Kodak F-2

Kodak’s 1928 primer prints three acid hardening fixing baths of the same family on two facing pages, and they differ almost entirely in their hardener. F-2 is the leanest of the three: against a given weight of hypo it carries a fifth of F-1’s sulfite and two-fifths of its alum. The primer says what it is for in five words — motion picture fixing bath — and does not say why it is built that way.

The hypo solution — the fixing bath itself

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	960 g	crystalline hypo
Water	4000 mL, added	Kodak prints "Water 4.0 liters" at the head of the formula and no make-up line, and the primer states its own convention for the difference: the term "cold water to make" is always given at the end of a formula where a definite final volume is intended, so that the concentration is known each time. There is no such line here, so this is water added and the finished volume is not published.
A tank formula on Kodak's four-litre tank basis, and the primer notes that motion picture work in practice needs larger volumes still.

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

Ingredient	Quantity	Form the source specifies
Sodium sulfite (anhydrous)	12 g	desiccated
Acetic acid (glacial)	72 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	24 g	powdered
Water	128 mL, added	The primer gives no temperature for this water, although the F-1a directions it sends the reader to specify about 52 degrees C for the same operation. What it does specify is that the hardener must be cool when it is added.
"Dissolve in the order given following the directions for mixing the stock hardener (Formula F-1a)": sulfite completely dissolved, then the acid, then the alum with constant stirring.

Mixed in this order — the working fixing bath

1. Start with the whole of The hypo solution — cool, and the hypo thoroughly dissolved first
2. Then add the whole of The hardener solution — cool, added slowly with stirring

When thoroughly dissolved add the following cool hardener solution slowly with stirring to the cool hypo solution.

Both words in "cool hardener ... cool hypo" are load-bearing. The primer's general rule for this family of baths is that a hardener added to warm or incompletely dissolved hypo throws a precipitate of sulfur. No final volume is stated for the mixture.

Purpose

To fix motion picture film. Kodak’s header is “Motion Picture Fixing Bath” and that is the entirety of the stated purpose; there is no descriptive paragraph, no time, no temperature and no capacity figure anywhere on the page.

What can be said about the intent comes from a different part of the same book. The primer explains that Eastman formulas are published on a one-litre basis for tray work and a four-litre basis for tank work, and that special purposes such as motion picture or photo-finishing work need larger volumes still. F-2 is a four-litre formula written for a bath that will be scaled up and run continuously, and the two things it economises on — sulfite and alum — are the two ingredients whose excess causes the troubles a long-running bath suffers from.

That last sentence is the course’s reading of Kodak’s own statements elsewhere in the book, not a claim Kodak makes here. The primer gives no reason for F-2’s proportions at all, and this page does not supply one it cannot source.

Recommended uses

Motion picture film, which is what it says. In a stills darkroom, the honest answer is that F-2 has no advantage over F-1 that the sources establish.

Where less hardening is wanted from an alum bath. Two-fifths of the alum per gram of hypo is a real and measurable difference and the primer’s own hardening test will show it. What Kodak does not publish is the melting-point band F-2 is compounded for, so a reader wanting known hardening should use F-1, whose band is published as 54 to 77 °C.

As the third term in a comparison. F-1, F-2 and F-14 hold the hypo constant and vary the hardener across a factor of five. Read together they are the clearest statement in the corpus of what each ingredient of an acid hardening bath is doing, which is why all three are in this formulary rather than only the general-purpose one.

When another formula is preferable

• For anything that is not a motion picture film, F-1, which is the general-purpose bath of the same book and the one with the published capacities and hardening range.
• For a deep tank of roll film, F-14, which is the tank formula of the family and buys sludging life with acid rather than by cutting the alum.
• Where no hardening at all is wanted, F-52 or a plain hypo bath.
• Where a hot darkroom is the problem, F-5, which is built for it. Cutting the alum is not a tropical strategy; the primer’s answer to heat is boric acid or a separate hardening bath, not a weaker one.
• Where the acid is wanted without any alum, the primer’s bisulphite fixing bath, which has nothing in it that can sludge.

Mixing

Two vessels, both cold when they meet.

