Restrainers and Antifoggants

A restrainer is the only ingredient in a developer that you add in order to make it work less well. It costs time, it costs density, and in the wrong quantity it costs the shadows outright — and Kodak’s 1928 primer lists omitting it among the mixing faults that make a developer fog.

The reason it earns its place is that development is not a switch but a race, and a restrainer lengthens the course for every runner. The exposed grain, which had a head start, still finishes. The unexposed one, which was going to arrive late and unwanted, now does not arrive at all inside the time you allow.

Bromide is the reaction’s own product, supplied in advance

Hurter and Driffield put the whole idea in a sentence of ordinary chemical dynamics: bromide is one of the products of the change that takes place in development, and the products of a reaction generally retard it. Every grain reduced releases one bromide ion — 0.74 g of bromide for every gram of silver, as the first page of this part computed. Adding potassium bromide at the start is putting the exhaust back in the intake.

That page also priced it. The potential of the silver bromide electrode falls by 59 millivolts for every tenfold rise in free bromide, so the four grams per litre in Kodak’s D-19b hold the silver couple about 0.28 V lower than a bromide-free bath does. That is a large thermodynamic statement and it is not by itself the mechanism, because the threshold a restrainer moves is kinetic. It applies to every crystal in the film equally, exposed or not, and what a photographer wants is an effect that falls on one and not the other.

⚗What the corpus establishes about the mechanism, and where it stops

Established. Bromide lowers the free silver ion concentration by the common-ion effect, and therefore lowers the potential the developing agent has to overcome, by a computable 59 mV per decade. Established too, from Kodak’s 1928 primer, that the effect is far larger on some agents than others: a small quantity of bromide affects hydroquinone strongly and Elon — metol — much less, which is the measurement behind the primer’s ranking of agents by “reduction potential”.

Not established here. Why the restraint falls harder on the unexposed grain than on the exposed one. The potassium bromide page says the same, and it is the honest position: the standard account is adsorption of bromide onto the crystal surface, raising the barrier to nucleating a new development centre far more than it raises the barrier to extending an existing one, and no Tier-1 source this course holds states it. What the course does hold is the closest available analogue, from Carroll and Hubbard’s work on organic antifoggants below, where the same adsorption argument is made explicitly by the researchers and is still hedged by them.

The distinction matters because it is the difference between a brake and a selective brake, and only the second is worth having.

What it looks like on the negative, measured in 1890

Hurter and Driffield measured this properly, and their Experiment 15 is the cleanest demonstration in the corpus of everything a restrainer does at once. One plate, one pyro-soda developer — sodium carbonate decahydrate 50, sodium sulfite 50 and pyrogallol 6 parts per thousand — five bromide levels, and three minutes of development for every strip.

Hurter and Driffield, Experiment 15: what bromide does to a whole curve

• No bromide
• 2 parts per thousand
• 8 parts per thousand
• 32 parts per thousand
• 128 parts per thousand

Show the numbers behind this plot

Five curves of density against log exposure, all developed for three minutes in the same pyro-soda developer and differing only in the potassium bromide added, measured by Hurter and Driffield. With no bromide the curve rises steeply from 0.565 at 1.25 candle-metre-seconds through 1.045, 1.530 and 1.940 to 2.255 at 20 and 2.500 at 40. With 2 parts of bromide per thousand the whole curve is displaced to the right and downwards: 0.120, 0.375, 0.790, 1.250, 1.700 and 2.020 at the same six exposures, so the highest density has fallen from 2.50 to 2.02 and the smallest exposures have almost nothing on them. With 8 parts the curve is lower still, reaching only 0.115 at an exposure of 5 and 1.190 at 40. With 32 parts nothing at all is developed below an exposure of 10, and the highest density reached is 0.290. With 128 parts the plate is essentially blank, rising from 0.060 to only 0.100 across the whole range, so development has been almost entirely prevented in three minutes. Not plotted, because it cannot go on a logarithmic exposure axis, is the unexposed density of each strip, which falls from 0.160 with no bromide to 0.090 with 2 parts and 0.060 with 8, 32 and 128 parts. That row is the antifogging action on its own, and it is complete by 8 parts per thousand while the image is still developing strongly.

