Re: Rumors......

David T. Ashley wrote:
"Robert Bolton" <robertboltondrop@xxxxxxx> wrote in message
news:13rae9vc0grvge5@xxxxxxxxxxxxxxxxxxxxx

I've heard an old wive's tale that windows in old buildings are
thicker at the bottom than the top due to really, slow, flow.
Don't
know if that's fact or fiction. Ditto the rumor about distortion
in
old glass being due to flow.

As best I know, this is true. Windows do get thicker at the bottom.

I seem to remember seeing such windows once, but I think they are
very old. It may have been during a tour of Henry Ford's residence
or
something similarly old.

I suspect that if you get a pane of glass, measure it with a
micrometer at a few places, and leave it in one position you'd be
able to detect the change within 2-5 years.

Google "does glass flow".

The mirror on the Hale telescope was at the time the largest single
piece of glass ever made, it went into service shortly after WWII, and
it is still accurate to a fraction of the wavelength of light. If
glass flowed enough to measure with a micrometer in 5 years then the
Hale would long since have ceased to function.

The myth of glass flowing originated when it was observed that the
window glass in very old buildings was thicker at the bottom. The
reality is that at the time the glass manufacturing processes used
resulted in a variation in thickness across the sheet and most
glaziers usually would put the thicker edge down. There are examples
of very old windows with the thicker edge at the side or top.

The making of inexpensive high quality window glass with polished
surfaces that are flat and parallel is very new--the process used
first went into service in 1959 and over the next decade or so pretty
much took control of the flat-glass market. Prior to that time glass
had to be ground on both sides to achieve a similar degree of quality
(Henry Ford developed a continuous flow process for doing that in the
'30s, _after_ his house was built).

--
--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)


.

Relevant Pages

  • Re: OT - recommend me a book
    ... I thought glass did flow which is why really old windows are thicker at the ... Or is it just me that's thicker at the bottom? ...
    (uk.games.video.misc)
  • Re: OT - recommend me a book
    ... I thought glass did flow which is why really old windows are thicker at the ... Or is it just me that's thicker at the bottom? ...
    (uk.games.video.misc)
  • Re: the net is falling and I want my mommy again
    ... GLASS IS NOT A LIQUID. ... The reason that 100-year-old windows are thicker on the bottom is ...
    (alt.sysadmin.recovery)
  • Re: Natl Geos Naked Science
    ... National Geographic Society, I'd guess. ... that the reason glass is thicker on the bottom of old windows is ... not because the glass flowed over time. ... This thicker part was generally placed at ...
    (sci.geo.earthquakes)
  • Re: Endfed wire ?
    ... ......so your old glass is thicker at the bottom ... look at the glass in mediaeval cathedrals and it is thicker ...
    (uk.radio.amateur)