1. The hypo solution. Dissolve 960 g of hypo in 4,000 mL of water. Note the wording: the primer says water 4.0 litres, not water to make 4.0 litres, and it explains its own convention elsewhere — “cold water to make” is the phrase it uses when a definite final volume is intended. There is no such phrase here, so the finished volume is more than four litres and Kodak does not say how much more.
2. Let it dissolve completely and cool.
3. The hardener solution. In 128 mL of water, dissolve in this order 12 g of sodium sulfite, then 72 mL of 28 per cent acetic acid, then 24 g of powdered potassium alum, with constant stirring. The primer sends you to F-1a’s directions for the detail, and the detail is that the sulfite must be completely dissolved before the acid arrives, and the alum last because it dissolves easily in an acid-sulfite solution and badly in a sulfite one.
4. Combine. Hardener into hypo, slowly, with stirring, both cool.

No temperature is given for the hardener’s 128 mL. F-1 and F-1a both specify about 52 °C for the same operation and F-2’s own entry does not. The course records the omission rather than filling it in; warm water dissolves alum and sulfite more readily, and the instruction that matters is that the solution be cold by the time it meets the hypo.

🧮The three baths, compared on the one basis that is exact

F-1 states its hypo as a make-up volume and F-2 and F-14 state theirs as a weight in a volume of water. The finished volumes are therefore not comparable and no source gives them. What is exactly comparable is the hardener carried per gram of hypo, because all three formulas state a hypo mass. Scaled to a common 960 g of hypo:

	Sodium sulfite	28 % acetic acid	Potassium alum
F-1	60 g	192 mL	60 g
F-2	12 g	72 mL	24 g
F-14	30 g	208 mL	60 g

F-2 carries a fifth of F-1’s sulfite, three-eighths of its acid and two-fifths of its alum. F-14 carries half the sulfite, slightly more acid, and the same alum.

Two different economies, then, from the same book. F-14’s is the one the primer explains — an excess of acid buys a long life before the aluminium sulfite sludge appears, at the cost of hardening. F-2’s is not explained anywhere in the source, and the course does not invent an explanation for it.

Behaviour

It fixes like any other 20-odd per cent hypo bath. Nothing in the reduced hardener changes what the thiosulfate does. The primer’s general figures apply: fixation is twice the clearing time, 30 to 40 per cent hypo fixes fastest, 18 °C is the recommended temperature and above 21 °C the bath is apt to precipitate sulfur.

It hardens less, and that is deliberate. Two-fifths of the alum against the same hypo is not a rounding. Whether it hardens enough for a given film is a question Kodak answers only by publishing the formula for one purpose.

Its sulfite reserve is small, so its margin against sulfurisation is small. Twelve grams per 960 g of hypo is the thinnest protection of the three baths, and the primer’s three causes of a sulfur precipitate — too much acid, too little or impure sulfite, too high a temperature — all become likelier as that reserve shrinks. A tub of sulfite that has oxidised to sulfate matters more here than in F-1.

Its sludging behaviour is not published and cannot be predicted from the corpus. It has less acid than F-1 per gram of hypo, which by the primer’s own account should shorten its life against the aluminium sulfite sludge; it also has less alum to precipitate. Which effect dominates is not something this course can say, and it is a good experiment.

Image characteristics

A fixer should have none, and F-2’s departures from that are the family’s.

A correctly working bath changes nothing except the removal of undeveloped halide.

A bath gone alkaline lets development continue and stains prints brown; the primer says so of acid baths generally.

A sulfurised bath fades the image years later, because the sulfur penetrates the gelatin.

Under-hardening is this bath’s characteristic risk rather than brittleness. A film fixed in F-2 and washed warm has less alum in its gelatin than one fixed in F-1, and frilling or reticulation is the consequence to watch for.

The mechanism

The chemistry is F-1’s and is set out there and on how fixer works. Thiosulfate complexes silver out of the halide:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

The whole of fixing

⚗What a smaller sulfite reserve actually means

The acid in a hardening fixer is always in the process of attacking the thiosulfate, and the sulfite is what it attacks instead:

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

The reaction the sulfite is there to forestall

SO₃²⁻ + 2 H⁺ → SO₂ + H₂O

The reaction that happens first, while there is sulfite left

F-2 has a fifth of F-1’s sulfite per gram of hypo and three-eighths of its acid. The ratio of acid to sulfite is therefore nearly twice F-1’s — 6 mL of 28 per cent acid per gram of sulfite against 3.2 — so the reserve is not merely smaller but is being drawn on faster relative to its size. That is arithmetic on Kodak’s numbers; whether it shows up as an earlier sulfur precipitate in practice is not recorded anywhere the course has read.

