Kodak F-6

One line of F-5 is different, and almost everything written about this formula gets that line backwards. Kodak did not take the boric acid out of F-6. It put in a salt that makes the boric acid in the tray, out of acetic acid the bath was carrying anyway — and it is the spending of that acetic acid, not the absence of the boric acid, that takes the smell of sulfur dioxide off the fixer. F-6 is the same five substances doing the same five jobs at a different acid balance, and every difference Kodak publishes between the two baths follows from that one fact.

Ingredient	Quantity	Form the source specifies
Water	600 mL	about 50 °C (125 °F)
Sodium thiosulfate	240 g	hypo
Sodium sulfite	15 g	desiccated
Acetic acid	48 mL	of a 28 per cent solution
Kodalk	15 g	sodium metaborate, Kodak’s trade name
Potassium alum	15 g
Cold water	to make 1000 mL

Purpose

To dissolve the silver halide the developer did not reduce, to arrest development while doing it, and to harden the gelatin — without putting a detectable quantity of sulfur dioxide into the air over the tray. Kodak’s own heading in the 1946 handbook is “Odorless Bath for Films, Plates, and Papers”, and its own statement of the problem, printed in both editions the course read, is this: in warm weather and in inadequately ventilated darkrooms the odour of sulfur dioxide given off by F-5 may be objectionable, and it can be eliminated almost entirely by omitting the boric acid and substituting twice its weight in Kodalk.

That is a formula written to solve a working condition rather than a photographic one. Nothing about the negative changes. What changes is the room.

Recommended uses

Warm weather, and any darkroom whose ventilation you do not trust. These are Kodak’s own two conditions, named in the same sentence as the formula. They are the conditions under which an acid fixing bath gives up the most sulfur dioxide: warmth drives the dissolved gas out of solution, and poor ventilation lets it accumulate.

Prints, and papers generally. J-1 adds a second reason that has nothing to do with the smell: F-6 “can also be used to advantage for fixing prints, since it washes out of photographic papers more rapidly than the baths which have a greater hardening action.” Read carefully, that sentence places F-6 among the baths of lesser hardening action, and it is the clearest statement Kodak makes about what the substitution costs. A less thoroughly tanned paper gives up thiosulfate faster, and the permanence of a fibre print is decided by how completely it does that.

Films and plates too. The 1946 heading covers all three materials, and it is the same 240 g of hypo per litre that F-5 uses. Note, though, J-1’s separate general recommendation: one fixing bath for films and plates and another for papers. A permission to use one formula for both is not an instruction to use one tray for both — paper carries far more developer per unit area into the fixer, and mixing the two shortens the life of both.

With a stop bath, always. This is not a refinement here; it is a condition Kodak attaches to the formula. The 1946 entry: the bath “should be used in conjunction with a stop bath such as Kodak SB-1 or SB-1a, or an acid hardening bath such as Kodak SB-3 to obtain the full useful hardening life,” and with a water rinse instead, “the life is about one-half that of Kodak F-5.” The 1977 entry repeats it with KODAK Indicator Stop Bath or SB-1 named. F-5 does not carry that warning, and the reason is in the chemistry below.

!Kodak publishes no fixing time for F-6

Neither printing gives this formula a duration. The 1946 handbook prints times for F-5 — films or plates 10 to 20 minutes in a fresh bath, prints 5 to 10 — and the 1977 printing gives F-5 5 to 10 minutes for both, which is itself a change Kodak made between editions and not a difference the course has introduced. F-6’s entry carries neither figure. The course does not transfer a number from one formula to another, so use the clearing-time measurement under Behaviour, which is what any published time was derived from in the first place.

