Experiment: The Development Time and Temperature Series

Every later decision this course asks you to make about a negative is read off one curve, and this is the session that makes it. What development time gives the contrast a diffusion enlarger wants? What does a push buy? How far off is your thermometer? Each is a line on a plot of contrast index against development time, and until you have measured that plot for your film in your developer, every answer is somebody else’s borrowed. Ten strips, one evening, and a table you will use for years.

Purpose

To develop one film in one developer for six times at one temperature and three temperatures at one time, read base plus fog, the speed point and the contrast index off every strip on the instruments Parts XIV and XV built, and produce the contrast-index-against-time curve, an empirical temperature coefficient with an uncertainty on it, and a personal processing table. The design is set out in the usual three parts — the hypothesis, the control, and the one variable that changes — the first of them stated twice, because there are two arms.

Hypothesis, the time arm. That contrast index rises with development time along a curve that flattens, and that the manufacturer’s published time falls inside the range this series spans. It fails if the curve does not flatten inside the range, meaning the series was too short; or if the published time lands outside the spread of your six points, meaning your process and the published one are not the same process.

Hypothesis, the temperature arm. That the contrast indices at 18 and 24 °C at one fixed time can each be matched to an equivalent time on the 20 °C curve, and that the coefficient derived from those two times is distinguishable from unity at the resolution the budget allows. It fails if the band covers no useful range — a real possibility on one strip per temperature, as the sizing below shows, and a finding about the apparatus rather than the chemistry.

The control. The manufacturer’s published time and temperature, developed three times over in the same session — for the course’s reference pairing, 11 minutes at 20 °C, ILFORD’s figure for FP4 Plus at EI 125 in Kodak D-76 diluted 1+1. Those three strips do three jobs: one rung of the time arm, the 20 °C leg of the temperature arm, and the spread between them is the only measurement of this process’s own repeatability the session has.

The one variable. In the time arm, development time and nothing else — same exposure batch, lamp, wedge, developer mix, dilution, vessel, volume, agitation script, temperature, fixer, wash and reader. In the temperature arm, temperature and nothing else, at the control time.

!What this page is not

It is not a repeat of Part XV’s characterisation assignment, which took three times and taught the shape. This one takes six spaced by a stated rule, adds an arm the eye cannot substitute for, and carries an uncertainty on every number. It is also not a calibration against the manufacturer: the published time is the control, not the right answer.

Learning objectives

By the end of the session and the reading that follows it you will be able to:

• choose the levels of a series by equal steps of the logarithm of the variable, and say what an arithmetic series spends its last strips on;
• run ten development cells from one exposure batch in one evening, in a randomised order, with the temperature logged at the start, middle and end of every cell;
• read a strip by named constructions — the course’s speed criterion and its contrast-index convention — and read a development time off your own curve for a target contrast index, with a tolerance taken from that curve’s slope;
• convert a contrast index measured at another temperature into an equivalent time, derive a coefficient and its band from two such times, and set it beside published figures that disagree with each other;
• separate the errors that move the contrast curve from those that move the exposure index, and issue the processing table as a dated, versioned record.

Prerequisites

Designing an experiment that yields a number owns the method this page executes, and nothing here re-derives it. Four curve families, measured should come first: it builds the budget on real strips and writes the resolution sentence this page assumes. Development kinetics is the mechanism. Gamma, contrast index and average gradient and film speed and exposure index own the two constructions. Calibrating the sensitometer and calibrating the densitometer are the hard prerequisites: both certificates on the bench, or there is no Type B column and no resolution sentence.

Safety classification

Level A. Standard home-darkroom controls, on a session that opens no jar of powder.

Every solution is diluted from something already made: the D-76 stock from Part VIII’s mixing lab, the fixer and wetting agent from bought concentrates. Under the course’s rubric that is gloves, eye protection whenever a solution is poured or agitated, dedicated labelled vessels, and the ordinary ventilation of a wet bench — the controls Part IX’s test-negative lab set for the identical operation.

What is not a hazard here, and why. No powder is weighed, so the inhalation route that makes metol and hydroquinone a Level B proposition in the mixing lab is not open here: both are present, but in solution at a developer’s working dilution, so the control is contact rather than airborne. Nothing is heated above 24 °C, so there is no thermal hazard. No concentrated alkali is handled, because D-76’s alkali is borax already dissolved. And a fixed, washed strip presents no chemical hazard: its silver is metallic and locked in hardened gelatin, and the soluble silver-thiosulfate complexes left in the wash. None of those absences is a general statement about the substances; each is about this operation, which is what a hazard assessment is.

One thing does get warm: the 24 °C bath, usually topped up from a kettle. Water at 24 °C scalds nobody; a kettle does. Bring the bath up before the session and keep the kettle off the wet bench.

Hazards

Hazard	Where it arises	Control
Skin sensitisation from metol and hydroquinone	Pouring, agitating and emptying ten cells of D-76 1+1	Single-use nitrile gloves throughout, changed if one is splashed inside. The glove page has the compatibility argument
Splashes to the eye, and working in darkness with liquids	Four lifts a minute for an hour, with the light off, over open vessels	Eye protection from the first pour to the last; vessels filled with headroom and lifts made slowly; the bench laid out in the light and not moved afterwards; one hand wet and one dry; the light switch found by touch first
Spent fixer, acidic and silver-bearing	Emptying the fixing tray	Its own labelled container to the silver stream, never combined with the alkaline developer waste
Hot water for the 24 °C bath, and loss of the exposure batch	Bringing the bath up; ten strips exposed in one sitting	Fill and stabilise the bath before the session and keep the kettle off the bench; the tin stays closed except during loading, and strips are notched by touch before the light goes on

Required PPE

Single-use nitrile gloves for every wet operation, donned and removed per the PPE SOP, because metol and hydroquinone are skin sensitisers and a sensitisation, once acquired, does not go away. Eye protection from the first pour to the last emptied vessel; splash goggles beat spectacles for this many lifts in the dark. And clean dry hands for the film, which protects the strips rather than you: handle every strip by its edges, because a fingerprint on a step is read as density.

No respiratory protection applies, for a specific reason rather than a reassuring one: nothing here is a powder and nothing is heated to a temperature at which the bath’s vapour changes. The particulate control belongs to the mixing lab.

Ventilation

Ventilation is not the control this page relies on, because nothing here evaporates or aerosolises at 20 to 24 °C: the developer is a dilute aqueous solution of an aminophenol salt, a phenol, a sulfite and a borate, and the fixer a thiosulfate at working strength. What the room needs is ordinary comfort ventilation for two hours in the dark, usually a light-tight vent, which Part XVI’s darkroom build solves. If the fixer is an acidic rapid type and the room is small, ventilate as the fixer mixing SOP sets out.

Materials

Item	Quantity	Note
Test strips from one exposure batch	10	135 mm strips, one emulsion batch, exposed in one sitting. Two spares if the tin allows
Opaque batch tin	1	Opened only with the light off
Transmission step wedge	1	The Stouffer T2115 or equivalent, the same one used for the whole batch
Card label slips	10	The eight-field slip specified by the test-negative lab. A strip without one is not data
Negative sleeves	5 pockets	Two 135 mm strips head to head in each
Measuring cylinders, 100 mL	4 to 6	One per simultaneous cell, standing in the bath, dedicated to developer
Water baths	3	One at each temperature. A deep tray, a bowl and an insulated box all work
Film clips and a drying line	10 clips	Still, dust-free air. Do not squeegee a test strip
Curve-plotting worksheet, lab notebook sheets and the formula version record	10 + 1 set + 1	One plotting sheet per strip; the version record names the bottle this session drew from

Chemicals

Nothing is weighed. Every solution is diluted from a stock or a concentrate, per the SOP for mixing from a stock.

