Rates, Temperature and Catalysts

An unexposed silver bromide crystal sitting in a developer is being asked to do something the chemistry positively favours: a reducing agent is present, silver ions are present, and the reaction releases energy. It nevertheless does not happen — not in ten minutes, and that is the whole reason photography works. Everything on the negative depends on a reaction being possible in one place and slow in another.

Possible is not the same as fast

Two questions look alike and are not.

Will it go? is thermodynamics, and the previous page answered it with potentials: put a strong enough reducing agent next to a silver ion and the electron transfer is downhill.

How fast will it go? is kinetics, and nothing in the potentials table answers it. Kodak’s 1928 primer, discussing what it called the reduction potential of developing agents, says so directly: that number “alone does not determine the speed with which the developer develops the image, because this depends chiefly upon the rate at which the developer diffuses into the film and on the quantity of developing agent and other substances in the developer.”

Photography lives in the gap between the two answers. The exposed grain and the unexposed grain sit in the same solution with the same thermodynamic prospects; what separates them is a rate. Development selectivity is a kinetic effect, not a thermodynamic one, and the honest way to say “an unexposed grain does not develop” is “an unexposed grain does not develop appreciably in the time you give it”.

The five things that change a rate

OpenStax’s chapter on the subject names them, and every one has a darkroom counterpart.

Factor	What it does	In a darkroom
Chemical nature of the reactants	different substances simply react at different rates	metol brings an image up quickly; hydroquinone brings it up slowly and then builds density
Physical state and subdivision	a two-phase reaction happens only at the interface, faster where the interface is larger	fine grain has more surface per gram of silver than coarse grain
Concentration	more reactant molecules per litre means more encounters	a developer at 1+3 works more slowly than the same developer at stock strength
Temperature	more of the molecules carry enough energy to react	the reason a development time is meaningless without a temperature
Catalysis	an alternative route with a lower barrier	the latent-image speck, and the trace-metal claim of the previous page

To which photography adds a sixth that a beaker of solution does not have: transport. The reaction happens inside a gelatin layer, so the reagent must arrive and the products must leave. That is the whole of the next page, and it is the reason agitation exists.

Two of those rows come with published numbers, and both are worth having.

Concentration. Ilford’s own table for ID-11 with Delta 100 Professional at EI 100 gives 8 minutes 30 seconds at stock strength, 11 minutes at 1+1 and 20 minutes at 1+3. Quartering the developer more than doubles the time. It does not quadruple it, which is itself informative: the rate is not simply proportional to the developing agent’s concentration, because at high dilution the reaction becomes limited by supply rather than by the chemistry, and because the alkali and the bromide have been diluted along with the agent.

Agitation. Ilford states this one as a coefficient: dish processing with continuous agitation “reduces the recommended development times by about 15%”, and the same 15 per cent appears again for rotary tube processors, which also agitate continuously. That is a manufacturer’s measurement of how much of the process is transport-limited, and it is a large number for a variable many people treat as a formality.

Collisions, and the barrier they have to clear

Collision theory says three things, and none of them needs mathematics. Molecules must collide; they must collide in the right orientation; and they must collide with enough energy to rearrange their bonds. The minimum energy that will do it is the activation energy, Eₐ.

Picture the energy of the reacting system as a landscape. Reactants sit in one valley, products in another, and between them is a hill. The height of the hill is the activation energy. Whether the product valley is lower than the reactant valley — whether the reaction releases energy — says nothing about how high the hill is. A downhill reaction can have a high hill in front of it, and it will then be slow. That is exactly the unexposed grain.

At any temperature the molecules in a solution carry a spread of energies, and only the ones in the high tail of that spread clear the barrier. Raise the temperature a little and you do not raise every molecule’s energy a little; you move a disproportionate number of them into the tail. That is why rates rise steeply rather than gently with temperature, and it is what the Arrhenius equation says:

k = A e^(−Eₐ/RT)

The Arrhenius equation

k is the rate constant, the thing that determines how fast the reaction goes at a given set of concentrations. Eₐ is the activation energy in joules per mole — the height of the hill. R is the gas constant, 8.314 J mol⁻¹ K⁻¹. T is the absolute temperature in kelvin. A is a constant related to how often molecules collide and how often they do so in a usable orientation. The whole of the temperature behaviour is in that exponential: T is in the denominator of a negative exponent, so a small rise in T makes the exponent less negative and k rises sharply.