When another formula is preferable

• When the darkroom is cool and well ventilated, F-5. It hardens more, it reaches its full published capacity after a plain water rinse, and the odour F-6 exists to remove is only a problem under the two conditions Kodak names.
• When you cannot guarantee a stop bath before the fixer — a travelling kit, a class, a sink with three trays — F-5 again. Halving a fixer’s capacity by leaving out a tray is a poor trade for a smell.
• When no hardening at all is wanted, F-24, printed on the same J-1 page, or Kodak Limited’s F-52. Toning, bleaching, redevelopment and any process that has to get a reagent into and out of the gelatin all go better on an unhardened layer.
• When paper is the whole job and hardening still matters, F-54, which is built by adding the F-53 hardener stock to a hypo solution rather than dissolving five things in one vessel.
• When fixing speed is the constraint, Kodak’s rapid bath F-7, of which J-1 says on the same page that it fixes much more rapidly than F-5 or F-6 and has a considerably greater useful capacity; F-9 is the same bath with ammonium sulfate in place of the ammonium chloride, for stainless steel tanks. Neither has an entry in this formulary yet, and J-1 attaches a caution to both that prolonged fixing in a rapid bath can bleach the image.
• For a printing-out process — salted paper, albumen, Van Dyke — an alkaline plain-hypo bath. Acid attacks the finely divided silver of a printed-out image, and this bath is still an acid bath.
• When you want to see the mechanism with nothing else in the tray, plain hypo.

Mixing

The order is Kodak’s and it is not a convenience. The 1946 entry says only “dissolve chemicals in the order given”, and J-1’s front matter states the general rule behind it: the ingredients of a formula are named in the order in which they should be dissolved unless the directions say otherwise. The 1928 primer explains what the order is protecting against.

1. Hypo, into water at about 50 °C, and dissolve it completely. Warm water is there because a large weight of hypo takes heat out of the solution as it dissolves. The primer’s warning is unambiguous: the hypo should be cool and completely dissolved before the hardener goes in, otherwise sulfur is likely to be precipitated.
2. Sulfite, so the defence is in the vessel before the attack arrives.
3. Acetic acid, into a cool solution.
4. Kodalk. This is the step with no counterpart in F-5, and it is a neutralisation: the metaborate meets the acetic acid and both are converted. Add it to the acid solution rather than the reverse, which is what Kodak’s order does, and no part of the vessel ever goes alkaline.
5. Potassium alum, last. The primer gives the reason and the failure: the alum dissolves more readily in the acid-sulfite solution, and if the order is reversed so that alum meets sulfite before the acid, a white sludge of aluminium sulfite forms which dissolves only with difficulty.

Both cool, if you mix the hardener separately. Kodak’s alternative route to this bath is the hardener stock F-6a, described under Variants: one part of the cool stock added slowly to four parts of cool 30 per cent hypo solution, stirring the hypo rapidly. The word cool appears twice in Kodak’s own sentence and the primer says what happens if you ignore it.

A practical note on the acid. 28 per cent acetic acid is not a bottle most readers own. Kodak’s 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. Substituting glacial acid volume for volume here would put roughly three and a half times the intended acid into the bath, which would destroy the whole point of the formula and probably the thiosulfate with it. Follow the mixing SOP.

🧮What the litre contains, and what the substitution actually did

240 g ÷ 248.19 g/mol = 0.967 mol/L

Thiosulfate, from the pentahydrate

15 g ÷ 126.05 g/mol = 0.119 mol/L

Sulfite, anhydrous

15 g ÷ 474.4 g/mol = 0.0316 mol/L, and so 0.0316 mol of aluminium

Potassium alum

Those three lines are identical to F-5’s. The bath is about 24 per cent w/v hypo, just below the 30 to 40 per cent the 1928 primer gives as the optimum for speed of fixation, so F-6 fixes at F-5’s rate and for the same reason.

Now the two lines that differ.

7.5 g ÷ 61.84 g/mol = 0.121 mol/L of boron, as an acid

Boric acid in F-5

15 g ÷ 137.86 g/mol = 0.109 mol/L of boron, as a base

Kodalk in F-6

Kodak’s “twice its weight” is very nearly a mole-for-mole swap of boron — 0.109 against 0.121, a tenth apart — because the tetrahydrate’s molar mass is 2.23 times boric acid’s. Whether Kodak intended that or simply rounded, neither document says, and the course does not guess. What matters is that F-6 ends up with almost exactly as much boron in it as F-5 does, in the same chemical form, once the acid has done its work.

