Kodak F-5a

Take F-5 apart and it falls into two halves that have nothing to do with each other. One half is hypo, which dissolves silver halide. The other half — sulfite, acid, boric acid and alum — does no fixing whatever; it exists to stop development, to protect the hypo from the acid, and to harden the gelatin. F-5a is that second half in a bottle at five times strength, and once it is in a bottle it turns out to have a second job that has nothing to do with fixing at all: it is the bath a sulfide-toned print goes through at the end of the sequence, two parts to sixteen of water.

Ingredient	Quantity	Form the source specifies
Sodium sulfite (anhydrous)	75 g	anhydrous in J-1, "desiccated" in the 1946 printing, which is the same salt under Kodak's older name for it
Acetic acid (glacial)	235 mL of a 28% solution	28 per cent; Kodak's own footnote makes it from three parts of glacial acetic acid to eight parts of water
Boric acid	37.5 g	crystals, and Kodak twice says so: powdered boric acid dissolves only with great difficulty and its use should be avoided
Potassium alum	75 g	fine granular, the dodecahydrate, in J-1's wording
Water	to make 1000 mL	at 50 °C; The water is used at two points and this field holds one of them. Kodak starts with 600 mL at about 50 °C (125 °F) and closes with "cold water to make 1.0 liter"; the 1,000 mL is the make-up volume and the 50 °C belongs to the first 600 mL alone. The same shortfall is recorded for F-1a and F-53 and in docs/FORMULARY-SCHEMA.md. G-23's 2006 reprint closes the table with "Water at 20 °C (68 °F) — 1 L" instead, which in Kodak's own typography is an added volume rather than a make-up volume; the two earlier printings both say "to make", and this entry follows them.

Purpose

To be measured into hypo, one part to four. Kodak’s own header is Stock Solution For Preparing Kodak Fixing Bath F-5, and the sentence that introduces it on the page above is equally plain: the hardener may also be mixed separately as a stock solution.

The gain is the same one every hardener stock offers. A hypo solution keeps, an acid–sulfite–alum solution keeps, and a bath that contains both keeps less well than either. The 1946 handbook’s own keeping table puts numbers on it for the made-up F-5: one week in a tray, one month in a gallon tank, three months in a full stoppered bottle and two weeks in a bottle half empty — the same four figures Kodak Limited publishes for its F-5 three years later. Splitting the bath into two bottles means the darkroom stores two stable things instead of one that is running down, and mixing a tray of fixer becomes one measurement instead of five.

And to be diluted with water alone, two parts to sixteen, as a hardening bath. This is the use the toning literature sends a reader here for, and it is not a fixing bath at all: at that dilution there is no thiosulfate in it. It is the last step before the wash for a fibre-base print that has been sulfide or polysulfide toned, and it exists because such a print was deliberately not hardened earlier.

Recommended uses

Whenever more than one tray of F-5 will be mixed. Five weighings and measurings become one measured volume of stock plus one weighing of hypo. Hypo is the cheap, bulky, slow-dissolving half and is best made in quantity; the hardener is the fiddly half and is made rarely.

Where the darkroom wants to choose its fixer at the tray. With a drum of hypo solution and a bottle of this stock on the shelf, the same two containers give a plain fixing bath when nothing is added and a hardening one when it is. That is a real choice for a printer, because the whole point of the toning literature’s advice is that a print destined for the toner should not meet a hardening fixer.

As the hardening bath after sulfide toning. Two parts of stock to sixteen of water, 2 to 5 minutes, fibre base, then the full wash. Kodak prints this for T-7a and for its polysulfide toner T-8, and offers its own packaged Liquid Hardener at 1 + 13 as the alternative for a reader who has not mixed the stock.

As the hardener half of Kodak’s rapid fixing bath. The 1946 handbook gives an alternative make-up for Rapid Fixing Bath F-7 built on this stock: 500 mL of water at about 50 °C, 360 g of hypo and 50 g of ammonium chloride dissolved completely, then 200 mL of F-5a and water to make a litre. One bottle, two fixing baths.

