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A glass scientist explained to me once that float glass is actually a "supercooled liquid" that never completely solidifies. "The glass in your windows is slowly flowing away," he said. "In fact, if you measure the glass in a 100-yearold window, you'll see that it is thickest near the bottom because it is flowing down." Few people would lose sleep over the fact that their window glass may melt away in a few hundred years. There is some concern, however, over the degradation and disappearance of newer elements in windows, namely the low-emissivity coating and argon gas filling which are responsible for the insulating power of highperformance glazings. How long does that low-e coating last? Is there a way to tell that it