Function of every ingredient

Sodium thiosulfate pentahydrate, 960 g in four litres of water. The fixing agent, and the only ingredient that does the job the tray exists for: it converts silver bromide into a soluble thiosulfato-argentate complex and lets it be washed away. More fixes faster up to Kodak’s 30 to 40 per cent optimum and Wall’s 40 to 45 per cent ceiling; less fixes slowly and risks leaving insoluble intermediate complexes that washing cannot remove. Note the form and the wording together: crystalline hypo, the pentahydrate, and 960 g dissolved in four litres rather than made up to four litres, so the strength is somewhat below 240 g/L and the source does not say how far below.

Sodium sulfite, 12 g. The preservative and the acid reserve, and the ingredient that most distinguishes this formula. Its job is to be consumed by the acetic acid so that the thiosulfate is not. At a fifth of F-1’s dose per gram of hypo it is the smallest reserve of the three baths in this family. More would lengthen the bath’s life against sulfurisation and shorten it against the aluminium sulfite sludge, since sulfite is one half of that precipitate. Less — or a stale tub that has oxidised to sulfate, which is not a preservative at all — is one of the primer’s three named causes of a sulfur precipitate, and there is little margin here. The desiccated salt is meant, not the heptahydrate crystals, which would need about twice the weight.

Acetic acid, 72 mL of a 28 per cent solution. The acid, doing the two jobs it always does: neutralising the developer’s alkali so that development stops at the tray, and holding aluminium(III) in solution so that the alum can harden rather than precipitate. More acid gives a longer sludging life and worse hardening — the primer states that trade explicitly — and less gives the reverse. The strength matters as much as the volume: substituting glacial acid volume for volume would put three and a half times the acid into the bath, and the primer names that as a common and consequential mistake.

Potassium alum, 24 g, powdered. The hardener: the aluminium(III) that tans gelatin and raises its melting point so that a film survives washing. At two-fifths of F-1’s dose per gram of hypo, this bath is compounded to harden distinctly less, and Kodak does not say what melting-point band it is aiming at. More alum hardens more and eventually induces brittleness; less leaves a film liable to frill or reticulate in a warm wash. The powdered form is specified because lump alum will not dissolve in a cold acid-sulfite solution in any reasonable time. Potash alum rather than chrome alum is the primer’s general preference for baths in long use, because a chrome alum bath loses its hardening whether it is worked or not.

Water, twice, and neither volume a make-up. Four litres for the hypo and 128 mL for the hardener, both printed as additions. Kodak’s own convention, stated in the same book, reserves “cold water to make” for the line that fixes a final volume; there is no such line in F-2, so no concentration can be computed exactly and none is printed here. The primer gives no temperature for either volume of water on this page, and the one temperature instruction it does give is negative: both solutions cold when they meet.

Interactions

With carried-over developer, which is what consumes the acid and eventually ends the bath. With less acid than F-1 per gram of hypo, F-2 has less to spend, so a rinse before the tray matters at least as much here — although the primer publishes the capacity gain for F-1 only.

With warmth, badly: faster fixing, faster sulfurisation, a bath that will not stay clear for more than a few days above 29 °C.

With undissolved hypo, which is the named cause of a sulfur precipitate at the moment of mixing.

With a second bath of itself, usefully: two-bath fixing is the standard defence against the insoluble complexes a tired bath leaves, and the rotation SOP has the practice.

With acid, in the waste bottle. See Incompatibilities.

Variants

F-1, the general-purpose bath of the same pages, at five times the sulfite, two and two-thirds the acid and two and a half times the alum per gram of hypo. It is the formula to use for anything F-2’s header does not name, and it is the one with published capacities and a published hardening range.

F-14, the deep tank bath for roll films, at half F-1’s sulfite, slightly more acid and the same alum. The other economy, and the one the primer explains.

F-1a, the hardener as a stock. F-2’s hardener is not available as a stock dilution — its proportions are its own — but F-1a’s mixing directions are the ones F-2 tells you to follow, which is why that page is a dependency of this one rather than a cross-reference.

F-16, the chrome alum bath on the facing page, for hot weather. Not written up in this formulary: it is a chromium(III) bath under the course’s chromium ruling, and the primer itself records that its hardening dies within days with or without use.

No course variant is offered. There is nothing here to make safer by adjusting, and adjusting the hardener is precisely what would turn F-2 into one of the other two published formulas.

Safety

Level B, for the same reason as the rest of the family: the acid at the mixing stage rather than the bath in the tray.