Chemical	Quantity	Form
D-76 stock from Part VIII’s lab — metol, hydroquinone, sodium sulfite, borax	450 mL	Solution, diluted 1+1 immediately before use and discarded after one strip, as Kodak’s J-78 sheet directs
Plain water rinse at the cell’s own temperature	1.2 L, two changes per strip	The course’s choice throughout Part IX; ILFORD note a water rinse may replace a stop bath but increases the risk of processing marks and stains
Rapid fixer, ammonium thiosulfate type	500 mL at 1+4	Fixed for twice the clearing time, per the clearing-time SOP
Wetting agent	500 mL at 1+200	ILFORD’s figure for ILFOTOL is 5 mL per litre

The silver bromide in the emulsion is a reagent rather than a substance you handle: it is what the developer reduces, and the bromide it releases ends up in the developer waste.

Equipment

The sensitometer from Part XIV with its certificate, used per the exposure-session SOP. The densitometer from Part XV with its certificate and its terminal logging to a file, checked per its SOP. A changing bag. One thermometer for the whole session, checked per the bench-check SOP — the reason it is one is in the Analysis. A stopclock reading seconds, three trays, and a spreadsheet or short script that can solve one equation by trial, because the contrast-index construction is a one-unknown root find. None of it is consumed, which is why none of it appears in the table below.

Estimated cost

££ on the planner’s bands, almost all of it capital bought earlier: the two instruments, the wedge, the cylinders, the thermometer and the changing bag. This session spends a third of a cassette of film and about half a litre of diluted developer.

Estimated consumables cost

One run, at ten strips:

Consumed	This session	Sourced price	Cost this session
35 mm film, one emulsion batch	10 strips of 135 mm, about one 36-exposure cassette	£6.37–£11.40 per 36-exposure roll	£6.37–£11.40
D-76 stock, from Part VIII	450 mL, diluted 1+1 and discarded strip by strip	Costed in Part VIII’s mixing lab	—
Rapid fixer concentrate	100 mL, to make 500 mL at 1+4	£21.05–£25.98 per 1 L of concentrate, diluted 1+4 for film	£2.11–£2.60
Wetting agent	2.5 mL, for 500 mL at 1+200	£28.70 per 1 L of concentrate, diluted 1+200	£0.07
Water for rinses and wash	about 6 L	Metered supply; the planner prices no water	—
Negative sleeves and card label slips	5 pockets, 10 slips	None. A named price gap: sleeving that passes the Photographic Activity Test	—
Squared paper for four plots	4 to 6 sheets	None. The planner carries no stationery line	—

The priced rows come to £8.55 to £14.07, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 4 of the 7 rows carry no dated price, so they count as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation. The film row prices a cassette because that is what the price file carries a dated figure for; a 30.5 m bulk tin yields around 225 strips and is far cheaper, and the file has no bulk-tin price, so the cheaper route is the one this table cannot cost. If the strips come from Part IX’s exposure batch instead, the floor drops to £2.18 to £2.67.

Waste streams

Three, kept apart, labelled per the container SOP and routed per the general waste SOP.

1. Spent developer, about 800 mL of D-76 at 1+1, alkaline, carrying metol, hydroquinone, sulfite, borate and the bromide from ten strips. It is one-shot by Kodak’s instruction, so all of it is waste.
2. Spent fixer and the first rinse after it, silver-bearing, to the silver stream.
3. Rinse and wash water: the two rinses after each development go with stream 1, the first change after fixing with stream 2, the rest with the general stream.

Streams 1 and 2 are never combined — one is a sulfite solution at a borax-buffered developer’s pH and the other is acidic, and mixing them spoils the silver recovery.

Alternative route

The session needs darkness for two operations and about twenty minutes: taking the strips from the tin and notching them, and the wet run itself, because the strips are lifted from open cylinders once a minute. Loading the sensitometer, fixing, washing, drying, reading and every piece of arithmetic happen in room light or inside a changing bag.

Route one: the daylight tank, which removes the darkroom entirely. Load each strip onto a 35 mm spiral in a changing bag, develop in a closed tank standing in the bath, and agitate by inversion. Two things improve — the agitation becomes the manufacturer’s own scheme, so the Analysis comparison becomes fair instead of approximate, and a closed tank holds temperature better than an open cylinder. Two get worse: one strip per run, because a tank cannot hold two clocks and a wet spiral cannot be reloaded, so ten cells become ten sequential runs over two evenings; and the developer temperature can only be read on the way in and on the way out. Log both, and treat the difference as the term the cylinder would have measured directly.

Route two: the orthochromatic route. If the batch is on ILFORD ORTHO Plus, the notching and the cylinders run under a deep red safelight — ILFORD’s 906 filter with a 15 W bulb at not less than 1.2 m. The cost is that the film is not the same film, so this series cannot be compared with a panchromatic one and the published times must be ORTHO Plus’s own. Route three: a windowless room at night, proved rather than assumed with the blackout leak test.

If none is available, the experiment cannot be run and the honest thing is to say so rather than substitute something that is not it. The analysis half still stands: read the archived Part IX strips, which include cells at 10 and 11 minutes, and plot the two points with a note that two points cannot show a shape. Then borrow. One evening in somebody else’s darkroom, or a processing service that will develop to a stated time on request, buys the six cells, and the reading, plotting and arithmetic are yours afterwards. A series developed by somebody else is weaker because the processing is not under your control — say so in the report — but it beats a table copied off a datasheet.

Preparation

About 60 minutes the evening before, plus the exposure session. The 180 minutes here is the wet run and the first pass of arithmetic; the strips cannot be read until bone dry, so the densitometer session is a third sitting. Every decision below is made before a number exists, because a decision made afterwards is a preference.

One film, one developer, and where the control comes from

The course’s reference pairing is ILFORD FP4 Plus in Kodak D-76 at 1+1: Parts VIII and IX already built and archived it, ILFORD publish a time for it, and Kodak publish the dilution instruction the session obeys. HP5 Plus in ID-11 or Tri-X in D-76 work identically.

Look the control up rather than taking it from here, because it depends on the meter setting. ILFORD’s FP4 Plus sheet gives Kodak D-76 at 1+1 as 9, 11 and 15 minutes for EI 50, 125 and 200 in a spiral tank with intermittent agitation; at the film’s own EI 125 that is 11 minutes.

Read what the makers say about their own numbers, because it is why this experiment is not redundant. ILFORD’s times are “intended as a guide” that “may be altered if a different result is needed” and “may need adjusting to suit individual processing systems and working practices”. Kodak are blunter: starting-point recommendations, and “for critical applications, run tests to determine the best development time”. This page is the test they are telling you to run.

✕Check the date on the sheet you compare with

The ILFORD datasheets this course reads carry a footer dated November 2018. ILFORD also publish a Film Processing Chart poster marked Version January 2025, whose table the course could not read — only its headings are in the PDF’s text layer — so it quotes no figure from it and cannot say whether the two agree. If the published time has moved since 2018, a disagreement with “the published time” may be a disagreement with a superseded one. Read the current sheet yourself and write its date in the notebook.

The six times: equal steps of the logarithm

The series must reach from clearly under-developed to where contrast index stops climbing usefully, and spend its six strips where they buy something. Both point at one rule.

t_k = t₀ × 10^k/10, for k = −3, −2, −1, 0, +1, +2

The ladder of development times

Each rung is 0.10 larger in the logarithm of the time than the one below, so the factors are 0.501, 0.631, 0.794, 1, 1.259 and 1.585 and six rungs cover a factor of 3.16. Kodak’s workbook notes that 0.1 is a convenient logarithmic interval and one third of the 0.301 that is a doubling, so each rung is a third of a stop of development time.

