phenolic

Well, before I go looking up the answers...

Phenolic is a plastic-like substance that was widely used as an electrical insulator (maybe not in theatrical lights) in the years of past. It was and is often used as a material for handles and knobs, which is where I think it is usually found in theatrical lighting, as is doesn't transfer heat very well. This means that the handle or knob will not heat up as much as a metal one when a fixture is hot. The problem is that over time, when constantly exposed to heat, phenolic can break down and become brittle. This is the main reason it not as widely used as an electrical insulator anymore.

Borosilicate I think has something to do with lenses, but I'm not entirely sure without looking it up.