Development Kinetics: Time, Temperature, Agitation and Exhaustion

Kodak’s sheet for D-76 says eight minutes at 20 °C for Tri-X in a small tank. Four other things are inside that sentence and none of them is printed next to it: an agitation scheme, a tank volume, a developer that has processed no film before this one, and a target contrast that Kodak chose on your behalf. Change any one and the eight minutes is wrong.

This page is about what actually sets the rate, and about the step almost nobody makes explicitly — turning a rate into the two things you can see on the negative, which are its contrast and the shadow detail it did or did not record. Part III owns the kinetics; this page spends them.

The shape, and why the shadows finish first

Development has three stages and you can watch them in a tray. There is an induction period in which nothing visible happens — ILFORD is precise about it for a correctly exposed fibre-base print, where the image begins to appear after 35 seconds. There is a rise, steepest where the exposure was heaviest because those regions carry the most developable crystals. And there is a flattening, which arrives in the shadows long before it arrives in the highlights.

Part III drew that pair of curves and the conclusion follows from their divergence rather than from either one: extending development raises the highlights much more than the shadows, so it raises contrast rather than raising exposure. ILFORD’s paper sheet gives the extreme case — after the recommended one to two minutes, a fibre-base print may be developed for six minutes “without any noticeable change in contrast or fog”, which is what development to completion means and why print times are quoted so casually.

Film is never developed to completion, and that is the whole reason this page exists. A film time is chosen part-way up the curve, and where you stop is a contrast decision.

Temperature, arrived at three ways

Development rate against temperature, from Kodak's own D-76 tables

• T-MAX 100 Professional
• TRI-X Pan
• PLUS-X Pan

Show the numbers behind this plot

Three rising sets of points showing relative development rate against temperature, computed as the reciprocal of Kodak's published small-tank development times for D-76 at full strength, each scaled so that its own 20 degrees C value is 1.00. T-MAX 100 Professional rises from 0.857 at 18 degrees through 1.00 at 20, 1.125 at 21 and 1.286 at 22 to 1.500 at 24. TRI-X Pan rises from 0.889 through 1.00, 1.143 and 1.231 to 1.455. PLUS-X Pan rises from 0.846 through 1.00, 1.100 and 1.222 to 1.467. All three are close together and all three are rising by roughly nine to ten per cent per degree, so six degrees of warming multiplies the rate by about one and a half. The three films are not identical, which is the second teaching point: the same developer at the same temperatures gives measurably different temperature coefficients on different emulsions.

Series	Developer temperature, °C	Rate relative to the same film at 20 °C
T-MAX 100 Professional	18.00	0.86
T-MAX 100 Professional	20.00	1.00
T-MAX 100 Professional	21.00	1.13
T-MAX 100 Professional	22.00	1.29
T-MAX 100 Professional	24.00	1.50
TRI-X Pan	18.00	0.89
TRI-X Pan	20.00	1.00
TRI-X Pan	21.00	1.14
TRI-X Pan	22.00	1.23
TRI-X Pan	24.00	1.46
PLUS-X Pan	18.00	0.85
PLUS-X Pan	20.00	1.00
PLUS-X Pan	21.00	1.10
PLUS-X Pan	22.00	1.22
PLUS-X Pan	24.00	1.47

Measured, in the sense that every point is the reciprocal of a time Kodak publishes in the J-78 data sheet for D-76 at full strength with agitation at 30-second intervals; the course has inverted and normalised them and fitted nothing. Kodak's times are rounded to the nearest quarter-minute, which is up to five per cent on a five-minute time, so the scatter between films is at the edge of what the tables can resolve.

Route one, the manufacturer’s rule. ILFORD instructs its users to increase the development time by 10 per cent for each 1 °C drop and decrease it by 10 per cent for each 1 °C rise, with a worked example repeated on the HP5 Plus sheet: 6 minutes at 20 °C becomes 4½ at 23 °C and 9 minutes at 16 °C. That is a published figure from a manufacturer, not a darkroom rule of thumb.

Route two, the manufacturer’s chart. ILFORD’s separate compensation chart tabulates the time at eight temperatures for every 20 °C time, and Part III showed that its ratios are the same all the way down, so a temperature change multiplies the time rather than adding to it.

Route three, the physics. Take the reciprocals of a published time table as relative rates, plot their natural logarithms against 1/T in kelvin, and the slope is −E_a/R.