Series	Log of the exposure, candle-metre-seconds	Density, inclusive of fog
No bromide	0.10	0.56
No bromide	0.40	1.04
No bromide	0.70	1.53
No bromide	1.00	1.94
No bromide	1.30	2.25
No bromide	1.60	2.50
2 parts per thousand	0.10	0.12
2 parts per thousand	0.40	0.38
2 parts per thousand	0.70	0.79
2 parts per thousand	1.00	1.25
2 parts per thousand	1.30	1.70
2 parts per thousand	1.60	2.02
8 parts per thousand	0.10	0.07
8 parts per thousand	0.40	0.09
8 parts per thousand	0.70	0.12
8 parts per thousand	1.00	0.33
8 parts per thousand	1.30	0.70
8 parts per thousand	1.60	1.19
32 parts per thousand	0.10	0.06
32 parts per thousand	0.40	0.06
32 parts per thousand	0.70	0.06
32 parts per thousand	1.00	0.09
32 parts per thousand	1.30	0.17
32 parts per thousand	1.60	0.29
128 parts per thousand	0.10	0.06
128 parts per thousand	0.40	0.06
128 parts per thousand	0.70	0.06
128 parts per thousand	1.00	0.06
128 parts per thousand	1.30	0.07
128 parts per thousand	1.60	0.10

Measured: every point is read from the table of Experiment 15 in the Hurter and Driffield Memorial Volume, with the exposures converted to their base-ten logarithms. Two things the plot cannot show. The unexposed density, which is the fog, is not on a logarithmic axis; it fell from 0.160 to 0.060 across the same series. And the image colour changed: at 128 parts the deposit was a yellowish fawn rather than black, and chemical analysis showed 2.43 times as much silver present for the same measured density, so the densities at the right-hand end of the series are not comparable with those at the left as measurements of silver.

Read the family from left to right and every claim on this page is in it.

The threshold moves. With no bromide the smallest exposure already carries 0.565 of density; with 2 parts per thousand it carries 0.120, and with 8 parts almost nothing until the exposure is eight times larger. The foot of the curve is where a restrainer bites hardest.

Fog falls, and falls first. The unexposed strip goes from 0.160 to 0.090 to 0.060, and it has reached its floor by 8 parts per thousand while the image is still developing strongly. That asymmetry — fog suppressed sooner than image — is the whole justification for the ingredient.

Contrast rises before it falls. Compare the no-bromide and 2-parts curves and the second is not merely lower, it is steeper across the middle, because the shadows have been held back further than the highlights. Push on to 32 parts and there is no contrast left because there is no image.

Nothing stops. H&D tested up to a 12 per cent solution with ferrous oxalate and with alkaline pyrogallol and never found the reaction cease. It is simply retarded, and if sufficient time is allowed the image will make its appearance in full force.

⁇Does bromide cost film speed?

Photographers commonly say it does, and two Tier-1 sources appear to disagree.

Hurter and Driffield say no. Having matched densities by extending the time — density 2.425 in 15 minutes with 90 parts of bromide against 2.50 in 3 minutes with none — they conclude that the retarding influence of bromide can be fully compensated by time of development, that the character of the negative is not materially altered in the end, and that the speed of the plate is not really altered by the addition of bromide.

Kodak’s 1928 primer says yes. Of a used developer it writes that the restraining action of the accumulated bromide and iodide “is analogous to cutting down the exposure, so that with an old developer it is not possible even on prolonged development to get the ultimate result out of an under-exposure.”

