Kodak R-4b

The same two chemicals as R-4a, in two trays instead of one, and the result is classed as a different kind of reducer. That is the whole interest of this formula: Kodak Limited’s own headers say cutting for the mixed bath and almost proportional for the separated one, and nothing changes between them except that the oxidation and the dissolving no longer happen at the same instant.

Solution A — the oxidising bleach, used alone

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	7.5 g
Water	to make 1000 mL	A 0.75 per cent w/v solution, and a tenth the strength of R-4a's Solution A. No temperature is given for making it up; the salt dissolves readily in cold water and a ferricyanide solution should not be warmed.
Kept in a dark or yellow bottle. Alone this is a bleach and not a reducer: it converts the image and removes nothing.

Solution B — the silver solvent, used afterwards

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	200 g	hypo
Water	to make 1000 mL	A 20 per cent w/v hypo solution - four fifths the strength of the fixing bath F-52 and a plain hypo solution in every other respect.
This bath is ordinary plain hypo. Nothing in it is peculiar to reduction, which is why the five minutes it is given is a fixing time rather than a reducing one.

Used in this order — reducing a negative, a step at a time

1. Solution A — 1 to 4 minutes — at 18–24 °C — With uniform agitation, and the length depending on the degree of reduction desired. This is the step that decides the result; nothing later can add to it.
2. Solution B — 5 minutes — No temperature is published for this bath. Five minutes is the whole instruction, and what happens in it is the removal of what the first bath converted.
3. Water — wash thoroughly — The handbook closes with washing rather than fixing, because the hypo bath has already done the fixing this process needs.

Treat the negatives in Solution A with uniform agitation for 1 to 4 minutes at 65-75 °F (18-24 °C), depending on the degree of reduction desired. Then immerse them in Solution B for 5 minutes and wash thoroughly. The process may be repeated if more reduction is desired.

The permission to repeat is the point of the whole arrangement. A mixed Farmer's bath is one continuous action that has to be stopped at the right moment; this is a measured step that can be taken again, and again, with a look at the negative between each.

Purpose

To lower density and contrast together, which is what an over-developed negative needs and what a cutting reducer cannot give. Kodak Limited’s header is “Two-bath Farmer’s reducer giving almost proportional reduction for lowering density and contrast of negatives”.

The 1928 primer supplies the classification behind that sentence. A proportional reducer acts on all parts of the negative in proportion to the quantity of silver present there, so it exactly undoes the action of development — and the primer draws the conclusion explicitly: a correctly exposed but over-developed negative should be reduced with a proportional reducer. A cutting reducer removes an equal quantity everywhere and therefore a larger proportion from the shadows, which raises contrast.

The primer also says, in as many words, that no single substance forms an exactly proportional reducer. Kodak’s word for this formula is almost, and this page keeps it.

Recommended uses

A correctly exposed but over-developed negative, which is the case the 1928 primer assigns to this class. Too much contrast, shadows that are present but sitting too high, highlights that are blocked.

Any reduction you want to take in steps. The handbook’s permission to repeat the process is what a mixed Farmer’s bath cannot offer: one to four minutes in the bleach, five in the hypo, a look at a dry-enough negative, and then the same again if it is not enough. Nothing about the first pass commits you to the second.

Where the shadows must survive. By Kodak’s classing this is the reducer that takes least from the thin areas, which is exactly where a cutting reducer takes most.

Not for fog. The handbook does give a further instruction for general fog and it cannot be read: the sentence breaks at a page turn in the copy the course holds. This entry publishes no fog dilution, because completing a manufacturer’s sentence from the shape of it is not a citation. For general fog use R-4 or R-4a, which the 1928 primer names for that job with its instructions intact.

