References: <3DC730B0.5FDAB60E@rica.net> <3DC738EC.44458C98@webaccess.net> <3DC7C3FF.7ABC2433@webaccess.net> <3DCB3310.C806A8AE@rica.net>
Subject: Re: O.T. Math help
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Date: Sun, 10 Nov 2002 16:43:39 GMT
NNTP-Posting-Date: Sun, 10 Nov 2002 11:43:39 EST
Organization: Cox Communications
"Jim Thompson" wrote in message
> On Fri, 08 Nov 2002 03:44:16 GMT,
> John Popelish ,
> In Newsgroup: sci.electronics.design,
> Article: <3DCB3310.C806A8AE@rica.net>,
> Entitled: "Re: O.T. Math help",
> Wrote the following:
> |Jim Thompson wrote:
> |> On Thu, 7 Nov 2002 19:32:37 -0600,
> |> "jcurtis" ,
> |> In Newsgroup: sci.electronics.design,
> |> Article: ,
> |> Entitled: "Re: O.T. Math help",
> |> Wrote the following:
> |> |My friend, glass does flow. Any competent glass worker will tell you
> |> |glass pane is somewhat thicker at the bottom than at the top, although
> |> |takes many years for this to be measurable.
> |> |
> |> Very true! My ancestors date back to the 1600's in Pendleton County,
> |> VA (now WV). Some of their homes still are standing. Some of the
> |> windows even show "ripples" from the flow.
> |I am pretty confident that those ripples were present the day the
> |windows were made.
> Neeeerp! These were German immigrants who knew how to make good float
> glass. After three hundred years the glass was clearly thicker on the
> bottom as well.
There's been a "pitch drop" experiment running in Australia since 1930,
measuring the viscosity of pitch (google for it - it's pretty interesting).
They estimate the viscosity of pitch to be around 2.3*10^8 Pa-s. In
contrast, the viscosity of glass, according to one site I found, is around
10^14 Pa-s at 800F. Another site lists viscosities of glass used in a glass
studio. They find clear float glass to be around 10^14 Pa-s at 500C (932F).
That's around 10^6 less viscous, at a much higher temperature than glass in
a window is ever going to see. Another way to look at it is that glass at
800F to 900F would have flowed, over the same 70 year period, about as much
as the pitch did in the first 37 minutes of the experiment.
At room temperature, the viscosity of glass is, by various estimates, over
10^20 Pa-s. That would put glass flow after 70 years equal to the first 2ms
of pitch flow.
Robert Brill, at the Corning Museum of Glass, has written a short discussion
of glass flow. He points out that the viscosity of lead is roughly 10^9
lower than glass, yet the lead doesn't flow out of old cathedral windows,
even though the windows themselves are said to be flowing.
He also states that the estimated time for a 1m tall, 1cm thick sheet of
glass to flow enough to thicken the bottom of the pane by 10A is roughly 10
-- Mike --