bath two, mix a fresh bath two, and keep going. Measure the clearing time in the second bath throughout. The prediction is that it barely moves, and the value of the experiment is that it turns a procedure everyone recommends into a number.

Add a fifth beaker, at half strength. ILFORD says a bath that is too dilute fixes poorly, and Part III’s arithmetic says capacity is very nearly proportional to thiosulfate. So a bath at 1+9 used on film should show a crossing at about half the area. That is a strong quantitative prediction with a cheap test, and a failure would be more interesting than a success.

Look at the last strip under a loupe, dry. The strips that cleared late are the ones most likely to carry the invisible compound. You will not see it — that is the point of the word — but comparing a strip cleared in fresh fixer with one cleared just before the crossing, both fixed for twice their own clearing time and washed identically, sets up exactly the sample pair Part XII’s residual-silver test will read.

Check your understanding

Question 1. Your test bath of 100 mL crossed twice its fresh clearing time after 1,320 cm² of undeveloped film had been through it, test strips included. What capacity does that give per litre, and what is the correct way to describe the figure?

• 2.4 films per litre; the bath is far worse than ILFORD claims
• 24 films of 135-36 equivalent per litre, which matches ILFORD's published figure — described as the course's own measurement under the course's own stated criterion, on undeveloped film
• 1,320 films per litre, since area converts directly
• It cannot be scaled, because capacity is not proportional to volume

Show the answer and why

Answer: 24 films of 135-36 equivalent per litre, which matches ILFORD's published figure — described as the course's own measurement under the course's own stated criterion, on undeveloped film

1,320 ÷ 550 = 2.4 film-equivalents in 100 mL, and ten times that is 24 per litre. Capacity does scale with volume, because the limit is stoichiometric — a litre holds ten times the thiosulfate of 100 mL and buys ten times the silver. The wording matters as much as the number: the figure is not an ISO capacity and does not certify anything, it is a measurement made under the doubling criterion the capacity lesson states in full, and it was made on undeveloped film, which sheds more silver per unit area than a processed negative does. Naming all three of those is what makes it usable by somebody else.

Question 2. The control beaker is never loaded, yet a third of the readings are taken in it. What exactly is it measuring, and what would you lose without it?

• Nothing useful; it is a formality that doubles the work
• Everything that changes across the afternoon except silver load — temperature drift, evaporation, variation between film scraps, and the observer improving at judging the endpoint — so without it a change in the test bath cannot be attributed to loading
• The rate at which fixer oxidises in an open beaker, which is the main cause of exhaustion
• The reproducibility of the timer

Show the answer and why

Answer: Everything that changes across the afternoon except silver load — temperature drift, evaporation, variation between film scraps, and the observer improving at judging the endpoint — so without it a change in the test bath cannot be attributed to loading

Any of those confounders on its own is large enough to matter: two degrees of drift is about nine per cent in clearing time, which is more than the whole change over the first two thirds of the exhaustion curve. The observer is the sneakiest of them, because getting better at spotting the endpoint makes clearing times fall, which looks like a fixer improving and would partly cancel the effect you are trying to see. A control that sits in the same tray, in the same fixer, judged by the same person under the same lamp, experiences all of that and none of the silver — which is precisely the definition of a control.

Question 3. Using Kodak's pair of 95 seconds at 18 °C and 60 seconds at 29 °C, what is the correction factor for a reading taken at 22.5 °C brought to a 20 °C reference, and what caution goes with it?

• About 1.11; and the caution is that two points define an exponential exactly while testing it not at all, so it is a working correction rather than a measurement
• About 0.89; and there is no caution, since Kodak measured it
• About 1.25; the relationship is linear and well established
• No correction is needed, since clearing time does not depend on temperature

Show the answer and why

Answer: About 1.11; and the caution is that two points define an exponential exactly while testing it not at all, so it is a working correction rather than a measurement

ln(95 ÷ 60) ÷ 11 = 0.042 per °C, so the factor is e^(0.042 × 2.5) = 1.11. The direction is worth checking by intuition rather than by algebra: a warm bath clears faster, so a time measured warm has to be scaled up to represent the same bath at 20 °C. And the caution is not pedantry — two points fit an exponential perfectly whatever the true relationship is, and the course has no third point to test it. Print the uncorrected numbers alongside the corrected ones so that a reader who disagrees with the model can redo the arithmetic.

Question 4. Why must the area of the test strips be added to the running total, and what happens if you leave them out?

• They need not be counted, since they are small and only immersed briefly
• They are undeveloped film immersed until it clears, so each one sheds its full silver charge into the bath exactly as a loading piece does; leaving them out overstates the capacity by whatever fraction of the total they represent
• They should be counted twice, since they are immersed twice
• They only matter for the control beaker

Show the answer and why

Answer: They are undeveloped film immersed until it clears, so each one sheds its full silver charge into the bath exactly as a loading piece does; leaving them out overstates the capacity by whatever fraction of the total they represent

A test strip does not know it is a measurement. Twenty strips of 15 cm² is 300 cm², which against a 1,320 cm² crossing is nearly a quarter of the loading — an error far larger than anything the temperature correction is worth. This is the general form of a real experimental trap: the instrument consumes the thing it is measuring. The alternative designs are worse. Reading fewer times loses the corner, and using smaller strips makes the endpoint harder to judge, which is where your scatter comes from in the first place.

Question 5. A reader objects that a bath loaded with undeveloped film is not a bath processing pictures. What is the strongest reply?

• The objection is wrong; a developed negative sheds the same silver
• The objection is right and the result is better for it: undeveloped film is the worst case, so the measured capacity is a lower bound on the capacity in ordinary use — and it is reproducible, where "a typical negative" is not, because the silver a negative sheds depends on how much of it was exposed
• The objection is right and the experiment is therefore not worth doing
• The objection is irrelevant, since clearing time does not depend on what has been through the bath

Show the answer and why

Answer: The objection is right and the result is better for it: undeveloped film is the worst case, so the measured capacity is a lower bound on the capacity in ordinary use — and it is reproducible, where "a typical negative" is not, because the silver a negative sheds depends on how much of it was exposed

The point of standardising on undeveloped film is precisely that it removes a variable nobody can control. Kodak's own method for estimating recoverable silver has to assume a percentage of exposure and picks a different figure for radiography and for graphic arts, which shows how much the answer moves. Choosing the worst case makes the experiment reproducible by somebody else, gives a bound rather than an estimate, and makes the direction of the error known. Reporting a bound and naming its direction is usually more useful than reporting an estimate whose direction you cannot state.

Question 6. Your curve is flat for the first two thirds of the loading, moving by less than the scatter you measured in phase one. What does that tell you about how a fixer should be monitored in ordinary use?

• That clearing time is a poor test and a silver estimator strip should be used instead
• That the test gives almost no early warning, so it must be run on a schedule tied to a capacity log rather than in response to something looking wrong — and that the log and the test answer different questions
• That the bath is not exhausting and can be used indefinitely
• That the scatter is too large for the test to be useful at all

Show the answer and why

Answer: That the test gives almost no early warning, so it must be run on a schedule tied to a capacity log rather than in response to something looking wrong — and that the log and the test answer different questions

The flatness is a real property of the chemistry, not a defect of the instrument: the free thiosulfate available and the free thiosulfate required converge only in the last part of the range, so almost nothing shows until almost everything does. The consequence for practice is that the running total is the early-warning system and the clearing time is the confirmation, which is why the capacity log has both columns. A silver estimator strip does not rescue this — Kodak's own publication puts its floor at 1 g/L, above the level that matters for fibre-base permanence.

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