It isn't really glass, it isn't really porcelain, what can it be made of? Porcelain generally doesn't have mold lines ir embossing. Doesn't look as "cement" as the previouslt posted one.

Update: After receiving this insulator, and giving it a good long soak in Oxalic acid, it became a nice snow-white and clean beayty! IT is white glass, but not white milk-glass.