As the American counterpart of F-53 for anyone reading Kodak’s London and Rochester handbooks side by side. They are not the same formula and neither handbook mentions the other.

When another formula is preferable

• When one tray is all that is wanted, mix F-5 straight through. It is the same bath and it saves storing a litre of stock.
• When the smell of a fixing bath is the problem, F-6 and its own stock F-6a. Kodak’s reason for F-6 is specific: in warm weather and in poorly ventilated darkrooms the odour of sulfur dioxide from F-5 may be objectionable, and omitting the boric acid in favour of twice its weight of Kodalk removes almost all of it.
• When no hardening is wanted at all — a print that is going to be toned, or any material whose maker says it needs none — F-24 or a plain hypo bath, and no hardener stock in the room.
• When the hardening must happen before the fixer rather than in it, the chrome alum hardening bath: a separate operation, a different metal, and a bath that announces its own death by changing colour.
• When the source you are working from is Kodak Limited’s, F-53, which is built on glacial rather than 28 per cent acid, carries no boric acid, and is dosed at 125 mL into a litre of finished bath rather than one part in five.

Mixing

Four solids and liquids, one order, and Kodak states twice that the order is the formula. Its general instruction is that most Kodak formulas are arranged so that the ingredients are named in the order in which they should be dissolved, and that constituents must be dissolved in the proper sequence to avoid undesirable reactions.

1. 600 mL of water at about 50 °C (125 °F). Warm, because both the sulfite and the alum are sluggish in cold water and because the boric acid is close to its cold-water limit.
2. 75 g of sodium sulfite. Dissolve it completely before anything else goes in.
3. 235 mL of 28 per cent acetic acid. This is the largest single component of the bottle by volume. If you are making the acid from glacial, Kodak’s footnote is three parts of glacial acid to eight parts of water — and the acid goes into the water, never the water into the acid.
4. 37.5 g of boric acid, crystals. Not powder: Kodak footnotes that twice, in 1946 and again in 1977, because powdered boric acid dissolves only with great difficulty.
5. 75 g of potassium alum, into the mixed acid–sulfite solution.
6. Cold water to make 1,000 mL, and let it cool before it goes anywhere near hypo.

Why the alum is last. The 1928 primer gives the reason in terms, for this whole family of stocks: the alum dissolves more readily in the acid–sulfite solution than in either alone. Reverse the last two steps and the alum meets a sulfite solution that is not yet acid, which throws a white sludge of aluminium sulfite that then dissolves only with difficulty when the acid finally arrives.

Why the water is warm at the start and cold at the end. These are two different jobs. The warm water dissolves; the cold water sets the volume and brings the stock down to the temperature at which it can meet hypo safely. Kodak’s direction for use says “cool” twice in one sentence — cool stock, cool hypo — and the primer explains what happens when either is not.

🧮Why the boric acid has to be 37.5 g and not 375 g

When the boric acid goes in, the vessel holds 600 mL of water plus 235 mL of acid plus 75 g of dissolved sulfite: call it 850 mL of solution.

37.5 g ÷ 850 mL = 4.4 g per 100 mL

The figure two Kodak printings give

375 g ÷ 850 mL = 44 g per 100 mL

The figure the 2006 reprint gives

Boric acid’s published solubility is 5.6 g per 100 mL at 20 °C, and the measured curve runs 4.7 per cent by weight at 20 °C, 10.3 at 50 °C and 27.5 at boiling. Those percentages are of the whole solution, so 27.5 per cent is about 38 g of boric acid per 100 g of water — the most that will ever dissolve, in water on the boil.

27.5 ÷ (100 − 27.5) × 100 = 38 g per 100 g of water

The most a boiling solution will hold

The first figure dissolves in warm water and stays dissolved when the make-up brings it down to 3.75 g per 100 mL. The second does not dissolve at any temperature this formula could be taken to.

Two more checks agree. Diluted one part to four as Kodak directs, 375 g/L would put 75 g of boric acid into a litre of fixing bath, against the 7.5 g that F-5 itself carries and against the maximum of 15 g per litre that Kodak’s own J-1 states for boric acid in an acid fixing bath. And Kodak’s companion stock F-6a is F-5a with 75 g of Kodalk where the boric acid stands — which is exactly twice 37.5 g, and Kodak describes F-6 as F-5 with the boric acid replaced by twice its weight of Kodalk. Four independent lines of evidence, one answer.

