Re: Smiley Faces!

["Paul Zitzewitz" <pzitzewi@sb-f1.umd.umich.edu>]

The proper terminology from the glass industry is "fused silica,"  
which is roughly 98% pure SiO2.  It has, indeed, a higher melting 
point.   It also has a much lower coefficient of thermal expansion. 

It is normally worked with a hydrogen-oxygen flame, whereas natural 
gas-oxygen is sufficient for pyrex.  The glassblowing shop gets very 
hot when you're working much fused silica.

One property that makes fused silica difficult to work is the 
extremely narrow range of temperatures for which it is soft and can 
be "worked."  You can't take the flame away and make small 
adjustments to the piece you've just worked, like you can with pyrex 
or soft glass. I made flexible bellows in fused silica during my 
graduate school days in the late '60s.  You would blow a spherical 
shape in a tube, then push the ends of the tube together to form a 
toroidal shape.  Once I was actually able to make three in a row 
without breaking one.  The tube could then flex a little, both in 
length and angle, to avoid cracking when we poured liquid nitrogen 
through it.  Fun stuff.

> Quartz is a mineral name, and like all minerals has a definite crystaline
> structure.  Melt the quartz and then cool it quickly so there is no long
> range crystaline structue and you have glass.  This is the definition of the
> term glass and it is how glass is made.  You don't have quartz any more.
> I guess I should be clear that you don't have to start with quartz to get
> glass, it is the lack of order that makes it a glass.  Maybe people are using
> the term quartz for SiO2, but SiO2 is silica.  A quartz rich rock has quartz
> crystals in it.  A silica rich rock has a high % of SiO2, but need not contain
> quartz.  You guys get all bent out of shape when the terminology of Physics is
> not used correctly, so you should be happy to learn correct usage of
> terminology from other sciences.
 
Paul Zitzewitz, Professor of Physics
Department of Natural Sciences
University of Michigan-Dearborn
4901 Evergreen Rd
Dearborn, MI 48128
(313) 593-5158 FAX (313) 593-4937