OpenStax gives the rule of thumb that comes out of this: “for many chemical processes, reaction rates are approximately doubled when the temperature is raised by 10 °C”. That is a general statement about chemistry, from a chemistry textbook, and the next section tests it against a photographic manufacturer who has actually measured the case in front of us.

What a compensation chart really claims

Ilford publishes a single sheet giving the development time at 18, 19, 20, 21, 22, 24, 25 and 27 °C for each recommended time at 20 °C, offered “as a quick and easy guide” and “a useful guide for all film/development combinations”. Times are rounded to the nearest fifteen seconds. Its own worked example: 8 minutes at 20 °C becomes 5 minutes 30 seconds at 24 °C.

Ilford's published compensation, for two starting times

• Recommended time at 20 °C is 12 minutes
• Recommended time at 20 °C is 9 minutes

Show the numbers behind this plot

Two falling curves of development time against temperature, read directly from Ilford's published compensation chart. The upper curve is for a film needing 12 minutes at 20 degrees C: it reads 14 minutes 45 seconds at 18 degrees, 13 minutes 15 seconds at 19, 12 minutes at 20, 10 minutes 45 seconds at 21, 9 minutes 45 seconds at 22, 8 minutes 15 seconds at 24, 7 minutes 30 seconds at 25 and 6 minutes 30 seconds at 27. The lower curve is for a film needing 9 minutes at 20 degrees C: 11 minutes 15 seconds at 18, 10 minutes at 19, 9 minutes at 20, 8 minutes at 21, 7 minutes 15 seconds at 22, 6 minutes 15 seconds at 24, 5 minutes 45 seconds at 25 and 5 minutes at 27. Both curves are steeper at the cold end than at the warm end, so the same one-degree change costs more time at 18 degrees than at 26. A horizontal guide is drawn at 5 minutes and labelled as the floor below which Ilford does not recommend developing, because of the risk of uneven development.

Series	Development temperature, °C	Development time, minutes
Recommended time at 20 °C is 12 minutes	18.00	14.75
Recommended time at 20 °C is 12 minutes	19.00	13.25
Recommended time at 20 °C is 12 minutes	20.00	12.00
Recommended time at 20 °C is 12 minutes	21.00	10.75
Recommended time at 20 °C is 12 minutes	22.00	9.75
Recommended time at 20 °C is 12 minutes	24.00	8.25
Recommended time at 20 °C is 12 minutes	25.00	7.50
Recommended time at 20 °C is 12 minutes	27.00	6.50
Recommended time at 20 °C is 9 minutes	18.00	11.25
Recommended time at 20 °C is 9 minutes	19.00	10.00
Recommended time at 20 °C is 9 minutes	20.00	9.00
Recommended time at 20 °C is 9 minutes	21.00	8.00
Recommended time at 20 °C is 9 minutes	22.00	7.25
Recommended time at 20 °C is 9 minutes	24.00	6.25
Recommended time at 20 °C is 9 minutes	25.00	5.75
Recommended time at 20 °C is 9 minutes	27.00	5.00

Every point is read from the published ILFORD chart, which rounds to the nearest 15 seconds; no curve has been fitted through them. Note that the chart stops at 18 and 27 °C, and that it is offered as a guide for all film and developer combinations rather than as a measurement of any one of them.

Three things that chart is claiming, and one it is not.

It claims a multiplicative rule. Divide every entry by the 20 °C entry in the same row and the ratio is the same all the way down the chart, to within the fifteen-second rounding: 1.23 at 18 °C, 1.11 at 19, 0.90 at 21, 0.81 at 22, 0.69 at 24, 0.63 at 25, 0.54 at 27. A temperature change multiplies the time rather than adding to it, which is why the correction for a 20-minute film is bigger in minutes than the correction for a 6-minute film.

It claims to apply to everything. Ilford says so, and that is a large claim: developing agents differ in their temperature sensitivity. Kodak’s 1928 primer states flatly that “a very little change in the temperature affects hydroquinone greatly and affects Elon very little” — Elon being metol. A single chart for all combinations is therefore an average, and a film datasheet’s own figures beat it. Ilford’s HP5 Plus sheet gives its own: 6 minutes at 20 °C becomes 4½ minutes at 23 °C and 9 minutes at 16 °C. Both sit close to the general chart, which is reassuring rather than surprising.

