Mineral deposit

Two federal agencies supply three words for what dissolved minerals leave behind. USGS writes that hard water is high in dissolved calcium and magnesium, and that “When hard water is heated, such as in a home water heater, solid deposits of calcium carbonate can form.” The compound in that sentence has its own entry, calcium carbonate. For glasses taken out of a dishwasher with spots or film on them, the USGS word is hard-water residue.

EPA's word is scale, and its secondary drinking-water standards define it directly: “Scale is a mineral deposit which builds up on the insides of hot water pipes, boilers, and heat exchangers, restricting or even blocking water flow.” The same standards list the noticeable effects of total dissolved solids above 500 mg/L as “hardness; deposits; colored water; staining”, a guideline for drinking water, not a figure for window glass.

Of the three, only hard-water residue is a USGS word for something on glass, and USGS wrote it about dishes. EPA's scale builds up inside heated plumbing, and the USGS calcium carbonate deposit forms when hard water is heated; neither describes a window. What the dishwasher example and a window share is reasoning, not a published statement: water left to dry on glass leaves behind what it carried. The USGS classes, from soft at 0 to 60 mg/L as calcium carbonate to very hard above 180, describe that water, not the glass.

The National Glass Association addresses dissolved solids for one technique only, the pure-water method its paper records, in which rinse water is generally allowed to evaporate. The water that carries minerals onto glass in the first place is hard water.

No source read for this page says whether a mineral deposit on glass can be removed. The one published line about glass past routine cleaning sits under NGA's heading for non-routine post-construction cleaning and restoration: “Glass that is improperly stored or left unprotected during construction may result in glass that cannot be successfully cleaned using routine cleaning procedures.” NGA names razor blades, chemical cleaning and mechanical polishing as techniques that may then become necessary, and it does not name mineral deposits as such a condition.

Whether a deposit can be polished off is answered at whether mineral deposits can be polished off glass, and why some spots outlast a wash is taken up at why regular cleaning does not remove the spots.