The concentration of dissolved multivalent minerals in water, principally calcium and magnesium, commonly expressed as an equivalent amount of calcium carbonate.
GlassRenu · Educational illustration
Meaning and boundaries
Hardness is a water-quality property, not a measurement of how firmly a deposit adheres or how deeply glass has been altered.
Why it matters in glass restoration
Water hardness influences scale formation, spotting, soap performance, and the composition of residues that may reach glass.