It claims a floor. Development times below five minutes “are not recommended due to the risk of uneven development”. Kodak’s D-76 sheet says the same in different words: “tank development times shorter than 5 minutes may produce poor uniformity.” That is not a rate statement at all — it is a transport statement, and it is why the chart’s warmest columns are dashes for short base times.

It does not claim to hold beyond its ends. The chart stops at 18 and 27 °C. Extrapolating it to 12 °C or 32 °C is guessing, and at the warm end there is a second reason to stop: gelatin softens, and the emulsion’s mechanical behaviour changes in ways the diffusion page takes up.

🧮The coefficient Ilford publishes, and the activation energy behind it

You do not have to infer the temperature coefficient, because Ilford states it. Its powder-developer sheet instructs the user to “increase the given development times by 10% for each 1ºC (2ºF) drop in temperature and decrease the given development times by 10% for each 1ºC (2ºF) rise in temperature”, and offers the same worked example as the HP5 Plus sheet: 6 minutes at 20 °C becomes 4½ at 23 °C and 9 minutes at 16 °C. Ten per cent per degree Celsius is a published manufacturer figure, not a rule of thumb passed round a darkroom.

The compensation chart agrees with it, and so does the chemistry. Take the chart’s ratios as relative rates by inverting them — a time of 1.23 is a rate of 0.81 — and plot the natural logarithm of the rate against 1/T in kelvin, which is the linear form of the Arrhenius equation. The eight points fall on a straight line with an R² of 0.997, and the slope gives an activation energy of about 67 kJ/mol. That activation energy in turn predicts a rise of 9.8 per cent per degree near 20 °C — Ilford’s ten — and a factor of about 2.5 for a ten-degree rise, where OpenStax’s general rule of thumb says approximately two.

Three numbers, three routes, one answer: the manufacturer’s instruction, the manufacturer’s chart, and the physical model. The activation energy is the course’s own arithmetic on Ilford’s published figures rather than a number Ilford states, and it inherits every limitation of the chart it came from — an average over films and developers, rounded to fifteen seconds, valid between 18 and 27 °C. It is offered as a way of understanding the ten per cent, not as a substitute for it.

Development is a rate process with a shape

A development time is not a switch. Watch a print in a tray and you can see the three stages.

An induction period. Nothing visible happens at first. Ilford’s paper developer sheet is precise about it: on a correctly exposed fibre-base print “the image will begin to appear after 35 seconds”.

A rise. The heavily exposed areas develop fastest, because they have the most crystals carrying latent-image specks, and density climbs steeply.

A slow approach to completion. As the developable crystals in a region are used up, the rate in that region falls away. Ilford’s statement of it is the one to remember: after the recommended 1 to 2 minutes, development of a fibre-base print “may be extended to 6 minutes without any noticeable change in contrast or fog”. Three times the time, no visible change. That is what development to completion means, and it is why print developing times are quoted so casually and film developing times are not.

Why doubling the time does not double the density

• A highlight: large exposure, many developable crystals
• A shadow: small exposure, few developable crystals

Show the numbers behind this plot

Two rising curves against development time. The lower curve is a shadow area, given a small exposure: it stays flat near zero for the first half minute, rises to 0.30 by four minutes, 0.43 by eight minutes and only 0.50 by sixteen minutes, so it is almost flat after the first few minutes. The upper curve is a highlight area, given a large exposure: it too is flat for the first half minute, then rises steeply to 0.84 at four minutes, 1.42 at eight minutes and 1.84 at sixteen minutes, still climbing at the right-hand edge. The vertical gap between the two curves, which is the contrast, grows throughout: it is 0.54 at four minutes, 0.99 at eight and 1.34 at sixteen. A shaded region at the left covers the first half minute and is labelled induction, nothing visible yet. The teaching point is that going from eight minutes to sixteen adds only about 0.4 to the highlight and 0.07 to the shadow, so it does not double either density; what it does is increase the difference between them, which is contrast.

