Kodak F-1

Two solutions, and the reason is not that the bath is unstable — it is that the two halves destroy each other if they meet in the wrong order or at the wrong temperature. Warm hypo plus acid gives sulfur. Alum before acid gives a sludge. Kodak’s F-1 is the American acid hardening fixing bath of the 1920s, and almost everything the 1928 primer says about how fixing baths behave, how long they last and how they go wrong, it says with F-1 as the example.

The hypo solution — the fixing bath itself

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	480 g	crystalline hypo
Water	to make 2000 mL	The primer gives no temperature for this water and one instruction about it that matters more than a temperature would: the hypo must be thoroughly dissolved and the solution cool before the hardener goes anywhere near it.
Twenty-four per cent hypo as made, and less than that in the finished bath, because the hardener adds its own volume afterwards and the primer states no volume for the whole.

The hardener solution — the acid, the preservative and the hardener, made separately

Ingredient	Quantity	Form the source specifies
Sodium sulfite (anhydrous)	30 g	desiccated, which is what Kodak's own tested sulphite was
Acetic acid (glacial)	96 mL of a 28% solution	28 per cent; the primer's footnote makes it from three parts of glacial acetic acid to eight parts of water
Potassium alum	30 g	powdered
Water	160 mL, added	at 52 °C; Water at about 125 degrees F, and the only warm water in the formula. The hardener is mixed hot and used cold: the primer's whole mixing rule is that both solutions are at room temperature when they meet.
Kodak's instruction is four words long - "dissolve in the order given" - and the order is the formula. Sulfite, then acid, then alum.

Mixed in this order — the working fixing bath

1. Start with the whole of The hypo solution — cool, and the hypo completely dissolved before anything is added to it
2. Then add the whole of The hardener solution — poured in slowly while the hypo is stirred rapidly

Then add the following hardener solution slowly to the cool hypo solution while stirring the latter rapidly.

The primer adds the failure this instruction exists to prevent: if the hypo is not thoroughly dissolved before the hardener is added, a precipitate of sulfur is likely to form. Neither the 1928 nor the 1924 printing states a final volume for the combined bath, which is why none is recorded here; the water and the acid alone come to 2,256 mL before 60 g of solids are added to it.

Purpose

To fix films, plates and papers, and to harden the gelatin while doing it. Kodak’s own header is “Acid Hardening Fixing Bath for Films, Plates and Papers”, and the breadth of that header is the point: F-1 is the general-purpose bath of the primer, the one the reducers send you to afterwards, the one the developer pages name at the end of their instructions, and the one whose capacity figures the book prints when it wants to say what a fixing bath is worth.

Three jobs in one tray. The hypo dissolves the undeveloped silver halide. The acetic acid neutralises the alkali carried in from the developer, so that development stops at the moment the film enters rather than continuing unevenly in the tray. The alum tans the gelatin so it can survive a wash. The sulfite is there so that the second job does not destroy the first.

Recommended uses

Anything the primer processes. F-1 takes films, plates and prints, which is more than most of the Kodak fixers in this formulary will: F-54 is for paper, F-5 for films and plates, and F-1 is for all three.

Where hardening is wanted and the darkroom is not hot. Potash alum is the hardener the primer prefers for long periods of use, because a chrome alum bath loses its hardening properties rapidly whether it is used or not.

Where the capacity has to be known in advance. This is the one bath in the corpus for which Kodak publishes four capacity figures rather than one, each tied to what precedes the tray. They are worth reading as a set: a third more films and a quarter more prints out of the same gallon, and the only variable is what the material passes through on its way from the developer.

As the standard against which the other 1928 fixers are read. F-2 and F-14 are F-1 with the hardener re-proportioned, and neither makes sense without it.

When another formula is preferable

• When hardening must not happen at all — before a toner, a reducer or an intensifier, or on a stain-image negative — the non-hardening F-52, or a plain hypo bath.
• When the darkroom is genuinely hot, F-5, which carries boric acid for exactly this case and is headed as a tropical bath. The primer’s own answer in 1928 was different and worse: a chrome alum bath, F-16, whose hardening dies in days.
• When the emulsion needs hardening before it is fixed rather than while it is, the chrome alum hardening bath used between development and fixation — which the primer prints as SB-3 and recommends in hot weather in conjunction with F-1. It also buys a third more capacity out of the fixer, which is recorded above.
• When you want the acid without the alum, the primer’s own bisulphite fixing bath: the same protective idea, no hardener, and nothing in it that can sludge.
• When you are mixing more than a tray at a time, F-1a, which is this hardener kept as a stock and added to plain hypo as required.