🧮Two manufacturers, six films, one activation energy

Part III fitted ILFORD’s chart and obtained an activation energy of about 67 kJ/mol with an R² of 0.997. Kodak’s D-76 tables are an entirely independent dataset — a different company, a different developer, five named films — and the same fit gives:

Film, in D-76 at full strength	Ea, kJ/mol	R²	per degree near 20 °C	Q10
T-MAX 100 Professional	69.4	0.990	10.2 %	2.56
T-MAX 400 Professional	60.6	0.988	8.8 %	2.27
TRI-X Pan	60.6	0.988	8.8 %	2.27
PLUS-X Pan	66.5	0.998	9.7 %	2.46
VERICHROME Pan	74.2	0.937	10.9 %	2.73

Every one of them lands between 61 and 74 kJ/mol, clustering on ILFORD’s 67, and every one predicts between nine and eleven per cent per degree — which is ILFORD’s published ten. The temperature coefficient over ten degrees, Q₁₀, comes out between 2.3 and 2.7, against OpenStax’s general rule of thumb of approximately two and Kodak’s own 1928 statement that a reaction whose time halves over 10 °C “is said to have a temperature coefficient of 2”.

The arithmetic is the course’s own, on figures Kodak and ILFORD publish, and it inherits every limitation of those tables: times rounded to the nearest quarter-minute, a range of only six degrees, and one developer. Read it as an explanation of the ten per cent rule, not as a substitute for it.

The coefficient belongs to the developer, not to development

Two of Kodak’s 1928 statements complicate the tidy picture above, and both are worth carrying.

The first: the temperature coefficient of development varies with the developing agent, being least with agents of high reduction potential such as metol and most with agents of low potential such as hydroquinone. Kodak draws the consequence explicitly for a mixed developer. At low temperature the hydroquinone is very inert while the metol is barely slowed, so the bath behaves as though it contained an excess of metol; at high temperature the hydroquinone gains far more than the metol, and the situation reverses. A cold MQ developer and a warm one are not the same developer run at two speeds; they are two different developers.

The second is the one that ends the argument for pushing the temperature: the fog reaction has a different and much higher temperature coefficient than development. A developer that gives good density with low fog at a normal temperature “may produce very bad fog if the temperature is high”. There is no compensation chart for that, because it is not a time you can shorten your way out of.

Kodak’s own process-control publication sets the tolerance that follows: temperature variations greater than 0.3 °C in the developer will affect process control and image quality. Not a degree — three tenths of one.

Agitation is a transport variable, and its size is published

Kodak’s 1928 primer already had the shape of it: the speed of development depends chiefly on the rate at which the developer diffuses into the film. Kodak’s modern process-control sheet says what agitation is for in one sentence that is transport from beginning to end — it maintains uniform solution activity by removing exhausted solution from the emulsion surface and replacing it with fresh solution.

Part III’s diffusion page owns the boundary layer this acts on, and its kinetics page owns the distinction between a reaction-limited and a transport-limited process. What belongs here is the number, and ILFORD publishes it: switching from its intermittent spiral-tank cycle — four inversions during the first ten seconds of each minute — to continuous agitation in a dish or a rotary tube requires the time to be cut by about 15 per cent.

That 15 per cent is the fraction of the development that was waiting for supply. It is not the whole reaction, so development is not purely transport-limited; it is far from nothing, so it is not purely reaction-limited either. And it means an agitation scheme is part of a published time, worth about a degree and a half of temperature.

✕Three ways a time is invalidated without touching the clock

Changing the pattern. Kodak’s D-76 sheet asks for agitation that is irregular or random, because currents flowing over the film in one direction consistently raise the density along their paths. It also specifies the initial burst and then five seconds at 30-second intervals, and notes that the number of cycles in each burst — two to five — is itself a contrast control.

Changing the volume. Writing about D-76 at 1:1, Kodak gives 473 mL for one 135-36 roll and 946 mL for two — and states that if you instead process one roll in a 237 mL tank, or two in a 473 mL tank, you must increase the development time by 10 per cent. The film is the same and the developer is the same; there is simply less of it per unit of emulsion, and it depletes faster. Halving the solution per roll costs a tenth of the development.