The acid is where the two baths genuinely part company:

48 mL × 0.28 g/mL ÷ 60.05 g/mol = 0.224 mol/L

Acetic acid, reading Kodak's 28 per cent as w/v

48 mL × 3/11 × 1.05 g/mL ÷ 60.05 g/mol = 0.229 mol/L

Acetic acid, reading Kodak's own 3-parts-glacial-to-8-of-water recipe

The two readings agree to within two per cent, so the choice between them does not matter. Call it 0.23 mol of acetic acid per litre — and note in passing that Kodak Limited’s London printing of F-5 specifies 17 c.c. of the glacial acid, which is 0.30 mol, about a third more. The F-5 this page is comparing against is the American one printed beside F-6, not the London one on this course’s F-5 page.

Of that 0.23 mol, the Kodalk consumes 0.109 mol before the bath is ever used. So:

	F-5 (American printing)	F-6
Acetic acid, free	0.23 mol/L	0.12 mol/L
Acetate	none	0.11 mol/L
Boric acid	0.121 mol/L, weighed in	0.109 mol/L, made in the bottle
Total acid available to meet carried-over alkali	about 0.35 mol/L	about 0.23 mol/L

F-6 therefore starts with roughly a third less acid than F-5 by this reckoning. Kodak’s own measurement is that its hardening life after a water rinse is about a half of F-5’s. Those are not the same number, and the course does not pretend they are — the sulfite is also a base and takes its share, and boric acid is far too weak to count as reserve down to the same endpoint as acetic acid. But they are the same direction and the same order of magnitude, and they explain why Kodak attaches a stop bath to this formula and not to F-5. This arithmetic is the course’s, not Kodak’s.

Behaviour

It fixes at F-5’s rate, because it is F-5’s hypo. Nothing in the substitution touches the fixing agent, its concentration or the pH range over which thiosulfate complexes silver. Where F-6 differs from F-5 is in how long it goes on doing it, and in how hard the emulsion is afterwards.

Fixing time is measured, not chosen. The 1928 primer’s rule stands for every bath on this site: the time for fixation is twice the time for the milkiness of the unreduced silver salts to disappear, and the clearing time depends on the strength of the hypo, the material, the temperature and the degree of exhaustion. Since Kodak publishes no duration for F-6, this is not a refinement here — it is the only honest way to time it. See the clearing-time SOP.

Its clock is the acid, and its clock runs faster than F-5’s. Every sheet brings alkaline developer in. That alkali is neutralised by the bath’s free acid, and when the free acid is gone the aluminium and the sulfite find each other. F-6 starts that race with less acid, and Kodak’s table is the measurement: 50 8 × 10 sheets per gallon after a water rinse against F-5’s 100, and 100 after SB-1, which is F-5’s figure exactly. A stop bath does not make F-6 better than it was; it stops the difference from mattering.

Exhaustion looks the same as in any alum-acid bath. The primer names frothing at the surface, milkiness through the solution, a sludge, and simply fixing so slowly that there is a danger of taking the material out before it is done. The measurable sign is the clearing time, and the primer’s rule is to discard when a slow-clearing film passes 12 to 15 minutes.

Hardening rises before it falls, here as everywhere. During the first stages of use the hardening properties of an alum bath increase slightly, after which they fall off rapidly, because the developer carried in is spending the acid. F-6 sits further along that curve from the start.

Temperature is a constraint at both ends, and this is the formula that has to say so twice. The primer: a film needing 95 seconds to clear at 18 °C would clear in about 60 at 29 °C, but it is dangerous practice to let the bath rise above 21 °C because it is apt to precipitate sulfur. The bath Kodak wrote for warm weather is still a bath that dislikes warmth; what F-6 removes is the smell of the sulfur chemistry, not the chemistry.

Image characteristics

A fixer used inside its life leaves no visible signature, and every visible signature it does leave is a fault. What F-6 changes is not the look of the negative but the state of the gelatin it hands to everything downstream.

It hardens less than F-5. That is Kodak’s own placement of it among “the baths which have a greater hardening action”, and it is the direction the acid arithmetic predicts. Consequences, in order of how often they matter: a print washes free of thiosulfate faster; a toner, a selenium bath or a dye reaches the image more evenly; and a wet emulsion is more easily marked, scratched or reticulated by a warm wash.

It stains exactly as F-5 stains, when it is finished. A bath whose acid is spent goes alkaline, and prints fixed in an alkaline bath are likely to stain brown. The primer says so of the whole class.

It has no effect on image tone or density in a bath used within its life, and neither Kodak printing claims one.

