Chrome alum

Chrome alum contains no aluminium, which its name does nothing to suggest, and the chromium in it is chromium(III), which matters more than anything else on this page. It is the strong hardener of the period formulary: three times as effective as potash alum by weight, and correspondingly harder to keep alive in a working bath.

In photography

A separate hardening bath. Kodak’s 1928 primer gives SB-3 as one line — 30 g of potassium chrome alum per litre of water — to be used after development and before fixation in hot weather, either in conjunction with the F-1 alum fixer or when the chrome alum fixer F-16 does not harden enough. The instruction that follows is the interesting part: agitate the negatives for a few seconds when they first go in, or a chromium scum forms. Where the bath is used, the water rinse may be skipped, although a few seconds of rinse still prolongs the hardener’s life.

A hardening fixer, briefly. F-16 puts the same salt into the fixing bath: solution A is 960 g of hypo with 60 g of sodium sulfite made to 3 litres, solution B is 60 g of chrome alum with 8 cm³ of concentrated sulfuric acid in 1 litre, and B is poured slowly into A while A is stirred. Kodak recommends it for hot weather and immediately undercuts the recommendation: it loses its hardening properties in a few days with or without use, so it should be used as soon as possible after mixing. Wall’s 1924 acid-chrome alum bath is the same idea at 20 g of chrome alum per litre, added last to a hypo and acid-sulfite solution.

Subbing, and the plate that stayed on its glass. The Image Permanence Institute’s account of the gelatin dry plate records that the glass support was coated with a substratum — a thin layer of gelatin hardened with a chrome alum solution — to receive the binder and prevent emulsion blisters, and that after development the binder itself was hardened with a potassium or chrome alum solution before fixing. The Getty Conservation Institute’s atlas finds the residue of that practice by X-ray fluorescence: small amounts of chromium detected in printing-out silver gelatin photographs are attributed to chromium alum used to harden the gelatin binder of the baryta layer. A hardener chosen in 1885 is still identifiable in the object in 2013.

The measure of its strength. Wall’s 1924 table Alums as Hardening Agents gives the weights that 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, anhydrous aluminium chloride 1.6 and chrome alum 2. Reilly puts the same point qualitatively from the gelatin side — additions of alum to gelatin solutions give gels that are harder and less permeable, while adding potassium chrome alum gives gels that are completely insoluble. Wall adds that basic chrome alum is more effective still, and gives its preparation: dissolve 100 g of chrome alum in 800 cm³ of hot water, add ammonia until a slight permanent precipitate persists after stirring, filter, and make up to a litre.

⚗The chemistry

Two colour changes on this page are diagnostic, and both are the chromium telling you what it has become. A cold solution of chrome alum is violet; Kodak records that it goes green on heating, “owing to the change in the composition of the salt” — the coordination around the chromium(III) centre changing, and with it the light it absorbs. In a working hardening bath the same signal runs slower: the bath is blue by artificial light when freshly mixed and turns yellowish-green with use, and Kodak’s rule is flat — it then ceases to harden and should be replaced.

The scum is the third chromium species. Alkaline developer carried in on the film precipitates chromium hydroxide at the emulsion surface, and once the film has dried that deposit cannot be removed. The course could not verify from a source meeting its standard how the chromium(III) ion binds gelatin at the molecular level; the period literature calls it tanning and stops, and this page stops there too.

Properties

Formula and mass. Kodak and Wall write the alums as double sulfates with twenty-four waters, K₂SO₄·Cr₂(SO₄)₃·24H₂O; PubChem’s record is the halved form, KCr(SO₄)₂·12H₂O, relative molecular mass 499.4. They are the same crystal. The anhydrous salt, which is what the ECHA notifications are filed against, has a relative molecular mass of only 283.22, so 1 g of the anhydrous salt would carry the chromium of 1.76 g of the crystals. No photographic formula in this course’s sources asks for the anhydrous salt, and every weight quoted here is of the dodecahydrate.

Form and solubility. Violet-red to black crystals; 1 part dissolves in 10 of cold water, and it is insoluble in alcohol. Wall notes that the solution looks purple by reflected light and reddish by transmitted light, which is a useful check that what is in the bottle is what the label says.

pH behaviour. Like every alum, acid in solution through hydrolysis of the trivalent metal ion, which is why the fixing bath built around it takes sulfuric acid rather than needing an alkali.

