Kodak R-4

One gram of potassium ferricyanide, and it is the whole formula. Everything else on this page is a litre of weak hypo and a warning about acid. Kodak printed this Farmer’s reducer in 1928 and a much stronger one in London in 1949, and the difference between them is not an error in either: it is the single control this bath has, published twice at two settings, and the weaker setting is the one a beginner should meet first.

Solution A — the oxidising bleach

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	1 g
Water	32 mL, added	Thirty-two millilitres of water, not water to make thirty-two millilitres: the primer prints the water as an amount alongside the salt. About 3 per cent w/v either way, and no temperature is given. Cold water dissolves the salt readily and a ferricyanide solution should not be warmed.
A cupful, not a bottle. This is the smallest made-up solution in the formulary, and its size is the point: it is one dose of oxidiser for one negative.

Solution B — the silver solvent

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	30 g	hypo
Water	1000 mL, added	A litre of water carrying 30 g of hypo, printed as an addition rather than as a make-up volume. That is a 3 per cent w/v hypo solution and it is far weaker than any fixing bath in this formulary - a tenth the strength of F-52.
This is a plain hypo solution and nothing else, at the strength the mixed reducer will have, because it is nearly all of the mixed reducer by volume.

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

1. Start with the whole litre of Solution B — the solution that receives
2. Then add the whole 32 mL of Solution A — added to it, and never the other way about

Add A to B and immediately pour over the negative to be reduced. The formula should be prepared immediately before using as it decomposes rapidly after mixing together the A and B solutions.

The primer states an order of addition and no ratio, which is why this is recorded as an order rather than as parts by volume. Reading it as the whole of A into the whole of B - the reading the R-4a entry already uses when it compares the two - gives 1032 mL of working reducer carrying 0.97 g/L of potassium ferricyanide and 29.1 g/L of hypo. That reading is stated here as a reading. What the source fixes beyond doubt is the direction of the pour: the small strong solution goes into the large weak one, never the other way about.

Purpose

To take density off a negative without taking the shadows with it. The 1928 primer’s own definition of what reduction is comes first, because it is the definition the whole category rests on: by reduction in photography is meant the removal of some silver from the image so as to produce a less intense image. The primer is unusually careful to add that the chemical action involved is oxidation, and that the English word “reduction” is unfortunate — other languages say weakening — because it collides with true chemical reduction, which is what happens in a developer.

Farmer’s is the primer’s type specimen of a cutting reducer: one that 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.

Recommended uses

A negative or lantern slide showing slight fog. The primer names this use first and it is the one this dilute version is best at. Fog is a thin even veil, which is exactly what an equal-depth cut removes before it touches anything you wanted.

Local reduction with a brush or a wad of absorbent cotton. The primer records the method in those words. At 0.97 g/L of ferricyanide the bath is slow enough that a brush stroke can be watched and stopped, which is the practical reason to reach for this printing rather than R-4a when the work is local.

A first reduction, on a negative you are not sure about. Nothing here is recoverable, so the argument for the weaker of two published baths is that it gives you more chances to stop.

Only with something to compare against. Density loss is hard to judge on a wet negative in a tray. Keep an untreated strip or a second frame from the same roll beside the dish.

When another formula is preferable

• When there is a great deal of density to remove, R-4a, whose working bath carries about two and a third times the ferricyanide at the same hypo strength. This one will get there eventually; that one gets there while you are still watching.
• When contrast must not climb, R-4b, the two-bath form, which Kodak Limited describes as almost proportional rather than cutting. Cutting always raises contrast — that is what cutting means — and no dilution of this formula changes that.
• When the negative is thin as well as dense-looking, neither. A cutting reducer takes the little shadow density that is left and makes the negative worse.
• When nothing is actually wrong with the negative, a different grade of paper. Reduction is irreversible and a paper grade is not.

Mixing

Two solutions, mixed in one direction only, and used at once.