When another formula is preferable

• When contrast must rise rather than fall, R-4a or R-4. Clearing a veiled negative is a cutting job.
• When the negative is thin as well as flat, no reducer at all. There is nothing above base plus fog to take away, and everything you remove comes out of the shadows you were trying to keep.
• When a truly proportional reducer is wanted, Kodak’s own R-5, which mixes a permanganate with a persulphate — and it is not in this formulary. Neither is R-1, the persulphate reducer that the 1928 primer classes as the only known flattening reducer, nor R-2, the permanganate bath. The reason is the same in all three cases and it is worth stating rather than leaving as an absence: the chemical encyclopaedia has no page for ammonium persulphate or for potassium permanganate, the encyclopaedia is complete at 129 entries, and a formula is not published here with an ingredient the reader cannot look up. The potassium persulfate page says the same thing from its own side — the reducers are the ammonium salt, and the course has fetched a hazard record only for the potassium one. R-4b is therefore the nearest thing this formulary has to a proportional reducer, and “nearest thing” is a fair description rather than a modest one.
• When the negative can simply be printed differently. A grade of paper is reversible; this is not.

Mixing

Two bottles that keep, and no bottle that contains both.

1. Solution A. Dissolve 7.5 g of potassium ferricyanide in water and make up to 1000 mL. Cold water; there is no reason to warm a ferricyanide and there is a reason not to. A dark or yellow bottle, because the solution photolyses to Prussian blue.
2. Solution B. Dissolve 200 g of hypo in water and make up to 1000 mL. This is a plain hypo solution and nothing else, at four fifths the strength of the fixing bath F-52.

There is no third step, and that is the formula’s architecture: the two solutions are never combined at all. Where R-4a has to warn that its stocks must not be mixed until the moment of use, R-4b has nothing to warn about, because the negative carries the chemistry from one tray to the other.

🧮Where the ferricyanide went

Per litre of bath actually used:

7.5 g/L of potassium ferricyanide, used undiluted

R-4b, Solution A

75 g/L × 1/32 = 2.34 g per litre

R-4a, working bath

1.0 g ÷ 1.032 L = 0.97 g per litre

R-4, working bath

So the two-bath form works at more than three times the oxidiser concentration of the strongest mixed Farmer’s in this formulary, and it is the gentler treatment. That is not a contradiction: in the mixed bath the silver leaves the film as fast as it is converted, so concentration and duration both drive an irreversible loss, while here the first bath only converts and the loss waits for the second tray.

The hypo goes the other way. R-4b’s second bath is 200 g/L; R-4a’s mixed bath works at 30 g/L. Once the conversion has been made and stopped, what is wanted is a fixing bath, and 200 g/L is a fixing-bath strength.

Rinse between the baths, even though the handbook does not say so, is not an instruction this page will give, because the handbook does not give it. What it says is: A, then B, then wash thoroughly. What it says elsewhere, in general, is that carrying one bath into the next is how a darkroom contaminates itself. The honest position is that Kodak publishes a two-tray sequence with no rinse between, and that a ferricyanide bath and a hypo bath are the two halves of a mixture the same handbook says will not keep.

Behaviour

The first bath is the control and the second is not. One to four minutes in Solution A, with uniform agitation, at 18–24 °C, depending on how much reduction is wanted. Five minutes in Solution B, flat, with no temperature published. Nothing you do in the hypo can take the reduction further than the bleach set it.

Repeatable. The handbook says the process may be repeated if more reduction is desired. This is the only reducer in the formulary that can be applied in measured doses.

Little visible happens in the first tray. At 0.75 per cent w/v and one to four minutes the negative is being converted rather than cleared, and the change to look for is a slight lightening and a faint yellowish cast, not the disappearance of density. The density goes in the second tray.

Kodak publishes no capacity and no keeping figure for either bath, and none is put here. What can be said from the corpus is negative and useful: a ferricyanide solution photolyses in light, and a hypo solution eventually sulfurs.

No temperature is published for the hypo bath. The 18–24 °C belongs to Solution A alone, and this entry does not stretch it across the sequence.

Image characteristics

Contrast falls with density. That is Kodak’s own header and it is the property that distinguishes this formula from every other reducer in this formulary.

The shadows keep more of what they had, which follows from almost proportional: the loss is more nearly in step with the silver already present, so a thin area loses a small absolute amount rather than the same absolute amount as a dense one.

“Almost” is doing real work in that word. The 1928 primer states that no single substance is an exactly proportional reducer and that the way to get closer is to mix a slightly cutting reducer with a flattening one. R-4b is not such a mixture; it is a cutting reducer’s chemistry rearranged in time, and Kodak’s claim for it is bounded accordingly.

