OK, this is not an insulator, but hang with me for a minute. Mold paste (mixture of beeswax & graphite) was used as a mold release coating in cast iron molds to prevent the sticky freshly molded glass from sticking to the inside of the mold. Sort of like oil spray for your skillet. This built up over time & clogged the engraving. Case in point this photo of a skillet embossing where the seasoning build up has clogged the engraved letters & logo even after wire brushing. Molds were not as visible as this, so a worker brushing a hot mold couldn't see if he got it all cleaned, and they were in a hurry to get the mold working again. This paste residue would produce an embossing with letters or portions of letters "missing" components.