Vitrite is a very low fusing point black glass mainly used for the insulation base of electric lamps.
[1] Vitrite can be called foam glass because the mineral from which it is obtained is a spongy mass varying in colour from white to black depending on impurities.
[2] To make the electric lamp, molten vitrite is poured into the shell up to a height of approximately a quarter inch (6 millimeters).
A plunger is brought down to force the plastic glass into shape and fix the contacts.
Vitrite is referred to as glass but in fact it is an enamel which contains a high proportion of lead (II) oxide to give fluidity, and considerable quantities of manganese oxide to produce a black colour.