🚀 go-pugleaf

billn@hpcvaac.cv.hp.com (bill nelson) writes:

>jeff@mama.minmet.uq.oz (Jeff Gates) writes:

>: >how window glass was made when these buildings were built.  The
>: >plates of glass were thicker near the bottom.  If you actually
>: >work out the calculations for how long it would take for glass
>: >to flow to this extent it turns out to be several centuries.

>Longer than that - try many thousands of years.

>: Fascinating!  I wonder if anyone else can support these calculations.
>: 
>: Now, since the subject of this article was "the Nature of Glass",
>: I would like to see if I can start a bit of controversy on the net!
>: 
>: It is often said that
>: 	" Glass is a liquid "
>: or that
>: 	" Glass is a supercooled liquid "
>: 
>: I have my own opinions on this, but first I want to see what comes out.

>Just take a look at a piece of obsidian. Why is it not flat? More than
>likely, it has been around for millions of years.

>Bill

I would like to follow up on Bill's comments:  Many of the tools made by
native Americans were made from obsidian.  These tools retain the sharp
edges that were formed by these ancient craftsmen for thousands of
years.  Surface tension should pull these sharp edges into a rounder
configuration and this should occur over time.  To my knowledge this has
not been observed.

If you look at ancient goblets, most are still standing reasonably
straight up.  On the other hand, when these same goblets are annealed
during the manufacturing process, care must be taken to preserve the
'straightness' of the stems on these goblets.  What I'm suggesting is
that centuries old goblets should never stand straight up and down as
many of them do, because they should tip due to viscous flow occurring
in the stem.

One other consideration for sagging windows:  Has anyone seen marks
where the glass has filled the mounting channels in the window frame and
then flowed out onto the lip of the channel?  If glass truly flows as is
the suggestion, then we should see evidence of the glass flowing out
over this mounting channel.

I have studied glass all of my professional life (~30 yrs).  I have
never seen any authority take this notion of glass flowing seriously.  I
have seen a picture of glass rods strongly bent and held in that
position for 26 years.  When one of the rods was eventually released, it
became perfectly straight within 24 to 48 hrs.  You would think that
permanent deformation of some type would have been evident after so long
a time.

	T. D. Taylor
	Dept. of Ceramic Engineering
	Clemson University
	Clemson, SC USA