Vitrite, also known as foam glass, is a very low fusing point black glass mainly used for the insulation base of electric lamps. It used to be made by the Vitrite Company.
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Process
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.
Composition
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 manganoc to produce a black colour.
References
Vitrite Wikipedia(Text) CC BY-SA