Going too fast. Both manufacturers set the same floor for a different reason from any of the above. ILFORD’s chart says times below five minutes are not recommended because of the risk of uneven development; Kodak says tank development times shorter than five minutes may produce poor uniformity. That is not a rate statement at all. Below about five minutes the time taken for the developer to soak in and distribute itself is no longer small compared with the development, so the parts of the film the solution reached first develop measurably further than the parts it reached last.

Four things run down, and the bath fails when the first of them does

Kodak’s 1928 primer lists what happens to a developer in use, and it is more than the obvious one: the agent and the sulfite are being oxidised by air; the agent is being consumed doing useful work; and oxidation products together with sodium bromide and iodide are accumulating. Add the acid load from the alkalis page and four independent things are moving.

• Developing agent, consumed by every silver ion reduced — and ILFORD points out that this depends on the pictures, not the film count: negatives of night shots that develop nearly clear use very little agent, while beach scenes in bright sun that come out well blackened use a great deal.
• Buffer capacity, consumed by the acid the same development releases and by anything carried in.
• Restrainer, which does not run down but accumulates, as the restrainers page sets out.
• The preservative, and with it the protection of everything else, whose loss is set by the headspace in the bottle rather than by the film put through.

Kodak’s primer gives the three symptoms that end a deep tank’s life, and each maps onto one of those: the time for complete development becomes excessively long; the solution stains or fogs badly; or the accumulation of by-products is so great that shadow detail is lost even with full development. That third one is the important one, because it is the failure that time cannot fix.

One-shot, reused, replenished

Three working methods, three sets of published numbers, and a genuine choice between them.

	Reused stock	Replenished	One-shot, diluted
ILFORD ID-11 or MICROPHEN	10 films per litre, time raised 10 % per film	replenisher withdrawn 2012	1+1 or 1+3, fresh each time
ILFORD PERCEPTOL	4 films per litre, not recommended beyond	—	1+1 or 1+3, fresh each time
Kodak D-76	16 sheets of 8 × 10 per gallon, or 4 per litre, time raised 15 % after every four	120 per gallon, at 22.2 to 29.6 mL of D-76R per roll, with no time increase	1:1, 8 sheets per gallon, discarded after one batch
Keeping, unused	6 months in a full bottle; ILFORD adds 1 month half full, 4 months in a deep tank with a floating lid	as the developer	24 hours at most, both makers

Replenishment buys a factor of seven and a half in capacity and, more valuable than that, holds the time constant, which is what makes a process controllable at all. Reuse buys economy and pays in exhaustion compensation that is admittedly approximate — ILFORD says so itself, that the compensation can only be an approximation across a range of circumstances.

ILFORD’s own summary of the trade is the clearest statement of it in the corpus, and it is worth having in front of you when the economy of a litre is tempting. Reusing developer lowers image quality slightly and increases the risk of physical damage; the developer oxidises with reuse and storage; precipitates may form; and tiny particles of emulsion from films processed previously may be held in suspension. One-shot processing is recommended when image quality, reliability and consistency are more important than economy. ILFORD does not recommend reusing diluted developers at all, and does not recommend push processing in a reused one.

Time becomes contrast, and contrast becomes speed

Everything above is a rate. What a photographer buys with it is a gradient.

Kodak’s definition of contrast index in its process-control publication is written for exactly this purpose: a measure of the degree of development that determines how well the density range of a normally exposed negative will print on a grade 2 paper. It is measured as the slope between two particular points on the characteristic curve, chosen to correspond to the minimum and maximum densities normally used, so unlike gamma it takes the shape of the toe into account. ILFORD expresses the same quantity as Ḡ and states the target its published times aim at: about 0.62. Part XIII owns both definitions properly; what belongs here is the curve that connects them to the clock.

Contrast index against development time

• Contrast index of the workbook example

Show the numbers behind this plot

A single curve rising from left to right and flattening as it goes, showing contrast index against development time at a fixed temperature, agitation scheme and developer. At five minutes the contrast index is 0.51, at six 0.55, at eight 0.62, at ten 0.67, at twelve 0.72 and at thirteen 0.73. The rise from five to eight minutes is 0.11 for three minutes; the rise from ten to thirteen is 0.06 for the same three minutes, so the curve is steadily losing its slope. A horizontal guide is drawn at 0.62, which is both the value this example reaches at eight minutes and the average gradient ILFORD states its own published development times aim at. The teaching point is that a fixed percentage error in time costs less contrast at long times than at short ones, and that halving the time does not halve the contrast index because the curve does not pass through the origin.