This is the course’s reading of a printed figure, not a transcription of it, and it is recorded as such in the entry’s provenance note.

🧮One part to four, and how exactly it reproduces F-5

The stock carries 75 g of sulfite, 235 mL of 28 per cent acetic acid, 37.5 g of boric acid and 75 g of alum per litre. One part of it in five is 200 mL per litre of finished bath:

75 g × 1/5 = 15.0 g per litre

Sodium sulfite

235 mL × 1/5 = 47.0 mL per litre

Acetic acid, 28 per cent

37.5 g × 1/5 = 7.5 g per litre

Boric acid

75 g × 1/5 = 15.0 g per litre

Potassium alum

Kodak’s own F-5 in the same two handbooks reads 15.0 g, 48.0 mL, 7.5 g and 15.0 g. Three of the four are identical and the fourth is within one part in fifty, which is Kodak rounding its own arithmetic. The stock is not an approximation of the bath; it is the bath.

The hypo is the interesting half. The 1946 printing calls the other four parts “cool 30% hypo solution (2½ pounds per gallon of solution)” — 1,134 g in 3.79 L, which is 300 g per litre of solution. Eight hundred millilitres of that carries 240 g, and 240 g is exactly what F-5 specifies. J-1 restates the same parenthesis in 1977 as “300 grams of sodium thiosulfate per liter of water”, and that is not the same statement: 300 g of crystals dissolved in a litre of water makes appreciably more than a litre of solution, so 800 mL of it would carry appreciably less than 240 g. The course reads the parenthesis the 1946 way, because it is the reading that reproduces the formula Kodak says the stock makes. The later wording is recorded here so that a reader who meets it knows which way it was read.

Behaviour

In the bottle it does nothing, which is the point of it. Nothing in this stock attacks anything else in it. What it will do over weeks is leave a deposit: Kodak Limited records that acid hardener stock solutions in general tend to grow a white incrustation of basic aluminium acetate inside the container, and says to ignore it because it usually does not impair the solution.

Its one failure at the mixing stage is visible before you use it. A hardener that comes out milky, with a sludge that settles, means a relative insufficiency of acid — the acid was under strength, or too much alum or sulfite went in. That is the 1928 primer’s diagnosis and it applies to every stock of this family.

Poured the wrong way, it makes sulfur. Hypo into hardener puts thiosulfate into a locally very acid solution, and thiosulfate in acid gives colloidal sulfur. Kodak’s direction is one-directional for that reason: the stock goes into the hypo, slowly, while the hypo is stirred rapidly, with both cool.

The bath it makes is the American F-5, whose published fixing times moved. The 1946 handbook gives films and plates 10 to 20 minutes in a fresh bath and a discard point above 20; J-1 in 1977 gives 5 to 10 minutes and a discard point above 10. Both give prints 5 to 10 minutes. The course’s F-5 page prints Kodak Limited’s London formula and its own ten-minute figure, which is a third statement again. Read the times off the printing you are following and do not average them.

Diluted with water alone it is a mild, short bath. Two parts in eighteen leaves 8.3 g/L of alum, 8.3 g/L of sulfite, 4.2 g/L of boric acid and about 26 mL/L of 28 per cent acid, and the print is in it for 2 to 5 minutes. Kodak states plainly that it does not change the colour or the gradation of the toned print.

Image characteristics

None of its own — it never touches a negative or a print except as a fifth of a fixing bath or a ninth of a hardening bath.

What it contributes to a fixing bath belongs on F-5’s page: development stopped on contact, gelatin tanned so a warm wet emulsion survives handling and washing, and a bath whose life depends on how much developer is carried into it.

The one image consequence that belongs here is what happens when the stock is mis-measured, because a stock concentrates that risk into a single cylinder. Too little and the fixing bath sludges early and stops hardening; too much and the emulsion can be over-hardened to brittleness, and — the reason the toning literature cares — made less receptive to a toner. Five weighings distribute an error; one measurement multiplies it by five.

