Fixing dissolves the undeveloped silver halides left in an emulsion after development, converting them into soluble complexes that the wash carries away. A fixer that looks clear and clears film in seconds can still leave a fibre-based print that yellows and fades within a few years, because the silver complexes it forms change character as the bath fills with dissolved silver. On paper, that change is the whole game, and a single bath cannot win it for long.

Paper Is the Hard Case, Film Is Not

The same litre of fixer behaves very differently depending on what you put through it, because the materials hold dissolved silver to wildly different degrees. Ilford’s technical data for Rapid Fixer puts the workable silver concentration of a film bath at 8-10 g/L before fixation suffers. Resin-coated paper tolerates 4-6 g/L, since its polythene base is sealed on both sides and the emulsion is a thin layer that releases its complexes readily. Fibre-based paper is the outlier: for high commercial permanence the bath must stay below 2 g/L, and for maximum long-term storage stability below 0.5 g/L.

The reason is the support. Film gelatin and the polythene of RC stock do not soak up the bath; the baryta-coated paper fibre underneath an FB emulsion does. It absorbs the fixing solution and holds the silver-thiosulphate complexes within the fibre, where they are slow to wash out and quick to decompose. Two-bath fixing exists to solve a problem that is almost entirely a paper problem.

What Happens Chemically as a Bath Ages

The fixing agent in Ilford Rapid Fixer is ammonium thiosulphate; it is a rapid fixer and contains no sodium thiosulphate (plain hypo). When the bath is fresh and thiosulphate is abundant, each dissolved silver ion picks up two or three thiosulphate ligands to form the highly soluble di- and tri-complexes, Ag(S₂O₃)₂³⁻ and Ag(S₂O₃)₃⁵⁻. These wash out cleanly.

As prints accumulate, dissolved silver rises and free thiosulphate is consumed. The equilibrium shifts toward the sparingly soluble mono-thiosulphate complex, AgS₂O₃⁻, which adsorbs onto paper fibre rather than dissolving away. Whatever is left behind — trapped mono-complex plus residual thiosulphate — eventually decomposes to silver sulphide, Ag₂S, the yellow-brown stain that conservators recognise as a failed fixing or wash. The conservation chemist James M. Reilly, founder of the Image Permanence Institute, frames the practical consequence in Care and Identification of 19th-Century Photographic Prints (Eastman Kodak, 1986): a bath approaching exhaustion forms complexes that cannot be washed out, so the first bath “does the bulk of the complexing, while the second insures that the complexes ultimately formed can be washed out.”

The Method, With Real Numbers

Ilford’s fresh-fixer minimum times at 20°C are short: general-purpose film 2-5 min at 1+4; RC paper 30 s at 1+4 or 1 min at 1+9; FB paper 1 min at 1+4 or 2 min at 1+9. Two-bath fixing splits that time across two baths of equal volume. Fix for half the recommended time in bath one, then transfer to bath two for the remainder. The finishing bath is always relatively fresh, so the last complexes a print forms are the soluble di- and tri-species.

A worked sequence for FB paper with Rapid Fixer at 1+4: bath one for 30 s, bath two for 30 s. Run prints through until bath one reaches your silver limit — about 40 8×10in sheets per litre for the 2 g/L commercial threshold, or about 10 sheets per litre for the 0.5 g/L maximum-permanence threshold. At that point discard bath one, promote bath two to first position, and mix a fresh second bath. The discipline is the point: throughput counting is only a guide, because how much silver a print dumps depends on how much of it is unexposed (a high-key print loads the bath far more than a near-black one), so treat the count as conservative and round down.

This is not the “two-bath” of Ansel Adams’ The Print (1983), which is a different procedure: an acid-hardener fixer for around three minutes, a rinse, then a second bath of plain near-neutral hypo for around three minutes to clear the acid before selenium toning. That sequence manages pH for toning; the Ilford method extends capacity and guarantees clean complexing. Do not conflate them.

Knowing When a Bath Is Done — and Testing the Result

For film, use the clearing-time test: fixing time should be twice the time a leader takes to clear, and you retire the bath when clearing time in used fixer exceeds twice the clearing time in fresh fixer (fresh film typically clears in 30-60 s). Paper gives no such visible signal, so test the finished, washed print. The Ilford residual-silver test dissolves 2 g sodium sulphide in 125 ml water, then dilutes that stock 1+9 for use; a drop on a washed white border should leave a barely visible cream tint. Any distinct yellowing means inadequate fixing or washing. The print must be washed first — the test will not work straight from the fixer — and silver-estimator strips are generally too insensitive to confirm the very low silver levels that optimum permanence demands.

Residual thiosulphate and residual silver are what the permanence standards actually measure. ISO 18917 sets out the methods (iodine-amylose, methylene-blue and silver-sulphide), while the stability specifications live in ISO 18901 for silver-gelatin films and ISO 18929/18920 for prints. Those limits are what “archival” means in numbers, rather than a feeling about how long a tray sat.

Fixing Only Counts If It Then Washes Out

A perfectly fixed print is still ruined if the complexes stay in the paper, so fixing and washing are one problem. FB paper needs 60 minutes in running water above 5°C on its own, against just 2 minutes for RC. The Ilford optimum-permanence sequence for untoned FB papers shortens that and improves it: fix in Rapid Fixer or Hypam 1+4 for 1 min, first wash 5 min in running water, 10 min in Ilford Washaid 1+4 with intermittent agitation, then a final 5 min wash, everything held at 18-24°C including the wash water.

Two warnings from that guide matter. Over-fixing and using exhausted fixer both make washing harder, not safer — more silver driven into the fibre is more to wash back out — which is exactly why two relatively fresh baths beat one tired one. And archival work demands a non-hardening fixer; Rapid Fixer qualifies and must not be used with a hardener, because hardened gelatin resists the wash. Reach for a washing aid (a hypo clearing agent such as Washaid), not a hypo eliminator. The old Kodak HE-1 formula — hydrogen peroxide, ammonia and potassium bromide — is no longer advised for modern materials: its oxidising action can attack paper and image silver, and residual ammonia even invalidates the residual-silver test. A stop bath before the fixer (Ilfostop, whose indicator dye shifts from yellow toward purple at exhaustion) cuts developer carry-over and prolongs the fixer, which keeps the whole capacity calculation honest.