White deposit noted during embedding after transferring tissue from unbuffered zinc formalin to buffered formalin is most likely due to which issue?

• A. Not washing before transfer
• B. Over-fixation
• C. Inadequate dehydration
• D. Too rapid cutting

Answer: (A)

The key idea is that white, chalky deposits during embedding after moving tissue from unbuffered zinc formalin to buffered formalin come from residual zinc fixative. Zinc ions left in the tissue can react with components of the buffered solution (for example phosphate) when the pH shifts, forming insoluble zinc salts that crystallize as white deposits. If the tissue isn’t washed thoroughly to remove these zinc ions before transfer, these salts crystallize and show up as white material during embedding, potentially interfering with paraffin infiltration and subsequent staining. The fixative residue needs to be washed away with an adequate rinse (often running water or an appropriate buffer) before moving into buffered formalin. Over-fixation would cause excessive cross-linking and hardening rather than crystalline deposits; inadequate dehydration would lead to moisture-related infiltration problems; and too rapid cutting would produce mechanical artifacts, not chemical white deposits.