Kodak R-4a

Two bottles that keep, and a reducer that does not. R-4a is the first entry in this formulary that its maker did not print as one solution, and the reason is not convenience: a ferricyanide-and-hypo mixture decomposes on standing, so a single bottle of Farmer’s reducer is a bottle of something that used to be Farmer’s reducer. Each stock has its own hazards — read Safety before you weigh anything — but it is the combination that has a shelf life measured in hours.

Stock Solution A — the oxidising bleach

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	75 g
Water	to make 1000 mL	A 7.5 per cent solution. The handbook gives no temperature; the salt dissolves readily in cold water and the solution should not be warmed, because heating a ferricyanide is one of the routes to hydrogen cyanide.
Kept in a dark or yellow bottle. This is the stock the 1928 primer describes as the working practice: a strong ferricyanide solution, kept, and drawn on when a reducer is wanted.

Stock Solution B — the silver solvent

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	240 g	crystalline hypo
Water	to make 1000 mL	A 24 per cent hypo solution - within a per cent of the hypo strength of the fixing bath F-52, and a plain hypo solution in every other respect.

Mixed in the ratio — the working reducer, mixed at the moment of use

1 part Stock Solution A + 4 parts Stock Solution B + 27 parts water

For use, take 1 part Solution A, 4 parts Solution B, then add water 27 parts. Pour the mixed solution at once over the negative to be reduced.

Thirty-two parts in all, so the working bath carries 2.34 g of potassium ferricyanide and 30 g of hypo to the litre. The handbook adds that the action is best seen when the solution is poured over the negative in a white dish, and that the negative is washed thoroughly before drying. Solutions A and B are not combined until they are to be used.

Purpose

To take density off a negative that has too much of it, and to open the highlights of a print that has been printed too dark. Kodak Limited’s own header is “Farmer’s reducer. A cutting reducer for correcting over-exposure and clearing shadow areas of negatives and high lights of prints”.

Cutting is the load-bearing word, and the 1928 primer defines it: a cutting reducer removes an equal quantity of silver from all parts of the image, and consequently removes a larger proportion of the image from the shadows than from the highlights. That is why it clears fog and veiled shadows before it touches the highlights, and why it raises contrast while it lowers density. A reducer that lowered density without touching contrast would be a different formula — Kodak’s R-2 — and a reducer that attacked the highlights hardest would be a third, Kodak’s R-1.

Recommended uses

A fogged or veiled negative. The 1928 primer names this first: Farmer’s is especially useful for clearing negatives or lantern slides which show slight fog. Fog is a thin, even deposit everywhere, which is exactly what a cutting reducer removes first.

An over-exposed negative that prints flat. Because the shadows lose proportionally more, the characteristic curve is lifted at the toe and the print gains contrast as it loses density.

A print with blocked highlights, treated locally. The primer records the traditional method: the solution applied with a brush or a wad of absorbent cotton rather than in a dish. Local reduction is the one job in this course where a bath is deliberately not used as a bath.

A negative you can watch. The handbook is explicit that the action is best seen when the solution is poured over the negative in a white dish, and to watch closely. There is no published time on this page because there is no published time in the source: the end point is judged by eye and the negative is pulled before it looks right, not when it does.

When another formula is preferable

• When contrast must not rise, Kodak’s R-2, the permanganate reducer, “for reducing density of negative materials without loss of contrast”. A cutting reducer always raises contrast; that is what cutting means.
• When contrast must fall, Kodak’s R-1, the persulphate reducer, which attacks the densest parts hardest. Wall’s 1924 handbook warns that persulphate’s action is much complicated by acid, chlorides and iron salts picked up from ordinary tap water, so it is the least predictable of the three.
• When the reduction must be gradual and controllable, Kodak’s R-4b, the two-bath Farmer’s, which puts the negative in a dilute ferricyanide bath first and a plain hypo bath afterwards, so the oxidation and the dissolving are separated in time and the process can be repeated a step at a time. Its quantities are under Variants.
• When nothing is actually wrong with the negative, print it differently. Reduction is irreversible, and a grade of paper is not.

Mixing

Two bottles, each made to its own litre, and neither of them mixed with the other until the negative is on the bench.

1. Stock Solution A. Dissolve 75 g of potassium ferricyanide in water and make up to 1000 mL. Cold water; it dissolves readily and there is no reason to warm a ferricyanide. Store it in a dark or yellow bottle — Kodak’s 1928 primer records that a potassium ferricyanide solution turns blue on prolonged exposure to light, through the formation of Prussian blue, and blue pigment is not something you want near a negative.
2. Stock Solution B. Dissolve 240 g of crystalline hypo in water and make up to 1000 mL. This is a plain hypo solution and nothing else; the same solid, at nearly the same strength, as the fixing bath F-52.
3. The working reducer, and only when you want it. One part A, four parts B, twenty-seven parts water. Pour it over the negative at once.