The mechanism

Why an insoluble salt dissolves. Silver bromide is insoluble because the lattice holds silver more tightly than water can. Thiosulfate holds it more tightly still, and in solution:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the bis(thiosulfato)argentate complex

Why the acid is needed, and why it is dangerous. The acid neutralises the alkali arriving on the film, which arrests development inside the fixing bath; and alum will not harden unless the bath is acid. But acid and thiosulfate are enemies. The 1928 primer sets the decomposition out in two steps: the acid converts the thiosulfate to free thiosulphuric acid, which is unstable and falls apart.

H₂S₂O₃ → H₂SO₃ + S

The 1928 primer's equation: thiosulphuric acid to sulphurous acid and sulfur

And this is where the smell comes from. The primer defines sulphurous acid in its opening chapter as the compound of water with sulfur dioxide:

SO₂ + H₂O → H₂SO₃

The primer's own definition, read in reverse in a warm tray

Read the two together and the odour of F-5 has a source and an address. Every acid alum fixer contains sulfur(IV) — as the sulfite that was weighed in, and as the sulphurous acid the slow decomposition of thiosulfate keeps making — and how much of it stands over the tray as sulfur dioxide gas is set by the acidity of the solution and by its temperature. Warm the bath, or make it more acid, and the same dissolved sulfur(IV) leaves as gas.

Why the Kodalk removes it. Sodium metaborate is the salt of boric acid: it is a base, and in an acid bath it does the only thing a base can do.

NaBO₂ + CH₃COOH + H₂O → H₃BO₃ + CH₃COONa

What happens the moment the Kodalk goes in

Boric acid has a pK_a of 9.27, so at the pH of a fixing bath essentially all of that boron finishes as undissociated boric acid — the same substance, in the same quantity, that F-5 has weighed into it as crystals. The boric acid is not missing from F-6. It is manufactured in the bottle, and paid for in acetic acid. What the bath has instead of 0.11 mol of free acetic acid is 0.11 mol of acetate, and an acid together with its own salt is a buffer: the bath sits at a higher and much flatter pH than F-5 does. Less acidity means less of the dissolved sulfur(IV) present as sulphurous acid and so as sulfur dioxide, and that is the odour gone.

⚗Neither Kodak document explains this

Both printings state the effect — omit the boric acid, put in twice its weight of Kodalk, the odour goes — and neither states a mechanism. The account above is the course’s reading, built from the primer’s own two equations and from the acid-base behaviour of a metaborate, both of which are established chemistry. Treat the effect as Kodak’s published finding and the explanation as this course’s interpretation, per Rule 7. What would settle it is a pH measurement of the two baths side by side, which is the first experiment at the foot of this page.

Why the sulfite is still the defence. Sulfite is the decomposition’s own product, so its presence pushes that equilibrium back; the primer puts it plainly, that the sulfite works in the opposite direction to the acid, and that we can prevent acid decomposing the hypo if we have enough sulfite present. It also reduces the oxidised developer carried in, which is what keeps the bath from browning.

Why the alum still hardens, and less well. Alum hardens only while the aluminium stays in solution, and it stays in solution only while the bath is acid. The 1928 primer measured the whole relationship: with a given quantity of alum, hardening increases with acetic acid to a maximum, beyond which it decreases until the solution does not harden at all — and separately, a bath containing an excess of acid lasts a long time before aluminium sulfite precipitates but does not harden as well as one with less. F-6 has less acid than F-5. Which way that moves the hardening depends on which side of the maximum F-5 sits, and no source read for this course says. What is not in doubt is the life: less acid, fewer sheets before the sludge, and Kodak’s own table measures the loss at half.

Why the boric acid was there in the first place. J-1’s general account of fixing baths states it directly: boric acid added to a bath of this type increases the hardening power of the alum and helps to prevent the formation of aluminium sulfite sludge. Kodak Limited’s handbook approaches the same problem from the developer end, noting that a Kodalk developer gives less tendency to precipitate aluminium sulfite from alum fixing baths. The boric acid page records the honest limit here, and it applies unchanged: no source read for this course states the mechanism by which boric acid specifically lengthens the life of an alum bath.