Series	Development time, minutes	Density above base plus fog
A highlight: large exposure, many developable crystals	0.00	0.02
A highlight: large exposure, many developable crystals	0.50	0.04
A highlight: large exposure, many developable crystals	1.00	0.14
A highlight: large exposure, many developable crystals	2.00	0.38
A highlight: large exposure, many developable crystals	3.00	0.62
A highlight: large exposure, many developable crystals	4.00	0.84
A highlight: large exposure, many developable crystals	6.00	1.18
A highlight: large exposure, many developable crystals	8.00	1.42
A highlight: large exposure, many developable crystals	10.00	1.58
A highlight: large exposure, many developable crystals	12.00	1.70
A highlight: large exposure, many developable crystals	16.00	1.84
A highlight: large exposure, many developable crystals	20.00	1.92
A shadow: small exposure, few developable crystals	0.00	0.02
A shadow: small exposure, few developable crystals	0.50	0.03
A shadow: small exposure, few developable crystals	1.00	0.06
A shadow: small exposure, few developable crystals	2.00	0.16
A shadow: small exposure, few developable crystals	3.00	0.24
A shadow: small exposure, few developable crystals	4.00	0.30
A shadow: small exposure, few developable crystals	6.00	0.38
A shadow: small exposure, few developable crystals	8.00	0.43
A shadow: small exposure, few developable crystals	10.00	0.46
A shadow: small exposure, few developable crystals	12.00	0.48
A shadow: small exposure, few developable crystals	16.00	0.50
A shadow: small exposure, few developable crystals	20.00	0.51

Drawn to teach the shape, not measured from a film: the course has no density series of its own, and Part XIII is where measured curves belong. What the shape asserts — a lag, a steep rise, and a flattening that arrives sooner in the shadows than in the highlights — is the accepted description, and it is what makes development time a contrast control. 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.

The consequence is the one every beginner gets wrong. Extending development raises the highlights much more than the shadows, so it raises contrast rather than raising exposure. A thin negative from underexposure cannot be rescued by longer development, because the shadow curve has already flattened; all you get is a thin negative with hard highlights. Part XIII gives that argument its proper form with measured curves, and Part VIII turns it into a working method.

Reaction-limited or transport-limited?

A sequence is only as fast as its slowest step, and development has two candidates.

• Reaction-limited. The developer is everywhere it needs to be, and the chemistry at the grain surface is what takes the time. Here temperature, concentration and the choice of agent set the rate, and agitation changes nothing.
• Transport-limited. The chemistry is fast enough that the grain has consumed the developer in its neighbourhood, and the rate is set by how quickly fresh developer arrives and spent developer leaves. Here agitation is a rate variable, and no amount of extra developing agent in the tank helps.

Real development sits between the two, and moves between them: a fresh, concentrated, warm developer tends towards transport-limited, and a dilute, cold one towards reaction-limited. This is why the same change to your agitation can make a large difference on one film-and-developer combination and none on another.

Kodak states the function of agitation in one sentence in its process-control publication, and it is a transport sentence from beginning to end: “agitation is necessary to maintain uniform solution activity by removing exhausted solution from the emulsion surface and replacing it with fresh solution.” Agitation is not a ritual, and it is not “mixing the chemicals”. It is the replacement of a depleted layer. The diffusion page takes that layer apart.

How much of development is transport-limited? Ilford answers the question with a number. Its intermittent agitation for a spiral tank is four inversions during the first ten seconds of each minute — so the film spends about a sixth of its time being agitated and five-sixths sitting still. Switch to continuous agitation, as a dish or a rotary tube gives, and Ilford instructs the user to cut the time by about 15 per cent. That 15 per cent is the part of the development that was waiting for supply. It is not the whole reaction, which tells you development is not purely transport-limited; and it is far from nothing, which tells you it is not purely reaction-limited either.

There is a second reason the figure matters: it means an agitation pattern is part of a published time. A datasheet time assumes an agitation scheme, and changing yours invalidates the time as surely as changing the temperature does — by, on Ilford’s own figure, about the same amount as a degree and a half of temperature.

Catalysis, and the smallest catalyst in photography

A catalyst raises the rate without being consumed. OpenStax is precise about the mechanism: it offers an alternative route whose rate-determining step has a lower activation energy. Two things follow that people constantly get wrong.

A catalyst does not change where the reaction ends up. The reactant and product energies are untouched; only the hill between them is lowered. A catalyst cannot make an unfavourable reaction favourable — it can only make a favourable one quick.