On a pyro-developed negative the stain is untouched, as with every ferricyanide reducer: the 1928 primer records that ferricyanide removes the silver and leaves the oxidation product of the pyro, so the negative appears to grow yellower as it reduces.

It is irreversible, one step at a time.

The mechanism

The chemistry of the two baths is the chemistry of R-4a’s two halves, separated.

⚗The same two reactions, in two trays

In Solution A, hexacyanoferrate(III) takes an electron from image silver, and with no solvent present the silver comes down where it stood, as the insoluble ferrocyanide. This is a bleach: the negative is converted and nothing has left it.

4 Ag + 4 [Fe(CN)₆]³⁻ → Ag₄[Fe(CN)₆] + 3 [Fe(CN)₆]⁴⁻

Solution A, alone, is a bleach

In Solution B, thiosulfate complexes that silver and carries it out of the gelatin. This is fixing, applied to a salt the first bath made.

Ag₄[Fe(CN)₆] + 8 S₂O₃²⁻ → 4 [Ag(S₂O₃)₂]³⁻ + [Fe(CN)₆]⁴⁻

Solution B removes what Solution A converted

What is different is when, not what. In a mixed Farmer’s bath these two reactions run at the same instant in the same tray, so a grain that is oxidised is dissolved before the bath reaches the one behind it. Here the whole of the conversion is done first, at a strength and for a time the worker chooses, and the removal happens afterwards in a separate tray where no further oxidation is possible.

⁇Why the classing changes, and what this course will not claim

Kodak Limited asserts the classing — cutting for R-4a, almost proportional for R-4b — and publishes no reason for it, on this page or anywhere else in the handbook. The 1928 primer defines the three classes and names Farmer’s as the type specimen of the cutting one, without discussing the two-bath arrangement at all. Nothing else in the course’s corpus explains the difference.

So this page states what it can source and stops. Fact: the same two reagents give a cutting reducer mixed and an almost proportional one separated, on the manufacturer’s own classification. Fact: the procedural difference is that the conversion is completed and timed before any silver is removed. Not established here: why that change of sequence changes which parts of the image lose most. There is a plausible account involving how far the oxidiser penetrates the depth of the emulsion before anything is carried away, and the course does not print it, because a plausible account of a manufacturer’s unexplained claim is exactly the kind of thing that gets repeated afterwards as though it were measured.

It is also, unusually, a question a reader can settle. See Experiments.

Why the hypo bath is so much stronger than the mixed formula’s. Once the conversion is over, the job of the second tray is to dissolve a silver salt completely and promptly, which is a fixing job, and 200 g/L is a fixing strength. In the mixed bath the hypo has to coexist with the ferricyanide it slowly destroys, and 30 g/L is what Kodak prints there.

Function of every ingredient

Potassium ferricyanide, 7.5 g to the litre of Solution A. The oxidiser, and in this formula the timekeeper: because Solution A carries no solvent, its only product is silver ferrocyanide sitting where the silver stood, so the minutes spent in this tray decide how much silver will afterwards be removed and nothing later can add to that decision. That is the difference between this entry and every other reducer here, where concentration and time drive an irreversible loss together. More of it, or longer in it, converts more; less converts less; and either way the negative comes out of Solution A looking almost unchanged. Wall’s 1912 dictionary gives the general rule that more ferricyanide means a stronger and quicker action, and it applies to the conversion rather than to the removal. The ruby-red crystals give a yellow solution. There is no hydrate to convert. Read its page before you open the jar: it is the reason this formula sits at Level B rather than Level A.

Sodium thiosulfate pentahydrate, 200 g to the litre of Solution B. The solvent, working alone and afterwards. It complexes the silver of the silver ferrocyanide into the soluble bis(thiosulfato)argentate ion and carries it out of the gelatin — the same ion and the same chemistry as fixing, at four fifths the strength of the F-52 fixing bath and ten times the strength of the hypo in R-4’s mixed bath. It has no oxidising power of its own, which is why it cannot overshoot: give it ten minutes instead of five and it removes the same silver. Less hypo, or an old and sulfured bath, leaves the pale ferrocyanide in the film and turns a reduction into a bleach. The crystals are the pentahydrate at 248.19 g/mol; 200 g of the anhydrous salt would be a substantially stronger bath and is not what the handbook prints.