What this course uses, and why. The two are reconcilable and the reconciliation is the useful part. H&D added bromide to a fresh developer and compensated with time; Kodak is describing an old one, which carries oxidation products and a fallen pH as well as bromide, and those are not compensated by time. So: fresh bromide retards and time restores; accumulated restraint does not restore, because bromide is not the only thing that accumulated. The potassium bromide page takes the same position. No source in this course’s corpus gives a figure for speed lost per gram of bromide, and none is offered here.

A developer restrains itself, and the manufacturers dose the cure

The formulas say where restraint is wanted, and the pattern is not subtle.

Developer	Potassium bromide	What it is for
D-76, D-23	none	fine-grain film developers at maximum speed
DK-20, DK-50	0.5 g/L	general-purpose film developers
DK-15, D-72	1.9 g/L	a tropical film developer and a rapid paper and plate developer
D-19b	4.0 g/L	high-contrast X-ray development
D-9, in the working bath	about 11 g/L	hydroquinone with caustic soda, developing in three minutes

Bromide is scarcest exactly where speed matters most, and heaviest where the alkali is most violent. D-9’s eleven grams per litre are not a contrast control; they are what stops a bath holding half a mole of free hydroxide from developing the entire frame.

Then a film goes in, and the developer starts dosing itself. ILFORD states the mechanism on its own powder sheet: as each film or batch is processed it releases halides and other by-products into the developer that act as a restrainer on the development of subsequent films. That is seasoning, and ILFORD answers it with arithmetic rather than chemistry.

Films of 135-36 through one litre of ILFORD stock	Development time
1st	the published time, N
2nd	N + 10 %
3rd	N + 20 %
4th	N + 30 %
5th	N + 40 %
10th	N + 90 %

ID-11 and MICROPHEN are rated at 10 films per litre on that schedule; PERCEPTOL at 4, with the fifth marked not recommended. Kodak answers the same question about D-76 in the same currency: 16 sheets of 8 × 10 per gallon — four per litre — with the development time raised 15 per cent after every four. Neither company publishes a bromide concentration, a pH drift or an agent consumption. They publish a compensation, which is the manufacturer’s way of answering the question without answering it.

!What a replenisher is doing, and what ILFORD's improvised one lets you infer

Replenishment is the alternative to compensation, and its object is different: to hold the bath at a fixed activity so that the published time keeps working. Kodak’s rate for D-76 is 22.2 to 29.6 mL of D-76R per 135-36 roll or 8 × 10 sheet, which raises the capacity from 16 sheets per gallon to 120 — a factor of seven and a half — with no increase in development time at all.

ILFORD withdrew its ID-11 replenisher in September 2012 and published two substitutes. The first is simply to compensate with time as above. The second is an improvisation with a lesson in it: take two packs, dissolve both part A sachets and one part B, make up to the pack volume, and discard the other part B. Part A is the smaller bag and goes in first, into warm water at about 40 °C.

Doubling A against B makes a replenisher, so whatever the working bath spends is in part A. This course will not say what that is: ILFORD publishes no ID-11 formula and this page does not reason from D-76’s to it. What the notice does establish, without disclosing anything, is the same fact the Kodak and ILFORD replenisher pairs establish in the alkalis lesson — that a working tank runs down unevenly, and that a replenisher is a different formula rather than more of the same one.

The organic antifoggants: a different mechanism, a hundred times the potency

The course keeps two words apart that many sources use interchangeably. A restrainer is a bromide or an iodide, working by mass action on a reaction whose own product it is. An antifoggant is an organic molecule working by adsorption, and the difference is measurable.

Carroll and Hubbard, at the United States National Bureau of Standards in 1932, made the measurement on 6-nitrobenzimidazole. They precipitated its silver compound, washed it, and put a silver electrode over a saturated solution of it at 30 °C. At pH 7.1 the compound proved more insoluble than silver bromide. At pH 3.5 the silver ion concentration over the same saturated solution rose to 6.4 × 10⁻⁶ N, because the silver is readily displaced by hydrogen — acid takes the antifoggant off the grain and hands it back.

