Here's your brief Chemistry lesson on the types of glass. This was touched on, last Monday.
Soft glass is a mixture of: 72% silica, 15% soda, 9% lime, 3% magnesia, and 1% alumina.
Hard glass (Pyrex) is a mixture of: 80% silicon dioxide, 14% boron oxide, 4% sodium oxide, and 2% aluminum oxide.
This type of glass takes a hotter flame. The nice thing about Pyrex is it strong and can withstand the thermal shocks. Such as letting it cool down quickly and heating it up in a flame quickly. Softer glass, will explode/shatter to any type of thermal shock.
Whey does Pyrex behave this way??....It is due to the COE (Coefficient of Expansion). The COE is the expansion and contraction as glass heats up and cools down. The lower the number, the less expansion and contraction occurs. Softer glasses (Bulls-Eyes glass) has a COE of 90, while Pyrex has a COE of 32. Being a Chemist, Pyrex is used since all of our glassware is made out of Pyrex.
To tell whether-or-not you have hard or soft glass, immerse the colorless glass in a solution of 16 parts methanol (MeOH) and 84 parts benzene. This solution has the same refractive index (the bending of light through a substance) as Pyrex. The Pyrex appears invisible, while the soft glass shows the form of the glass.
OK, enough Chemistry today........