A catalyst lowers the barrier in both directions. It speeds the reverse reaction by the same factor, which is why a catalyst reaches equilibrium sooner rather than reaching a different equilibrium.

The hill, with and without a lower path

1. 1Reactants — developing agent and silver ion, at the same energy on both paths
2. 2Uncatalysed barrier — high: the reaction is favourable but slow
3. 3Catalysed barrier — lower: the same reaction, a different route, far faster
4. 4Products — silver metal and the oxidised agent, at the same energy on both paths

A catalyst changes only the middle of the picture. Both ends are where they were, which is why catalysis is a kinetic idea and not a thermodynamic one.

The photographic case is the latent-image speck. A few atoms of silver on the surface of an exposed crystal make the reduction of that crystal go within the time you allow, while the identical crystal beside it, without a speck, does not. That is catalysis in the sense that matters here: a site that lowers the barrier. The physics of how it does so belongs to Part IV, which takes up the latent image and development as amplification.

The unwelcome catalysis is the one the previous page discussed and declined to assert: trace metals speeding the oxidation of a developer by air. It is very widely claimed; the course has found no source it has read that establishes it for photographic developers, and says so rather than repeating it.

Temperature in a real darkroom

Manufacturers state tolerances, and they differ by an order of magnitude depending on what is being controlled.

Statement	Tolerance	Context
Kodak, process control for black-and-white film	variations greater than ±0.3 °C in the developer “will affect process control and image quality”	a monitored production process with control charts
Ilford, paper development	20 °C ±1 °C recommended	dish processing to completion
Ilford, film in spiral tanks with the ID-11 family	20 °C recommended, 20–24 °C usable with times reduced	manual amateur processing
Ilford, all process solutions against each other, ID-11 family	within ±1 °C of the temperature in use	manual amateur processing
Ilford, stop bath and fixer	18–24 °C	steps that are not contrast-critical

Those are not in conflict. They are different answers to different questions: how tightly must this be held for a measurable process, and how far can it wander before a print changes visibly.

For your own tank, the useful conversion is the one the chart gives you: one degree Celsius is worth about ten per cent of the development time. Being 1 °C warm and developing for the nominal time is the same error as developing 10 per cent too long. Whether that matters depends on where you are on the density curve — in the highlights it is a visible change in contrast, and in a print developed to completion it is very nearly nothing, which is exactly what Ilford’s “extend to 6 minutes without noticeable change” is telling you.

The practical enemy is drift. A tray of developer at 20 °C in an 18 °C darkroom is losing temperature all evening, and a tank held in the hand is gaining it. Measure the developer, not the room, and measure it at the end as well as the beginning.

🧪Measure the drift before you argue about the tolerance

Put a thermometer in a working tray of developer and record its temperature every ten minutes for an hour of ordinary printing, along with the room temperature. Most darkrooms drift further than their owners believe. If your tray moves 2 °C over the session, you have a 20 per cent change in development rate between the first print and the last, and you now know whether to fix the temperature or to develop to completion so that it does not matter.

Deliberately hot, deliberately cold

Not every process is trying to hold 20 °C.

Emulsion making runs hot, because gelatin must be liquid and because crystal growth is a rate process the maker is steering on purpose; Part V is where that happens.

Papers are developed to completion, which is a decision to run past the region where time matters so that time stops being a variable. Ilford’s 1 to 2 minutes with a 6-minute latitude is exactly that design.

Stand and semi-stand development run long, dilute and almost without agitation. Described honestly, that is a decision to make development transport-limited on purpose, so that local exhaustion does the compensating. The mechanism belongs to the diffusion page and the practice to Part VIII; this page only insists that it is a rate decision and not a recipe, and that its results follow from the physics rather than from the number of minutes.

Reading a chart properly, and what it cannot tell you

A time-and-temperature chart answers exactly one question: for this film in this developer at this dilution, how long at a different temperature to reach the same contrast?

Notice the last three words. Ilford states what contrast its own tabulated times are aiming at — “negatives of normal contrast, typically around a Gbar of 0.62” — and adds that the figures “are only a guide and may need to be adjusted to suit individual processing systems, working practices and preferences”. A compensation chart preserves whatever contrast the base time was giving you. It cannot supply one.