The mechanism

Kodak states the division of labour in a fixing bath in one paragraph, and every ingredient of this stock is in it: the alum is added to harden the gelatin of the emulsion in order to prevent excessive swelling or softening in the wash water; the acid serves to provide the best conditions for efficient hardening action; and the sodium sulfite is necessary to prevent the hypo from being decomposed by the acid.

⚗Three ingredients that must not meet in the wrong order

Sulfite and acetic acid make 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

The alum needs that pH before it arrives. In a sulfite solution that is not yet acid, aluminium(III) comes down as aluminium sulfite — the white sludge — and redissolves slowly. In an acid–sulfite solution it stays dissolved and can be carried into a fixing bath, where it cross-links gelatin.

And the whole bottle is a delivery system for what must not happen too early. Acid straight into hypo gives sulfur:

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

Why the stock goes into the hypo slowly, stirred, and cold

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

What the boric acid adds, and where the answer stops. Kodak’s claim is specific and this page repeats only that much: boric acid added to a bath of this type increases the hardening power of the alum and also helps to prevent the formation of aluminum sulfite sludge. The boric acid page sets out what is established underneath that claim — with a pK_a of 9.27 it is about three thousand times weaker than acetic acid, so it adds a large reservoir of undissociated acid while barely moving the pH — and states the limit: no source read for this course explains by what mechanism boric acid specifically strengthens an alum bath. Treat the buffering as well supported and the hardening claim as Kodak’s, unexplained.

⚗Why hardening and long life pull against each other

The 1928 primer measured the trade-off that governs every alum fixer, and it is the reason a hardener stock’s proportions cannot be improved by eye.

With a fixed quantity of alum, hardening rises as acetic acid is added, reaches a maximum, and then falls away until the bath does not harden at all. But the minimum quantity of acid needed to give the bath a long useful life before aluminium sulfite precipitates is usually greater than the quantity that hardens best.

So a long-lived alum fixing bath is a slightly worse hardener than a short-lived one, and the primer’s practical consequence follows: with a bath containing a minimum of acid, add about half as much acid again as it originally contained as soon as a slight precipitate appears. F-5a’s 235 mL of 28 per cent acid per litre is Kodak’s chosen point on that curve, not an arbitrary quantity.

Function of every ingredient

Sodium sulfite, 75 g in a litre. The preservative, and in a hardener stock specifically the reserve of base that the acetic acid attacks in preference to the thiosulfate once the two are in one tray. It also has a mechanical job during the mixing: it is the solution the alum has to dissolve in, and it must be completely dissolved before the acid arrives or the acid finds solid sulfite rather than sulfite ion. More lengthens the finished bath’s life against sulfurisation and shortens it against the aluminium sulfite sludge, because sulfite is half of that precipitate. Less, or an old tub that has oxidised to sulfate, is one of the primer’s three named causes of a sulfur precipitate. At 75 g in 600 mL of warm water it is well inside the published solubility of 22 g per 100 mL at 20 °C, which is why it dissolves before anything else needs it to.

Acetic acid, 235 mL of a 28 per cent solution, and by volume the largest thing in the bottle. Two jobs. In the finished fixing bath it neutralises the alkali carried in on the film or print and so arrests development on contact; in this bottle, and before that, it sets the pH at which aluminium(III) will stay in solution instead of sludging. More gives a bath with a longer sludging life and worse hardening; less gives better hardening and earlier sludging, and past a point a milky stock. Note the strength: the formula wants 28 per cent, made by Kodak’s footnote from three parts of glacial acid to eight of water. Substituting glacial acid volume for volume would put roughly three and a half times the acid into the bottle, and the primer names the opposite mistake — 28 per cent used where glacial was meant — as a classic cause of an aluminium sulfite sludge.

