What is the film on my windows?

The glass trade already uses the word film for something specific: a layer applied to the surface of the glass. The National Glass Association writes, "For glass with window film applied to the surface, care must be taken not to scratch the film." That is a product, installed on purpose, with its own care rules.

The other kind is a dullness that built up on the glass itself, with nothing installed, and that is the film this page is about. What an applied film tolerates, including why NGA sends the ammonia question to the film supplier, is covered on how exterior glass gets cleaned.

Coastal air carries salt, and the window makers' own guide names it only on aluminum frames and on screens, never on the glass itself. The same salt reaching the glass beside them has no name in any maker's or association's sentence; salt film is the everyday name for it.

USGS defines hard water plainly: "Hard water is high in dissolved minerals, largely calcium and magnesium." Its household example is the spots or film on glasses taken out of a dishwasher, which it calls "more hard-water residue." USGS writes about dishes, not windows, though water dries the same way on a pane. It also notes that in hard water, soap reacts with calcium to form soap scum, a line written about hands, hair and laundry. When hard water dries on glass, what stays is a mineral deposit in the plain sense: the minerals the water carried.

A film can also be leftover cleaning product rather than dirt at all. PGT, CGI and WinDoor's own care guide warns owners twice: "Rinse and dry any run-down quickly to prevent the cleaning solution from leaving a residue on your glass," and "Rinse your glass thoroughly with clean water to prevent the cleaning solution from leaving a film on your glass." The National Glass Association's quick-reference list carries the same instruction as a flat DO NOT: allow water or cleaning residue to remain on the glass or adjacent materials. Solution that is not fully removed before it dries can also leave a line rather than a film, and that mark is streaking.

None of the published sources above names a color, a texture or a test that tells salt, hard-water residue and unrinsed solution apart. The honest next step is looking at the glass itself, not guessing from how a film is described.

• A mark that survives a proper cleaning rather than wiping away is a separate question, covered on why some spots don't come off with regular cleaning.
• Glass that seems past routine cleaning altogether is covered on can windows be too dirty to clean.
• A haze sitting between two panes rather than on the outside face is a different thing entirely, covered on why windows go cloudy between the panes.