Series	Development time, minutes	Contrast index
Contrast index of the workbook example	5.00	0.51
Contrast index of the workbook example	6.00	0.55
Contrast index of the workbook example	8.00	0.62
Contrast index of the workbook example	10.00	0.67
Contrast index of the workbook example	12.00	0.72
Contrast index of the workbook example	13.00	0.73

Every value is published, in Kodak's H-740 sensitometry workbook, but the workbook names neither the film nor the developer — it calls them XYZ and A — so this is a taught shape rather than a measurement of any material, and the plot is labelled accordingly. The horizontal guide is a separate manufacturer's statement: ILFORD publishes its development times as targeting a Gbar of about 0.62, so the two independent sources meet at the same number for a normally developed film. 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.

Read the curve as a control law. Between five and eight minutes the contrast index climbs 0.11; between ten and thirteen it climbs 0.06 for the same three minutes. A short development time is a twitchy one, which is a second reason for the five-minute floor beyond uniformity. Kodak’s own advice follows the same logic: if negatives are consistently flat, increase the time slightly, by 10 to 15 per cent; if too contrasty, decrease it by the same.

Kodak’s workbook is also explicit that the same curve is moved by four factors, not one: time, temperature, agitation and developer. Three of them are the subject of this page, and the fourth is the subject of the rest of this part.

!Why a developer changes a film's speed, and what the course may call it

A film’s speed is not a property of the film alone, and Kodak says so twice in its own words.

Its sensitometry workbook notes that different developers yield different film speeds, and that the developer specified by the standard method is similar to D-76 — which is why D-76 is the reference developer of the whole subject and why a speed figure is meaningless without one. Its process-control publication is blunter still about the speed value it asks a technician to plot: it “is an arbitrary number that relates only to the control strip; it is not an ISO/ASA speed”.

That distinction is the course’s terminology rule made concrete. Film speed is the manufacturer’s figure, determined by a standard method under a standard development. Effective film speed is what you actually get, in your developer, at your contrast, and it is measured rather than looked up. This course never writes “ISO speed” for a figure it has determined itself; it says the figure was measured under a stated criterion and names the standard as the thing that criterion is modelled on. Part XIII writes that criterion once, and every later part cites it.

ILFORD’s own tables show the effect without describing it. For HP5 Plus it publishes times in ID-11 and MICROPHEN at EI 400 and upwards, and in PERCEPTOL only at EI 250 and 320 — so the fine-grain developer’s cost appears as meter settings ILFORD is prepared to quote rather than as a figure. For MICROPHEN it does state one: a speed increase of up to half a stop with most films, and more with the fast ones.

Reading a manufacturer’s chart critically

Six questions, in the order they bite.

What agitation does this time assume? ILFORD’s times assume four inversions in the first ten seconds of each minute; Kodak’s assume five seconds at 30-second intervals with two to five cycles. Continuous agitation is 15 per cent shorter. If your scheme is neither, the time is a starting point.

What contrast is it aiming at? ILFORD says Ḡ of about 0.62 and that higher or lower may be preferred. A time is an answer to a question about contrast, and if you want a different contrast it is the wrong answer.

Is the developer fresh? All of these times are for a bath that has processed nothing. The compensation schedules exist precisely because the published time is the first film’s.

What volume, in what tank? Kodak’s 10 per cent for a small tank is a real correction that has nothing to do with chemistry beyond the ratio of solution to emulsion.

Does the chart claim more than it measured? ILFORD offers its compensation chart as “a useful guide for all film/development combinations”, which is a large claim given that Kodak measured different temperature coefficients for different agents in 1928 and that the table above shows a spread across five films in one developer. A film’s own sheet beats a general chart.

Where does the chart stop? ILFORD’s runs from 18 to 27 °C and Kodak’s tables from 18 to 24 °C. Outside those, you are extrapolating — and at the warm end there is a second reason to stop, because gelatin softens and the fog reaction’s higher temperature coefficient is waiting.

📓What to record, so that next time is not a guess

For every film: the developer and its dilution, the batch date, how many films that batch has already done, the temperature at the start and at the end, the agitation scheme in words, the total time, and the compensation you applied and why.

