Discoloration associated with metal or metal compounds deposited on or reacting at a glass surface.
GlassRenu · Educational illustration
Meaning and boundaries
Metal staining may come from runoff, corrosion products, tool transfer, grinding debris, or contact marks. Color alone cannot identify the metal.
Why it matters in glass restoration
Different metals and oxides respond differently to chemicals and abrasion, and embedded debris may conceal pits or scratches.