C₇H₅N₃O₂ + Ag⁺ → AgC₇H₄N₃O₂ + H⁺

Carroll and Hubbard's silver nitrobenzimidazolate, and the proton whose return reverses it

Then the number that separates the two classes: in emulsions near neutrality the additive lowers the silver ion concentration only a little, but mol for mol it produces about ten times the effect of soluble bromide in delaying after-ripening. Carroll and Hubbard were scrupulous about what that did not prove — something more than the decrease in silver ion concentration must be involved, they wrote, and another method of attack would be needed to decide what — and they pointed at adsorption without claiming it.

Benzotriazole is the compound that took the job over, and its numbers follow the same shape. The IUPAC compilation gives its ring N–H a pK_a of 8.64 at 20 °C, so in a paper developer at 10.5 it is almost entirely the benzotriazolate anion; and PubChem records that silver is determined gravimetrically by precipitation with benzotriazole, which is as strong a statement of insolubility as analytical chemistry makes.

C₆H₅N₃ + Ag⁺ → AgC₆H₄N₃ + H⁺

The reaction an analyst uses to weigh silver, and a print developer uses to protect a highlight

That pK_a explains where it works and where it does not. Between about pH 3 and pH 7 it is almost all neutral molecule, so it does very little; above pH 9 it is almost all anion, and armed. A paper developer at 10.4 is where the ion is, which is where the compound is used.

✕The dose this course will not give you

Organic antifoggants are used in fractions of a gram where a bromide is used in grams, and Carroll and Hubbard’s factor of ten explains the ratio. No source in this course’s corpus gives a concentration per litre for benzotriazole in any developer, and none is invented here. Go to the supplier’s sheet for the product you have.

Two claims commonly attached to it are also unsourced here and are marked as such: that it cools the image tone of a warm-tone paper, and that it delays the first appearance of the image. Printers report both. The course holds no measurement of either for this compound.

Carroll and Hubbard did measure one cost, and it is worth knowing before reaching for an organic antifoggant in an emulsion rather than a developer: their additive selectively depressed the sensitivity conferred by sensitising dyes, more for filtered exposures than for white light, and unevenly across erythrosin, pinaflavol and pinacyanol in that order. Their explanation is the same adsorption argument — a compound that binds silver ions competes with the basic dyes for the same sites, so the acid dye suffers least.

Iodide, which restrains and sometimes does the opposite

Kodak’s primer names the restrainer as “the bromides and iodides of sodium or potassium”, and iodide earns the mention twice over. As a developer ingredient it is rare but real: the primer’s tropical developer D-13 takes 2.1 g/L of potassium iodide where nearly every other formula takes bromide, and Wall’s 1924 formulary gives it a narrower job still, 2 g per litre to obviate the black hair-like abrasion marks that glossy development papers show, with the note that “the iodide is the active agent”. As a by-product it arrives unasked: an iodobromide emulsion releases iodide as well as bromide, and the primer lists both among the accumulating restraints in a used bath.

Iodide should restrain harder than bromide for a reason already derived in this part: silver iodide is the least soluble of the three halides, so the silver iodide electrode sits about 200 mV below the bromide one. It is also the ion the emulsion holds most tightly, which is why iodide in the crystal is a different subject from iodide in the bath.

🕰The Lainer effect, given without an explanation

Eder records that Alexander Lainer found that under certain conditions the action of the developer is accelerated by adding potassium iodide — in contrast to potassium bromide — and that the technical literature named this the Lainer effect. The same ion that restrains can therefore also speed development up.

The course has found no sourced statement of which conditions, and gives the effect without a mechanism. It is here because a reader will meet the name, and because it is a useful corrective: the halide ions are not a single lever with a single sign.

Speed, fog, contrast and tone: what you are trading

Four consequences, in the order a photographer meets them.