Mixing

Two vessels, and both of them cold when they meet.

1. The hypo solution. Dissolve 480 g of hypo in water and make up to 2,000 mL. Let it cool completely and make sure not a crystal remains. The primer is explicit about why: if the hypo is not thoroughly dissolved before the hardener is added, a precipitate of sulfur is likely to form.
2. The hardener solution. Into 160 mL of water at about 52 °C, dissolve in this order 30 g of sodium sulfite, then 96 mL of 28 per cent acetic acid, then 30 g of powdered potassium alum. Let it cool.
3. Combine. Pour the hardener slowly into the hypo while the hypo is stirred rapidly. Not the other way round, and not quickly.

Why that order and not another. Sulfite first means the acid arrives to find its buffer already present. Alum last means it dissolves into an acid-sulfite solution, in which it goes easily, rather than into a sulfite solution, in which it forms a white sludge of aluminium sulfite that redissolves only with difficulty. The primer says both things in terms, and adds that a milky hardener means a relative insufficiency of acid — the acid was under strength, or too much alum or sulfite went in.

🧮What the bath is actually made of, and what it is not

Kodak prints the hypo half as “water to make 2.0 liters” and the hardener half as a set of quantities added to it. No volume is stated for the mixture, and there is no honest way to compute one, because dissolving 60 g of solids does not add a known volume of liquid. What can be said is a lower bound:

2000 mL + 160 mL + 96 mL = 2256 mL

Liquid in the finished bath, at least

So the hypo strength, printed as 24 per cent w/v in the hypo solution, is at most about 21 per cent in the tray. The primer states elsewhere on the same subject that 30 to 40 per cent hypo fixes most rapidly. F-1 is therefore not built for speed, and its capacity figures and its hardening range are the properties it was compounded for instead.

This is the schema’s known gap for a combined bath, recorded in docs/FORMULARY-SCHEMA.md: a multi-solution formula has no field for a make-up volume that belongs to the mixture rather than to a solution, and here there is no such volume to record in any case.

A note on the acid. 28 per cent acetic acid is not a bottle most readers will own. Kodak’s own footnote makes it from three parts of glacial acid to eight parts of water; the course’s standing practice, set in Part X, is to buy acid at a stated dilution and compute the volume rather than handle the glacial acid, and the SB-1 page carries that arithmetic.

Behaviour

It stops development on contact and it does not stop being acid until the developer has used it up. Every print that goes in carrying carbonate spends a little of the acetic acid. That is the bath’s real clock, and it is why the four published capacities differ only in what happens before the tray.

Its exhaustion has three visible signs and one measurable one. The primer names frothing at the surface, milkiness, and a sludge through the whole solution. The measurable one is the clearing time: when a slow-clearing film takes more than 12 to 15 minutes to clear, the bath is finished. The clearing-time test is the procedure.

Its hardening rises before it falls. During the first stages of use the hardening properties increase slightly, after which they fall off rapidly. A bath that has been used a little hardens more than a fresh one; a bath that has been used a lot hardens less and eventually not at all, because the acid is gone and the alum has come down as aluminium sulfite.

It has a temperature ceiling of 21 °C in use and 29 °C on the shelf. Above 21 °C the primer calls it dangerous practice, because the bath is apt to precipitate sulfur; above 29 °C it will not stay clear more than a few days however carefully it was mixed. The rate of fixing rises with temperature — a film that clears in 95 seconds at 18 °C clears in about 60 at 29 °C — and that speed is not worth having.

Hardening is a measurable property and Kodak measured it. F-1 is compounded to give a hardening of 54 to 77 °C, determined by putting a strip of the fixed and washed film in water and heating it slowly until the gelatin flows away from the support. That is an experiment anybody can repeat, and it is under Experiments below.

Image characteristics

A fixing bath should have none, and the ways F-1 acquires them are all faults.

A correctly working bath changes nothing. It removes what was never developed. The image is silver, and silver is not what thiosulfate is for.

An exhausted bath stains prints brown. The primer states the outcome directly: prints fixed in a bath that has gone alkaline are likely to become stained brown. The alkali is carried-over developer, and the brown is development continuing in the fixing tray.

A sulfurised bath fades the image later. If a bath that has thrown sulfur is used, the sulfur is apt to penetrate the gelatin and may afterwards cause fading. This is the one fixing fault whose consequence arrives years after the mistake.

Over-hardening makes a brittle negative. A certain minimum of alum is needed for hardening and an excess produces too much of it and induces brittleness. F-1 is at the sensible end of that range; a bath that has had extra alum tipped into it is not.