Boric acid, 37.5 g in a litre, five times the 7.5 g the diluted bath will carry. This is the ingredient that separates F-5 from the older acid hardening baths such as F-1a’s, and Kodak names two benefits for it: better hardening, and less tendency to precipitate aluminium sulfite. Mechanistically what is established is that it is a very weak acid in quantity — capacity without strength — so it replaces hydrogen ions as carried-over developer consumes them without ever making the bath acid enough to attack thiosulfate. More is not free and Kodak sets the ceiling itself: above about 15 g per litre in the bath, boric acid stops being the answer and the surplus is just a solid the wash water has to deal with. Less, and the bath’s pH leaves its window sooner. The form is specified and repeated in every printing: crystals, not powder, because powdered boric acid dissolves only with great difficulty. This ingredient is also the main reason this page carries a safety level rather than none.

Potassium alum, 75 g in a litre, the fine granular dodecahydrate. The hardener, and the reason the formula has the name it has. It supplies the aluminium(III) that tans gelatin, raising the temperature at which the layer softens so that a film can be washed in running water for an hour, or a fibre print for thirty minutes, without the emulsion leaving its support. More hardens more and eventually too much, giving brittleness and — in a print about to be toned — an emulsion the toner cannot get into. Less gives frilling and reticulation in a warm wash. The granular form is specified because lump alum in a cool acid–sulfite solution is a long wait. Potash alum rather than chrome alum is a deliberate choice on keeping grounds: a chrome alum bath loses its hardening power rapidly whether it is used or not, so potash alum is preferred wherever a bath will be in service for a long period.

Water, at two temperatures and two points. 600 mL at about 50 °C to dissolve four things that would be reluctant cold, then cold water to make 1,000 mL. The cold water is not a formality: the stock has to be cool when it meets hypo, and finishing the make-up cold is how it gets there.

The hypo is not in this formula, which is the whole architectural point. The four parts the stock goes into are the reader’s own 30 per cent hypo solution, and the bottle on the shelf is indifferent to whether they are ever made.

Interactions

With hypo, one part to four, both cool, stock into hypo. That is the formula’s entire external behaviour as a fixer, and every way of getting it wrong — warm, fast, or hypo poured into hardener — gives sulfur. Kodak’s wording puts the agitation on the receiving vessel: stir the hypo rapidly.

With undissolved hypo. The primer’s warning is specific and easy to trip over when a large batch of hypo has been made in a hurry: if the hypo is not thoroughly dissolved before the hardener is added, a sulfur precipitate is likely. A few crystals on the bottom of the tank are enough.

With carried-over developer, which is what consumes it. Every sheet brings alkali into the bath, and that alkali is spent against the acetic acid first and the boric acid reserve after it. When both are gone the alum and the sulfite are still there, and they combine as aluminium sulfite. Kodak’s remedy for the general case is a stop bath such as SB-1 or an acid rinse rather than a plain water rinse; the primer’s remedy once a slight precipitate has appeared is more acid, not more of this stock.

With carbonate developers and a poor rinse, at the print. Carbonate meeting acid evolves carbon dioxide inside a soft emulsion, which is the primer’s account of blisters. The hardener is what makes the gelatin able to resist that, and an insufficient rinse is what makes the gas.

With a toner, indirectly and importantly. G-23’s advice is that a hardening fixer is not recommended for prints intended for toning, because it makes the paper emulsion less receptive to the toner solution. That is why the same publication then sends the reader here after the toning is finished. The course reads the sequence as a deliberate rearrangement rather than a contradiction: the hardening a fibre print needs before an hour in the wash is given at the end instead of the beginning, so that the toner meets an unhardened emulsion and the wash meets a hardened one. Kodak states the two halves of that and does not join them up; the joining is this course’s reading.

With sodium bisulfite or hypo clearing agent in the toning sequence. J-1 puts one of those between the polysulfide toner and this hardening bath. Neither is in the formula and neither interferes with it; the order is the instruction.

Variants

F-5 mixed directly, without a stock. Kodak prints both on the same page and plainly regards them as the same bath; the arithmetic above shows they are, to within a millilitre of acid in forty-eight. The course’s F-5 page prints Kodak Limited’s London formula, which uses 17 mL of glacial acid where the American F-5 uses 48 mL of 28 per cent. Read against Kodak’s own footnote — 28 per cent acid being three parts of glacial to eight of water — 48 mL of it carries about 13 mL of glacial acid, so the London bath carries roughly a third more acid than the American one. That comparison is the course’s arithmetic on two published formulas, not a statement either handbook makes, and it is enough to say that the two F-5s are near relatives rather than the same solution.