Rung	Factor	Exact time	Set the clock to
k = −3	0.501	5.51 min	5:30
k = −2	0.631	6.94 min	6:55
k = −1	0.794	8.74 min	8:45
k = 0, the control	1.000	11.00 min	11:00
k = +1	1.259	13.85 min	13:50
k = +2	1.585	17.43 min	17:25

Round to five seconds and write the time you actually ran. Both makers bound the bottom rung: ILFORD’s chart says times below five minutes are not recommended because of the risk of uneven development, and Kodak’s D-76 sheet says tank times under five minutes may produce poor uniformity. If your published time is 10 minutes or less, shift the ladder up one rung, to k = −2 … +3.

!In what sense a degree of temperature costs more at a short time

That five-minute floor is a transport statement, and it is where the common claim that a temperature error hurts more at short times is actually true. Proportionally the coefficient is the same everywhere: nine or ten per cent of the time per degree at five minutes and at seventeen. What changes is that every fixed-size error — filling and draining, the time a cold strip takes to reach bath temperature, the transient of the pour — is a larger fraction of a short run. Thirty seconds of equalisation is nine per cent of 5:30 and three per cent of 17:25.

In contrast terms the claim is not true as usually stated. The effect of a degree on contrast index is the coefficient multiplied by the slope of the curve, and on Kodak’s published shape that product is about 0.022 per degree at five minutes, 0.027 at eight and eleven and 0.013 at thirteen — largest in the middle. Respect the floor for uniformity and dead time, not for contrast sensitivity.

What the spacing rule buys: equal steps of log time give equal steps of contrast

• Kodak H-740's six published contrast indices
• Six levels spaced by equal time: 1.6 minutes apart
• Six levels spaced by equal log time: a factor of 1.21 apart

Show the numbers behind this plot

Kodak's six published contrast indices are drawn as a rising line from 0.51 at five minutes through 0.55 at six, 0.62 at eight, 0.67 at ten and 0.72 at twelve to 0.73 at thirteen minutes, steep at the left and nearly flat at the right. Two sets of six marker points are laid on that same line, both running from five to thirteen minutes and both starting and finishing at the same two contrast indices. The square markers are spaced by equal steps of time, 1.6 minutes apart, and land at contrast indices of 0.510, 0.571, 0.625, 0.665, 0.705 and 0.730; the gaps between successive squares are 0.061, 0.054, 0.040, 0.040 and 0.025, so the first pair is more than twice as far apart in contrast as the last pair. The triangular markers are spaced by equal steps of the logarithm of time, a factor of 1.21 apart, and land at 0.510, 0.552, 0.597, 0.642, 0.689 and 0.730; their gaps are 0.042, 0.045, 0.045, 0.047 and 0.041, which are equal to within a hundredth. The drawing's point is that both sets spend six strips over the same range, but the equal-time set spends its last two on a difference of 0.025 that a home bench may not be able to resolve, while the equal-log-time set buys about 0.045 of contrast at every rung.

Series	Development time, minutes	Contrast index
Kodak H-740's six published contrast indices	5.00	0.51
Kodak H-740's six published contrast indices	6.00	0.55
Kodak H-740's six published contrast indices	8.00	0.62
Kodak H-740's six published contrast indices	10.00	0.67
Kodak H-740's six published contrast indices	12.00	0.72
Kodak H-740's six published contrast indices	13.00	0.73
Six levels spaced by equal time: 1.6 minutes apart	5.00	0.51
Six levels spaced by equal time: 1.6 minutes apart	6.60	0.57
Six levels spaced by equal time: 1.6 minutes apart	8.20	0.63
Six levels spaced by equal time: 1.6 minutes apart	9.80	0.67
Six levels spaced by equal time: 1.6 minutes apart	11.40	0.70
Six levels spaced by equal time: 1.6 minutes apart	13.00	0.73
Six levels spaced by equal log time: a factor of 1.21 apart	5.00	0.51
Six levels spaced by equal log time: a factor of 1.21 apart	6.06	0.55
Six levels spaced by equal log time: a factor of 1.21 apart	7.34	0.60
Six levels spaced by equal log time: a factor of 1.21 apart	8.89	0.64
Six levels spaced by equal log time: a factor of 1.21 apart	10.77	0.69
Six levels spaced by equal log time: a factor of 1.21 apart	13.00	0.73

Kodak's six values are published, for a film and developer its sensitometry workbook declines to name and calls XYZ and A; the two sets of marker points are this course's own arithmetic, interpolated along those published values to show where each spacing rule would land. Neither set describes a material you can buy, and your own curve will have a different shape - which is exactly why the spacing rule, and not the numbers, is what transfers.

Both spacings cost six strips over the same range. The equal-time set buys 0.061 of contrast index between its first two levels and 0.025 between its last two; the equal-log-time set buys about 0.045 at every rung. If your resolution allows differences of 0.03 and above, the equal-time series has spent its last strip on a difference it may not report. Kodak’s own bracket for finding a control-strip time is arithmetic — 4, 6, 8, 10 and 12 minutes — which is a different job: it hunts for the single time landing a contrast index within ±0.02 of a known aim and stops when it finds one. An arithmetic bracket is right for finding a point and wrong for drawing a curve.

The temperature arm, and the cell that does three jobs

Three more strips, all at the control time, at 18, 20 and 24 °C — the coldest and warmest columns of Kodak’s own published D-76 tables, so the arm measures exactly the interval two manufacturers publish across. ILFORD state a usable range of 20 to 24 °C for their own powder developers while publishing compensation figures down to 18 °C, so 18 °C is the cold end of what is published and below the range one maker recommends for its own product, and the notebook says so beside the result. The 20 °C leg is not a fourth strip; it is the control cell.

Ten strips, two arms, one shared cell

1. 1Arm one: six times at 20 °C — spaced by 0.10 in log time — a third of a stop each — with everything else held
2. 2The control cell, 11:00 at 20 °C — the manufacturer's published time, developed three times over
3. 3Arm two: three temperatures at 11:00 — 18 and 24 °C are the ends of Kodak’s own published D-76 table
4. 4The shared leg — the 20 °C leg is the control cell, not a fourth strip

Ten strips, one exposure batch, one evening. The design's whole economy is in the shared cell: three strips at one condition pay for the control, for the middle of the temperature arm, and for the resolution sentence the analysis is measured against.

Sizing the temperature arm before spending film on it

The temperature arm is the marginal measurement of this part. The arithmetic below is a sizing exercise on Kodak’s published numbers, not a prediction, done in advance so that it cannot be adjusted afterwards.

