Fixing dissolves the undeveloped silver halides left in an emulsion after development, converting them into soluble complexes that the wash carries away. The step is often treated as a formality, yet the long-term stability of a print depends on it more directly than almost any other stage. A fixer that looks clear and works quickly can still leave a print that will stain and fade, because the chemistry of fixation changes as the bath ages.

Why a Single Bath Exhausts

A working fixing bath dissolves silver halide by forming thiosulfate complexes with the silver ion. While the bath is fresh and thiosulfate is abundant, the dominant product is a highly soluble complex that washes out readily. As prints accumulate, dissolved silver builds in the bath and free thiosulfate is consumed. The equilibrium shifts toward less soluble argentothiosulfate complexes. As the conservation chemist James Reilly describes in The Albumen and Salted Paper Book, a bath approaching exhaustion “loses its ability to form soluble silver-thiosulfate complexes,” and the complexes that do form become difficult to remove. These compounds lodge in the paper fibers of fiber-based stock and resist washing, later decomposing to release sulfur that stains and attacks the image.

How Two Baths Ensure Complete Fixation

Two-bath fixing addresses this by dividing the work. The print passes first through bath one, which carries most of the silver and performs the bulk of the complexing, then through a fresher bath two. The second bath always contains ample free thiosulfate, so any complexes formed there are the soluble variety that washes cleanly out. In Reilly’s terms, the first bath “does the bulk of the complexing, while the second insures that the complexes ultimately formed can be washed out.” When bath one is retired, bath two is promoted to first position and a fresh second bath is mixed, so the final bath a print ever sees is never near exhaustion.

Tracking Capacity by Silver Concentration

Capacity is best expressed as dissolved silver concentration. Ilford’s technical data for Rapid Fixer states that for commercial permanence with fiber-based papers the silver level should not exceed 2 g/L, roughly 40 prints of 20.3 by 25.4 cm per litre of working solution. For archival permanence the limit drops to 0.5 g/L, about 10 such prints per litre. A two-bath regime keeps bath two well below these thresholds throughout, while residual-silver and residual-thiosulfate tests confirm that washing has succeeded.