The mechanism

Fixing. Thiosulfate takes silver out of the halide as a soluble complex — the same chemistry set out in full on how fixer works and on the sodium thiosulfate page.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

What the hypo is for

Stopping. The developer’s alkali meets a weak acid in quantity, and the developing agents stop being developing agents the moment the pH falls. That is the same argument as the stop bath’s, made in the fixing tray instead of before it.

Hardening. Aluminium(III) cross-links gelatin, raising its melting point from the temperature of a warm wash to somewhere between 54 and 77 °C. The potassium alum page carries what the course can source about the mechanism; the page does not claim more than that, and neither does this one.

⚗The two ways this bath destroys itself

Acid on thiosulfate. Thiosulfate is stable in alkali and decomposes in acid, throwing colloidal sulfur and sulfur dioxide.

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

Why a fixing bath needs a sulfite reserve

The sulfite is what keeps this from happening: it holds a reserve of sulfite ion that the acid consumes first, and the primer’s three causes of a sulfur precipitate — too much acid, too little or impure sulfite, and too high a temperature — are three ways of running out of that reserve.

Alum without acid. Aluminium(III) needs a low pH to stay in solution. As the developer’s alkali eats the acetic acid, the pH climbs, and the aluminium comes out with the sulfite as a white gelatinous sludge of aluminium sulfite. That sludge is the end of the bath’s hardening, and it is the reason the primer’s capacity figures depend on the rinse: every gram of carbonate that does not enter the tray is acid the bath keeps.

Function of every ingredient

Sodium thiosulfate pentahydrate, 480 g made up to two litres. The fixing agent, and four-fifths of the formula by weight. It converts silver bromide, which is insoluble and light-sensitive, into a thiosulfato-argentate complex, which is soluble and is not. More of it fixes faster up to about 30 to 40 per cent, which is where the primer puts the optimum and above F-1’s own 24 per cent; Wall’s 1924 handbook sets the ceiling at 40 to 45 per cent, above which a bath is not improved. Less of it fixes slowly and risks leaving the insoluble intermediate complexes that no amount of washing removes. The crystalline pentahydrate is what “hypo” means in these books; the same 480 g of the anhydrous salt would be a substantially stronger bath and is not what Kodak printed.

Sodium sulfite, 30 g. The preservative, and in this bath specifically the acid reserve. Its job is to be attacked by the acetic acid so that the thiosulfate is not: it is the buffer between the two ingredients that cannot be in the same tray without it. More of it lengthens the life of the bath against sulfurisation but pushes it towards the aluminium sulfite sludge, because it is the sulfite half of that precipitate. Less — or an old tub that has oxidised to sulfate, which the primer warns is not a preservative at all — is one of the three named causes of a sulfur precipitate. The form is the desiccated salt: Kodak’s tested sulphite was the dried salt at about 92 per cent, not the heptahydrate crystals, and the crystals would need roughly twice the weight.

Acetic acid, 96 mL of a 28 per cent solution. The acid, doing two separate jobs that are easy to confuse. It arrests development, by neutralising the alkali the film brings in. And it holds the aluminium in solution, which is the condition under which alum hardens rather than precipitates. More acid gives a longer sludging life and, counter-intuitively, worse hardening — the primer states the relationship explicitly, that a bath with an excess of acid may be used a long time before the aluminium sulfite precipitates but does not harden as well. Less acid gives better hardening and a bath that sludges sooner. The primer’s remedy when a slight precipitate first appears is to add about half as much acid again as the formula originally contained; it also warns that using 28 per cent acid where a formula asks for glacial gives less than a third of the intended concentration, which is the commonest way of getting this ingredient wrong.

Potassium alum, 30 g, powdered. The hardener. It supplies the aluminium(III) that tans gelatin and lifts its melting point into the 54 to 77 °C band Kodak compounds this bath for, so that a film can be washed for an hour in running water without the emulsion softening off its support. More alum hardens more and eventually too much, giving brittleness. Less gives a film that frills or reticulates in a warm wash. Kodak specifies the powdered form because it dissolves; lump alum in a cold acid-sulfite solution is a long wait. Potash alum rather than chrome alum is a deliberate choice on the primer’s part, and the reason is keeping: a fresh chrome alum bath loses its hardening properties rapidly whether it is used or not, so potash alum is preferred for long periods of usage.