🧮What one strip per temperature can and cannot settle

One, measure the contrast index at 18 and 24 °C at the control time. Two, read each back off the 20 °C curve as an equivalent time; its uncertainty is the contrast index’s divided by the slope of your own curve there:

Δt = Δ(CI) ÷ (dCI/dt)

Turning a contrast uncertainty into a time uncertainty

Three, the ratio of the two equivalent times is the ratio of the rates. On ILFORD’s published ratios, 11 minutes at 18 °C is equivalent to about 9.0 at 20 °C and 11 at 24 °C to about 16.0 — a ratio of 1.77 over six degrees, or 10.0 per cent per degree, or a Q₁₀ of 2.60. Now the band, at a resolution of 0.02 in contrast index (Kodak’s own acceptance window), a slope near nine minutes of about 0.025 per minute, and ±0.4 °C on the interval:

The curve at the warm equivalent time	Strips per temperature	The band on the coefficient
Still climbing, about 0.025 CI per minute	1	7.0 to 13.5 % per degree — Q10 1.97 to 3.56
Still climbing, about 0.025 CI per minute	3	8.0 to 12.4 % per degree — Q10 2.16 to 3.21
Flattened, about 0.010 CI per minute	1	5.8 to 15.1 % per degree — Q10 1.75 to 4.07
Flattened, about 0.010 CI per minute	3	7.3 to 13.2 % per degree — Q10 2.02 to 3.46

The arm confirms an order of magnitude and cannot refine one: one strip a temperature distinguishes ten per cent a degree from five or twenty, not eight from twelve. Presentation can hide an uncertainty — the same result reads as “10.0 per cent per degree, between 7 and 13.5” or as “a Q₁₀ of 2.60, between 1.97 and 3.56”, because Q₁₀ raises the ratio to the power 10 ÷ ΔT, so quote both. And the slope at the warm end decides, which is checkable from the time arm before the temperature strips are read: state the slope, and if it is below about 0.015 per minute, report the coefficient with its widened band and name the term that did the damage.

The agitation script, and the fork you have to take

ILFORD’s published times for a spiral tank assume four inversions during the first ten seconds, and four more during the first ten seconds of each further minute. Part IX’s cylinder script — gentle movement for thirty seconds, then lift, drain two seconds, re-immerse each minute — is a scaled-down version of Kodak’s large-tank procedure, and is not what ILFORD’s eleven minutes assumes. Cylinders buy throughput, four cells at once, and a thermometer standing in the developer throughout; a tank buys a comparison with the published time that is fair rather than approximate. What is not defensible is changing scheme partway, or not recording which you used.

Size the difference before choosing. ILFORD state that continuous agitation calls for reducing spiral-tank times by up to 15 per cent, and Kodak advise a 10 to 15 per cent change to correct negatives that are consistently flat or contrasty. A lift-and-drain script sits between the two, so the plausible agitation term is nought to about 15 per cent of the time — 1.7 minutes on 11, roughly 0.04 in contrast index on Kodak’s published slope. That is comparable with the whole resolution budget, which is why the scheme is recorded in words: lower the strip in and move it gently up and down for the first 30 seconds; then at the start of each subsequent minute lift it clear, drain for a count of two, and re-immerse; stagger the starts within a batch by 30 seconds; drain at the stated time and go straight into the rinse.

Run order, the data table, and the developer volume

Draw the run order from a hat. Everything that drifts through a session enters in a sequence, and if the sequence follows the variable, none of it can be separated from it afterwards. Shuffle ten cards, record the order before the session, and follow it even when it puts the shortest cell last. Randomise the reading order separately.

Draw the data table in full, because a table drawn afterwards acquires exactly the columns that turned out to be interesting. Ten rows — T-1 to T-6, C-2, C-3, K-18, K-24 — and columns for run order, intended and actual time, bath and developer temperature at start, middle and end, base plus fog, speed point, contrast index and deviations.

Fix the analysis now, in writing. The constructions are Part XIII’s and no others. Base plus fog comes from each strip’s own masked patch, read the same day on the same instrument as the strip beside it. The exclusion rule for a spoiled strip is stated in physical terms — a drying mark on a step, a lift missed, a temperature excursion — not as “it looked wrong”.

Check the developer volume against the maker’s own minimum. Kodak state that one 135-36 roll, given as 80 square inches, is developed in 473 mL of diluted D-76, and that using 237 mL calls for a 10 per cent time increase. A 135 mm strip is 7.32 square inches, 9.2 per cent of a roll, so Kodak’s rate implies about 43 mL. The 80 mL per cylinder this page uses is 1.85 times that, the margin that lets it ignore the correction. Ten cells is 800 mL of working solution and 400 mL of stock; take 450.

Procedure

Six stages after the exposure batch: preflight and the dummy temperature run, 40 minutes; allocating the strips in darkness, 15; the time arm, 55; the temperature arm, 20; wash and file, 30; and the reading session on another evening, 90.

Stage 0 — The exposure batch

1. Expose all ten strips, plus two spares, in one sitting on the sensitometer from one lamp at one distance through one wedge, per the exposure-session SOP, logging the minutes since switch-on against every strip. Every strip carries the masked patch the test-negative lab specified, because base plus fog comes from each strip’s own patch — which is what makes ten strips enough.
2. Load in a changing bag, store the exposed strips in the closed tin, and develop them as soon as the calendar allows. ILFORD’s instruction for FP4 Plus is to process exposed film as soon as practical, and the course has no figure for what months of keeping do to a latent image. Days are fine; months are a term you cannot size.

Stage 1 — Preflight, and the dummy temperature run

3. Check the thermometer against a reference per the bench-check SOP and write the offset down. Set up three water baths at 18, 20 and 24 °C, brought up before the session and given twenty minutes to settle with the cylinders standing in them.
4. Run the baths for fifteen minutes with the thermometer in a cylinder of water, logging every minute. This dummy run measures what your bath holds tonight, and the spread is the temperature term in the budget. Kodak state that a developer temperature varying by more than 0.3 °C affects process control and image quality; ILFORD ask that all process solutions be within 1 °C. If your bath cannot hold 0.3 °C, say so now and carry the larger figure through. A coefficient derived from temperatures you did not hold is not a result.
5. Lay the bench out in the light in the order it will be used, find the light switch by touch, make up the fixer at 1+4 and the wetting agent at 1+200, and bring the rinse water to the temperature of the cell it will follow. Measure the clearing time on a scrap per the clearing-time SOP; the fixing time is twice it.

Stage 2 — Allocate the strips, in darkness

6. With the light off, nip each of the ten strips along the top edge from the datum notch — one nip for T-1 through six for T-6 — then one to four nips on the lower edge for C-2, C-3, K-18 and K-24, so no strip is ambiguous by touch. Close the tin and put it away before the light goes on, and lay each strip beside its cylinder in the drawn run order.

Stage 3 — The time arm

One evening, ten cells: an example draw

T-5 · 13:50 · 20 °C

develop 13:50rinsefix

T-1 · 5:30 · 20 °C

develop 5:30rinsefix

C-3 · 11:00 · 20 °C

develop 11:00rinsefix

T-6 · 17:25 · 20 °C

develop 17:25rinsefix

T-3 · 8:45 · 20 °C

develop 8:45rinsefix

T-4 · 11:00 · 20 °C · control

develop 11:00rinsefix

T-2 · 6:55 · 20 °C

develop 6:55rinsefix

C-2 · 11:00 · 20 °C

develop 11:00rinsefix

K-18 · 11:00 · 18 °C

develop 11:00rinsefix

K-24 · 11:00 · 24 °C

develop 11:00rinsefix

0 min20406075

Four cells at a time, one minute apart, is the most a single pair of hands can lift on a one-minute cycle; the temperature arm runs last as a pair because two baths need two thermometer readings a minute. The order shown is one draw from the hat and is not a prescription — yours will be different, and the difference is the point.

7. Batch one, four cells, started a minute apart in the drawn order, each cylinder filled with at least 80 mL of freshly diluted D-76 1+1 immediately before its cell begins, on the written agitation script.
8. Log the developer temperature at the start, midpoint and end of every cell — the developer, with the thermometer in the cylinder, not the bath. This column decides whether the temperature arm is a result, and it is the easiest to skip in the dark.
9. Lift each strip at its own mark, rinse for one minute in two changes of plain water at the cell’s own temperature, and fix for twice the clearing time.
10. Batch two, the remaining four twenty-degree cells, cylinders rinsed and refilled, fresh developer for every cell. Kodak’s instruction for the diluted solution is that it is discarded after one batch and neither reused nor replenished, and a bath that has already developed a strip is a different bath — the subject of the exhaustion experiment, which must not leak into this one.