1. Solution A. Dissolve 1 g of potassium ferricyanide in 32 mL of cold water. There is no reason to warm a ferricyanide and there is a reason not to; see Safety. Keep it in a dark or yellow bottle, because a ferricyanide solution turns blue on prolonged exposure to light through the formation of Prussian blue.
2. Solution B. Dissolve 30 g of hypo in a litre of water. This is an ordinary plain hypo solution at about a tenth the strength of the fixing bath F-52.
3. Add A to B, not B to A, and pour it over the negative immediately.

🧮What the two published Farmer's reducers actually differ by

The primer gives an order of addition and no ratio. Taking the whole of A into the whole of B — which is the reading the R-4a entry already uses when it compares the two — the working bath is 1032 mL:

1.0 g ÷ 1.032 L = 0.97 g per litre

Potassium ferricyanide

30.0 g ÷ 1.032 L = 29.1 g per litre

Sodium thiosulfate

Against Kodak Limited’s R-4a at 1 + 4 + 27, which works out at 2.34 g/L of ferricyanide and 30 g/L of hypo. So across twenty-one years Kodak held the solvent almost exactly constant and changed the oxidiser by a factor of 2.4.

Put as a ratio of the two active ingredients, R-4 is about 1 part ferricyanide to 30 of hypo and R-4a is about 1 to 13. Wall’s 1912 dictionary says what that ratio buys: with a little ferricyanide the action proceeds slowly and the gradation does not suffer, while larger quantities eat away the shadows and increase the contrasts. The two Kodak printings sit on either side of that sentence.

Never make it as one bottle. The primer’s own words are that the formula should be prepared immediately before using, as it decomposes rapidly after mixing the two solutions.

Behaviour

It is slow, and that is what it is for. Nothing in the primer gives a time, because there is no time to give: the end point is judged by eye against an untreated comparison and the negative is pulled before it looks right rather than when it does.

It announces its own exhaustion by colour. Wall’s 1924 handbook gives the rule for the bath in general: a pale yellow mixture is best, the colour disappears rapidly in use, and that is the sign of exhaustion. A fresh mixture is applied rather than an old one allowed to act. At this dilution the yellow is faint to begin with, so watch the negative rather than the tray.

Dilution is the control, and this formula is already most of the way down it. Wall’s rule is that the weaker the solution the more even the action, that is to say the less the shadows are attacked. There is very little room left below 0.97 g/L before the bath stops doing anything you can see.

There is no capacity figure and none is meaningful. The bath is mixed for one negative and thrown away.

Image characteristics

Contrast rises as density falls. Equal silver off everywhere is proportionally more off the thin areas, so the toe of the characteristic curve lifts faster than the shoulder.

Fog and veil go first, which is why the primer names slightly fogged negatives and lantern slides as the use.

On a pyro-developed negative it appears to make matters worse. The primer is explicit: in a pyro negative the image is partly the oxidation product of the pyro sitting with the silver; ferricyanide removes the silver and leaves the stain, so the negative appears to grow yellower as it reduces. The reducer is not making the colour, it is uncovering it. The primer adds the contrast that proves the point — permanganate attacks the stain image in preference to the silver and makes such a negative less yellow.

It is irreversible. Shadow detail taken off is gone rather than lightened.

The mechanism

Two reactions in sequence in the same tray, and the reason the two solutions cannot share a bottle.

⚗Oxidise, then dissolve

Hexacyanoferrate(III) takes one electron from metallic silver.

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

Solution A's only job

Left to itself the silver ion meets the ferrocyanide it has just made and comes down as an insoluble pale salt, still in the gelatin. That is a bleach and 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. The primer states the chemistry in one clause — the hypo dissolves the silver ferrocyanide — and the ion it forms is the one that does all the fixing in this course.

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

Solution B's job: the ferrocyanide the bleach made goes into solution as the fixing complex

Why the ratio is the control and the hypo is not. The hypo is in large excess in both Kodak printings: at 29 g/L there is far more thiosulfate than the ferricyanide can ever give it silver to carry. So the rate is set entirely by the oxidiser, which is why the two published formulas differ in the ferricyanide and agree in the hypo. Adding more hypo to this bath does not make it reduce faster; it makes a more expensive bath.