Water, 160 mL of it warm and the rest cold. The 160 mL at 52 °C is the hardener’s own solvent and the only warm water in the formula, and it is warm for one reason: alum and sulfite dissolve reluctantly cold. Everything else about water here is a prohibition. The hypo solution must be cool before the hardener meets it, or the bath sulfurises; the hardener must be cool too. Kodak states no volume for the mixture, and the silver bromide this bath is built to remove is unaffected by the question, but the reader computing a strength should know that the tray holds rather more than two litres.

Interactions

With the developer, through everything the film carries in. This is the interaction that sets the capacity. Kodak’s four figures move by a third for films and by a quarter for prints, and the only variable in either pair is what happens between the developer and the fixer.

With a rinse, in the film’s favour. A thorough water rinse is the baseline. Two to three minutes in the SB-3 chrome alum hardening bath raises the film capacity from seventy-five to a hundred sheets per gallon. The SB-1 acid rinse raises the print capacity from a hundred to a hundred and twenty-five and, in the primer’s words, makes the bath not sludge so rapidly.

With warmth. Everything about this bath is worse warm: faster fixing, faster sulfurisation, a shorter shelf life and a softer emulsion.

With a second bath of itself. Two-bath fixing is the standard defence against the insoluble intermediate complexes a tired bath leaves behind; the rotation SOP records the practice.

With anything acid, in the waste bottle. See Incompatibilities.

Variants

F-1 mixed from the F-1a stock, which is Kodak’s own alternative and is a page of its own: the same four hardener ingredients kept as a four-litre stock and added one part to eight parts of a 25 per cent hypo solution. The two routes are not quite identical — the stock route arrives about four per cent leaner in sulfite and alum and about six per cent leaner in acid, per gram of hypo — and the arithmetic is set out on that page.

F-2, the motion picture bath. Per gram of hypo it carries a fifth of F-1’s sulfite, three-eighths of its acid and two-fifths of its alum. Kodak gives no reason for the difference. (Per gram of hypo is the only exact comparison available: F-1 states a make-up volume and F-2 and F-14 state a weight of hypo dissolved in a volume of water, so the finished strengths are not comparable and no source gives them.)

F-14, the deep tank bath for roll films. Per gram of hypo, the same alum as F-1 with half the sulfite and eight per cent more acid — which is, by the primer’s own account of the trade, a bath bought a longer sludging life at the cost of some hardening.

F-16, the chrome alum bath, printed on the facing page for hot weather. It is not written up in this formulary: it is a chromium(III) bath, and the course’s chromium ruling and its own warning that the bath’s hardening dies within days make it a poor recommendation as well as a regulated one.

The 1924 printing is the same formula stated differently: the hardener alone, with the instruction to add it to 64 oz of the plain hypo bath F-11. It gives no temperature for the hardener’s water and no metric column. It is recorded here as corroboration rather than as a variant, because nothing about the composition differs.

No course variant is offered. There is nothing to make safer by weakening. The hazards of this bath are in the mixing order and in the 28 per cent acid, and both are addressed by following Kodak’s own instructions rather than by changing the quantities.

Safety

Level B, and the level is set at the bench rather than in the tray.

The acid is the reason. Even at 28 per cent, acetic acid is corrosive enough to matter, and the glacial acid the primer’s footnote tells you to dilute is worse: read the chemical page before opening a bottle of it. Buying acid ready-diluted to a stated strength and computing the volume, as Part X sets out, removes the hazardous operation from the session entirely.

The finished bath is unremarkable and its waste is not. Neither hypo nor potassium alum carries an agreed GHS classification, which is an absence of a classification rather than a finding of safety, and the alum page records the workplace exposure limit that applies to soluble aluminium dust.

The hazard the bath can be made to produce is sulfur dioxide. Thiosulfate and sulfite both give it when a strong acid is added. Nothing acidic beyond the formula’s own acetic acid goes into this tray, and nothing from this tray goes into a bottle that has held acid.

Ventilation as for any darkroom, with the extra provision belonging to the mixing stage rather than the tray: dilute acid where the air moves, and add acid to water.

Storage

Two bottles or one, and the choice has consequences. Mixed, F-1 is a working bath rather than a stock: the primer publishes no keeping figure for it at ordinary temperatures and one warning for warm ones, that above 29 °C it will not remain clear longer than a few days however correctly it was mixed. Kept as F-1a plus plain hypo, both halves keep far better, which is the whole reason that formula exists.

Glass or plastic, never metal, stoppered, out of the light and out of the warm. Label with the formula, the strength and the date per the labelling SOP — and add a tally, because this bath’s remaining life is a number of sheets and a bottle with no count on it has an unknown capacity left.

