Kodak F-1a

A hardening fixing bath is a hypo solution and three other things, and the three other things keep far better on their own than they do in the hypo. F-1a is Kodak’s bottle of the three: four litres of sulfite, acid and alum that turn any plain hypo solution into F-1 at the moment you want one, one part to eight.

Ingredient	Quantity	Form the source specifies
Sodium sulfite (anhydrous)	480 g	desiccated, which is the form Kodak sold as tested sulphite
Acetic acid (glacial)	1500 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	480 g	powdered
Water	to make 4000 mL	at 52 °C; The formula's water is used at two points and this field can hold only one of them. Kodak starts with 1700 c.c. at about 52 degrees C for the sulfite and closes with "cold water to make 4.0 liters"; the 4,000 mL is the make-up volume and the 52 degrees C belongs to the first 1,700 mL alone. The gap is recorded in docs/FORMULARY-SCHEMA.md, and the same shortfall appears on Kodak Limited's F-53.

Purpose

To be measured into plain hypo. Kodak’s header is “Acid Hardener Stock Solution”, and the primer states the reason for the arrangement in its general instructions rather than in the formula: prepare the acid hardening solution as a separate stock solution and add this to the hypo solution as required.

The advantage is keeping. An acid hypo solution goes milky on standing — that is the primer’s own observation, made about bisulphite baths and true of this one — whereas a hypo solution keeps and an acid-alum-sulfite solution keeps. Splitting them means the darkroom stores two stable bottles instead of one unstable tray.

The second advantage is arithmetic. Once the hardener is a stock, a fixing bath is one measurement instead of four, and the same bottle serves any volume of bath you happen to want.

Recommended uses

Whenever more than one tray of fixer will be mixed. This is the whole case for the formula. Four weighings become one measured volume.

Where the hypo is bought or mixed separately. Hypo is cheap, dissolves slowly and is usually made in quantity; the hardener is the fiddly part and is made rarely.

As the American counterpart of F-53 when reading Kodak’s two handbooks side by side. F-53 is Kodak Limited’s acid hardener stock, built on glacial acetic acid and a different sulfite-to-alum ratio, and the pair are worth comparing: the same idea, twenty-one years and an ocean apart.

Where the reader wants the acid and the preservative without committing to alum in every bath. F-1a cannot be split — the alum is in the bottle — but having it as a separate bottle at least means a plain or bisulphite bath can be run out of the same hypo without contamination.

When another formula is preferable

• When one tray is all that is wanted, mix F-1 directly. It is the same bath in slightly different proportions and it saves storing four litres of stock.
• When the source you are following is Kodak Limited’s, use F-53, which is the stock its F-54 is built from and which carries glacial rather than 28 per cent acid.
• When hardening is not wanted, no hardener stock at all: F-52 or a plain hypo bath.
• When the emulsion needs hardening before fixing rather than during it, the chrome alum hardening bath, which is a separate operation and a different metal.

Mixing

Four ingredients, one order, and the order is not negotiable.

1. 1,700 mL of water at about 52 °C. Warm, because sulfite and alum are reluctant in cold water.
2. 480 g of sodium sulfite. Dissolve it completely before anything else goes in. The primer says so twice.
3. 1,500 mL of 28 per cent acetic acid, added to the fully dissolved sulfite and mixed thoroughly.
4. 480 g of powdered potassium alum, added to the mixed sulfite-acid solution with constant stirring.
5. Cold water to make 4,000 mL.

Why alum last. Kodak gives the reason in terms: the alum dissolves more readily in the acid-sulfite solution. Reverse the last two steps and you get a white sludge of aluminium sulfite that dissolves with difficulty once the acid is finally added. The primer records an alternative for anyone who prefers it — dissolve the alum and the sulfite in separate solutions, cool both, add the acid to the sulfite solution and then add the alum solution — which is the same chemistry arranged so that the alum never meets a sulfite solution that is not already acid.

Then cool it, and use it cold. The stock goes into the hypo slowly, with stirring, and both solutions must be at room temperature. Warm hypo plus acid is a sulfur precipitate.