Acetic acid at 28 per cent demands gloves and eye protection, and the glacial acid the primer’s footnote sends you to in order to make it demands considerably more — read the chemical page first. The course’s standing practice, from Part X, is to buy acid at a stated dilution and compute the volume so that no glacial acid is handled in the session.

The solids are unremarkable and not unclassified for good reasons. Neither hypo nor potassium alum carries an agreed GHS classification, which is an absence rather than a finding; the alum page records the exposure limit that applies to soluble aluminium dust, and sodium sulfite gives sulfur dioxide in acid.

Sulfur dioxide is the gas this tray can be made to produce, and with the smallest sulfite reserve of the three baths it is the one most easily provoked. Nothing acidic beyond the formula’s own acid goes into it.

Ventilation as for any darkroom, with the extra care belonging to the dilution of the acid.

Storage

No keeping figure is published for F-2 and none is stated here. What the primer publishes about acid fixing baths in general is a warning rather than a figure: above 29 °C the bath will not remain clear more than a few days however correctly it was mixed.

Glass or plastic, never metal, stoppered, cool and dark. Label with the formula, the strength and the date per the labelling SOP, and keep a tally of what has been through it — because with no published capacity, a count of sheets and a clearing time are the only record of this bath’s remaining life that exists.

If the bath will be made repeatedly, keep the hardener as F-1a instead and mix F-1 rather than F-2, unless the reduced hardening is specifically wanted. Two stable bottles beat one tray with a shelf life nobody has measured.

Incompatibilities

Strong acids, which liberate sulfur dioxide and colloidal sulfur from a thiosulfate-sulfite bath; the incompatibilities page treats this as the most likely serious accident in a home darkroom.

Developer, in either direction. One pair of tongs per tray.

Ferricyanide baths and their waste, because acid on ferricyanide is the documented route to hydrogen cyanide.

Alkalis, which raise the pH past the point where aluminium stays in solution and end the bath at once.

Sulfide toners, whose waste gives hydrogen sulfide when it meets an acid.

Chrome alum baths, a separate chromium(III) waste stream that must not be mixed into an aluminium one.

Waste

Silver-bearing, as every spent fixer is. F-2 carries dissolved silver as the thiosulfate complex, along with sulfite, acetate and aluminium.

Collection rather than treatment at darkroom scale, per Part XII and the silver-bearing waste SOP. Keep it away from every acid waste and every ferricyanide waste.

The disposal ruling governs, and local regulation decides. This course cannot tell you what it says where you are.

Troubleshooting

Milky the moment the hardener went in. Warm hypo, undissolved hypo, or the hardener poured in too fast. A pale yellow slow-settling precipitate is sulfur and does not redissolve.

Milky after a week in a warm room. Expected above 29 °C, per the primer. Mix fresh and keep the tray cooler.

A white gelatinous sludge. Aluminium sulfite; the acid has been consumed by developer carried in. The primer’s remedy at the first sign is to add about half as much acid again as the bath originally contained.

Fixing slower than it was. Time a clearing test; past 12 to 15 minutes for a slow film, discard.

Negatives frilling or reticulating in the wash. The likeliest complaint about this particular bath. F-2 hardens less than F-1 by design; either wash cooler, or use F-1, or harden separately before fixing.

Prints stained brown. The bath has gone alkaline. Rinse before fixing.

A sharp smell over the tray. Sulfur dioxide. Find the acid that got in.

Experiments

Measure the hardening Kodak did not publish. Fix identical strips in F-1 and in F-2, wash both, then heat each in water until the gelatin flows off the support. F-1’s band is published as 54 to 77 °C; F-2’s is not published anywhere, and two-fifths of the alum ought to show. This is the single most useful missing number on this page.

Race the two sulfite reserves. Run F-1 and F-2 side by side at 25 °C, adding a measured dose of carbonate solution to each at intervals to stand in for carried-over developer, and record which throws a precipitate first and which kind. The arithmetic above says F-2’s acid-to-sulfite ratio is nearly twice F-1’s; nothing in the corpus says what that does.

Find the capacity nobody printed. Fix 8 by 10 sheets in a measured volume, timing a clearing test every few sheets, and stop at 12 to 15 minutes. Kodak gives that figure for F-1 and F-16 and not for F-2. The clearing-time experiment has the method.

Test the wording. Make up 960 g of hypo in 4,000 mL of water and measure the resulting volume, then compare it with 960 g made to 4,000 mL. The difference is what separates Kodak’s “water” from its “cold water to make”, and it is the reason this page declines to print a percentage.