Stage 4 — The temperature arm

11. Run K-18 and K-24 together, a minute apart, in their own baths, at the control time. Two cells at once is the limit, because each needs its own thermometer reading every minute.
12. Bring each cell’s rinse water to that cell’s temperature: ILFORD ask that all process solutions be within 5 °C of the developer, and a strip taken from 24 °C developer into 18 °C water has had a thermal shock that belongs to nothing in the design.
13. Log the temperatures as before. If either cell drifts outside the tolerance set in stage 1, write the excursion down and finish the cell anyway. A recorded excursion is data; a cell abandoned halfway is nothing.

Stage 5 — Wash, dry, label and file

14. Wash all ten strips for 5 to 10 minutes in running water within 5 °C of the process temperature, final-rinse in wetting agent at 1+200, and hang them in still, dust-free air. Do not squeegee a test strip: a drying mark on a step is read as density.
15. While they dry, complete a card slip for every strip — all eight fields — and record the formula version code of the developer stock, issued per the versioning SOP. A curve tied to “D-76, the bottle on the shelf” is a curve nobody can check.
16. When they are bone dry — not before — sleeve them in pairs and file them. Do not read them tonight: a strip that is not quite dry reads high.

Stage 6 — The reading session, on another evening

17. Run the densitometer’s warm-up and daily check per its SOP, recording the calibration step before the first strip and after every third. A drift caught at the third strip costs three strips; caught at the end it costs the session.
18. Read all twenty-one steps and the masked patch of every strip, in the separately drawn reading order, logging to a file rather than transcribing. Read three of them twice, lifting each off the stage and replacing it between readings: that replaced-sample spread is your Type A term.

Expected observations

This says what the design expects and what would show the expectation wrong. It does not say what your numbers will be, because a page that reports the result has pre-empted the experiment.

In the trays. The six time-arm strips come out visibly different, and the difference is concentrated at the dense end, because heavily exposed steps have many developable crystals while lightly exposed steps run out early. If they look equally different all the way down, the fault is in the exposure rather than the development, and the place to look is the wedge and the lamp. The 24 °C strip should look denser than the control and the 18 °C strip thinner; if the 24 °C strip is denser and greyer in its masked patch, you have seen the fog reaction, whose coefficient Kodak’s 1928 primer says is much higher than development’s. Read the patch rather than judging it.

Four things that would falsify the design rather than the chemistry. The three control strips spreading further than the resolution sentence allows; the bottom rung failing to give a readable curve; the top rung sitting on the one below it; and the 24 °C equivalent time landing beyond the top of the measured range, which would force an extrapolation the plan forbids.

One observation that is not a fault. The strips will not agree with the published time. Almost nobody’s do, the makers say so on the sheets, and the disagreement is the subject of the Analysis rather than a reason to repeat the session.

What is happening chemically

Why time is a contrast control and barely a speed control

Development is the reduction of silver ions to metallic silver, paid for by the oxidation of the developing agent. For hydroquinone in an alkaline bath, and for the halide the emulsion holds:

C₆H₆O₂ + 2 AgBr + 2 OH⁻ → C₆H₄O₂ + 2 Ag + 2 Br⁻ + 2 H₂O

Two silver ions reduced per molecule of hydroquinone consumed, with bromide released into the bath

The reaction starts at the latent-image specks the exposure made and is autocatalytic: the silver already deposited catalyses the reduction of more. A crystal with a large speck begins almost at once; one with a marginal speck takes longer to start at all. So extra time adds much density where there were many developable crystals and little where there were few, and the difference between a heavily and a lightly exposed region grows. That difference is the slope contrast index measures. The toe, where the speed point lives, is made of crystals that barely have a speck: exposure places the shadows and development sets the slope.

Why temperature is not simply time

The compensation charts claim that a change of temperature can be exactly undone by a change of time. That claim is what makes the equivalent-time construction legal, and it is a claim rather than a law. Behind it is the Arrhenius relation, which Part III sets out: if development were a single reaction whose extent depended only on rate multiplied by time, halving the rate and doubling the time would give the same negative.

Development is at least four things in series — the developer diffuses into the swollen gelatin, reaches the crystal surface, transfers electrons, and has its oxidised product carried away while fresh agent arrives — and whichever step dominates decides what an Arrhenius fit to a table of times returns. Such a fit gives an apparent activation energy for the composite rather than for any step, which is why this course quotes none for a named developing agent: it holds no peer-reviewed measurement of one, and Part III’s 67 kJ/mol is its own fit to ILFORD’s published chart, marked as such. Kodak’s 1928 primer said the first part outright — the speed of development depends chiefly on the rate at which the developer diffuses into the film — and drew the consequence: because a small temperature change affects hydroquinone greatly and metol very little, a cold MQ developer and a warm one are not the same developer run at two speeds.

So the arm’s honest test is not “does the coefficient come out at 2.5”. It is whether the temperature strips can be matched to the 20 °C curve at all — whether one equivalent time describes each, or whether the 18 °C strip matches on contrast and fails on the speed point and the shape of its toe. Matching on one and not the others means temperature changed the developer and not just its clock.

What the fog reaction does at 24 °C

Kodak’s second 1928 statement ends the argument for warming a developer to save time: the fog reaction has a different and much higher temperature coefficient than development, and there is no compensation chart for fog because it is not a time you can shorten your way out of. This session measures it for free, because base plus fog comes from every strip’s own masked patch. Plot it — three points against temperature at one time, six against time at one temperature — and if base plus fog at 24 °C has risen past your resolution, say so in the result: a contrast index measured on a strip whose base has moved has had its construction’s zero moved with it.

Data to record

Copy this into the notebook before the session.

Session header. Date; who; room temperature at start and end; the three bath set points; every instrument with its checked offset; the sensitometer and densitometer certificate dates; wash water temperature. Two certificate dates on the page with the result are what make it quotable a year later.

Exposure batch block. Film, format, emulsion batch number; the sensitometer’s lamp, warm-up and cadence; the wedge part number and whether it is calibrated; the exposure time and date; the interval between exposure and development.

Developer block. The formula version of the D-76 stock, its mixing date, its measured pH and that measurement’s temperature; the dilution; the volume per cylinder; the agitation script in full words.

The dummy run log, with the spread computed — the term the temperature arm rests on, measured on the night rather than assumed. One row per cell, from the table drawn in Preparation, plus base plus fog, speed point, contrast index and the local slope after the reading. The reading session: reading order, the calibration step before the first strip and after every third, the replaced-sample spread, and the file the log went to.

Deviations. A lift missed. A cylinder knocked. A bath at 24.6 °C at the ninth minute. The thermometer 0.4 °C low at the end. Write them at the time, because a deviation remembered afterwards is a deviation reconstructed to suit the result.

Analysis

1. The resolution sentence, restated before anything is plotted

Take the three control strips — T-4, C-2, C-3 — and compute the spread of their contrast indices, speed points and base-plus-fog readings. That spread is a Type A term in NIST’s sense, against the Type B figures off two certificates. Combine them by Part II’s rule: add for a bound, and compute the root-sum-of-squares too if you like, but say which rule gave which figure and never print the smaller one bare. Then write:

The smallest difference in contrast index this session may report is ____. The smallest shift in the speed point is ____ log H. The smallest change in base plus fog is ____ D.

Three anchors sanity-check it and none replaces it: Kodak’s tightest action limit on contrast index is 0.07 either side of aim and their acceptance window for choosing a time is ±0.02; X-Rite publish ±0.01 D repeatability and ±0.02 D linearity for a commercial desktop transmission densitometer; and the course’s contrast-index convention is expected to agree with the straightedge version to about 0.02, the floor on any comparison with a figure drawn with a straightedge, a manufacturer’s included.

