RE: [ATM] re: heating glass

["Donald Good" <donald.good@comcast.net>]

> -----Original Message-----
> From: Bill Kelley [mailto:flex@commspeed.net] 
> Sent: Thursday, January 27, 2005 6:50 PM
> To: Donald Good; 'Ellen Mackenzie'; 'ATM'
> Subject: Re: [ATM] re: heating glass
> 
> 
> Also, for the record, the following information does not 
> apply to high index leaded glass.
> There seems to be some difference in the rate of microwave 
> heating of most glasses, probably due to traces of metals, 
> notably iron.
> Bill
> 

Quite right.  One of the primary physical characteristics of metals is the
existence of "free" electrons - electrons which are only loosely bound to a
particular atomic nucleus.  These outer shell electrons are free to move
from one metal nucleus to another.  This gives metals their low resistance
to electricity and high reflectivity to light (most electromagnetic
radiation) giving them that silvery color.  In the case of glasses with
metal content (in the form of metal oxides), these normally free electrons
can no longer move from metal nucleus to nucleus due to the oxide bonding
and the insulating effect of the glass.  These electrons still respond to
electromagnetic radiation to a lesser degree, but this time the result is
absorption and heating.

Common mirror glass is low in metal oxides.  Plate glass (e.g. soda lime,
type 0080) is <30% metal oxides and Pyrex (e.g. borosilicate, type 7740) is
only 6% metal oxides.  On the other hand, lead glass can be as high as 80%
metal oxides.

Don

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