Kodak Hardener F-6a, printed on the facing page of the 1946 handbook, is F-5a with 75 g of Kodalk standing exactly where the 37.5 g of boric acid stands, everything else unchanged. It makes F-6, the odourless bath, and Kodak’s reason for it is ventilation: the smell of sulfur dioxide from F-5 can be objectionable in warm weather and in a poorly ventilated darkroom.

Rapid Fixing Bath F-7 built on this stock. 500 mL of water at about 50 °C, 360 g of hypo, 50 g of ammonium chloride dissolved completely, then 200 mL of F-5a and water to make a litre. Kodak warns that with a rapid fixing bath the fixing time must not be prolonged for fine-grained emulsions or for any paper print, because the image may bleach — particularly with warm-tone papers.

F-1a, the older American hardener stock, is the direct ancestor: 120 g/L of sulfite, 375 mL/L of 28 per cent acid and 120 g/L of alum, no boric acid, dosed one part in nine into a 25 per cent hypo solution. F-5a is weaker in all three and adds the fourth ingredient. Kodak’s own claim for the change is on the F-5 entry: better hardening and less sludging than the older type of bath.

F-53, Kodak Limited’s stock, is the London answer to the same problem — glacial acid, no boric acid, a different alum-to-acid ratio, and 125 mL into a litre of bath.

No course variant is offered. The proportions of a stock are only meaningful against the dilution that follows them, and weakening one half without the other would produce a bath Kodak never published.

Safety

Level B, and the level is set by two of the four ingredients.

235 mL of 28 per cent acetic acid is the bulk of the bottle. At that strength it is corrosive enough to require gloves and eye protection, and the glacial acid Kodak’s footnote sends you to in order to make it is considerably worse. 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. If you must dilute glacial acid, acid into water, never water into acid.

37.5 g/L of boric acid is five times the concentration F-5 carries, and boric acid’s one agreed hazard class is reproductive toxicity — the aggregated GHS classification is H360, may damage fertility or the unborn child, with no acute toxicity or corrosivity entry beside it. The boric acid page carries the classification, the notification counts behind it and the safety card’s additional observations. The practical consequences here are ordinary and non-negotiable: weigh it with the dust down, do not eat or drink in the mixing area, and treat this bottle as one that a person who is pregnant or trying to conceive should not be mixing.

The solids are otherwise the mild half. Neither sodium sulfite nor potassium alum carries a serious agreed classification; the alum page records the workplace exposure limit for soluble aluminium, and the sulfite’s own hazard is what it gives off in acid.

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

The diluted hardening bath is much milder than the stock — roughly a ninth of everything — but it is still an acid bath containing boric acid, and it is used on prints that are handled wet. Gloves, and tongs dedicated to it.

Storage

In small bottles rather than one large one. Kodak’s own advice about stock solutions applies here directly: when stock is drawn from a large bottle the air space increases each time, and it is best to store stock solutions in small bottles — leaving a small air space so the solution volume, varying with temperature, does not loosen the stopper or burst the bottle.

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

No printing of this formula publishes a keeping time or a capacity, and none is stated here. What can be said from the sources is that the stock arrangement exists because the two halves keep better apart than together, so a dated bottle and a note of when it was last drawn from 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 somebody who finds it will most need to know. Store it away from anywhere food is handled: the boric acid is the reason.

Incompatibilities

Alkalis and carbonates, which it will neutralise noisily and which will spend the acid the formula is counting on. Boric acid in particular is converted to a borate by carbonates and hydroxides, which destroys the acid reserve.

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

Potassium ferricyanide baths and their waste. Acid on ferricyanide is the documented route to hydrogen cyanide, and this is a bottle of acid. It must never stand near a Farmer’s reducer tray, a rehalogenating bleach, or their waste containers — which matters here more than on most hardener pages, because the toning sequence this stock ends puts a ferricyanide bleach in the same room.