Function of every ingredient

Sodium thiosulfate, 240 g of the pentahydrate. The fixing agent, and the only ingredient doing the job the bath is named for: it complexes silver out of the undeveloped halide, two thiosulfates to each silver ion. More fixes faster up to the primer’s 30 to 40 per cent optimum, and also means more thiosulfate to wash out afterwards, which is a permanence cost paid at the far end of the process. Less fixes more slowly, exhausts sooner, and crosses into the region where the primer’s insoluble silver-thiosulfate compound forms — invisible, and it does not wash out. Kodak’s line names the crystalline salt; 240 g of the anhydrous salt would be a bath a third stronger than the formula.

Sodium sulfite, 15 g, desiccated. The preservative, doing two separate jobs. It suppresses the acid decomposition of the thiosulfate by being that reaction’s own product, and it reduces the oxidised developer carried in on each sheet so the bath does not brown. Less, or a tub that has oxidised to sulfate on the shelf, and the bath goes milky with sulfur — the primer names insufficient or impure sulfite as one of three causes. More is not free: sulfite is the other half of the aluminium sulfite sludge that ends the bath’s life, and this bath has less acid than F-5 holding that reaction back. Note that “desiccated” is Kodak’s word for the anhydrous salt; the crystalline heptahydrate would need roughly twice the weight.

Acetic acid, 48 mL of a 28 per cent solution. The acid, and in this formula it has three jobs rather than two. It arrests development by neutralising the alkali the film brings in; it holds the aluminium in solution so the alum can harden; and — uniquely here — about half of it is consumed by the Kodalk at the moment of mixing, which is the reaction that makes the bath odourless. More acid would lengthen the life and bring the smell back, which is to say it would move the formula towards F-5. Less would give a bath that goes alkaline within a handful of sheets, sludges and stains prints brown. Getting the strength wrong is the classic error: the primer warns that using 28 per cent acid where a formula asks for glacial gives less than a third of the intended concentration, and the reverse mistake here would be worse.

Kodalk, 15 g. Sodium metaborate, Kodak’s trade name, and the ingredient that makes this formula a formula rather than a footnote. It is the conjugate base of boric acid, so it does two things at once: it removes 0.11 mol of free acetic acid from the litre, raising and flattening the bath’s pH, which is what suppresses the sulfur dioxide; and it becomes the 0.11 mol of boric acid that F-5 carries as a solid, which is why F-6 still has an acid reserve at all. More would take the bath further towards neutrality, where alum stops hardening and dichroic fog becomes possible — the primer names a bath that does not contain acid as a cause of it. Less would leave sulfur dioxide over the tray, which is the one thing this formula exists to prevent. The hydrate matters: Kodalk is the tetrahydrate, molar mass 137.86 against the anhydrous salt’s 65.80, so 15 g of the anhydrous metaborate bought under a chemical name rather than a trade name would put more than twice the intended alkali into the bath.

Potassium alum, 15 g. The hardener, supplying 0.032 mol of aluminium per litre — the same as F-5. It tans the gelatin so that a wet emulsion survives handling and drying, and the property is measurable: the primer’s baths of this class are compounded to give a hardening of 54 to 77 °C, found by heating water around a fixed and washed strip until the gelatin flows off its support. More alum overshoots into brittleness, which the primer names. Less falls below the minimum that hardens at all. It is worth saying what this ingredient is not responsible for here: the reason F-6 hardens less than F-5 is not the alum, which is unchanged, but the acid the Kodalk consumed.

Water, 600 mL at about 50 °C, then cold water to make 1000 mL. Kodak’s two water lines do two different jobs. The warm 600 mL dissolves a large weight of hypo, which cools the solution sharply as it goes into solution; the cold water at the end fixes the final volume and therefore every concentration on this page. Do not let the bath be warm when the acid and the alum go in — warmth at that moment is the primer’s third named cause of a sulfur precipitate. J-1’s general instruction that packaged fixing baths are mixed at a temperature not exceeding 26.5 °C is the same caution in another form.

Interactions

Acetic acid against metaborate, in the mixing jug. The interaction unique to this formula, and it happens once, before any film goes near the bath. Add the Kodalk to the acid solution as Kodak’s order requires; adding acid to a metaborate solution instead would take a part of the vessel through the pH range where aluminium sulfite forms, and the alum is not yet in, but the hypo is, and a locally alkaline hypo solution is not what the next step wants either.