Fog. This is the one the ingredient is for, and it is the one H&D’s unexposed row measures: 0.160 down to 0.060, complete by 8 parts per thousand. Every other consequence is a price paid for it.

Speed. Retarded in a fresh bath and recoverable by time, on H&D’s measurement; not recoverable in a seasoned one, on Kodak’s, because bromide is not the only thing that has accumulated. The course quotes no figure per gram.

Contrast. Raised at small additions, because the toe is held back further than the shoulder, and destroyed at large ones. Kodak’s D-52 paper developer is the practical version: the 1928 primer takes one stock solution and gives four different additions of a 10 per cent bromide solution — 4, 8 or 16 mL per litre of working solution, which is 0.4 to 1.6 g of salt — one for each grade of Vitava paper. The bromide is the grade control.

Image colour. H&D measured it and it is startling: at 128 parts per thousand the deposit was a yellowish fawn rather than black, and analysis found 2.43 times as much silver for a given measured density. The silver is being deposited in a different physical form. Wall’s 1924 formulary gives the practical version for chloride papers — the longer the exposure, with a corresponding increase of bromide, the warmer the tones.

!What ILFORD's paper-developer sheet does and does not say about tone

This page’s research asked for the effect of bromide on image tone from ILFORD’s paper developer sheet. The sheet does describe image tone for each product — MULTIGRADE neutral with most papers, PQ UNIVERSAL and BROMOPHEN slightly warm of neutral, BROMOPHEN particularly recommended for the warmest tone on MULTIGRADE Warmtone — but it attributes none of that to bromide, or to any other ingredient, and it publishes no formulas.

So the tone claim on this page rests on Wall’s 1924 account of chloride papers and on H&D’s measured colour change at a bromide level no one would use, and the modern manufacturer’s sheet is evidence for the existence of tone differences between developers and for nothing about their cause.

Dilute, standing, and the developer that has nothing left to restrain it

The last case is the one where the restrainer is not a chemical you add but a condition you create. Dilute a developer and reduce the agitation, and the solution immediately around a heavily exposed region is not replenished; it accumulates the bromide that region has just released and runs out of agent. That is local exhaustion, and Part III’s diffusion page owns the transport half of it. It is the mechanism behind compensating and edge effects, and acutance-adjacency-and-compensation owns the consequences.

The point that belongs here is that historical practice added a restrainer as well. The 1906 British Journal Photographic Almanac prints Hübl’s directions for stand development with concentrated glycin: one ounce of stock in 80 to 90 ounces of water, plus 80 minims of ten per cent bromide, in which a properly exposed plate should appear in fifteen or twenty minutes and reach full density in several hours — with caustic soda added for under-exposure and more bromide for over-exposure. A bath left standing for hours is a bath given hours in which to fog, and the deliberate restrainer is what buys the time.

⁇How good the evidence for stand development actually is

The course’s own open questions record that stand and semi-stand development may have no Tier-1 or Tier-2 evidence base at all. What exists is a century of practitioner accounts, one of which is Hübl’s above, and a large modern community literature that this course may not cite for chemistry.

Teaching it as an experiment is defensible; teaching it as established practice is not. Where this part returns to it, the page will say plainly that the practice is community-documented and that the student’s own measurement is the evidence.

⚗The antifoggant nobody weighs out

Kodak’s primer records a remedy for aerial fog in Elon–hydroquinone developers that is worth the space because it names a fifth restrainer. Add about 5 per cent of old developer to the freshly mixed bath — and the primer adds that this is more effective than increasing the concentration of bromide above the normal quantity, because the oxidised developer probably acts as an antifogging agent.

The oxidation products the sulfite page spends its length trying to prevent are therefore doing useful work as well. That is why a developer’s first film and its fifth are not merely slower and faster but different in character, and it is one more reason the course’s answer to “is this bath still good?” is a fogged-strip test rather than a calculation.