Sulfides. Acid plus a sulfide gives hydrogen sulfide, and the sulfide toner that this bath follows is exactly such a bath. The 2 + 16 hardening bath comes after a thorough rinse for that reason, and its tray is not the toner’s tray.

Metals. Kodak’s own instruction is that aluminium, zinc and galvanized iron are not used with developers or fixing baths, and boric acid attacks metals with the evolution of hydrogen. Glass, hard rubber, polyethylene, enamelled steel or stainless steel are the materials Kodak names for photographic solutions.

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

Waste

Unused stock is an acid waste, not a silver one, and 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 the silver-bearing waste SOP describes; recovery comes before anything else.

The spent 2 + 16 hardening bath is neither. It has never held thiosulfate and never touched undeveloped halide, so it is not a silver waste; what it is is a dilute acid solution containing aluminium and borate. Borate is the reason it is not a sink disposal by default: it is persistent, the related borax is recorded as harmful to aquatic organisms, and boric acid is a registered pesticide. It does not go on the garden.

Bottle the three streams separately, label each with what is in it, and take the routes the general chemical waste SOP and the disposal ruling set out. What any of them may lawfully do at the end of that route is decided by local regulation, which this course cannot state for you.

Troubleshooting

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

A white sludge appeared when the alum went in. The acid–sulfite 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.

Undissolved crystals on the bottom that will not go. Almost always the boric acid, and almost always because powdered boric acid was used instead of crystals, or because the water had cooled too far before it went in. Kodak footnotes the powder problem twice. Warm the solution gently 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. The primer adds a fourth cause worth knowing in a hot darkroom: a bath above 29 °C will not stay clear for more than a few days even when it was mixed correctly.

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

The finished bath sludges after only a few sheets. Not the stock’s fault: the acid is being consumed by developer carried in. Use a stop bath or an acid rinse rather than a water rinse, and read the capacity figures on F-5’s page, which move by a large factor on that variable alone.

Blisters on prints coming out of the fixer. Carbon dioxide from carbonate developer meeting the acid inside a gelatin layer too soft to resist it. Rinse better, and check that the hardener went in at all.

The toned print will not take the toner evenly, and it was hardened first. That is the sequence inverted. The hardening bath belongs after the toner, not before it, and a print already fixed in a hardening fixer should skip this bath entirely — which is what Kodak’s own footnote instructs.

Experiments

Mix the same fixing bath both ways and compare them. One tray of F-5 mixed straight through, one of four parts 30 per cent hypo to one of this stock. Fix identical strips, measure the temperature at which the gelatin lets go, and run both to exhaustion counting sheets. The arithmetic above predicts a difference of about two per cent in the acid and nothing else; whether two 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 appear. Then add the acid and time how long it takes to clear. This is the fastest way to understand why Kodak states the order twice on one page and why it has not changed in eighty years.

Test the boric acid reading yourself. Weigh 37.5 g of boric acid crystals into 850 mL of water at 50 °C and watch it dissolve; then try 375 g in the same volume at the same temperature. The second is a demonstration, not a formula, and it is the cheapest possible check on a printed figure. Discard both as borate waste.

Titrate the acid reserve. Make the stock, dilute one part in five with water rather than hypo, and titrate against a standard alkali to find how much base the bath will absorb before its pH collapses. Then make the same bath with the boric acid omitted and repeat. Kodak claims boric acid increases the hardening power of the alum and reduces sludging; the acid-capacity half of that claim is measurable in a home laboratory, and the corpus contains no published number for it.

Time the toning hardener. Tone two identical fibre prints in T-7a, give one the 2 + 16 bath for 5 minutes and the other none, wash both for thirty minutes and compare them for surface damage, for wet handling strength and for image colour. Kodak states the bath does not change the colour or gradation; that is a claim a ring-around can check.

Compare the three hardener stocks. F-5a, F-1a and F-53, each dosed into hypo at its own published rate, measured for hardening and for sludging life. Three houses, two continents, thirty years apart, and no published comparison anywhere in this course’s corpus.