Which step is most likely omitted if a silver-stained slide shows a black precipitate after storage?

• A. Sodium thiosulfate
• B. Dehydration
• C. Rehydration
• D. Counterstain

Answer: (A)

In silver staining, after the developer creates the image by reducing silver at target sites, there is often unreacted silver salts still present. A fixative step with sodium thiosulfate is used to dissolve and remove those unbound silver salts, effectively stopping any further development and preventing background or storage-induced precipitation. If this fixing step is omitted, the remaining silver salts can react over time (for example with environmental sulfides or light exposure) and form a black precipitate, which obscures the stain and degrades the result. Dehydration, rehydration, and a counterstain don’t address that residual silver issue, so they wouldn’t prevent the dark precipitate from forming during storage.