It cannot tell you the right contrast for your negative, your paper or your subject. It cannot tell you what happens below its coldest column or above its warmest. It cannot tell you whether your agitation matches the one the times were measured with — and an Ilford or Kodak time assumes a stated agitation pattern, so changing yours invalidates the time before the temperature does. It cannot tell you what a different developing agent’s temperature sensitivity is, since a general chart is an average. And it cannot tell you that five minutes at 24 °C will develop evenly, which is why both makers put a floor under it.

Use the film’s own datasheet where one exists, the general chart where one does not, and your own records over both.

Recap

Thermodynamics says whether a reaction can go and kinetics says how fast, and photography lives in the gap: the unexposed grain would develop eventually, and does not in ten minutes. Rate is set by the nature of the reactants, their state and surface, concentration, temperature and catalysis, plus, in an emulsion, the transport of reagent to the grain. Collision theory explains temperature: only molecules above the activation energy react, and warming moves a disproportionate number of them over the barrier, which the Arrhenius equation expresses as an exponential in −Eₐ/RT. Fitting Ilford’s published compensation chart to that form gives about 67 kJ/mol, which means roughly 10 per cent per degree and a factor of about 2.5 per ten degrees — near, and a little above, the textbook rule of doubling. A compensation chart claims a multiplicative rule, claims generality it can only approximate, and puts a floor at five minutes for a reason that is about evenness rather than rate. Development itself has a shape: an induction period, a steep rise, and a flattening that arrives earlier in the shadows than the highlights, so extending development raises contrast rather than density in general. Where the chemistry outruns the supply, development is transport-limited and agitation becomes a rate variable; Kodak defines agitation exactly that way. A catalyst lowers the barrier without moving either end of the reaction, and the latent-image speck is the case that makes the picture.

Next: the transport that this page kept deferring — how a developer reaches a grain buried in gelatin, how thiosulfate finds its way out again, and why washing is a matter of changes of water rather than of time under a tap.

Check your understanding

Question 1. A datasheet gives 9 minutes at 20 °C. Using Ilford's compensation chart, the time at 24 °C is 6 minutes 15 seconds. What assumption have you just made, and what was that chart measured on?

• That your film-and-developer combination has the same temperature sensitivity as the average the chart represents — Ilford offers it as a guide for all combinations, not as a measurement of yours, and Kodak's own primer notes that hydroquinone is far more temperature-sensitive than metol
• None; the chart is exact for every film
• That the developer is fresh
• That the film is 35 mm

Show the answer and why

Answer: That your film-and-developer combination has the same temperature sensitivity as the average the chart represents — Ilford offers it as a guide for all combinations, not as a measurement of yours, and Kodak's own primer notes that hydroquinone is far more temperature-sensitive than metol

The chart is a single set of ratios applied to every combination. That is a real assumption, because developing agents differ: the 1928 Kodak primer says a very little change in temperature affects hydroquinone greatly and Elon (metol) very little, so a hydroquinone-heavy developer should move more than the chart says and a metol-heavy one less. Where the film's own datasheet gives temperature figures, use those instead. Freshness and format matter too, but they are not what the chart assumed away.

Question 2. Development is extended from eight minutes to sixteen. Predict what happens to shadow density, highlight density and contrast, and say which is limited by something other than time.

• All three double
• Nothing changes, because development is complete at eight minutes
• Shadow density doubles and highlights stay the same, so contrast falls
• Shadow density rises a little, highlight density rises substantially but well short of doubling, and contrast rises — the shadows are limited by how few crystals there carry a developable speck, which is a matter of exposure and not of time

Show the answer and why

Answer: Shadow density rises a little, highlight density rises substantially but well short of doubling, and contrast rises — the shadows are limited by how few crystals there carry a developable speck, which is a matter of exposure and not of time

The shadow regions have few crystals carrying a latent-image speck, so their density curve flattens early: no amount of extra time creates developable crystals that exposure did not make. The highlights have many, so they keep building for longer. The gap between the two is contrast, and it grows. This is why development time is a contrast control and exposure is a shadow-density control — the old formula about exposing for the shadows and developing for the highlights is a statement about these two curve shapes.

