Glass is made from silica, a mineral found in sand, but to enable it to cope with thermal shock (so that it does not shatter in extreme cold or heat), it is usually processed, and compounds are added to strengthen it. Here are some of the terms used to describe processed glass, with an explanation of what they mean:

Tempered Glass: Pyrex ware in the USA, Bormioli Rocco glassware and other brands of glass storage containers use tempered glass for their glass storage bowls. The glass is heated to just below it’s liquifying point and then before it softens, it is subjected to jets of extremely cold air to cool it very fast. This hardens the outside and inside surfaces of the glass and makes it resistant to thermal shock. The other advantage of tempered glass is that when it shatters, it breaks into small round edged pieces, instead of into sharp shards, which means you are less likely to cut yourself. Tempered glass is sometimes known as toughened glass. Most glassware that can go in the oven and microwave is made from tempered glass.

Soda-lime glass: This is glass made from about 75% silica, 12% soda, 10% lime, plus other minerals to color the glass. This type of glass is used to make cookie jars and most other types of glass bottles. Kinetic Glasslock and Snapware Glasslock containers are made from tempered soda lime glass – in other words, the soda lime glass is put through the tempering process detailed above to enable it to cope with high and low temperatures and allows the glass containers to go in the freezer and microwave – though you should note that Kinetic Glasslock and Snapware containers can not go in the oven.

Borosilicate Glass: This is a mix of silica and boron oxide which makes the glass resistant to thermal shock. Borosilicate glass was invented by Pyrex (who are owned by Corning) in the early part of the 20th century, and vintage Pyrexware is made of it. Borosilicate glass can usually cope with ovens as well as microwaves. Pyrex made in the USA is no longer made of borosilicate glass, instead they use tempered glass. Borosilicate glass is now mainly used for manufacturing laboratory glass ware, though you occasionally get some glass storage containers made of it for example the Innobaby rectangle container set.

Please see the links on the right for a list of all the glass containers featured on this site.