Matt, imaged glass tools are of two varieties. The most common is   
purchased from Kodak coated with emulsion just like polyester film is   
coated.  You may plot on it with a flat bed plotter, or contact print it.   
 Then to process it you generally lay it flat in a tray, emulsion up and   
pour developer chemistry onto it.  Swoosh it around for a few minutes,   
pour it off, add stop bath for a few seconds, pour it off then add fixer   
for a few minutes, rinse well and dry.  (The data sheet will have exact   
chemistry, temperature and times.)
Imaged plates of this variety are dimensionally stable but are expensive   
and easily damaged.

I have also purchased glass plates with a layer of chromium on them   
coated with photoresist.  This can be contact printed from silver   
artwork, developed, then chemically etched to remove the chrome from the   
non image areas and resist stripped.  This makes a scratch resistant,   
imaged glass plate.  Still expensive and easily broken.

What is much more common today is to purchase clear glass books and use   
vacuum or tape to hole film artwork in place.  These have been referred   
to as glass phototools and may be what you are referring to.  They are   
available from several companies like Precision Art Coordinators, Byers   
Inc. or C.A. Picard.  The caveat with glass books is that often the glass   
is very flat and the product can be less than flat.  The resulting   
off-contact can cause defects.

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