The 28 per cent acetic acid keeps indefinitely and should be stored as the corrosive it is: cool, closed, away from bases and oxidisers and away from anything that has held a fixer.

Incompatibilities

Strong acids. Sulfuric, hydrochloric, or a concentrated stop bath tipped into the tray or the waste bottle liberates sulfur dioxide and colloidal sulfur. The incompatibilities page treats this as the most likely serious accident in a home darkroom.

Developer, in the other direction. One pair of tongs per tray; a splash of fixer in a paper developer bleaches the print.

Ferricyanide baths and their waste. Acid plus ferricyanide is the documented route to hydrogen cyanide, and this bath is an acid one. Farmer’s reducer waste and fixer waste do not share a bottle.

Alkalis. Carbonate carried in is what ends this bath; a bottle of sodium carbonate solution tipped in ends it at once, and past the point where the alum will stay in solution.

Sulfide toners. Acid meeting a sulfide gives hydrogen sulfide. Different tray, different waste stream.

Chrome alum baths, mixed in by accident: a chromium(III) stream is a different waste route and keeping the two apart is what keeps that ruling’s containers honest.

Waste

This is the silver stream. Spent F-1 carries dissolved silver as the thiosulfate complex, together with sulfite, acetate and aluminium.

Recovery before disposal where it is available. The 1928 primer already treated an exhausted bath as a resource rather than a waste and listed the period’s methods; for a home darkroom the practical route is collection, and Part XII sets out what is realistic at that scale.

Never down a drain on the argument that it is only salty. Collect it, label it, keep it away from every acid waste and every ferricyanide 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 bath went milky the moment the hardener went in. The hypo was warm, or it was not completely dissolved, or the hardener was poured in fast. A pale yellow precipitate that settles slowly is sulfur and does not redissolve; mix a fresh bath.

The hardener itself was milky before it was added. A relative insufficiency of acid, which in practice means acid under strength — 28 per cent used where glacial was meant gives less than a third of the intended concentration — or too much alum or sulfite.

A white gelatinous sludge after a while in use. Aluminium sulfite: the acid has been consumed by carried-over developer. The primer’s remedy at the first sign is to add about half as much acid again as the bath originally contained. Once the sludge is general, the bath is finished.

Fixing is taking much longer than it did. Time a clearing test. Past 12 to 15 minutes for a slow film, discard.

Froth on the surface. An exhaustion sign in its own right, per the primer.

Prints coming out stained brown. The bath has gone alkaline. A rinse bath before it is the answer for next time, and it buys twenty-five sheets of capacity as well.

Negatives brittle, or cracking at the edges as they dry. Over-hardening. Check that the alum was weighed and not estimated, and that nothing has been topped up.

Emulsion soft or frilling despite the alum. Check the bath’s age rather than the alum: hardening rises slightly with early use and then falls away rapidly once the acid is spent.

A sharp smell over the tray. Sulfur dioxide, which means acid has got in from somewhere. Find it.

Experiments

Measure the hardening the way Kodak did. Fix and wash two strips of the same film, one in F-1 and one in a plain hypo bath, then put each in water and heat it slowly until the gelatin flows off the support. Kodak compounds F-1 for 54 to 77 °C. This is a published number you can check with a thermometer and a beaker, and very few numbers in this formulary are.

Test the capacity claim, and the rinse claim with it. Fix identical 8 by 10 sheets in a measured volume of F-1, timing the clearing of a test strip every few sheets, and stop when clearing passes 12 to 15 minutes. Then repeat with an SB-1 rinse before the fixer. Kodak says the second run should reach a quarter further; the clearing-time experiment gives the method.

Watch the hardening rise and then fall. Take a hardening measurement on a fresh bath, again after a few sheets, and again near exhaustion. The primer claims hardening increases slightly at first and then falls off rapidly, which is a shape rather than a number and has never been plotted in this course.

Provoke the sulfur precipitate deliberately, once, in a small beaker. Add a little of the hardener to warm hypo instead of cold and watch the pale yellow cloud form. Doing it on purpose in 50 mL is how you learn to recognise it in two litres. Do it with the ventilation on: the same reaction gives sulfur dioxide.

Compare the three 1928 baths, scaled to a common weight of hypo. F-1, F-2 and F-14 carry three different hardeners against the same hypo — per 960 g of it, 60/192/60, 12/72/24 and 30/208/60 grams of sulfite, millilitres of 28 per cent acid and grams of alum. Fix identical strips in each, measure the hardening, then run each to exhaustion counting sheets. Kodak publishes no comparison; the three formulas are an experiment already designed.