I agree with everything that Ralph says, as usual. Where's the check,  Ralph. 
One condition I have never not encountered in older glass  when  removing it 
is the inescapable variation in thickness, which, fabrication aside,  occurs, 
so I am told by those much older (few left) and wiser (of which there  are 
legions) that due to the liquid nature of glass, it "creeps", flowing  somewhat 
to the lowest point in an assembly, resulting in somewhat thicker glass  at the 
lower end of a light than at the top. Is is also more brittle.
 
V.I.

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