[Boatanchors] RE: 2X2 question

[mikea]

On Sun, Apr 09, 2006 at 06:51:21PM -0400, J. Forster wrote:
> B Riches wrote:
> 
> > Interesting - I guess we forget that glass is a "liquid". In 100+ year old
> > homes that have the original window glass the panes are thicker at the
> > bottom due to the flow of the glass by gravity.
> 
> I used that argument a few days ago, but now question it. Apparently, before the
> 'float' process, glass had a wedge to it and practice was to install it thick
> end down. I've no idea which is true.

I'm told that while glass is indeed a liquid, it flows only _v_e_r_y_ 
slowly indeed -- too slowly for the glass in a window to sag to the 
bottom over only a thousand years or so. 

But I've had some experience, through association with a flame-chemist
friend, with helium diffusing through glass. It's a tiny atom, and it
goes right through rubber and the like all too readily, a little less 
so through glass, and about the only way to keep it in is a knife-edge
metal-to-metal seal. 

See, among others, 

<http://www.exo.net/~pauld/popularerrors/popularerrors.html>.

"popular error: Glass is a liquid

 The original report noted that glass seemed to be thicker at the
 bottom of ancient windows. It then went on to speculate that the
 thickening was caused by the flow of the glass under gravity.
 
 Later research showed that some ancient windows were thicker on top or
 on the sides. So the original observations were incorrect.
 
 The Corning Museum of Glass website contains a great resource of
 information on glass. <http://www.cmog.org/page.cfm?page=77>

 It notes that room temperature glass has a viscosity of 1022 poise.
 The viscosity of a liquid controls how fast it flows under gravity.
 (SAE 30 motor oil has a viscosity of about 1 poise, water is 0.01
 poise.) The viscosity of glass is so high that you could wait the
 entire age of the universe and see no measurable thickening of the
 glass under earth gravity."

The Corning Museum of Glass is a wonderful place. Visit it if you can!

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin