A Hemingray CD 106 Photo # 1 To see photos # 2 [id=277300879]
What I think causes the streaking effect on the surface of the glass, is carbon burning off and leaving a residue or imprint in the glass.
In this insulator there is a cloud of carbon particles inside the glass that is at least 1/8 inch inside the glass that then flows at an angle to the surface and then out along the surface; however it is no longer in the form of carbon but has left behind a residue.
It appears that as the glass was being poured into the mold, carbon was present either on the surface of the glass or on the mold (possibly in the form of graphite to lubricate the mold) then as the glass was pressed in to the mold the glass either picked up carbon off the inside of the mold or the carbon already on the surface was folded in such a way that part of the carbon was trapped inside the glass and part was between the glass surface and the mold. The part on the surface of the glass then burned off leaving the marks on the surface of the glass.