Magnesium oxide (MgO) is a compound of a metal and a non-metal. It is held together with double ionic bonds (two positively charged Mg ions bonded with two negative oxygen ions) which makes it non-reactive and tightly bound together. Because of this strong attraction, MgO has a very high melting point of nearly 3,000°C (over 5,000°F), which makes it suitable for lining furnaces, fire-proofing building materials, and for crucibles, those small, white vessels you may have used to melt other substances in your lab. You ask about industrial ovens--in addition to its very high melting point, MgO is also very non-reactive, meaning that when used to line ovens, no particles of MgO will be released from the lining and incorporated into any food baked inside. MgO is also a very strong refractory material; it maintains its structural properties at very high temperatures, again, making it desirable for use when extremely high temperatures are generated.