From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: O.T. Math help
References: <3DC730B0.5FDAB60E@rica.net> <3DC738EC.44458C98@webaccess.net> <3DC7C3FF.7ABC2433@webaccess.net>
Date: Thu, 07 Nov 2002 00:52:41 GMT
NNTP-Posting-Date: Wed, 06 Nov 2002 16:52:41 PST
George Gonzalez wrote:
> A few yrs later in college the Physics Prof told us the reason old glass
> windows are irregular is
> due to the glass flowing....
> Then just last week my son came home with the story that glass flows......
> It's in the textbook,
> teacher's head, worksheets, quizzes... too late to stamp this out!
Glass is not really a solid. In its rigid state, typical glasses have a
viscosity of about 10^13 Poise at ordinary temperatures. For comparison,
water is about 10^-2 Poise and honey about 10^4 Poise. Glass, if cooled
improperly from a melt may become a solid and cooling rate for glasses
strongly prone to devitrification is controlled. The most important
properties of glass are lost if crystalization occurs during cooling.
The effects of glass slowly flowing are not really seen in old windows.
The viscosity is so high that the flow of a window, even over a couple
of hundred years, is really too small to notice. A common way of making
window glass was to pour glass onto a polished iron sheet. Pouring was
often done in the middle and the glass would flow outward and then cool
to rigidity. This left waves in rings in the middle.
The first paragraph is based on the "Schott Guide to Glass" by Heinz G.
Pfaender and revised and expanded by Hubert Schroeder, 1983, Van
Nostrand Reinhold, NY. See chapter 2 in particular. The second paragraph
has information learned from artisans at Old Sturbridge Village, MA.
Schott Glaswerke is the oldest maker of technical glass products
extending all the way back to the 19th century. They are still one the
important makers of technical glass products.
Oh, the teacher was partly right about glass. It does flow at room
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org