Water, to make each solution up to its own litre. Both solutions are made to a volume rather than carrying an added volume, which is Kodak Limited’s usual typography and is what fixes each strength. The 0.75 per cent w/v of Solution A is the low one in this formulary, and it is low on purpose: this bath is meant to be entered and left several times rather than raced.

Interactions

With acid, of any kind, Solution A is dangerous rather than merely spoiled. See Safety.

Between the two trays, and this is the interaction the formula is made of. The negative carries ferricyanide into the hypo bath, where the two react as they do in a mixed Farmer’s — which is why Solution B is a working bath rather than a stock that lasts, and why a hypo tray used for this should not go back to fixing prints afterwards.

With iron. The 1928 primer warns that a trace of iron in a ferricyanide bath, from a chipped enamelled tray in its example, throws blue spots of ferric ferrocyanide onto the work. Plastic or glass trays; no steel tongs; and two sets of tongs, because this formula has two trays.

With a stop bath or acid fixer carried in on the negative. Wash the negative before it goes into Solution A. See SB-1 for what must not be in the room.

With a pyro stain, which it will not touch.

With light. A ferricyanide solution turns blue on prolonged exposure through the formation of Prussian blue, which is why Solution A lives in a dark bottle.

Variants

Kodak R-4a, the mixed bath, on the same handbook page: 75 g/L of ferricyanide and 240 g/L of hypo, mixed 1 + 4 + 27 at the moment of use. Same chemicals, one tray, and Kodak classes the result as cutting. It is the formula to reach for when the job is fog or veiled shadows.

Kodak R-4, the 1928 mixed bath, weaker again at 0.97 g/L of ferricyanide in the working solution. Kodak’s three Farmer’s reducers make a useful set: two mixed baths at two strengths, and this one, which is the same chemistry taken apart.

The bleach on its own. Solution A is a plain ferricyanide bleach with no bromide in it, which is a different thing from the rehalogenating ferricyanide-bromide bleach the toning entries use. Without a halide the oxidised silver has nothing to become but the ferrocyanide, so the image is converted to something a hypo bath dissolves rather than to something a sulfide bath can tone.

Kodak’s own R-1, R-2 and R-5, the persulphate, permanganate and proportional reducers of the same handbook. They are named on this page as alternatives and are not published in this formulary; the reason is under When another formula is preferable, and it is a missing encyclopaedia entry rather than a missing source.

No course variant is offered. Kodak already publishes the dose control this formula needs, in the form of a repeatable step, and there is nothing to weaken.

Safety

Level B, from potassium ferricyanide, exactly as for the mixed Farmer’s reducers. The hazard is not the reduction; it is the ferricyanide meeting an acid.

Never bring acid near Solution A, its bottle, its tray or its waste. Princeton University’s guidance for photography states that potassium ferricyanide releases hydrogen cyanide gas if heated, if hot acid is added, or on exposure to strong ultraviolet light such as a carbon arc, and records that cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid. This formula is Farmer’s reducer.

Two trays means two chances to get it wrong. The particular risk of a two-bath process is the third tray somebody puts on the bench beside it. No stop bath, no acid fixer, no acid rinse in this sequence at any point.

No warming of Solution A, and no hot water used to dissolve the salt.

Weigh the ferricyanide with the dust down; Kodak’s 1928 mixing instructions single out the salts whose dust should not be raised.

Gloves, eye protection and ordinary ventilation, and hypo at 20 per cent w/v is unremarkable on its own beyond being a strong salt solution.

Storage

Solution A in a dark or yellow bottle, out of the light. A stock that has gone green or blue has already made pigment and will spot the negative.

Solution B as any plain hypo solution, in a stoppered bottle, dated. At 200 g/L it is a fixing-strength solution and it goes off the way one does, by sulfuring.

Neither keeping figure is published and neither is invented here. The handbook’s keeping-properties table covers developers only, and its one keeping sentence on these pages — “the stock solutions keep well but the mixture should be used immediately” — is printed under R-2, about a permanganate and a sulphuric acid stock, and is not evidence about these two.