🧮What one plus four plus twenty-seven actually gives you

Thirty-two parts in all. Per litre of working solution:

75 g/L × 1/32 = 2.34 g per litre

Potassium ferricyanide

240 g/L × 4/32 = 30 g per litre

Sodium thiosulfate

The hypo figure is worth holding on to, because Kodak’s much earlier R-4 — one gram of ferricyanide in 32 mL of water, added to 30 g of hypo in a litre — arrives at the same 30 g of hypo per litre and less than half the ferricyanide, about 0.97 g/L against 2.34 g/L. Twenty-one years apart, Kodak kept the solvent constant and roughly doubled the oxidiser. Which of the two you want is a real choice and is discussed under Variants; what the arithmetic settles is that they are two formulas, not two printings of one.

Never make it as one bottle. The handbook’s closing sentence is the formula’s real instruction: solutions A and B should not be combined until they are to be used, and they will not keep long when mixed. The 1928 primer says the same thing in stronger words — the mixture decomposes rapidly once combined.

Behaviour

It announces its own exhaustion by colour. Wall’s 1924 handbook gives the working rule: a pale yellow mixture is best, and the colour of the solution rapidly disappears in use, which is the sign of exhaustion. A fresh mixture should be applied rather than an old one allowed to act. This is one of the few baths in the formulary whose end point you can see without a test.

Dilution buys evenness. Wall again: the weaker the solution, the more even the action — that is to say, the less the shadows are attacked. Kodak’s 1+4+27 is already a considerable dilution of both stocks, and diluting further is the standard way to slow a reduction that is running away.

There is no capacity figure and no time. Neither is published, because neither is meaningful for a bath that is mixed for one negative and thrown away. What the handbook publishes instead is a procedure: pour, watch closely, use a white dish, and wash thoroughly when it has gone far enough.

The stocks keep and the mixture does not. That asymmetry is the whole architecture, and it is the reason this entry needed a schema that could hold two solutions.

Image characteristics

Contrast rises as density falls. Equal silver off everywhere means proportionally more off the thin areas, so the toe of the curve lifts faster than the shoulder. Wall’s 1912 dictionary puts the control in the operator’s hands: with a little ferricyanide the action proceeds slowly and the gradation does not suffer; with larger quantities the shadows can be eaten away and the contrasts increased.

Fog goes first, which is what makes it the standard treatment for a slightly veiled negative or lantern slide.

It is irreversible and it is not selective by tone alone. Taken too far, the shadow detail is gone — not lightened, gone — and no amount of printing brings it back.

On a pyro-developed negative it appears to make things worse before it makes them better. The 1928 primer records that ferricyanide removes the silver but not the stain, so such a negative appears to grow yellower as it reduces. The reducer is not making the colour; it is uncovering it.

The mechanism

Two reactions in sequence, in the same tray, at the same moment. That is the whole formula, and it is why the two halves cannot share a bottle.

⚗Oxidise, then dissolve

The iron in hexacyanoferrate(III) is iron(III). It takes one electron from metallic silver and becomes hexacyanoferrate(II), the ferrocyanide.

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

Solution A's only job

Left alone, the silver ion meets the ferrocyanide it has just made and comes down as the insoluble salt — a pale image still sitting in the gelatin. That is a bleach, not a reducer.

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

What Solution A alone would leave behind

Solution B is what makes it a reducer. Thiosulfate complexes the silver out of the silver ferrocyanide and carries it into the bath, so the silver leaves the film altogether.

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

Solution B's job, and the reason the density goes

Why the mixture will not keep. The two solutions are not inert towards each other. Thiosulfate is a reducing agent and ferricyanide is an oxidising one, and in the same bottle they slowly consume one another whether or not there is a negative present — which is why the yellow colour fades in use, and why it fades on the shelf as well. A bottle of premixed Farmer’s is a bottle in which the oxidiser has already been spent on the solvent.

Why it cuts rather than proportions. The oxidation happens at the surface of each silver grain, and the rate depends on how much grain surface the solution can reach, not on how much silver is stacked behind it. Thin areas present nearly all of their silver to the bath; dense areas shield most of theirs. So an equal depth comes off everywhere, which is an equal quantity everywhere, which is a larger proportion where there was least to begin with.