Two of those are the ones people leave out and later need. The film count turns a puzzling thin negative into an arithmetic error you can correct. The end temperature catches the tank that started at 20 °C in a 16 °C darkroom and finished at 18. Kodak’s tolerance is 0.3 °C; a tank losing two degrees over ten minutes is a quarter of a stop of development you did not intend.

Recap

• A published time hides four variables: agitation, tank volume, developer history and a target contrast the manufacturer chose.
• Development has a shape, not a duration. The shadows flatten first, so extending development raises contrast rather than exposure, and a thin negative from under-exposure cannot be developed out.
• Ten per cent per degree is a manufacturer’s published figure, and the Arrhenius fit to Kodak’s own D-76 tables gives 61 to 74 kJ/mol across five films — clustering on the 67 kJ/mol Part III obtained from ILFORD’s chart — with Q₁₀ between 2.3 and 2.7.
• The coefficient belongs to the developer. A cold MQ bath behaves as if it held excess metol and a warm one as if it held excess hydroquinone, and fog’s coefficient is higher than development’s, which is why warm processing has a ceiling.
• Agitation is transport, and ILFORD prices it at 15 per cent, worth about a degree and a half.
• Four things run down at different rates and the bath fails when the first does; the failure that matters is loss of shadow detail, because time does not fix it.
• One-shot, reuse and replenishment are three published methods with three sets of numbers; replenishment is the only one that holds the time constant.
• Time buys contrast index, and the curve flattens, so short times are twitchy and long ones are forgiving. A developer changes a film’s effective speed, and the course keeps that phrase apart from the manufacturer’s figure.

Check your understanding

Question 1. Kodak publishes 9 minutes for Tri-X Pan in D-76 at 18 °C and 5½ minutes at 24 °C, both in a small tank. What activation energy does that imply, and why should the number be treated cautiously?

• About 20 kJ/mol; cautiously because development is not a single reaction
• About 61 kJ/mol, from the slope of ln(1/t) against 1/T; cautiously because the times are rounded to the nearest quarter-minute over a six-degree range, and because it describes one film in one developer
• About 200 kJ/mol; cautiously because the tables are old
• It cannot be estimated from two times

Show the answer and why

Answer: About 61 kJ/mol, from the slope of ln(1/t) against 1/T; cautiously because the times are rounded to the nearest quarter-minute over a six-degree range, and because it describes one film in one developer

Take rates as reciprocals of times, plot their logarithms against 1/T in kelvin, and the slope is −Ea/R with R = 8.314 J per mol per K. Kodak’s five-point Tri-X row gives 60.6 kJ/mol at R² 0.988, which predicts 8.8 per cent per degree near 20 °C and a Q₁₀ of 2.27. The cautions are real and worth carrying: quarter-minute rounding is up to five per cent of a five-minute time, the range spans six degrees, and the five films in the same table spread from 61 to 74 kJ/mol. What survives all of that is the agreement with ILFORD’s published ten per cent per degree and with Part III’s independent fit at 67 kJ/mol.

Question 2. A photographer moves from a spiral tank with ILFORD’s standard inversion cycle to a rotary processor and keeps the same time. What happens, and by roughly how much?

• Nothing: agitation affects evenness but not the amount of development
• The negatives develop further than intended, by roughly the 15 per cent ILFORD instructs users to remove for continuous agitation — about the same as a degree and a half of extra temperature
• The negatives develop less, because rotary processors use less solution
• The effect depends only on the film

Show the answer and why

Answer: The negatives develop further than intended, by roughly the 15 per cent ILFORD instructs users to remove for continuous agitation — about the same as a degree and a half of extra temperature

Agitation acts on the boundary layer, replacing solution the emulsion has already depleted, and Kodak’s process-control sheet defines it exactly that way. ILFORD prices the whole of that contribution at about 15 per cent: it instructs a 15 per cent reduction for dish processing with continuous agitation and the same for a rotary tube without a pre-rinse. The fact that the figure is 15 per cent rather than 50 or 2 is itself informative, because it says development is neither purely transport-limited nor purely reaction-limited. Note also the direction of the volume correction, which is separate: a small tank needs 10 per cent more time, not less.

Question 3. Kodak’s workbook gives a contrast index of 0.51 at 5 minutes, 0.62 at 8 and 0.73 at 13. A darkroom timer is consistently 30 seconds fast. Where does that error cost most, and why?