Recap

• Bromide is the reaction’s own product, added in advance; it lowers the free silver ion concentration and with it the potential the agent must overcome, by 59 mV per decade.
• Why the restraint falls harder on the unexposed grain is not established by this course’s sources. The standard adsorption account is named and marked as unverified.
• H&D’s Experiment 15 shows the whole effect at once: the threshold moves right, fog reaches its floor first, contrast rises at small additions and vanishes at large ones, and nothing ever stops.
• Fresh bromide is compensated by time; accumulated restraint is not, because a seasoned bath has lost pH and gained oxidation products too.
• Film developers carry little or no bromide and high-energy developers carry grams, which is the clearest statement in the formulary of what the ingredient is for.
• Organic antifoggants work by a different route and about ten times harder mol for mol, they depend steeply on pH through their own pK_a, and the course publishes no dose.
• A used developer restrains itself, and the manufacturers answer that with a time compensation or a replenisher rather than with a concentration.

Check your understanding

Question 1. In Hurter and Driffield’s Experiment 15, adding 2 parts of potassium bromide per thousand dropped the highest density from 2.50 to 2.02 in the same three minutes, and dropped the unexposed density from 0.160 to 0.090. Which of those two numbers is the argument for using a restrainer?

• The fall in maximum density, because a lower maximum means finer grain
• The fall in unexposed density, because that is fog removed; the fall in maximum density is the price paid for it
• Neither: the effect is too small to matter
• Both equally, because the whole curve moves together

Show the answer and why

Answer: The fall in unexposed density, because that is fog removed; the fall in maximum density is the price paid for it

A restrainer is bought for the fog and paid for in everything else. Read the whole series and the asymmetry is the point: the unexposed density reaches its floor of 0.060 by 8 parts per thousand, while the image at the highest exposure is still developing to 1.19 at the same concentration. Fog is suppressed sooner than image. That difference in rate of surrender is what makes the ingredient worth its cost, and it is why bromide belongs in a high-energy developer such as D-9, whose 11 g per litre exist to stop a caustic hydroquinone bath developing the entire frame.

Question 2. A litre of ILFORD ID-11 stock has developed four films and is about to develop a fifth. What does ILFORD tell you to do, and what does that instruction reveal about what it knows?

• Discard it: four films is the capacity
• Add 40 per cent to the development time, and the instruction reveals that ILFORD measures the loss of activity empirically rather than publishing a bromide concentration or a pH
• Add fresh stock to restore the volume
• Nothing: stock developer is used until it fails

Show the answer and why

Answer: Add 40 per cent to the development time, and the instruction reveals that ILFORD measures the loss of activity empirically rather than publishing a bromide concentration or a pH

The published schedule is N, N+10 %, N+20 %, N+30 %, N+40 % for the first five films and N+90 % by the tenth, with ten films per litre for ID-11 and MICROPHEN and four for PERCEPTOL. Notice what it is not: a concentration of accumulated bromide, a pH, or a figure for agent consumed. Kodak answers the same question the same way, at 15 per cent after every four rolls or sheets per gallon of D-76. Replenishment is the alternative that keeps the time fixed instead — 22 to 30 mL of D-76R per roll raises the capacity from 16 sheets per gallon to 120 with no time change at all.

Question 3. Benzotriazole has a ring N–H with a pKa of 8.64. Why does it work in a paper developer at pH 10.4 and do very little in a stop bath?

• Because it decomposes in acid
• Because the active species is the anion, which dominates above about pH 9 and is nearly absent between pH 3 and 7; and Carroll and Hubbard showed for a related compound that acid displaces the silver from the antifoggant altogether
• Because a stop bath contains no silver
• Because it is insoluble in acid solutions

Show the answer and why

Answer: Because the active species is the anion, which dominates above about pH 9 and is nearly absent between pH 3 and 7; and Carroll and Hubbard showed for a related compound that acid displaces the silver from the antifoggant altogether

The mechanism is the formation of an extremely insoluble silver salt on the grain surface — so insoluble that analysts weigh silver by precipitating it with benzotriazole. That reaction needs the anion, and the pKa says where the anion is: almost all neutral molecule between pH 3 and 7, almost all anion above 9. Carroll and Hubbard measured the reverse for 6-nitrobenzimidazole and found the silver readily displaced by hydrogen at pH 3.5, which is the same statement from the other end. An antifoggant is therefore a pH-gated device, and a developer is the only bath in the sequence that arms it.

