Magnesium oxide

Magnesium oxide, MgO, also called magnesia, is a white powder or ceramic made of magnesium and oxygen. It has one of the highest melting points of any common material, about 2,800 °C, and it blocks electricity while passing heat fairly well.

Burning magnesium metal does make magnesium oxide, in a brilliant white flash, but that would be wasteful. Instead it comes from minerals and seawater, where magnesium is already bound to oxygen. Magnesite, magnesium carbonate, is mined and heated in a kiln, driving off carbon dioxide. Or magnesium hydroxide is made from seawater or salty brines and heated to drive off water. The heating temperature controls how reactive the powder is.

In display glass, a small amount of magnesium oxide joins calcium oxide and strontium oxide to adjust how the glass melts and flows and to keep it from crystallizing. Its biggest uses are heat-proof bricks that line steel furnaces and kilns, and the packed insulating powder around the wire inside heating elements, like those in ovens. Its close relative, magnesium hydroxide, is the antacid milk of magnesia.