[78-L] glass record mystery

[Joe Salerno]

Recently I acquired a number of glass based records. Or I think they are 
glass. Some say they are glass. They are flat like glass. They weigh 
like glass, sound like glass when you tap them lightly with your 
fingernail, and looking at the edge of the center hole, I don't see 
anything that looks shiny like aluminum. Dates are 1944, which would be 
the right time frame for glass.

But when I attempt to shine a bright flashlight or a red laser thru 
them, they are opaque, even in a very dark room at night.

Is there some reason why a glass based disc would not pass light?
-- 
Joe Salerno