2. Contrast index against time, and speed against contrast index

Plot the six contrast indices against the actual times, with the three control values shown as three points rather than a mean so the plot carries its own scatter, and interpolate between neighbours rather than fitting a curve through six points. Then say which of three descriptions the shape fits: still climbing at the top rung; flattening; or flat, with the top two rungs inside your resolution. Kodak’s workbook names the plot’s purpose in one line — to find the development time for a desired contrast index — and the shape decides how far up it that operation stays honest.

Then plot the speed point against contrast index: contrast index is what you choose and the speed point is what it costs. Expect the speed axis to move far less and its uncertainty to be larger, for an arithmetic reason — a density uncertainty becomes an exposure uncertainty by division by the local gradient, and the speed point sits on the toe where that gradient is small. A spread of 0.01 D on a straight line of gradient 0.6 is 0.017 in log exposure; on a toe of gradient 0.15 it is 0.067, nearly a quarter of a stop, so compute the multiplier per strip. Then convert the speed point at your chosen contrast index into a personal exposure index under the course’s criterion, with criterion, process, batch and certificate date all named — never “the ISO speed is”, which is a claim the course does not make anywhere.

3. The equivalent-time bridge, and the coefficient

Reading a temperature back onto the time axis

• Your own contrast index against time, from the six-rung ladder

Show the numbers behind this plot

A single rising curve of contrast index against development time at twenty degrees Celsius, climbing from 0.40 at five minutes through 0.49 at seven, 0.55 at eight and three quarter minutes, 0.62 at eleven and 0.68 at thirteen and five sixths, to 0.72 at seventeen and a half, steeply at the left and flattening towards the right. Three horizontal guide lines cut it. The lowest, at a contrast index of 0.55, is labelled as the value measured on the eighteen degree strip at the control time, and it meets the curve at eight and three quarter minutes, which is marked as the eighteen degree equivalent time. The middle guide at 0.62 is the control itself at twenty degrees and eleven minutes and meets the curve at eleven minutes. The highest, at 0.68, is labelled as the value measured on the twenty-four degree strip and meets the curve at thirteen and five sixths minutes, marked as the twenty-four degree equivalent time. The construction the drawing shows is that a contrast index measured at another temperature is carried horizontally onto the twenty degree curve and read down as the time that would have produced it, so that the ratio of the two equivalent times becomes the ratio of the two rates. On these illustrative figures that ratio is 13.85 divided by 8.75, or 1.58 over six degrees, which is about 8 per cent per degree.

Series	Development time at 20 °C, minutes	Contrast index
Your own contrast index against time, from the six-rung ladder	5.00	0.40
Your own contrast index against time, from the six-rung ladder	6.00	0.45
Your own contrast index against time, from the six-rung ladder	7.00	0.49
Your own contrast index against time, from the six-rung ladder	8.75	0.55
Your own contrast index against time, from the six-rung ladder	11.00	0.62
Your own contrast index against time, from the six-rung ladder	13.85	0.68
Your own contrast index against time, from the six-rung ladder	17.40	0.72

Drawn to show the construction, not measured from any film: the curve is a plausible shape and the three contrast indices on it are illustrative, chosen so that they do not land on the published expectation. Your own curve, your own three values and your own two equivalent times are the whole of the deliverable, and if they reproduce these numbers it is a coincidence. 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.

Carry each temperature strip’s contrast index horizontally onto the 20 °C curve and read the time under it, recording the slope there. Then take the ratio of the two equivalent times: equal contrast index means equal degree of development, and degree of development is rate multiplied by time, so the rates stand in inverse proportion to the times that produced it.

k(24 °C) ÷ k(18 °C) = t₂₄ ÷ t₁₈

The rate ratio from two equivalent times

Those are the equivalent times at 20 °C, not the eleven minutes both strips ran. Convert both ways:

per degree = (t₂₄ ÷ t₁₈)^1/ΔT − 1 and Q₁₀ = (t₂₄ ÷ t₁₈)^10/ΔT

Per degree, and over ten degrees

ΔT is the measured interval from your logged developer temperatures, not the nominal six: at 18.3 and 23.7 °C it is 5.4, and the exponent changes by more than ten per cent. Then put the band on it by the sizing arithmetic, using your own resolution and your own two slopes.

!One error cancels here and one does not, and they are the same thermometer

In the temperature arm a zero offset largely cancels, because the coefficient depends on the interval between two cells: 18 and 24 read as 17.5 and 23.5, and the interval is still six. In the time arm it does not cancel at all — every cell was developed half a degree colder than recorded, so the whole curve shifts, in exactly the way a chemical difference would, and at nine to ten per cent of the time per degree half a degree is about 33 seconds on 11 minutes. What cancels in neither is a scale error, reading differently at 18 than at 24, which is why ILFORD advise checking a probe against a good-quality liquid-in-glass thermometer.

4. Your number, beside published ones that disagree with each other

Source	What it is	Per degree near 20 °C	Q10
ILFORD, powder-developer sheet	A published instruction: change the time by 10 % per °C	10 %	—
ILFORD, compensation chart, 8-minute row	The course’s arithmetic on a table offered for all combinations	10.0 %	2.60
Kodak F-4017, TRI-X 400 in D-76 stock	The course’s arithmetic on a published table, 18 to 24 °C	9.1 %	2.38
Kodak F-4017, TRI-X 400 in D-76 1+1	The same table, the dilution this session uses	5.6 %	1.73
Kodak F-4017, TRI-X 400 in XTOL 1+1	The same table, another developer at the same dilution	5.5 %	1.71
Kodak J-78, TRI-X Pan in D-76 1+1	A second Kodak document, an older film	5.5 %	1.70
Kodak J-78, T-MAX 100 in D-76 1+1	The same document, a different film at the same dilution	9.3 %	2.44
Kodak J-78, VERICHROME Pan in D-76 1+1	The same document again	10.6 %	2.75
Watkins, printed by Wall in 1924	A historical figure for a metol–hydroquinone developer	—	1.9

Every Q₁₀ above except the last is this course’s arithmetic on times the makers publish, computed as the 18 °C time divided by the 24 °C time and raised to the power ten over six; the makers publish times, not coefficients, apart from ILFORD’s ten per cent rule. The last row is Watkins’s figure as Wall printed it in 1924, for a developer class rather than a product, determined by factorial development timed from the appearance of the image — a method nothing else in the table shares.

Read the spread before reading any row. For what is nominally one quantity the published record runs from about 1.7 to about 2.75: ILFORD’s chart claims one ratio for every film and developer, and Kodak’s tables give ratios differing between films in one document and between two of their own. Three cautions apply to all — the times are rounded to the quarter minute, the span is six degrees, and a published time is a starting-point recommendation rather than a measurement of a rate. That spread is the reason the arm is worth running, because no lookup will tell you what a degree is worth on your bench to better than about half.

⁇Does dilution lower the temperature coefficient? The evidence points both ways

The rows for D-76 and XTOL at 1+1 in Kodak’s F-4017, and for TRI-X Pan in J-78, all land near 1.7 — markedly below the 2.3 to 2.6 of the same developers at stock strength in the same documents. A tidy mechanism is available: in a stock bath the surface reaction limits the rate, while in a dilute one-shot bath the agent near a grain is locally depleted and must be replenished by diffusion, whose temperature dependence is much weaker, so the apparent coefficient falls towards the diffusion value.

