Hard water

The United States Geological Survey keeps its own definition short: “The simple definition of water hardness is the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, largely calcium and magnesium.” USGS adds that water picks those minerals up as it moves through soil and rock.

USGS then grades hardness by how much is dissolved: “General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard.” Those are general guidelines, and no figure for a Naples address is printed here. USGS routes that question elsewhere: “information that is directly relevant to water hardness and other chemical properties at a home or immediate locale should be provided by the local health agency or local water utility, or by the vendor of a local water-softening system.”

Water reaches a window from more than the kitchen tap, and why sprinklers spot windows is answered on its own page.

USGS reaches for a kitchen example: “One of the most common causes of cloudy dishes and glassware is hard water.” Its longer version names the moment: “Have you done a load of dishes in the dishwasher, taken out the glasses, and noticed spots or film on them? This is more hard-water residue.” USGS writes that about dishes and never mentions windows. The reasoning carries over anyway: water left to dry on any glass leaves behind the minerals it carried.

Hard water also changes how soap behaves. USGS says that in hard water, soap reacts with the calcium to form soap scum, and that more soap or detergent is needed to get things clean. The National Glass Association's cleaning paper handles dissolved minerals from the water side instead, recording a technique whose wash and rinse water must be low in dissolved solids; that specification is set out in the pure-water section of the window cleaning page.

Pilkington's glass cleaning bulletin, ATS-166, names hard water for one product, its Activ self-cleaning glass. It describes dried water spots or deposits that can appear when that glass dries after hosing with water that was too hard, and it says “The water used to rinse Activ self-cleaning glass should not be ‘hard’.” That is one maker's line about one coated product, and nothing in it is written for other glass.

Past that one product, what carries over is reasoning, not a rule: minerals dissolved in water stay behind when a drop dries on glass, and the visible result is spotting. Whether a given dried spot is a mineral deposit or something else, and why some marks survive regular cleaning, are questions with pages of their own.