Function of every ingredient

Potassium ferricyanide, 75 g to the litre of Stock Solution A. The oxidiser, and the only ingredient that decides whether silver leaves. Kodak’s 1928 primer states its value in photography in one sentence: it oxidises the silver image and forms silver ferrocyanide from it, so that a negative left in a plain ferricyanide solution is slowly bleached. More of it, at a given hypo strength, means a faster and less even reduction that goes for the shadows — Wall’s 1912 dictionary is explicit that the more ferricyanide added, the stronger and quicker the action. Less means a slow action that spares the gradation. Note the form: there is no hydrate to convert, so 75 g is 75 g whatever the jar says, and the ruby-red crystals give a yellow solution, which catches out people who expect the bath to look red.

Sodium thiosulfate pentahydrate, 240 g to the litre of Stock Solution B. The solvent, and the ingredient that turns a bleach into a reducer. It complexes the silver of the silver ferrocyanide into the soluble bis(thiosulfato)argentate ion and removes it from the gelatin — the same chemistry, and the same ion, as fixing. More hypo does not reduce faster; it only makes sure the ferrocyanide never has a chance to sit in the film as silver bromide’s pale cousin. Less risks a bleached rather than a reduced negative. The crystals are the pentahydrate at 248.19 g/mol; 240 g of the anhydrous salt would be a substantially stronger solution and is not what the handbook prints.

Water, twenty-seven parts of it. Not a diluent that happens to be there: the dilution is the control. Wall’s rule — the weaker the solution, the more even the action — makes those twenty-seven parts as much a part of the formula as the two stocks, and they are the first thing to change when a reduction is running too fast to watch.

Interactions

With acid, of any kind, this bath is dangerous rather than merely spoiled. See Safety below; it is not a question of the reduction failing.

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, and no steel tongs.

With a residual stop bath or acid fixer carried in on the negative. Wash the negative before reducing. Acid in the tray is the hazard above, and acid in the waste bottle is the same hazard later.

With a pyro stain, which it will not touch. See Image characteristics.

With light. A ferricyanide solution turns blue on prolonged exposure to light through the formation of Prussian blue, per the 1928 primer, which is why Stock A lives in a dark bottle.

With subsequent fixing. Kodak’s R-1 and R-2 both end with the negative in an acid fixing bath; R-4a does not, because the hypo is already in the bath. The handbook’s instruction after R-4a is washing, not fixing — and given the acid in most fixing baths, that difference is worth respecting.

Variants

Kodak R-4, the 1928 formula. Solution A is 1.0 g of potassium ferricyanide in 32 mL of water; Solution B is 30 g of hypo in 1 litre; the direction is to add A to B and immediately pour over the negative. The working strength is about 0.97 g/L of ferricyanide against R-4a’s 2.34 g/L, at the same 30 g/L of hypo. The earlier bath is gentler and more even by Wall’s rule and slower to run away; the later one is faster and better suited to work with a lot to remove. They are separate formulas rather than two printings of one, and this course does not merge them.

Kodak R-4b, the two-bath Farmer’s. A 7.5 g/L ferricyanide solution and a 200 g/L hypo solution, used one after the other rather than mixed: 1 to 4 minutes in A with uniform agitation at 18–24 °C, then 5 minutes in B, and the pair repeated if more reduction is wanted. Kodak describes the result as almost proportional rather than cutting, which is a different job — separating the oxidation from the dissolving in time changes which parts of the image lose most. It is the formula to reach for when contrast must not climb.

Wall’s 1924 working practice, which is not a formula at all and is worth knowing as the way the bath was actually used: a little 10 per cent ferricyanide solution added to a 20 per cent plain hypo solution, judged by eye to a pale yellow. It is R-4a’s architecture with the ratio left to the operator.

A course variant is not offered. There is nothing to make safer by weakening: the dilution is already the control, and any reader who wants a gentler bath uses more water and Kodak’s own instruction to watch closely.

Safety

Level B. The hazard is not the reducer working as intended; it is the reducer meeting an acid.

Never bring acid near a ferricyanide bath, its bottles or its waste. The international chemical safety card for potassium ferricyanide states that it reacts with acids and that this generates a toxic hazard. Princeton University’s guidance is more specific and more alarming: potassium ferricyanide will release hydrogen cyanide gas if heated, if hot acid is added, or if exposed to strong ultraviolet light such as a carbon arc, and it records that cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid. That sentence names this formula. It is the reason this page exists at Level B rather than Level A and the reason the waste rule below is absolute.

Practically, that means: no stop bath tray on the same bench run, no acid fixer poured into the reducer’s waste, no acid used to “clear” a stained tray that has held ferricyanide, and no warming of Stock A.

