Strontium oxide

Strontium oxide, SrO, is a white powder made of the soft metal strontium bonded to oxygen. It belongs to the alkaline earth oxides, the same family as lime (calcium oxide) and magnesia, and it reacts with water to form a strongly alkaline solution.

Although strontium and oxygen are its building blocks, it is not made by burning the metal, which would be costly. Strontium is mined as the mineral celestine, strontium sulfate. That is converted, usually by roasting with coal and then treating the result in water, into strontium carbonate. Heating the carbonate drives off carbon dioxide and leaves the oxide. Glassmakers usually add strontium carbonate to the batch, and it becomes strontium oxide in the furnace.

In display glass, a small amount of strontium oxide is mixed with related oxides such as calcium oxide and magnesium oxide. Together they fine-tune how the glass melts and flows, and help stop it from crystallizing as it is formed into huge thin sheets. Its biggest use used to be the glass faceplates of old tube TVs, where strontium helped block X-rays made inside the tube.