Vitrite, also known as foam glass, is a very low fusing point black glass mainly used for the insulation base of electric lamps. It was formerly made by the Vitrite Company.
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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 oxide to give fluidity, and considerable quantities of manganoc (probably a manganese oxide) to produce a black colour.