Orange, red, brown, or yellow discoloration associated with iron-bearing material or iron corrosion products on glass.
GlassRenu · Educational illustration
Meaning and boundaries
Color is suggestive but not conclusive. Other metals, organic material, mineral deposits, and coatings can produce similar colors.
Why it matters in glass restoration
Iron residue may be removable while the underlying surface has separate abrasion, pitting, coating damage, or chemical alteration.