Why the mixture will not keep. Thiosulfate is a reducing agent and ferricyanide an oxidising one, and in the same vessel they consume one another whether or not a negative is present. That is why the yellow 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.

Function of every ingredient

Potassium ferricyanide, 1.0 g in the 32 mL of Solution A, 0.97 g/L in the working bath. The oxidiser, and the only ingredient in this formula that decides anything. The 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 fixed 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, and that larger quantities eat away the shadows and increase the contrasts. Less means an action so slow it cannot be judged. There is no hydrate to convert, so one gram is one gram whatever the jar says, and the ruby-red crystals give a yellow solution, which catches out people expecting the bath to look red. Read its page before opening the jar: this substance is the reason this formula is Level B rather than Level A.

Sodium thiosulfate pentahydrate, 30.0 g in the litre of Solution B, 29.1 g/L in the working bath. 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 carries it out of the gelatin — the same chemistry and the same ion as fixing, at about a tenth the concentration of the plain hypo fixing bath. More hypo does not reduce faster, because it is already in large excess; less, past a point, risks a bleached rather than a reduced negative, with the pale ferrocyanide left sitting in the film. The crystals are the pentahydrate at 248.19 g/mol; 30 g of the anhydrous salt would be a substantially stronger solution and is not what the primer prints.

Water, and there is a great deal of it. Not a diluent that happens to be there. Wall’s rule — the weaker the solution, the more even the action — makes the litre as much a part of the formula as the gram, and it is the first thing to change if the reduction is running faster than it can be watched. It is also why Prussian blue spotting is worth guarding against: a bath this dilute takes long enough that a negative sits in contact with whatever the tray is made of.

Interactions

With acid, of any kind, this bath is dangerous rather than merely spoiled. See Safety. 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 own example — throws blue spots of ferric ferrocyanide onto the work. Plastic or glass trays, no steel tongs, no paperclips.

With a stop bath or acid fixer carried in on the negative. Wash the negative before reducing it. Acid in the tray is the hazard below; acid in the waste bottle is the same hazard later. Keep the stop bath tray out of this bench run entirely.

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

With light, which turns a ferricyanide solution blue through the formation of Prussian blue.

With subsequent fixing, which is not wanted. The hypo is already in the bath and the primer’s instruction after reduction is washing, not fixing.

Variants

Kodak R-4a, the 1949 London printing, at 75 g/L of ferricyanide and 240 g/L of hypo mixed 1 + 4 + 27. Its working bath is 2.34 g/L of ferricyanide against this formula’s 0.97 g/L, at essentially the same hypo. The two are separate formulas rather than two printings of one, and this course does not merge them. Which you want is a real choice: this one for fog, for local work and for anything you want to be able to stop; that one when there is a lot to remove.

Kodak R-4b, the two-bath form, in which the oxidation and the dissolving are separated in time — a 7.5 g/L ferricyanide bath, then a 200 g/L hypo bath. Kodak calls the result almost proportional rather than cutting, which is a different job rather than a gentler version of this one.

Wall’s 1924 working practice, which is not a formula and is worth knowing as how the bath was actually used: a little 10 per cent w/v ferricyanide solution added to a 20 per cent w/v plain hypo solution, judged by eye to a pale yellow. That is this formula’s architecture with the ratio left to the operator, and the 1928 primer describes the same habit — a strong ferricyanide solution kept in stock, a few drops of it added to a hypo solution when a reducer is wanted.

No course variant is offered. There is nothing to make safer by weakening. The dilution is already the control, this is already the weaker of Kodak’s two printings, and a reader who wants a gentler bath adds water and watches.

Safety

Level B, from potassium ferricyanide. 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. Princeton University’s guidance for photography states that potassium ferricyanide will release hydrogen cyanide gas if heated, if hot acid is added, or on exposure 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 by name. The dilution does not soften it: a litre of weak reducer holds the same ferricyanide the acid would act on, and the small strong Solution A is the worst thing in the room to spill on an acid-wet bench.

