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Glass is not porous and would not be able to soak up any excess moisture which would travel through the stem and nasty tobacco juice into your mouth. Usually they are made of briar, meerschaum, and hardwoods. Glass would be a great insulator thus getting Very hot! Perhaps @Glassman could elaborate as well.
I would think you would get to much nasty baccy juice from a glass pipe since there wouldn't be anything to absorb it before it gets to your mouth. And to make one thick enough to not get hot while smoking it for 20-40 minutes would be a pretty heavy pipe I would think depending on the style of it.