Potassium alum

A fixing bath does its real work on the silver. Alum is in it for the gelatin: it is the ingredient that stops the swollen, warm, half-dissolved binder from lifting off the base, and the reason a 1930s negative could be washed for an hour in summer and survive it.

In photography

The hardening fixer. Kodak’s 1928 primer names it outright — the commonest hardening agent is potassium alum, the alums having the property of tanning gelatin. The acid hardening bath F-1 is 480 g of hypo made to 2 litres, to which a separately made hardener is added: 30 g of sodium sulfite, 96 cm³ of 28 per cent acetic acid and 30 g of powdered potassium alum in 160 cm³ of water at about 52 °C. The deep-tank bath F-14 uses 60 g of alum per 4 litres of bath, the motion-picture bath F-2 only 24 g. The stock hardener F-1a carries 480 g of alum with 480 g of sulfite and 1,500 cm³ of the same acid in 4 litres, to be added one part to eight of a 25 per cent hypo solution.

What hardening buys, in a number. The primer does not leave “hardening” as an adjective. Baths such as F-1, it says, are compounded to give a hardening of 54 to 77 °C, and it gives the test: fix and wash a strip of film, immerse it in water, and heat the water slowly until the gelatin flows away from the support. That temperature is the property being bought. Below it the emulsion melts off the base; above it the coating stays put while the tray sits in a warm darkroom.

More and less. A minimum quantity of alum is needed for that hardening, and an excess overshoots into brittleness. The acid is entangled with it in a way worth knowing: 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. Meanwhile the minimum acid needed to give the bath a long life before it precipitates aluminium sulfite is usually more than the quantity that hardens best. A long-lived alum fixer is therefore a slightly worse hardener than a short-lived one, and the primer records the consequence: in use the hardening first rises as developer is carried in and neutralises acid, then falls away rapidly.

⚗The chemistry

Alum dissolved in water is not neutral. Mike Ware writes the hydrolysis of the hydrated aluminium ion as the source of the acidity:

[Al(H₂O)₆]³⁺ → [Al(H₂O)₅(OH)]²⁺ + H⁺

Why an alum solution is weakly acid

That liberated hydrogen ion is what a plain hypo bath cannot tolerate, because it converts thiosulfate to colloidal sulfur:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Acid attacking thiosulfate

And when the acid runs out instead, the aluminium and the sulfite find each other:

2 Al³⁺ + 3 SO₃²⁻ → Al₂(SO₃)₃

The white sludge that ends an alum fixer

Kodak’s primer names that white gelatinous precipitate as aluminium sulfite and treats its appearance as the end of the bath’s useful life. What the course could not verify from a source meeting its standard is the molecular account of the bond between aluminium and gelatin itself. That the effect is real is well attested — Reilly states that additions of alum to gelatin solutions give gels that are harder and less permeable — but the period sources call it “tanning” and leave it there.

A toner, by way of that acidity. Kodak’s G-23 gives Hypo Alum Sepia Toner T-1a: 480 g of sodium thiosulfate pentahydrate in 2.8 litres, to which 120 g of potassium alum dissolved in 640 cm³ of water at 70 °C is added, then 4 g each of silver nitrate and sodium chloride in 64 cm³ of water, the whole made up to 4 litres and worked at 49 °C for 12 to 15 minutes. The 1928 primer explains the mechanism: a hypo-alum bath held at the point where it is about to precipitate sulfur may be considered as carrying free sulfur in solution, and prints immersed in it at about 49 °C have their image converted directly to silver sulfide, with no bleaching step. It gives the working range as 49 to 52 °C and warns that much above 54 °C the print blisters and the image bleaches, and it explains the third solution: a fresh bath weakens the print by eating out the highlights unless a little silver is added, preferably as silver chloride — which is what the silver nitrate and the sodium chloride make when they meet. Here the weak acidity that ruins a fixer is the whole point of the formula, and a used bath works better than a fresh one.

Clearing, and paper. Wall’s 1912 dictionary gives potash alum for rendering gelatine films less liable to mechanical injury and for clearing them of stains, and adds that its recommendation as a hypo eliminator is of doubtful benefit — a nineteenth-century claim already being marked down in a period reference book. His 1924 formulary prints a clearing bath of 200 g of alum with 65 g of citric acid per litre, and a 10 per cent alum bath among the hardening baths for hot weather, while noting that acid chrome alum fixing baths had made such separate baths largely obsolete.

Relative potency. Wall’s 1924 table Alums as Hardening Agents gives the weights that will render 100 parts of dry gelatine insoluble in hot water: potash alum 6 parts, ammonia alum 5.6, aluminium nitrate 4.5, aluminium sulfate 4.2, chrome alum 2 and anhydrous aluminium chloride 1.6. Potash alum is the weakest of the six by weight, and chrome alum three times as efficient — which is why chrome alum turns up wherever hardening had to be fast, and potash alum wherever the bath had to last.