Stability. This is where it differs sharply from potash alum. A plain aqueous solution keeps its hardening power indefinitely. Add sulfite and it does not: Kodak’s 1924 and 1928 primers both record that a chrome alum solution loses hardening power in the presence of sodium sulfite at a rate that depends on the concentrations of both, that a 1 to 2 per cent chrome alum fixing bath is useful only immediately after mixing, and that a 5 to 10 per cent one holds for two or three days. Even a plain hardening bath falls off in use, because developer carried over on the plates brings sulfite with it. The 1928 primer draws the practical conclusion: for long periods of use a potash alum fixing bath is to be preferred.

Handling

The aggregated ECHA notifications give Warning, the GHS07 irritant pictogram, and three statements: skin irritation and serious eye irritation, each notified by 100 per cent of reports, and respiratory irritation by 84.9 per cent. They are filed against the anhydrous salt because the hydrate’s PubChem record carries no GHS section at all, and the note in this page’s hazard block says so.

What is not there is as important. This is chromium(III), not chromium(VI). HSE’s EH40 sets chromium(III) compounds, measured as chromium, at 0.5 mg per cubic metre over eight hours, and chromium(VI) compounds at 0.01 mg per cubic metre, fifty times lower, with the Carc, sen and BMGV notations attached — carcinogen, respiratory sensitiser, and a biological monitoring guidance value. Chrome alum carries none of those notations and none of the corresponding hazard statements. Dichromate is a different substance with a different page, and the two are not interchangeable in an assessment because they share a metal.

Why Level B. Against the course rubric, the classification itself would sit inside Level A: irritant is the ceiling Level A allows. Two things move it up. Level B names “fine powders that must not be inhaled”, and a salt notified for respiratory irritation by 84.9 per cent of reports, with a workplace exposure limit expressed in milligrams of dust, is exactly that; and Level B’s control of chemical splash goggles rather than glasses is what a 100 per cent notification of serious eye damage or irritation calls for. The waste is the third consideration: a chromium-bearing solution is not rinse water, and Level A’s waste criterion does not cover it. It does not reach Level C, whose chromium trigger is explicitly chromium(VI) compounds.

⚠Safety

Goggles, gloves and a particulate mask for weighing; no hurried pouring of the crystals from a height. Mix F-16 in the order Kodak gives, and add the sulfuric acid slowly down the side of the stirred vessel — never water into the acid. Keep the salt away from sulfite solutions, which destroy the hardening you are paying for, and keep the bath away from carried-over developer, which deposits a chromium scum you cannot remove after drying. Collect the used solution; it does not belong in a drain.

Waste

The stream is a chromium(III) solution, usually with sulfate and acid in it and, where the salt was used in a fixing bath rather than a separate hardener, with dissolved silver as well. Recover the silver where there is any, then bottle and label the chromium-bearing residue on its own.

How that residue classifies depends on which chromium it contains, and the Environment Agency’s WM3 guidance shows the machinery: its list of waste defines a “heavy metal” as a compound of a named set including chromium(VI), while chromium in general appears instead in the separate definition of “transition metals”, and mandatory group entries exist for chromium compounds where no substance-specific entry applies. Nothing about that assessment is done by eye at a sink. Route it through a household hazardous waste service — the UK government publishes a finder for the local one, and ILFORD’s guidance for domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern.

History

Chrome alum arrives in photography with the gelatin dry plate and for the dry plate’s own problem. Wall’s 1912 dictionary gives its use in one clause — hardening gelatine, “for example, in the preparation of emulsions for dry plates to prevent frilling” — frilling being the emulsion lifting from the edges of the plate in the wet baths. The substratum described by the Image Permanence Institute is the same answer applied one layer down: gelatin hardened with chrome alum, laid on the glass so that the emulsion has something to hold on to.

By 1924 Wall could write that separate hardening baths had been made largely obsolete by chrome alum fixing baths, and by 1928 Kodak was explaining why that had not lasted either — the sulfite in the fixer kills the hardener within days, so the potash alum bath, weaker but stable, is the one to use for a working darkroom. That is the whole arc: the better hardener lost to the better keeper. What remains of it is in the objects, where the Getty’s X-ray fluorescence still picks the chromium out of a baryta layer coated more than a century ago.