Question 3. Fitting Ilford's chart to the Arrhenius form gives about 67 kJ/mol, implying roughly 10 per cent per degree Celsius. A tank of developer is at 22 °C instead of the intended 20 °C, and you develop for the nominal time. Roughly how over-developed is the film, and what does the chart itself say?

• About 2 per cent over
• About 20 per cent over, and the chart agrees: its 22 °C ratio is 0.81, so it wanted only 81 per cent of the nominal time
• About 100 per cent over
• Not over-developed at all, since 2 °C is within tolerance

Show the answer and why

Answer: About 20 per cent over, and the chart agrees: its 22 °C ratio is 0.81, so it wanted only 81 per cent of the nominal time

Ten per cent per degree, compounded over two degrees, is about 21 per cent faster. Ilford says the same thing three ways: its powder-developer sheet instructs the user to change the time by 10 per cent for each degree, its compensation chart calls for 0.81 of the 20 °C time at 22 °C, and 1 ÷ 0.81 = 1.23. Whether 20 per cent matters depends on the material: on a film it is a visible contrast change, and on a paper developed to completion Ilford says three times the time makes no noticeable difference. Note which direction the correction goes — warmer means less time, and the commonest error is to remember the coefficient and forget the sign.

Question 4. Two darkroom workers disagree about whether more vigorous agitation makes a film denser. One finds a clear effect and the other none. How can both be right?

• One of them is measuring wrongly
• Agitation only matters for sheet film
• Their processes sit at different points between reaction-limited and transport-limited: where fresh developer is arriving faster than the grain can use it, agitation changes nothing; where the grain has locally exhausted its supply, agitation sets the rate
• Agitation only matters above 24 °C

Show the answer and why

Answer: Their processes sit at different points between reaction-limited and transport-limited: where fresh developer is arriving faster than the grain can use it, agitation changes nothing; where the grain has locally exhausted its supply, agitation sets the rate

A sequence runs at the speed of its slowest step. A concentrated, warm developer tends to be limited by supply, because the chemistry at the grain is quick enough to strip the neighbourhood; a dilute, cool one tends to be limited by the chemistry itself. Kodak defines agitation in exactly these terms — removing exhausted solution from the emulsion surface and replacing it with fresh — which is a statement about transport, not about mixing. The next page is where that layer at the surface gets its name and its physics.

Question 5. Why do both Ilford and Kodak refuse to recommend development times shorter than five minutes, even when their own charts produce such a number at high temperature?

• Because the objection is not about rate at all: a short time gives too little opportunity for uneven arrival and departure of solution to average out, so the risk is uneven development rather than wrong density
• Because the chemistry becomes unpredictable above 24 °C
• Because the developer oxidises too quickly
• Because thermometers are not accurate enough

Show the answer and why

Answer: Because the objection is not about rate at all: a short time gives too little opportunity for uneven arrival and departure of solution to average out, so the risk is uneven development rather than wrong density

Ilford words it as "risk of uneven development" and Kodak as "may produce poor uniformity", and both are transport statements. Filling a tank, agitating and draining all take a fixed number of seconds, and the shorter the total the larger a fraction of the process those non-uniform moments become. Two other consequences worth noticing: a short time also magnifies timing errors as a percentage, and it leaves less scope to use time as a contrast control.

Question 6. Name one photographic reaction that is thermodynamically favourable but usefully slow, and say what would happen if it were not slow.

• The dissolution of silver bromide in water; if it were fast the emulsion would wash away
• The neutralisation of a developer by a stop bath; if it were fast, the stop bath would not work
• The oxidation of sulfite by air; if it were fast, developers would keep better
• The reduction of an unexposed silver halide crystal by the developing agent; if it were fast, every crystal would develop and the film would come out uniformly black rather than carrying an image

Show the answer and why

Answer: The reduction of an unexposed silver halide crystal by the developing agent; if it were fast, every crystal would develop and the film would come out uniformly black rather than carrying an image

This is the single most important kinetic fact in photography. The developing agent is a strong enough reducing agent to reduce silver ions, and nothing in the thermodynamics distinguishes an exposed crystal from an unexposed one — both would go, given long enough. What separates them is a barrier that the latent-image speck lowers on one and not the other. Development selectivity is entirely a matter of rate, which is also why over-development and too-active a developer both produce fog: they give the slow reaction enough encouragement to happen after all.

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