Acid against thiosulfate, restrained by sulfite. The central three-body problem of every acid fixer. F-6 sits at a lower free-acid concentration than F-5, which pushes this equilibrium the safe way — less sulfur, less sulfur dioxide — and pushes the next one the unsafe way.

Alum against sulfite, refereed by acid. The second triangle, and the one that sets the bath’s life. While free acid remains, the aluminium stays in solution; when it is gone, aluminium sulfite comes out as a white sludge and hardening stops. F-6 has about a third less free acid to spend, and Kodak’s capacity table is that sentence with numbers on it.

The developer’s alkali, through the film, and what the stop bath does about it. Every sheet brings carbonate or metaborate alkalinity in on its gelatin. An acid stop bath spends its own acid on that alkali instead of the fixer’s, and for F-6 that is the difference between 50 and 100 sheets per gallon. SB-1 and SB-1a are the two Kodak names in the 1946 entry; the 1977 entry names its Indicator Stop Bath or SB-1.

Carbon dioxide, and a blistering risk this formula reduces at both ends. The primer’s account of blisters is that when the sodium carbonate of the developer is neutralised by the acid in the fixing bath, carbon dioxide is evolved and lifts the softened gelatin, and that the risk is worst when the bath holds an excess of acid, the film is insufficiently rinsed and the weather is hot. F-6 holds less excess acid than F-5 does. Kodak Limited makes the complementary point from the developer side: a film developed in a Kodalk developer will not blister in an acid fixing bath even at high temperatures, because that alkali evolves no carbon dioxide on acidifying. The two facts are independent, and a tropical worker could have both.

With the wash, and everything after it. A less hardened layer gives up thiosulfate faster, which is the argument for this bath on paper. It is also a layer with less mechanical protection in a warm wash. See the sulfite washing aid and HE-1 for what follows the fixer, and the residual hypo and silver tests for how you find out whether it worked.

Variants

F-5 is the formula this one modifies, and Kodak numbered it separately rather than calling it a version, which is why this page is a verified entry of its own rather than a variant. Read the two together: they differ in one line and agree in five.

A caution about which F-5 you are comparing. The F-5 that F-6 modifies is the American printing — 48 mL of 28 per cent acetic acid — which is what both documents on this page carry. The F-5 published on this course’s own F-5 page is Kodak Limited’s London metric column, whose acid line reads 17 c.c. of the glacial acid, about a third more acid in molar terms. Kodak’s two national organisations printed the same formula number with different acid lines, and the substitution described on this page is arithmetic on the American one.

Kodak Hardener F-6a, printed immediately below F-6 on Formulas page 50 of the 1946 handbook, is Kodak’s alternative route to the same bath: water at about 50 °C 600 mL, sodium sulfite (desiccated) 75 g, 28 per cent acetic acid 235 mL, Kodalk 75 g and potassium alum 75 g, cold water to make 1 litre, dissolved in that order. One part of the cool stock is then added slowly to four parts of cool 30 per cent hypo solution while the hypo is stirred rapidly. It is a five-times concentrate of everything in F-6 except the hypo, which is exactly what a hardener stock is, and it exists because a large user makes hypo solution by the drum and hardener by the bottle. F-6a is not in this formulary’s register and has no entry, so its quantities are recorded here rather than on a page of their own; the same is true of its F-5 counterpart, F-5a, which the register lists as planned but which is not yet written.

The course publishes no variant of its own. The obvious one — F-6 with more acid, to lengthen its life without bringing the smell back — is not a safer version of this formula but an untested one sitting somewhere between it and F-5, and Rule 6 does not permit the course to invent a bath and print it as though somebody had measured it. If you want F-5’s capacity, mix F-5.

Safety

Level B, and one ingredient sets it.

Kodalk is why this page cannot be Level A. The aggregated ECHA notifications for sodium metaborate tetrahydrate carry the signal word Danger, with H319 (serious eye irritation) in 82.6 per cent of the reports, H360 (may damage fertility or the unborn child) in 45 per cent and the weaker H361 in 26.2 per cent; the chemical page records that the sample behind those percentages is small and that the notifiers disagree with one another, and that the course designs controls around the more severe reading. The controls are dust suppression at the balance, nitrile gloves, eye protection and strict hygiene, because the route that matters for a borate is ingestion.

