Re: mass extinctions and DDT

[Jeff Hecht <jeff.hecht@sff.net>]

At 3:39 PM +0100 12/30/01, David Marjanovic wrote:
>  > >Aren't there some big windows in a chaple in Europe that are now thicker
> >  >at the base because glass is just a really really viscous liquid?  Or is
>  > >that science babble?
> >
>
> Some windows are thicker at the base after a mere 100 years (depending on
> the sort of glass AFAIK). Very old windows tend to become white -- the glass
> finally crystallizes = freezes into a solid.

Glass does not flow over even geologic time scales, although in 
theory it is an extremely viscous liquid. Extremely means that the 
predicted relaxation time at room temperature for window glass is 
10E34 years, longer than the 15 billion year age of the universe. 
(Edgar Dutra Zanotto, "Do Cathedral Glasses flow," American Journal 
of Physics, V. 66, 392-395 (May 1998). It is likely that cathedral 
window panes are thicker at the bottom than the top because the 
glaziers installed it that way.

Glasses do degrade and crystallize over very long times; tektites are 
an example. The degradation times depend on their composition.

-- Jeff Hecht