The rest of the same evidence refuses the generalisation. In J-78 the D-76 1+1 rows for T-MAX 100, PLUS-X and VERICHROME Pan come out at 2.44, 2.19 and 2.75. The course therefore states the observation and not the mechanism; the experiment that would settle it is the first entry under Further experiments.

Two texts this course has not read. Mees’s 1942 volume and Haist’s Modern Photographic Processing very probably carry measured coefficients for MQ developers, and the course holds neither; under its own sourcing rule it will not paraphrase a figure it has not read. If you have either book, add its figure with its chapter, temperature range and method, because those three decide what it means.

5. Comparing with the published time, and shortlisting the cause

Read your target contrast index off your own curve and compare the time with the manufacturer’s. The difference is a measurement, and the useful move is to sort the candidates by which axis they move.

Candidate	Moves the contrast curve?	Moves the exposure index?	Plausible size
Agitation scheme differing from the maker’s	Yes, strongly	Slightly	Up to 15 % of the time, from ILFORD’s continuous-agitation figure
Thermometer offset	Yes, and it looks like chemistry	Slightly	About 9 to 10 % of the time per °C
Developer strength: mixing, age, oxidation	Yes	Yes	Not sized here; ILFORD publish pH 8.60 to 8.70 for fresh ID-11 stock and advise users to measure their own
Water supply	Unknown	Unknown	Unquantified. Record it and carry it as a systematic term
Film emulsion batch	Yes	Yes	Unquantified between batches; zero within one, which is why the session is one batch
Camera meter calibration	No	Yes	Whole stops are possible
Shutter accuracy	No	Yes	Whole stops are possible
Sensitometer lamp and wedge	No	Yes	Part XIV’s budget, about 0.047 log H uncalibrated

The last three rows are the point of the table. A meter reading a stop out and a shutter running slow are serious problems and are not candidates for a disagreement about development time: the strips were exposed by a sensitometer through a wedge, and a contrast index is a slope that does not care where on the exposure axis it sits. Those errors move your exposure index and your negatives in the field, and this session is blind to them. Keeping the two lists apart turns “my times disagree with ILFORD” from a complaint into a diagnosis. Then size what you found: a time 15 per cent short of the maker’s, on a lift-and-drain script where they assume four inversions a minute, is accounted for by agitation alone; at 40 per cent it is not, and the thermometer, the developer’s strength and the emulsion batch are the next three places to look.

6. The personal processing table

The deliverable. One page, dated, filed with the negatives, and re-issued rather than edited.

Field	What goes in it
Film and emulsion batch	The name and the batch number, because a batch is a material
Developer and its version code	The STEM-INITIALS-SEQUENCE code from the versioning SOP, its mixing date and its measured pH
Dilution and volume per strip	As used, with the one-shot instruction stated
Temperature	The one the times are for, with the tolerance you actually held
Agitation	The script in full words, not an adjective
Vessel	Cylinder or tank, and its size
Time for CI ____	Your enlarger’s aim, read off your own curve
Time for CI ____ − 0.15	A grade softer, for a contrasty subject
Time for CI ____ + 0.15	A grade harder, for a flat subject
Personal exposure index	Under the course’s criterion, in the course’s form
Tolerance on each time	From the slope of your own plot
Instrument certificates	Sensitometer and densitometer, by date
Date, and when it is next re-checked	With a control strip, not with faith

Three rows need their sources said aloud. The aim comes from Kodak’s process-control publication, 0.58 for a diffusion enlarger and 0.43 for a condenser; Kodak’s Tri-X datasheet states its starting-point times are intended to produce 0.56; ILFORD say their times give normal contrast, typically around a Ḡ of 0.62, which is a different construction and sits beside the contrast indices rather than in the same column. The ±0.15 step is this course’s inference, not either maker’s advice: Kodak’s two aims differ by 0.15 and ILFORD quantify the same enlarger difference as about one paper grade. If the softest row falls below the shortest time the ladder ran, the table says so and leaves it blank — dilution rather than time is the lever there. The tolerance comes off your own plot: Kodak accept a time landing the contrast index within ±0.02 of aim, which at 0.03 per minute is ±40 seconds and at 0.01 per minute is ±2 minutes.

What cannot be concluded from this session

Not a property of the developer: every number belongs to your film batch, bottle, water, thermometer, agitation and evening, and only the direction generalises, because that comes from mechanism. Not an ISO speed, and not a claim of conformance to any standard. Not a settled temperature coefficient for D-76: two strips at two temperatures give a number with a wide band, among published values that do not agree with each other. And not a table that stays true — it is valid for the batch, the bottle and the bench that made it, and is re-checked with one control strip rather than trusted, on a date written on its own face.

Troubleshooting

What you see	Likely cause	What to do
The three control strips spread further than the certificates predict	The process, not the instruments: a lift missed, a stagger collapsed, a bath drifting	Do not narrow the resolution sentence to fit. Widen it to the measured spread, say so, and check the deviations column
The top two rungs sit on each other	The curve has flattened, which is a result and not a fault	Report it. To locate the top of the curve, add one strip at k = +3 rather than re-running the ladder
The bottom rung has almost no curve on it	The ladder was pitched too low, or the developer is weaker than the published time assumes	Report the point as it is and add a rung at the top, never below five minutes
Streaks running down from the dense steps, or a pale band across one end of every strip	An inconsistent lift or too little solution for the first; a shifted wedge or uneven illumination for the second	The first excludes one strip under the rule written in advance; the second is an exposure fault that invalidates the batch, and sends you back to the sensitometer’s uniformity test
The 24 °C masked patch is visibly greyer	The fog reaction, whose coefficient is higher than development’s	Read it rather than judging it. If base plus fog has moved past the resolution, that strip’s contrast index has had its zero moved and the result is qualified
The 24 °C equivalent time lands beyond the top rung	The ladder does not reach far enough to interpolate	Do not extrapolate. Add a rung at k = +3, or report the coefficient from the 18 to 20 °C pair with its much larger band, and say which you did
Your time for the maker’s aim is more than 40 % from theirs	Something bigger than agitation	Check the thermometer against a reference, then the developer’s age and pH, then the batch and the meter setting the published time is quoted for
Densities read high on strips read early	The strips were not bone dry	Re-read the next day. A drying term correlated with reading order is indistinguishable from instrument drift, which is why the reading order is randomised

Clean-up

Empty each cylinder into the developer waste container as its cell finishes rather than at the end; ten cylinders standing about in the dark is how one gets knocked over and how one gets re-used by mistake. Rinse each before refilling. Wash the trays, clips and thermometer, and put the thermometer back in its case, because tonight’s offset is only worth having if the instrument survives. Leave the strips hanging: they are the session.

Storage

The strips, once bone dry, go into sleeves in pairs with their card slips and into the Part IX archive: the re-check in six months compares a new control strip with these, and the developer comparison will want the control condition to compare against. Store cool, dry and dark, in sleeving that passes the Photographic Activity Test where you can get it.

The developer stock goes back capped and full, dated, with its version code legible; a part-full bottle oxidises faster, which is Kodak’s own reason for the difference between their six-month and two-month storage figures. The diluted working solution is not stored. The paperwork is filed together and referenced from the formula version record, so the bottle and the table can find each other in two years.

Disposal considerations

Spent developer, about 800 mL, is alkaline and holds sulfite, borate, metol, hydroquinone, their oxidation products and the bromide from ten strips. Sulfite is an oxygen scavenger and exerts an oxygen demand on whatever receives it; the aminophenol and the phenol are what a water authority is likely to care about. Spent fixer and the first rinse after it are acidic and carry dissolved silver as thiosulfate complexes — the stream with recoverable value, routed to the silver route — and the two are never combined, because mixing them compromises that recovery.

