Ferric Ammonium Sulphate (Iron Alum) in Gordon and Sweets protocol is used for what purpose?

• A. To cover aldehyde groups with Fe ions to prepare for silver binding to retic fibers
• B. To fix tissue
• C. To dehydrate
• D. To stain nuclei

Answer: (A)

Ferric ammonium sulfate acts as a mordant in this protocol. After oxidation (which creates aldehyde groups on components of the tissue), the ferric ions bind to those aldehyde groups, forming iron-aldehyde complexes. These complexes provide sites that strongly attract and anchor silver ions, so that silver deposits selectively on reticular fibers during the development step. In short, this iron alum step prepares the aldehyde-rich reticulin network to take up silver, producing the characteristic reticulin stain. It isn’t a fixative, a dehydrant, or a nuclear stain.