1893

Borosilicate Glass

Friedrich Otto Schott (1851–1935)

Glass is mostly silicon dioxide, but the number of different additives and recipes for it is beyond counting. The most common glass is the so-called “soda-lime” variety, which contains salts of calcium and sodium (slightly different recipes are used for window glass versus bottle glass). These additives make the glass easier to melt and manipulate, while keeping the product hard and colorless, but there are some disadvantages. One big problem is thermal stability: sudden heating and cooling cycles almost invariably cause an object made of soda-lime glass to crack.

That’s not very good for laboratory work, where glass is otherwise an almost ideal material for storing and reacting many chemicals. There are reagents that will degrade it (hydrofluoric acid and strong hydroxide base solutions, for example), but many other nasty compounds can be stored safely in glass containers for years. If these containers are at risk of suddenly shattering when they’re warmed up or cooled down, however, that’s not a safe storage solution at all.

Enter borosilicate glass, which is made by adding boron oxide to the silica mixture. It was first sold in 1893 by Friedrich Otto Schott, a well-known German glass chemist of his time, and varieties of it quickly cropped up in other countries. It’s even harder and more chemically unreactive than standard glass varieties, and it has tremendous thermal stability. This is the type of glass used in baking dishes—making them oven-, dishwasher-, and microwave-safe—and in labware. Most chemists have never seen an Erlenmeyer or round-bottom flask that wasn’t borosilicate—virtually all laboratory glassware is made of it.

Although borosilicate is more difficult to soften and work than soda-lime glass, the benefits are worth the trouble. It’s not impervious to thermal shock, but it takes some work to get it to break, and under normal lab usage it’s very durable indeed. The only problem is that chemists get used to glass behaving so well and sometimes get surprised in the kitchen with other varieties!

Laboratory glassware is almost always made from borosilicate mixtures; it has to go through too many temperature changes for most other kinds of glass to have a chance.