Note what this does and does not change against F-5. F-5’s own Level B is set by boric acid, whose classification is H360 with reproductive toxicity as the only hazard class in it. Swapping boric acid for sodium metaborate does not remove a reproductive-toxicity classification from the formula; it exchanges one borate for another. Anyone who chose F-6 over F-5 on health grounds has chosen wrong, and the reason to choose it is the one Kodak gives — the air in the room.

The acid is milder here than on some pages of this formulary, but it is still acid. Kodak specifies the 28 per cent solution rather than the glacial acid. The classification on the acetic acid page describes the concentrate; a 28 per cent solution is not the same hazard, but it will still sting eyes and damage them, and if you make it yourself from glacial acid you handle the concentrate to do so. Acid into water, never the reverse.

⚠An odourless fixer is not a safer fixer

This is the most important sentence on the page. What F-6 removes is the smell of sulfur dioxide, not the requirement for ventilation and not the sulfur chemistry, which is still in the tray: the same 0.12 mol of sulfite, the same slow acid attack on thiosulfate, and the same violent release of sulfur dioxide if anything strongly acid ever reaches this bath or its waste bottle.

Kodak’s own safe-handling publication J-98A names sulfur dioxide, with acetic acid and ammonia, as a common potential indoor air contaminant of photographic processing, and states that these may be eye- and respiratory-tract irritants depending on airborne concentration. Its indicator for elevated levels is irritation, not odour, and its controls are general dilution ventilation and covers over open tanks. Keep both. Do not let a bath that no longer announces itself talk you out of the extract fan.

Ventilation is therefore the ordinary darkroom provision — an extract that changes the room’s air, a covered tray between sheets, and a door that opens — exactly as it would be for F-5. See the ventilation check.

Storage

Kodak publishes keeping figures twice and the two printings do not entirely agree. The 1946 handbook’s F-6 row gives 1 week in a tray, 2 months in a gallon tank, 3 months in a full stoppered bottle and 3 weeks half full. The 1977 table gives 1 week in a tray, 1 month in a gallon tank and 2 months in a bottle, with a single bottle column where the earlier table has two. The tray figure is the same; the tank figure differs by a factor of two; the bottle figure sits between the earlier handbook’s full and half-full numbers. The course records both rather than picking one, and notes that the schema has no way to say two sources disagree.

Both tables carry the same caveat and it matters more here than elsewhere: the keeping figures are for solutions held at 18 to 21 °C and are proportionately less at higher temperatures. The bath written for warm darkrooms keeps worst in exactly the darkrooms it was written for.

Glass or plastic, never metal, and never a bottle that has held acid. Label with the formula, the strength, the date and the sheet count, per the labelling SOP: a fixer’s remaining life is a number of sheets, and this bath’s number depends on whether a stop bath preceded it.

Store the Kodalk dry, closed and cool. Its chemical page records a melting point of 53.5 °C and that metaborate crystals take up carbon dioxide from the air to become sodium carbonate and borax, so an open tub slowly turns into a different alkali — and in this formula the alkali is a measured reagent, not a buffer to be topped up by eye.

Incompatibilities

Any acid stronger than its own. Sulfuric, hydrochloric or concentrated stop bath tipped into this tray or its waste bottle liberates sulfur dioxide and colloidal sulfur. F-6 is the bath on which this mistake is easiest to make, because the usual early warning — the smell — has been deliberately removed. See the incompatibilities page.

Developer, in the other direction. One pair of tongs per tray. Fixer in a paper developer dissolves halide out of the paper and fogs or bleaches it.

Alkalis. Raising the pH of a thiosulfate bath is what ends it, and this bath starts closer to that edge than F-5 does.

Oxidising agents. Thiosulfate is a reducing agent and its oxidation product, tetrathionate, attacks image silver. Persulfate and permanganate reducers, hypochlorite bleaches and hydrogen peroxide belong in other bottles.

Sulfide toners. Acid meeting a sulfide gives hydrogen sulfide. Sulfide toner waste stays away from this bath and from every acid stream.

