Windows That Wash Themselves
Given people's aversion to washing windows, it's no wonder that at least two companies, Pilkington and PPG Industries, now make glass that cleans itself. The secret ingredient is titanium dioxide, a metallic compound that's permanently embedded in the surface of the molten glass during manufacture but doesn't affect its transparency. When exposed to the sun's ultraviolet rays, the titanium dioxide kicks off a chemical reaction that disintegrates organic dirt, such as tree sap, pollen, and dead bugs. The coating also makes the glass hydrophilic; that is, water doesn't bead up but spreads out in sheets that slice off loosened debris like an invisible squeegee. "It doesn't leave glass sparkling like it came out of the dishwasher," says Chris Barry, director of technical services for Pilkington, "but it's still quite clean." Windows with self-cleaning glass cost about 20 percent more than ordinary windows but need cleaning only about half as often.

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