A fixed print is not yet a permanent one. Fixer dissolves the unexposed silver halides that would otherwise darken in light, but the spent fixer itself becomes a threat. Thiosulfate and the soluble silver-thiosulfate complexes formed during fixing, if left in the paper, slowly attack the image silver and yellow the highlights. Resin-coated paper holds little chemistry and washes quickly. On fibre-based paper the chemistry penetrates the absorbent baryta-coated cotton base, and removing it is the central problem of archival printing.

Why fixer must leave the base

Residual thiosulfate decomposes over time into compounds that react with image silver to form silver sulfide, producing the characteristic stain and fading of poorly washed prints. Residual silver complexes are equally damaging, remaining as an unstable salt rather than stable metallic image silver. Fresh, properly diluted fixer used for the minimum effective time is the first defence, because an exhausted or over-long fix loads the base with silver-thiosulfate complexes that are far harder to wash out than thiosulfate alone. The goal is a base carrying so little residual chemistry that no reaction of consequence can occur over a long storage life.

Plain washing is slow because thiosulfate diffuses out of the fibre base only as fast as fresh water replaces the laden water around it, and cold water slows diffusion further; washing is markedly less effective below roughly 15 degC.

The role of a hypo clearing agent

A hypo clearing agent, or washing aid, accelerates this removal by ion exchange rather than by simple dilution. A solution of a salt such as sodium sulfite displaces thiosulfate held in the gelatin and base, substituting ions that themselves wash out readily. Ilford describes its Washaid as a hypo-eliminator that aids the removal of the thiosulphate by-products of fixation by ion exchange, and notes it is particularly beneficial where a hardening fixer has been used, since hardened gelatin releases chemistry more reluctantly.

The practical payoff is shorter washing and far less water. Ilford’s optimum permanence sequence for fibre papers, intended to combine maximum longevity with minimum water use, runs at 18–24 degC: a short fix in Rapid Fixer at 1+4 for one minute, a first wash of five minutes in running water, ten minutes in Washaid at 1+4 with intermittent agitation, and a final wash of five minutes, extended to about thirty minutes where the print is selenium toned. Without a clearing agent, fibre prints may require continuous cold washing for an hour or more to reach a comparable result.

Wash sequence and water economy

Two principles govern an efficient wash. First, the laden water must be exchanged, not merely circulated; a wash that leaves prints in a static bath of their own thiosulfate accomplishes little. Archival washers that flush from the bottom and overflow from the top, with periodic complete water changes, exchange water more effectively than a continuous trickle. Second, prints should not be over-fixed or left in the wash indefinitely, as prolonged immersion can soften the emulsion and leach image tone with no archival gain. The clearing-agent step does most of the work, so the washes around it can be short. Fixing fresh and brief reduces the fixer load at its source and repays itself many times over at the wash stage.

Testing for residual silver and hypo

Sequences are guidance; the print itself is the evidence. Two qualitative spot tests published by Kodak remain the standard check. The residual silver test, Kodak ST-1, uses a sodium sulfide solution—a 2-percent stock diluted one part in nine—applied to a clear margin. Any brown or tan stain beyond the faintest cream indicates silver left by inadequate fixing or washing. The residual hypo test, Kodak HT-2, is an acidified silver nitrate solution—in the standard formula, silver nitrate and 28-percent acetic acid in water—that reacts with residual thiosulfate to form a brown stain; the spot is left two minutes, blotted, and compared against a reference such as the Kodak Hypo Estimator, only a near-imperceptible stain being acceptable.

For quantitative work, archives rely on standardised laboratory methods. The methylene-blue and silver-densitometric procedures of ISO 18917 (formerly ANSI PH4.8) measure residual thiosulfate directly as a mass per unit area, allowing a print to be held against a defined permanence limit rather than judged by eye.