Practically: no stop-bath tray on the same bench run, no acid fixer into the reducer’s waste, no acid used to clean a tray that has held ferricyanide, and no warming of Solution A.

Weigh the ferricyanide with the dust down. One gram is a small weighing and a light powder; Kodak’s 1928 mixing instructions single out the salts whose dust should not be raised.

Gloves, eye protection and ordinary ventilation as for any bench chemistry. Hypo at 3 per cent w/v is unremarkable on its own.

Storage

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

Solution B as any plain hypo solution, stoppered. At 3 per cent w/v it is a weak solution and there is little point keeping it long; make it with the negative in mind.

No keeping figure is published for either solution and none is invented here. What the 1928 primer does say is that the usual practice was to keep a strong ferricyanide solution and draw on it as needed, which tells you a ferricyanide stock keeps and does not tell you for how long. Date both bottles.

The mixture is not stored at all. Mix it, use it, discard it.

Label both bottles with the formula, the solution letter, the strength and the date. Two nearly colourless solutions on a shelf are indistinguishable at arm’s length.

Incompatibilities

Every acid there is, and hot or concentrated acid worst of all: stop baths, acid fixers, acetic acid, citric acid, and the acid somebody keeps for cleaning trays. Hydrogen cyanide. See incompatibilities.

Heat. The international chemical safety card for potassium ferricyanide states that the solid decomposes on heating, producing toxic gases including hydrogen cyanide.

Strong ultraviolet, including a carbon arc.

Iron and steel — trays, tongs, clips, 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.

Waste

A litre and a bit of it per negative, and all of it hazardous. What comes off the dish is dissolved silver as the thiosulfate complex, the ferrocyanide the reduction made, and whatever ferricyanide the negative did not use — which at this dilution is most of the gram.

It gets its own labelled container, and no acid stream goes into it. This is the 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, though at this dilution it is thin: one negative’s worth of silver in a litre.

Bottle it on its own, label it, keep it clear of every acid stream, 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

Nothing is happening. Most likely nothing much will: at 0.97 g/L this is the weaker of Kodak’s two Farmer’s reducers, and on a genuinely dense negative it can take longer than your patience. Check the tray colour first — if the pale yellow has gone, the bath is exhausted and Wall’s rule applies, which is to mix a fresh one rather than let an old one act. If the bath is still yellow and the negative is not moving, you want R-4a.

The negative went too far. Reduction is irreversible; there is no recovery. Next time dilute further and work in a white dish, which is where the change is visible.

The reduction is uneven or blotchy. The solution was poured rather than flowed over the negative, or the dish was not rocked. Keep the solution moving.

Blue spots on the negative. Iron, from a tray, a clip or a pair of tongs, giving ferric ferrocyanide. The 1928 primer names the mechanism; the fix is to keep metal away from this bath.

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

A pyro negative looks yellower than when it went in. Expected. The stain is not silver.

Any smell at all, and particularly a sharp almond-like one. Leave the room, ventilate it, and do not go back in to investigate. Something acid has reached the ferricyanide, and finding out what is a job for afterwards.

Experiments

Put the two Kodak Farmer’s reducers side by side. Two identical step wedges, one in this bath and one in R-4a, each timed to the same loss of maximum density. Read both. If the classing is right, the two curves should have the same shape and differ only in how long they took to get there — the ferricyanide concentration is a rate control and not a character control. That is a claim, and this is how you find out whether it survives your densitometer.

Measure what “cutting” means. Reduce a read step wedge to a visible change and read it again. A cutting reducer takes 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. It is the cheapest demonstration in the course of the difference between density and contrast.

Find the floor. Dilute this bath further — a half and a quarter of the published strength — and time each to the same visible change. Wall says the weaker bath is the more even one. Establish where “so even it never finishes” begins for your hand, your eye and your water.

Test the excess. Make the bath once as published and once with the hypo halved, and reduce identical strips. If the reasoning above is right, the halved-hypo bath should behave almost identically until it starts leaving a pale deposit, at which point you have found the edge of the excess by experiment rather than by assertion.