Chrome alum baths. Kodak’s SB-3, which the 1946 entry offers as an alternative to a stop bath before this fixer, is a chromium(III) bath and a different waste stream. If you use it, keep the containers separate.

Waste

This is the silver stream, and it is the most valuable waste a darkroom makes. Spent F-6 carries dissolved silver as the thiosulfate complex, together with sulfite, acetate, borate and aluminium. Recovery first, disposal second; for a home darkroom the practical route is collection.

Borate is still in it. Replacing boric acid with metaborate does not take boron out of the waste bottle — the arithmetic above shows there is very nearly as much of it — and borate does not degrade. Spent fixer does not go on the garden.

Collect it, label it, keep it apart from acid wastes, and follow the silver-bearing waste SOP. The disposal ruling governs and the jurisdictional caveat is not a formality: local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

A pale yellow precipitate that settles slowly, soon after mixing. Sulfur. The primer’s three causes are too much acid, too little or impure sulfite, and too high a temperature. On this formula add a fourth: the acid was glacial where 28 per cent was meant, or the Kodalk was left out or under-weighed. A sulfur precipitate does not redissolve; remix.

A white sludge, and a bath that no longer hardens. Aluminium sulfite. The acid has been spent. On F-6 this arrives sooner than on F-5 and much sooner if there was no stop bath — Kodak’s own figures say after about half as many sheets.

Prints or negatives staining brown. The bath has gone alkaline. Same cause, one step further on.

Fixing taking much longer than it did. Clear a test strip and time it. Past 12 to 15 minutes for a slow-clearing film, discard.

Poor hardening from a fresh bath. The primer’s three causes are inferior alum, too much acid or sulfite, and too little alum. On this formula there is a fourth: too much Kodalk, which takes the bath towards the flat end of the acid-hardening curve. Weigh it.

A smell of sulfur dioxide from a bath that should not have one. Something has made it more acid than the formula: an over-strength acid, a missing or under-weighed Kodalk, or acid carried in from an over-strength stop bath. Do not carry on and do not sniff it deliberately; ventilate, and mix again.

Blisters or tiny crater-like pits on dry film. Carbon dioxide from a carbonate developer meeting the fixer’s acid, per the primer, worst in hot weather with insufficient rinsing. Rinse longer, or move to a Kodalk-based developer, which evolves none.

Yellowish-green by reflected light, reddish-pink by transmitted light. Dichroic fog, which the primer attributes to a fixing bath that does not contain acid or that is old and loaded with dissolved silver. On F-6 the first of those two arrives earlier than on F-5.

Experiments

Measure the pH of F-5 and F-6 side by side. This is the experiment that tests the mechanism this page proposes, and it needs a calibrated meter and two 250 mL batches — see the pH meter SOP. The course’s prediction is that F-6 reads higher and that the difference is roughly what a half-neutralised acetic acid buffer would give. If it does not, the explanation on this page is wrong and Kodak’s published effect still stands, which is exactly the distinction Rule 7 asks you to keep.

Reproduce Kodak’s halved capacity. Two litres of F-6, two identical sets of sheets, one series preceded by a plain water rinse and the other by SB-1, clearing time recorded against sheet count in both. Kodak’s table predicts the water-rinse series will reach the 12-to-15-minute discard point at about half the sheet count of the stop-bath series. This is the single most useful measurement on this page, because it is the one that decides whether you can afford the third tray.

Titrate them. Take 25 mL of fresh F-5 and 25 mL of fresh F-6 and titrate each with standard sodium hydroxide to a stated endpoint. The arithmetic in the maths callout predicts F-6 will take about a third less alkali. A number from your own burette is worth more than a number from this page’s table.

Test the hardening rather than assuming it. The primer’s own method: fix and wash two strips, one in each bath, immerse each in water and heat the water slowly until the gelatin flows away from the support. Kodak places F-6 among the baths of lesser hardening action but publishes no temperature for it, so this is a genuinely open measurement. Do it at Level B with eye protection and treat the hot water as the hazard.

Watch the washing difference on paper. Fix two identical fibre prints, one in F-5 and one in F-6, wash both for the same time and run the residual hypo test on each. Kodak’s claim is that F-6 washes out of paper more rapidly. Record it in the lab notebook with a formula version code on both baths.