🚀 go-pugleaf

woyansky@flamingo.ece.cmu.edu (George J. Woyansky) writes:

>I remember doing a calculation in one of my first materials courses at NU
>about the subject of "flowing" glass and if I remember correctly it would
>take tens of thousands or maybe millions of years to see an effect of thicker
>glass at the bottom of a window pane.  

Depends on the glass.  Among other adventures, we own a glass art 
studio (my wife is the artist, I'm the process control guy).  I can
assure you that just the glasses we use vary markedly as to hardness,
slump temperature, viscosity at a given temperature and so on.  I can
easily believe that some of the very soft, very low softening point
glass we use could flow over periods measured in centuries.

An "obvious proof" that glass does not flow is to note that telescope
and camera lens, even those hundreds of years old, have not flowed.
An "obvious disproof" is the fact that almost every piece of old 
decorative or glazing glass I've placed a micrometer on that is over
a hundred or so years old is a bit thicker at the bottom.  Even when
glass from the same sheet, as evidenced by the striations and bubbles,
is placed in random orientation.  Something appears to be happening to
this window glass but I'm not going to even wager a guess as to what.

>Interesting note-- many old churches with large stained-glass panes are
>thicker on the bottom than at the top.  They were actually made this
>way intentially so that the bottom of the pane could support the weight of
>the glass.  This may be where some of the folklore comes from. 

None that we've ever worked with.  Art glass is made by one of two 
methods.  Blown glass comes a large cylindrical vessel is that is hand blown, 
the ends are cut off, the cylinder is split longitudinally, and the two
half-cylinders are slumped flat in a kiln.  The other, more common kind
is made by either extruding or rolling molten globs of glass onto
metal plates.  The plates and/or the loose material placed on the plates
are chosen to render the desired surface pattern.  No vendor I know
of makes glass intentionally non-uniform in thickness.  It would drive
the artist batty trying to fit the different lead cames together for one
thing.  Blown glass DOES vary in thickness but that is considered 
an unavoidable defect.

Large windows are reenforced by either attaching external bracing to the 
came and/or by using came that has been extruded around a steel wire and/or
by using zinc came in critical areas.
cut off. 

John

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