13-03-2016, 07:06 AM
(11-03-2016, 12:47 PM)boater sam Wrote: I was told years ago that glass is a supercooled liquid, whatever that is, and that it continues to move. Certainly really old windows are thicker at the bottom than the top.
It's more complex than that! Room temperature glass doesn't flow - if it did, the annealing operation would be unnecessary as stresses would just disappear themselves.
Old lenses and mirrors in telescopes and cameras don't flow out of shape making the equipment useless.
Old windows may well be thicker at the bottom than the top, but before the float glass process, making sheets of glass involved spinning a molten blob of the stuff, and it didn't give a uniform thickness. So when cut up for windows, the thicker region was put at the bottom - well, you would, wouldn't you?