Ventilation, gloves and eye protection as for any bench chemistry, and hypo is unremarkable on its own. Weigh the ferricyanide with the dust down; Kodak’s 1928 mixing instructions single it out among the salts whose dust should not be raised.

Storage

Stock A in a dark or yellow bottle, out of light, because the solution photolyses to Prussian blue. A stock that has gone green or blue has already made pigment.

Stock B as any plain hypo solution, in a stoppered bottle. The handbook publishes no keeping figure for either stock, and none is invented here. It does print “the stock solutions keep well but the mixture should be used immediately” on the same page — but under R-2, about a permanganate and a sulphuric acid stock, so it is not evidence about these two. What can be said from this page’s own sources is negative and useful: a ferricyanide stock photolyses in light, and a hypo stock eventually sulfurs. Date both bottles.

The mixture is not stored at all. Mix what the negative needs, use it, discard it. A bottle of premixed Farmer’s reducer is the one storage decision this formula forbids.

Label both bottles with the formula, the solution letter, the strength and the date. Two clear colourless-to-yellow solutions on a shelf are indistinguishable at arm’s length, and mixing them up here means pouring 240 g/L of hypo where 75 g/L of ferricyanide was meant.

Incompatibilities

Acids — stop baths, acid fixers, acetic and citric acid, and above all hot or concentrated acid. Hydrogen cyanide. See incompatibilities.

Heat. The safety card states that the solid decomposes on heating, producing toxic gases including hydrogen cyanide.

Strong ultraviolet, including a carbon arc, per Princeton’s guidance.

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

Developer, in either direction, as for every bath in the darkroom. One pair of tongs per tray.

Alkalis, which have no place in the reduction and shift the bath away from the mildly neutral condition it is designed for.

Waste

Two hazards in one bottle, and they must not meet a third. The used reducer carries dissolved silver as the thiosulfate complex and it carries ferrocyanide and unreacted ferricyanide.

It gets its own labelled container, and no acid stream ever goes into it. This is the single most important line on the page: acidifying ferricyanide waste is one of the documented routes to cyanide poisoning in a darkroom.

Silver recovery first where it is available. The solution is silver-bearing, and 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.

Collect it, label it, keep it away from every acid waste, and follow the silver-bearing waste SOP and the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

The negative went too far before you could stop it. The bath was too strong, or it was not being watched. Dilute further next time — more water, not less ferricyanide — and work in a white dish where the change is visible. There is no recovery: reduction is irreversible.

Nothing is happening. The mixture has gone colourless, which is exhaustion, not patience. Wall’s rule: apply a fresh mixture rather than let an old one act.

The reduction is uneven, or blotchy at the edges. Too strong and too fast, or poured rather than flowed over the negative. The remedy is the same: dilute, and keep the solution moving.

Blue spots on the negative. Iron, from a tray, a clip or a pair of tongs. The 1928 primer names the mechanism; the fix is to stop using metal near this bath.

The negative has gone pale but the density has not really dropped. The hypo side is not working — Stock B was omitted, badly under-measured, or is old and sulfured. What you have is a bleach, and the pale salt is silver ferrocyanide still in the gelatin.

A pyro negative looks yellower than when it went in. Expected. The stain is not silver and the reducer does not remove it.

A sharp, almond-like or bitter smell, or any smell at all. Leave the room and ventilate it. Then find the acid, because there is one.

Experiments

Measure what “cutting” means. Expose and develop a step wedge, read its densities, reduce it in R-4a to a visible change, and read it again. Plot the two curves. A cutting reducer should take a roughly constant density off every step, which appears on the plot as one curve translated downwards — and therefore steeper in relative terms at the toe. This is the cheapest way in the whole course to see the difference between density and contrast.

Put the three reducers against each other. The same step wedge, reduced in R-4a, in a permanganate bath and in a persulphate bath to the same loss of maximum density, then all three read. Kodak’s classification says the three curves should differ in shape, not just in height. Nothing in the corpus shows you the plot; you can make it.

Find the dilution where the action becomes controllable. Mix 1+4+27 as printed, then 1+4+59 and 1+4+123, and time each to the same visible change on identical strips. Wall says the weaker bath is the more even one. Establish for your own hand and eye where “too fast to watch” begins.

Separate the two reactions. Run R-4b’s two-bath sequence on one strip and R-4a’s mixed bath on its twin, to the same density loss, and compare the shape of the two curves. This is Kodak’s own claim — cutting against almost proportional — put to the test with the same two chemicals in both trays.

Watch a premixed bottle die. Mix a working solution, use half of it at once and stopper the rest. Try the remainder on an identical strip at one hour, one day and one week, and record the colour with the result. The handbook’s “will not keep long” is not a number; make it one for your own conditions.