A visible discoloration or appearance change associated with material on, in, or chemically interacting with a surface.
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Meaning and boundaries
Stain is an appearance description, not a diagnosis. A stain may be removable residue, transferred metal, a corrosion product, coating change, or permanent substrate alteration.
Why it matters in glass restoration
Using 'stain' without identifying the mechanism can lead to the wrong chemical, abrasive, or replacement decision.