🧮One part to eight, and how close that is to F-1

The stock holds 120 g of sulfite, 375 mL of 28 per cent acetic acid and 120 g of alum per litre. Diluted one part into eight parts of a 25 per cent hypo solution, nine litres of working bath carry:

120 g × 1/9 = 13.3 g per litre

Sodium sulfite

375 mL × 1/9 = 41.7 mL per litre

Acetic acid, 28 per cent

120 g × 1/9 = 13.3 g per litre

Potassium alum

250 g × 8/9 = 222 g per litre

Hypo

The honest comparison with F-1 is per gram of hypo, because F-1 states no volume for its finished bath. Scale the figures above to F-1’s 480 g of hypo and the stock route delivers 28.8 g of sulfite, 90 mL of acid and 28.8 g of alum, against F-1’s own 30 g, 96 mL and 30 g.

So F-1 mixed directly is about four per cent richer in sulfite and alum and about seven per cent richer in acid than the same bath assembled from this stock. That is well inside the tolerance of a bath whose capacity is quoted in whole sheets, and it is a good deal closer than the two printings of SB-1 are to each other. It is the course’s arithmetic on Kodak’s published numbers, not a statement Kodak makes.

Behaviour

It is a stock and not a bath, and it behaves like a stock. Nothing happens in the bottle except a slow deposit: Kodak Limited records that acid hardener stocks stored for several weeks tend to form a white incrustation of basic aluminium acetate inside the container, and says it should be ignored because it does not usually impair the solution’s useful properties.

Its only failure at the mixing stage is visible. A milky hardener means a relative insufficiency of acid — the acid was under strength, or too much alum or sulfite went in. The primer names the commonest version of that mistake: using 28 per cent acid where a formula asks for glacial gives less than a third of the intended concentration. This formula asks for 28 per cent, so a reader who has glacial acid must dilute it and not merely measure less of it.

It behaves badly if it is poured the wrong way. Hypo into hardener, rather than hardener into hypo, puts the thiosulfate into a locally very acid solution. That is the condition sulfur comes out of.

It buys keeping and costs storage. Four litres of stock, plus the hypo, occupy a shelf. The gain is that neither bottle is doing anything to the other while it sits there.

Image characteristics

None of its own — it never touches a negative except as one ninth of a fixing bath. What it contributes to that bath’s behaviour is on F-1’s page: stopped development, hardened gelatin in the 54 to 77 °C band Kodak compounds for, and a capacity that depends on the rinse before the tray.

The one image consequence that belongs here rather than there is what happens when the stock is mis-measured. Too little hardener and the fixing bath sludges early and stops hardening; too much and the negative can be over-hardened to brittleness. Both are consequences of a single measuring cylinder, which is the risk a stock solution trades for its convenience.

The mechanism

Nothing in this bottle reacts with anything else in it, which is the point of it.

⚗Three ingredients that must not be assembled in the wrong order

Sulfite and acid are a buffer pair. Acetic acid protonates sulfite to bisulfite, and the solution settles at a pH where aluminium(III) is soluble and thiosulfate is not yet attacked.

SO₃²⁻ + CH₃COOH → HSO₃⁻ + CH₃COO⁻

What the first two ingredients make together

Alum needs that pH before it arrives. In a sulfite solution that is not yet acid, aluminium(III) precipitates as aluminium sulfite - the white sludge - and redissolves only slowly. In an acid-sulfite solution it stays in solution and can be carried into the fixing bath, where it cross-links gelatin.

And the whole bottle is a delivery system for what must not happen too early. Put the same acid straight into hypo and you get sulfur:

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

Why the stock goes into the hypo slowly, with stirring, cold

The sulfite in the stock is what prevents this in the finished bath. The slow stirred addition is what prevents it during the mixing, when a local excess of acid is possible even though the finished proportions are safe.

Function of every ingredient

Sodium sulfite, 480 g in four litres — 120 g/L. The preservative, and in a hardener stock specifically the reserve of base that the acetic acid will attack in preference to the thiosulfate once the two are in the same tray. It also does a mechanical job at the mixing stage: it is the solution the alum has to dissolve in, and it must be completely dissolved before the acid goes in or the acid finds solid sulfite rather than sulfite ion. More of it lengthens the finished bath’s life against sulfurisation and shortens it against the aluminium sulfite sludge, because sulfite is half of that precipitate. Less is one of the primer’s three named causes of a sulfur precipitate, along with an old tub that has oxidised to sulfate — which the primer warns is not a preservative at all. The form is the desiccated salt Kodak sold as tested sulphite, about 92 per cent pure; the heptahydrate crystals would need roughly twice the weight for the same effect.