Question 4. Kodak says a used developer’s restraint is "analogous to cutting down the exposure" and that prolonged development will not rescue an under-exposure in it. Hurter and Driffield say the speed of the plate is not really altered by adding bromide. How does the course reconcile them?

• Kodak is wrong; H&D measured it
• H&D added bromide to a fresh developer and compensated with time, while Kodak is describing an old bath that has also lost pH and gained oxidation products, and those are not compensated by time
• H&D used plates and Kodak used film
• They are describing different halides

Show the answer and why

Answer: H&D added bromide to a fresh developer and compensated with time, while Kodak is describing an old bath that has also lost pH and gained oxidation products, and those are not compensated by time

Both are Tier-1 and both are right about what they measured. H&D matched densities by extending the time — 2.425 in fifteen minutes with 90 parts of bromide against 2.50 in three with none — and concluded that the retarding influence is fully compensated by time and the character of the negative not materially altered. Kodak is describing seasoning, which is bromide plus accumulated oxidation products plus a fallen pH. The practical rule that follows is worth more than either statement alone: a fresh bath with a known bromide addition is predictable and correctable; a seasoned one is neither, which is why one-shot working exists.

Question 5. At 128 parts of bromide per thousand, H&D found the deposit was a yellowish fawn and that a given measured density corresponded to 2.43 times as much silver as a black deposit of the same density. Why does that matter beyond the curiosity?

• It means the bromide was contaminated
• It means density measurements across that series are not comparable as measurements of silver, because the deposit’s physical form — and so its optical properties — changed with the restrainer
• It means the plate was under-fixed
• It means the silver was not fully reduced

Show the answer and why

Answer: It means density measurements across that series are not comparable as measurements of silver, because the deposit’s physical form — and so its optical properties — changed with the restrainer

Density is an optical measurement, and it stands in for silver only while the silver keeps the same physical form. H&D report that the deposit forms very slowly in the presence of large amounts of bromide and is evidently of a totally different physical character, changing gradually as the bromide rises. That is why their own plot of Experiment 15 has to be read as densities inclusive of fog and not as a silver assay, and it is a warning that recurs whenever a developer changes the form of the deposit rather than the amount — which is exactly what the solvent and staining developers later in this part do on purpose.

Question 6. Kodak reports that adding about 5 per cent of old developer to a freshly mixed Elon–hydroquinone bath prevents aerial fog more effectively than raising the bromide above the normal quantity. What does that tell you about the oxidation products?

• That old developer is more concentrated
• That the oxidation products act as antifoggants in their own right, so a developer’s by-products both restrain it and protect it, and a bath’s behaviour changes in character rather than merely in speed as it seasons
• That the bromide in the old developer is doing the work
• That aerial fog is caused by fresh developing agent

Show the answer and why

Answer: That the oxidation products act as antifoggants in their own right, so a developer’s by-products both restrain it and protect it, and a bath’s behaviour changes in character rather than merely in speed as it seasons

The primer says it directly — the oxidised developer probably acts as an antifogging agent — and the comparison with bromide is what makes it informative: a class of restraint that bromide cannot substitute for. Read alongside the same book’s statement that accumulated bromide, iodide and oxidation products all restrain development, it explains why seasoning is not simply a slowing down. The first film in a bath and the fifth meet chemically different solutions, which is the argument for the fogged-strip activity test rather than for arithmetic on elapsed capacity.

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