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

• A. 4 mg/cm2
• B. 6 mg/cm2
• C. 2 mg/cm2
• D. 10 mg/cm2

Answer: (C)

Residual fixer, especially thiosulfate, on film can continue to react during storage and cause long-term damage to the image and supporting gelatin emulsion. To protect archival quality, the amount of thiosulfate left per unit area must be kept very low. The standard limit used in archival testing is 2 mg per square centimeter, which reflects a practical balance between effective fixer removal and feasible washing procedures while minimizing the risk of chemical change over decades. Higher values—such as 4, 6, or 10 mg/cm2—indicate more residue and a greater chance of deterioration as the film ages, so they are not acceptable for archival standards.