Acetic acid, 1,500 mL of a 28 per cent solution — 375 mL/L, and by far the largest single component of the bottle. Two jobs. In the finished fixing bath it neutralises the alkali carried in from the developer and so arrests development on contact. In this bottle, and before that, it sets the pH at which aluminium(III) will stay in solution. More acid gives a fixing bath with a longer sludging life and worse hardening; less gives better hardening and earlier sludging. The primer states that relationship explicitly and gives the working remedy: when a slight precipitate first appears in a bath containing a minimum of acid, add about half as much acid again as it originally contained. Note the strength: this formula wants 28 per cent, made by Kodak’s own footnote from three parts of glacial acid to eight of water, and substituting glacial volume-for-volume would put three and a half times the acid into the bottle.

Potassium alum, 480 g in four litres — 120 g/L, and the reason the formula is called a hardener at all. It supplies the aluminium(III) that tans gelatin, raising its melting point so a film can be washed for an hour in running water without the emulsion leaving its support. More hardens more and eventually too much, giving brittleness; less gives frilling and reticulation in a warm wash. The powdered form is specified because it dissolves in a reasonable time; 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, on keeping grounds: a chrome alum bath loses its hardening properties rapidly whether it is used or not, so potash alum is preferred where a bath will be in use for a long period.

Water, at two temperatures and two points. 1,700 mL warm at about 52 °C to dissolve the sulfite and the alum, then cold water to make 4,000 mL. The cold water is not a formality: the stock must be cool when it meets the hypo, and starting the make-up cold is how it gets there. The hypo it will be diluted into is not part of this formula and is not in the table above, which is the whole architectural point — the eight parts it goes into are the reader’s own plain hypo solution.

Interactions

With hypo, one part to eight, and only when both are cold. That is the formula’s entire external behaviour, and getting it wrong in either direction — warm, or fast, or hypo into hardener — throws sulfur.

With undissolved hypo. The primer’s warning is specific: if the hypo is not thoroughly dissolved before the hardener is added, a precipitate of sulfur is likely to form. A few crystals on the bottom of the tank are enough.

With a fixing bath that already contains a hardener. Adding more does not help a tired bath. Once the acid has been eaten by carried-over developer, extra hardener stock adds alum to a bath that will immediately precipitate it. The primer’s remedy for early sludging is acid, not more of this.

With glassware that has held an alkali. The bottle is strongly acid; a carbonate residue will fizz and consume acid the formula is counting on.

Variants

F-1, the same bath mixed directly, without a stock. The composition differs by a few per cent, computed above. Kodak prints them on the same page as alternatives and clearly regards them as the same bath.

F-53, Kodak Limited’s acid hardener stock. Fifty grams of sulfite, 75 mL of glacial acetic acid and 100 g of alum per litre against F-1a’s 120, 375 mL of 28 per cent acid and 120. F-53 is the more concentrated in alum relative to its acid, and it is dosed differently: 125 mL into a litre of finished bath rather than one part in nine. The two are not interchangeable volume for volume and neither handbook mentions the other.

The 1924 printing is the same formula in avoirdupois, with the direction for use pointing at the plain bath F-11 rather than naming a percentage. It is recorded as corroboration, not as a variant.

Kodak’s own alternative mixing method, printed in the primer’s general instructions: dissolve the alum and the sulfite as separate solutions, cool both, add the acid to the sulfite solution, then add the alum solution. Same formula, same order of chemical events, more washing up.

No course variant is offered. The formula is a stock whose proportions are only meaningful against the one-to-eight dilution that follows it, and weakening either half without the other would produce a bath Kodak never published.

Safety

Level B, and the whole of the level is in the acid.

1,500 mL of acetic acid is the bulk of this bottle. At 28 per cent it is corrosive enough to demand gloves and eye protection; the glacial acid the primer’s footnote sends you to in order to make it is worse, with a flash point of 39 °C and a vapour that reaches a harmful concentration in a small room by evaporation at ordinary temperatures. The acetic acid page carries the classification and the exposure limits, and Part X sets out the course’s standing practice: buy the acid at a stated dilution and compute the volume, so that no glacial acid is handled in the session.

Add acid to water, never water to acid, if you are diluting glacial acid to make the 28 per cent.

The solids are the mild half. Neither sodium sulfite nor potassium alum carries a serious agreed classification; the alum page records the workplace exposure limit that applies to soluble aluminium dust, and the sulfite’s own hazard is what it gives off in acid. Weigh both with the dust down.

A bottle of this stock plus a bottle of hypo is a sulfur dioxide generator if they are combined carelessly, and so is this stock plus any spent fixer in a waste container. Keep the two streams apart until you mean to combine them, cold and slowly.

