Tempered glass

The National Glass Association (NGA) sorts architectural glass by strength in its cleaning paper before it says how to clean any of it: “Glass can be of various strengths, i.e., annealed, heat-strengthened or fully tempered.” When the paper turns to the glass types a cleaner has to identify, it gathers the last two under one word, heat-treated, and attaches its caution to that word.

Cardinal Glass Industries describes the treatment itself. Its glossary calls tempered glass “a type of safety glass that has been thermally treated to significantly increase its strength compared to standard annealed glass,” produced by heating the glass to a high temperature and then rapidly cooling it to induce surface compression. Annealed glass, first on NGA's list, is the baseline in Cardinal's comparison.

Laminated glass, the sister term, is defined by construction instead: layers of glass with a plastic interlayer between them, in Cardinal's definition, and NGA's cleaning rules for it are written about that interlayer's exposed edge rather than about heat treatment. The construction side is the subject of laminated glass.

Two lines in NGA's paper shape how tempered glass is handled, and both lean toward caution. One is about identification: in NGA's account, heat-treated glass may carry no logo that can be seen, so a pane without one may still be heat-treated. The other is about tools: NGA ranks a razor blade as “the last possible option,” especially on heat-treated glass.

Together, the two lines mean a look at the pane may not settle whether it is heat-treated. NGA's own checklist tells a cleaner to “be aware of and follow the glass supplier's specific cleaning recommendation,” which is where a question about a particular pane goes.

Blades are held back further still. Routine cleaning, in NGA's paper, has no place for razor blades or metal scrapers, and on coated glass surfaces NGA's word for a blade is never; whether a blade belongs on window glass at all is argued at is it safe to scrape windows with a razor blade, and how grit caught under any tool leaves fine scratches is explained in can window cleaning scratch the glass. Whether a pane carries a coating at all is the question behind whether Low-E windows need special cleaning.