The routes are jurisdictional. The disposal page explains why the course describes chemistry and general practice and gives no universal instruction. Check your local regulations; they govern, and they differ between countries and often between municipalities.

Questions

1. Your ladder is 5:30, 6:55, 8:45, 11:00, 13:50 and 17:25. A colleague argues that 5, 8, 11, 14, 17 and 20 minutes is simpler and covers more range. Give the strongest argument for their version and the strongest against, then say which you would run and what you would lose.
2. The three control strips give contrast indices of 0.61, 0.63 and 0.68. What is the resolution sentence, what should you do about the third strip, and why does the answer depend on something you wrote down before the session?
3. Your 18 °C strip reads back as an equivalent time of 9.4 minutes and your 24 °C strip as 15.1, and the logged temperatures average 18.2 and 23.8 °C. Compute the per-degree figure and Q₁₀, then say which you would put on your processing table and why.
4. Your thermometer is later found to read 0.6 °C low at every temperature. Which arm is affected, by roughly how much, and what would you have to re-run to fix it?
5. Your curve gives 9 minutes for a contrast index of 0.58 where ILFORD publish 11. List the candidate causes in the order you would test them, say which your session’s own records can rule out tonight, and name the one candidate this experiment is structurally incapable of detecting.

Further experiments

Run the temperature arm at stock and at 1+1 on the same evening. This settles the question the Analysis had to leave open. Six strips — three temperatures at each of two dilutions, at each dilution’s own published time — fits one session, and is a measurement nobody outside a manufacturer’s laboratory appears to have published.

Repeat the two temperature cells three times each. The sizing table says what it buys: about a third off the width of the coefficient’s band, for four extra strips.

Add a fourth and fifth temperature. With 18, 20, 22 and 24 °C you can plot the logarithm of the equivalent time against the reciprocal of absolute temperature and see whether the points are straight. A straight line supports the single-rate-process picture the compensation charts assume; a curve says the composite is changing across the range.

Turn the control cell into a control chart. Every session in this part develops at least one strip at the control condition. Plot their contrast index and base plus fog against date with action lines from your own certificate, the way ILFORD’s process-control introduction describes.

Check your understanding

Question 1. Why are the six development times spaced by equal steps of the logarithm of the time rather than by equal numbers of minutes?

• Because contrast index rises quickly at short times and slowly at long ones, so equal time steps buy large differences at the bottom of the range and differences too small to resolve at the top
• Because logarithms are the natural units for anything sensitometric
• Because the manufacturers publish their development times as logarithms
• Because it makes the plot a straight line, which is easier to read

Show the answer and why

Answer: Because contrast index rises quickly at short times and slowly at long ones, so equal time steps buy large differences at the bottom of the range and differences too small to resolve at the top

Interpolating Kodak H-740’s six published contrast indices, six levels spaced 1.6 minutes apart between five and thirteen minutes buy gaps of 0.061, 0.054, 0.040, 0.040 and 0.025 in contrast index, while six levels spaced by a constant factor of 1.21 buy 0.042, 0.045, 0.045, 0.047 and 0.041. Both cost six strips. The equal-time set spends its last strip on a difference of 0.025, which a home bench with a resolution of 0.03 may not report at all. The rule generalises: space the levels by the expected size of the effect, not by equal intervals of the variable.

Question 2. The temperature arm develops three strips for the SAME time at three temperatures. How does a rate ratio come out of that?

• Each strip’s contrast index is read back off the 20 °C time curve as the time that would have produced it, and the ratio of those two equivalent times is the ratio of the rates
• The ratio of the three contrast indices is the ratio of the rates
• The ratio of the three base-plus-fog readings gives it, since fog tracks rate
• It cannot: a fixed-time series carries no rate information at all

Show the answer and why

Answer: Each strip’s contrast index is read back off the 20 °C time curve as the time that would have produced it, and the ratio of those two equivalent times is the ratio of the rates

Contrast index is not proportional to rate, so the ratio of two contrast indices means nothing as a rate ratio. What the time arm provides is a ruler: it maps contrast index onto time at one temperature. Carrying a contrast index measured at 18 °C horizontally onto that curve gives the time at 20 °C that would have produced the same degree of development, and since degree of development is rate multiplied by time, equal contrast at two temperatures means the rates stand in inverse proportion to the times. That is why the two arms are one experiment and why the temperature arm cannot be run without the time arm first.

Question 3. A thermometer reads 0.6 °C low at every temperature. Which statements are true of this session?

• The temperature coefficient is largely unaffected, because it depends on the interval between two cells and an offset is common to both
• The whole contrast-index-against-time curve is displaced, because every cell ran colder than recorded
• The displacement of the time curve looks exactly like a difference in developer activity
• The speed points are unaffected, since the speed criterion is defined above base plus fog

Show the answer and why

Answer: The temperature coefficient is largely unaffected, because it depends on the interval between two cells and an offset is common to both, The whole contrast-index-against-time curve is displaced, because every cell ran colder than recorded, The displacement of the time curve looks exactly like a difference in developer activity

An offset cancels in a difference and does not cancel in a level. The temperature arm measures an interval, so 18 and 24 read as 17.4 and 23.4 and the interval is still six degrees. The time arm measures levels: every cell was 0.6 °C colder than the label, which at roughly nine to ten per cent of development time per degree is about six per cent of the time, and it shifts the whole curve in the direction a weaker developer would. That is the dangerous case, because nothing on the plot says the cause was a thermometer. The fourth option is wrong for a different reason: the speed point is read off the same displaced curves, so it moves too, though far less, because development is a contrast control and barely a speed control.

Question 4. Your curve gives 9 minutes for the contrast index ILFORD’s published 11 minutes is meant to reach. Which of these can this session’s own records rule out as the cause?

• A camera shutter running fast
• A hand-held meter reading a stop out
• The developer having been mixed to the wrong strength
• The agitation scheme differing from the one ILFORD’s times assume

Show the answer and why

Answer: A camera shutter running fast, A hand-held meter reading a stop out

The strips were exposed by a sensitometer through a step wedge, so no camera and no meter was anywhere near this experiment; both are ruled out at once, and neither could move a contrast index in any case, because a contrast index is a slope and does not care where along the exposure axis it sits. Those two errors are real and serious — they move your personal exposure index and your negatives in the field — and this session is structurally blind to them. The other two remain live: the developer’s strength and the agitation scheme both move the contrast curve, and ILFORD’s own up-to-15-per-cent figure for continuous agitation makes the fourth the first one to size.

Question 5. The published temperature coefficients this course could assemble run from about 1.7 to about 2.75 for what is nominally the same quantity. What follows for the number you write on your processing table?

• That your own measured figure, with its band and its conditions written beside it, is worth more for your bench than any single published value, and that the disagreement is itself information about how well the quantity is known
• That the published figures are unreliable and should be ignored in favour of your own
• That the mean of the published figures, about 2.2, is the best available estimate
• That the quantity is not well defined and should not be quoted at all

Show the answer and why

Answer: That your own measured figure, with its band and its conditions written beside it, is worth more for your bench than any single published value, and that the disagreement is itself information about how well the quantity is known

The spread is not evidence that anybody is wrong. ILFORD’s chart is offered as a guide for all combinations and says so; Kodak’s tables are film-specific and dilution-specific starting points rounded to the quarter minute; Watkins’s 1924 figure was determined by factorial development on plates of that era. They are answers to slightly different questions, measured by different methods over different ranges. Averaging them mixes those questions together. What the spread does establish is that no lookup will tell you what a degree is worth on your bench to better than about fifty per cent, which is precisely the gap a measurement with a stated band fills — and why the band has to be quoted with the figure rather than dropped from it.

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