Storage

In small bottles rather than one large one. Kodak Limited’s advice about stock solutions applies directly: when stock is drawn from a large bottle the air space increases each time it is opened. Four litres in four one-litre bottles loses nothing and keeps three of them full.

Expect a white crust and ignore it. Basic aluminium acetate forms on the inside of the container over several weeks, and Kodak says it does not usually impair the solution. What would impair it is a general milkiness, which is a mixing fault and not an ageing one.

Neither primer publishes a shelf life for this stock, and none is stated here. What can be said from the sources is that the arrangement exists because the two halves keep better apart than together, so a dated bottle and a note of when it was last used is the honest substitute for a figure.

Glass or plastic, never metal, tightly stoppered, cool, away from bases and away from the fixer shelf. Label it with the formula, the strength and the date per the labelling SOP, and label it as an acid, because that is what a reader who finds it will most need to know.

Incompatibilities

Alkalis and carbonates, which it will neutralise noisily and which will end its usefulness.

Hypo, except deliberately, cold and slowly. The whole formula is designed around one controlled meeting with thiosulfate; every uncontrolled one gives sulfur and sulfur dioxide.

Ferricyanide baths and their waste. Acid on ferricyanide is the documented route to hydrogen cyanide, and this is a bottle of acid. Never near a Farmer’s reducer tray or its waste container.

Sulfides. Acid plus a sulfide gives hydrogen sulfide. Sepia toning waste is a different stream.

Chrome alum solutions, which are a chromium(III) waste route and must not be mixed into an aluminium one.

Bare metal — trays, funnels, tongs. An acid alum solution will attack them and pick up iron.

Waste

Unused stock is an acid waste, not a silver one, and that distinction matters because the two must not share a container. It carries no silver at all until it has been in a fixing bath.

Spent fixing bath made from it is the silver stream, and goes the route F-1’s page and the silver-bearing waste SOP describe.

Aluminium is the ingredient that makes even the unused stock a matter for the waste rules rather than the sink: it is the reason a hardening fixer’s residues are treated differently from a plain one’s. Follow the general chemical waste SOP and the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

The stock is milky as mixed. A relative insufficiency of acid. Either the acid was under strength — the classic case is 28 per cent used where glacial was meant — or too much alum or sulfite went in. Remix; do not try to correct a milky batch by adding acid to it, because the proportions are then unknown.

A white sludge appeared when the alum went in. The sulfite-acid solution was not thoroughly mixed, or the alum arrived before the acid. Aluminium sulfite dissolves slowly in the acid that follows it; stir and wait before deciding the batch is lost.

The alum will not dissolve. The solution had cooled too far, or lump alum was used instead of powdered. Warm it gently — the formula’s own water is at 52 °C — and stir.

The fixing bath went milky when the stock was added. The hypo was warm, or not completely dissolved, or the stock went in too fast, or the two were poured the wrong way round. That precipitate is sulfur and it does not redissolve.

A white crust inside the storage bottle. Basic aluminium acetate. Kodak says ignore it.

The finished bath sludges after only a few sheets. Not this stock’s fault: the acid is being consumed by developer carried in. Use a rinse bath, and read the capacity figures on F-1’s page, which differ by a quarter on that variable alone.

Experiments

Mix the same fixing bath both ways and compare them. One tray of F-1 mixed directly, one of eight parts 25 per cent hypo to one part of this stock. Fix identical strips, measure the gelatin melting point of each, and run both to exhaustion counting sheets. The arithmetic above predicts a difference of a few per cent; whether a few per cent is detectable is a question the sources do not answer.

Get the order wrong on purpose, in 100 mL. Alum into sulfite before the acid, and watch the white sludge. Then add the acid and time how long it takes to clear. This is the fastest way to understand why Kodak repeats the order twice on one page.

Test the keeping claim. Bottle the stock in a full bottle and a half-empty one, and test both at intervals by making a small fixing bath and measuring its hardening. Kodak publishes no shelf life for F-1a; this course would like one.

Weigh the crust. If a bottle has stood long enough to grow basic aluminium acetate, filter, dry and weigh the deposit and compute what fraction of the alum has left the solution. Kodak says the incrustation does not impair the solution; that is a claim with a number behind it that nobody has printed.

Compare it with F-53. Make both stocks, dose each into plain hypo at its own published rate, and measure hardening and sludging life. Two companies, two continents, two answers to the same problem, and no published comparison anywhere in the corpus.