Properties

One substance, two formulae. Kodak and Wall both write the alums as a double sulfate with twenty-four waters, R₂SO₄·M₂(SO₄)₃·24H₂O, where R is an alkali metal or ammonium and M is aluminium or chromium. PubChem’s record for the same material is the halved form, KAl(SO₄)₂·12H₂O, with a relative molecular mass of 474.4. They describe the same crystal; the period formula is simply doubled, and a period weight given against it is still a weight of this salt.

Form and solubility. Colourless or white, odourless, and soluble in water. Kodak notes that the potassium and ammonium alums crystallise in large clear crystals but are sold as very fine ones, to make weighing easier and to stop the caking that powdered alum suffers. The course could not verify a numerical solubility from a source meeting its standard: the line in Wall’s 1912 entry that carries one has been destroyed by optical character recognition in the copy available here, and rather than guess at the digits the page leaves the gap open.

pH behaviour. Weakly acid in solution, by the hydrolysis above, and that acidity is the whole of its behaviour towards thiosulfate and towards sulfite. Stability. Dry and closed it keeps indefinitely. A made-up acid hardening fixer does not: its life ends when the acid has been neutralised by carried-over developer and the aluminium sulfite sludge appears.

Handling

The aggregated ECHA notifications held by PubChem carry no classification: 43 of 43 reports say the substance does not meet GHS hazard criteria, and none of the notifications supplies a hazard code. That is a thin evidence base, and it is not the end of the assessment. HSE’s EH40 lists soluble aluminium salts at 2 mg per cubic metre averaged over eight hours, with no short-term limit — a figure that exists precisely because a soluble aluminium salt is a powder someone weighs out. EH40 also states that absence from its list does not indicate that a substance is without risk.

Why Level A. Against the course rubric, potassium alum sits inside Level A on every criterion that applies. There is no classification above the rubric’s ceiling of irritant, harmful if swallowed or corrosive; nothing is heated above 50 °C in a fixing bath; and the waste is silver-bearing fixer, which is the stream Level A anticipates. Two operations on this page step up. Hypo-alum toning is Level B, because the rubric puts the holding of solutions above 50 °C there and Kodak’s working range for this bath, 49 to 52 °C, crosses that line in an open tray. And mixing a concentrated acid hardener from acetic acid is a Level B operation for the acid, not for the alum.

⚠Safety

Gloves and eye protection for solutions; goggles and unhurried weighing for the crystals, against EH40’s dust limit. The order of mixing is a safety instruction and not only a recipe: dissolve the sulfite first, add the acid, then the alum, and add the finished hardener slowly to a cool, fully dissolved hypo solution while stirring. Reverse it and you get either a sludge of aluminium sulfite or, if the hypo is warm or undissolved, a bath precipitating sulfur. Never acidify a hypo bath by hand to “help” the hardener along.

Waste

The stream is spent hardening fixer: dissolved silver as the thiosulfate complex, plus aluminium, sulfite and acetate from the hardener. Recovery comes first, because the silver is the part with value and the part that matters environmentally; the aluminium rides along with it. Kodak’s J-300 guidance for amateurs states that septic systems are not designed to treat photographic processing solutions and lists household hazardous waste collection among the routes for used fixer; ILFORD tells domestic users in the UK to bottle wastes separately, label them and take them to a household waste and recycling centre, and the UK government publishes a finder for the local service. A hypo-alum toner bath is the same stream with more aluminium and added silver in it. Check your local regulations; they govern.

History

Alum reached photography already old. PubChem’s ChEBI description lists its roles as mordant, astringent and flame retardant, all of them older than the camera, and all of them the same underlying behaviour — a trivalent metal ion binding a protein or a fibre.

It was in the paper before it was in the fixer. Ware records that British photographic papers of the 1850s were sized with gelatin hardened by an acidic solution of alum, while French papers used an alkaline starch-and-resin size, and that the difference decided which process each suited: French sheets for the waxed paper process, British for the standard calotype recipes. The practice survives in papermaking, where Ware notes that a hardener such as alum is usually incorporated in the papermakers’ sizing bath for tub-sized sheets.

Its darkroom career ran from the acid hardening fixer of the 1900s to the end of the twentieth century, and it is over. ILFORD’s current sheet for HYPAM states that modern camera films are sufficiently hardened when manufactured, that the general use of a fix hardening agent is no longer recommended, and that for paper processing it is not recommended at all — leaving four cases where it still is: process temperatures above 30 °C, poor drying performance, a need for shorter drying times, and a risk of physical damage such as a roller transport processor. That list is the 1928 primer’s hot-weather problem, still on the sheet, now confined to a corner of it.