Unusually for a reducer, there is no mixture to store, because the solutions are never combined. What has to be disciplined instead is the used Solution B, which now holds both silver and ferricyanide carried in on the negative and is not a fixing bath to be kept.

Label both bottles with the formula, the solution letter, the strength and the date.

Incompatibilities

Every acid there is, and hot or concentrated acid worst of all — with a particular warning for a two-tray process, where the acid arrives on the third tray somebody added. Hydrogen cyanide. See incompatibilities.

Heat, against the solid: its international chemical safety card records decomposition on heating with toxic gases including hydrogen cyanide among the products. Nothing in this formula is warmed.

Strong ultraviolet, including a carbon arc.

Iron and steel — trays, tongs, clips, chipped enamel. Blue spots.

Developer, in either direction. One pair of tongs per tray, and this process has two.

A fixing bath used for anything else. Solution B after use is a contaminated hypo solution, not a fixer.

Waste

Two streams that must be treated as one and kept away from a third. Used Solution A carries ferricyanide and ferrocyanide; used Solution B carries the same, brought in on the negative, plus dissolved silver as the thiosulfate complex.

One labelled container for both, and no acid stream ever goes into it. Acidifying ferricyanide waste is one of the documented routes to cyanide poisoning in a darkroom.

Silver recovery first where it is available. Kodak’s own guidance names a ferricyanide bleach as one of the cases its standard desilvering advice does not cover, so the practical route in a home darkroom is collection rather than treatment.

Both trays go into one labelled container, kept clear of every acid stream, and the silver-bearing waste SOP and the disposal ruling govern what happens to it. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Nothing happened in the first tray. Expected. Solution A converts and removes nothing; the negative should look very slightly lighter and faintly yellowish and no more. Judge the result after the hypo bath, not before it.

Nothing happened after the hypo bath either. The time in Solution A was at the bottom of the range for the density you have. Repeat the pair — the handbook expressly permits it — rather than extending the hypo, which cannot help.

Too much came off. Reduction is irreversible. Next time work at one minute in Solution A rather than four, and take two passes.

A yellowish deposit that will not wash out. Solution B is exhausted, sulfured or was too short. What is left in the gelatin is silver ferrocyanide, and a fresh five minutes in fresh Solution B is the thing to try.

Blue spots. Iron, from a tray, a clip or a pair of tongs, giving ferric ferrocyanide. Keep metal away from Solution A.

The contrast went up instead of down. Something behaved like a mixed Farmer’s bath. The likeliest cause is hypo carried into the first tray — from a wet negative that was not washed after fixing, or a shared tray — which turns Solution A into a dilute R-4a.

A pyro negative looks yellower. Expected. The stain is not silver and no ferricyanide reducer touches it.

Any smell at all, and particularly a sharp almond-like one. Leave the room and ventilate it before doing anything else. An acid has reached the ferricyanide tray, and in a two-tray sequence the likeliest source is a tray or a graduate that was doing something else an hour ago.

Experiments

Settle Kodak’s classification for yourself, which is the one experiment on this page that answers an open question. Two identical step wedges, one reduced in this two-bath sequence and one in R-4a, each taken to the same loss of maximum density. Read both and plot them against the untreated wedge. A cutting reducer should give a curve translated downwards by a roughly constant density; an almost proportional one should give a curve rotated about its toe. Kodak asserts that the two formulas fall on opposite sides of that distinction and publishes no evidence; nothing in this corpus shows the plot. You can make it.

Calibrate the first bath. Four identical strips at one, two, three and four minutes in Solution A, all given the same five minutes in Solution B. Plot density lost against time in the bleach. Kodak ties the time to “the degree of reduction desired” and leaves the mapping to you; this is that mapping, for your negatives and your water.

Test the claim that the second bath cannot overshoot. Two strips given the same time in Solution A, then five minutes and thirty minutes respectively in Solution B. If the reasoning under The mechanism holds, the two should read the same, and any difference is telling you something about the hypo bath this page has not accounted for.

Take one reduction in two passes and one in a single long pass to the same total density loss, and compare the curves. The handbook permits repetition; whether the result is the same as doing it all at once is a question the handbook does not answer.