• At long times, because the absolute error is the same but the density is higher
• At short times, because the contrast-index curve is steepest there — 0.11 of contrast index across three minutes at the short end against 0.06 across three minutes at the long end
• Equally at both, since the error is a fixed time
• It never matters, because contrast index is set by the film

Show the answer and why

Answer: At short times, because the contrast-index curve is steepest there — 0.11 of contrast index across three minutes at the short end against 0.06 across three minutes at the long end

The curve flattens, so the same slice of time buys progressively less contrast as development proceeds: 0.11 between five and eight minutes, 0.06 between ten and thirteen. A fixed timing error is therefore a larger contrast error at the short end. That is a second and independent argument for the five-minute floor both manufacturers publish — the first being uniformity, since below five minutes the time the developer takes to soak in stops being small compared with the development itself. Long dilute times are forgiving of the clock and unforgiving of everything else.

Question 4. A litre of ID-11 stock has developed nine films on ILFORD’s compensation schedule and the tenth is due. A friend suggests topping the bottle up with fresh stock instead. What is wrong with that?

• Nothing: it restores both volume and activity
• It restores volume and dilutes the accumulated bromide, but restores no ratio — the result is a weaker version of a used developer rather than a fresh one, which is why a replenisher is a different formula
• Fresh and used developer cannot be mixed
• It will fog the film

Show the answer and why

Answer: It restores volume and dilutes the accumulated bromide, but restores no ratio — the result is a weaker version of a used developer rather than a fresh one, which is why a replenisher is a different formula

Four things have moved independently in that bottle — agent consumed, buffer spent, restrainer accumulated, preservative oxidised — and adding working-strength developer moves all four back by the same fraction, which is not the fraction any of them needs. A replenisher is designed to put back what the bath spends at the rate it spends it, which is why D-76R carries the same sulfite as D-76, half again as much agent and ten times the borax. Kodak’s replenished capacity is 120 sheets per gallon against 16 unreplenished, with no time increase at all; ILFORD, having withdrawn its replenisher in 2012, tells users to compensate with time instead and admits the compensation is an approximation.

Question 5. Kodak’s process-control publication says of the speed value plotted on a control chart that "it is an arbitrary number that relates only to the control strip; it is not an ISO/ASA speed". Why does this course quote that?

• To show that Kodak’s method is unreliable
• Because it is the same distinction the course draws between film speed and effective film speed, and it licenses a working figure that is measured under a stated criterion rather than borrowed from a standard
• Because control strips are obsolete
• Because ISO speeds only apply to colour film

Show the answer and why

Answer: Because it is the same distinction the course draws between film speed and effective film speed, and it licenses a working figure that is measured under a stated criterion rather than borrowed from a standard

A speed figure means nothing without the development that produced it — Kodak’s own workbook says different developers yield different film speeds and that the developer the standard method specifies is similar to D-76. So there are two different quantities with one name, and this course keeps them apart: film speed is the manufacturer’s determination under a standard development, effective film speed is what your developer at your contrast actually delivers. The practical consequence is visible in ILFORD’s own tables, which quote HP5 Plus at EI 250 and 320 in PERCEPTOL and at 400 and above in ID-11 and MICROPHEN, without ever printing a speed loss.

Question 6. Kodak’s 1928 primer says the fog reaction has a much higher temperature coefficient than development. What follows for someone processing at 27 °C to save time?

• Nothing, provided the time is shortened by the compensation chart
• Fog rises faster than image density with temperature, so the compensation chart holds the image density but not the fog, and the negatives lose shadow separation to a raised base
• The developer will exhaust faster but the negatives will be identical
• The film will reticulate, which is the only risk

Show the answer and why

Answer: Fog rises faster than image density with temperature, so the compensation chart holds the image density but not the fog, and the negatives lose shadow separation to a raised base

A compensation chart is a statement about one rate — the image-forming one. If a second, unwanted reaction has a steeper temperature dependence, then correcting the time to hold image density constant necessarily lets the fog run ahead. Kodak states the outcome directly: a developer that gives good density with low fog at a normal temperature may produce very bad fog if the temperature is high. That is why both manufacturers’ charts stop where they do, and why the softening of gelatin, which is the fault people expect at high temperature, is only the second reason.

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