Why are electric bulbs made up of glass.

Consider the reason that electric bulbs are manufactured in the first place: They are used to shed light on their world, to illuminate the darkness wherever they may be, to spread their warm reassuring glow for the benefit of all who may pass by.

An electric bulb uses a very thin wire, which heats to a high temperature and glows brightly when electric current passes through it. That wire is the strength of the electric bulb, but also its fatal weakness. For if the wire were surrounded by air when it heated and glowed, it would instantly burn up, and its glow would be extinguished forever. In order to keep the bulb glowing, air must not be allowed to reach it. This means that the wire must be sealed inside some sort of an enclosure that can be sealed so tight that even air cannot penetrate it.

The next question is: What to use for an air-tight enclosure ? It is said that Mr. Edison (the inventor of the electric light-bulb) tried more than 400 different ways to manufacture his invention, before he found one that was dependable enough to use in mass production. Edison himself claimed that the 400 failed experiments were trials of different materials for the filament ... the thin wire inside the bulb. But I suspect that many of those experiments involved the search for the best material to use to keep the air out, and prevent the thin wire from burning out. This relates exactly to the question you're asking.

I believe that Edison must have tried bulbs enclosed in steel, clay, salami, aluminum, stone, leather, wood, egg shell, cardboard, bone ... who knows what else. He eventually realized a critical related discovery: The enclosure for the fine wire not only needed to prevent air from entering the bulb, it also needed to allow light to get OUT ! I'm sure that as soon as this realization hit him, he rushed to his laboratory, tried a bulb surrounded by GLASS, and the rest became history.

b.a device that inputs heat into a cool object