For film archival, thiosulfate levels calculated from fixer retention testing should be ____ mg/cm2.

• A. 2
• B. 4
• C. 6
• D. 10

Answer: (A)

Keeping fixer residues extremely low is essential for archival film integrity. Thiosulfate from fixer can remain trapped in the gelatin or base and, over time, react with environmental sulfur compounds or with the silver image itself, leading to image degradation, staining, or accelerated deterioration of the emulsion.

The standard measure is the amount of thiosulfate left per unit area of film, expressed as mg per square centimeter. For archival purposes, the acceptable limit is 2 mg/cm2. This threshold represents a balance between practical processing and long-term stability; staying at or below this level minimizes the risk of chemical reactions that could compromise the archival quality of the film.

Higher residual levels, such as those corresponding to 4, 6, or 10 mg/cm2, would indicate more fixer remaining and greater potential for future deterioration, so they do not meet archival safety standards.