The Chemistry of Glass
The Composition and Properties of Different Types of Silica Glass
Glass is really a super-cooled liquid with a long history and fascinating chemistry.
Other than plastic, one of the materials every person uses most is glass. Its transparency makes it ideal to use in windows, its rigidity makes it useful for many types of container and it can be melted and reformed into many different shapes. What is it made of and why is it so special?
Super-cooled Liquid
The technical term, “glass” actually refers to a state of matter rather than a particular chemical. It can be defined as an amorphous solid that has cooled to rigidity without crystallizing. Essentially it is a super-cooled liquid and it can be considered to be a very viscous liquid. What is going on here?
Soda-Lime-Silica
Ordinary commercial glass is made from sand which is silica or silicon dioxide. Silica has a giant crystalline structure, it is made of silicon atoms covalently bonded to oxygen atoms, which are covalently bonded to other silicon atoms and so on. It is a very hard structure which requires very high temperatures to melt because the covalent bonds need to be broken. This melting point can be reduced by adding a chemical called a flux. In normal glass the flux is sodium carbonate which becomes sodium oxide or soda in the glass. Calcium carbonate or lime is added to glass as a stabilizer which prevents the glass from dissolving in water.
Amorphous Structure
When the soda-lime-silica mix is melted it become amorphous. This means that it has irregular, random arrangements of its atoms. This mixture is cooled quickly so that this amorphous structure does not have time to form regular structure or crystals. This structure contributes to many of the important properties of glass. Most importantly, it has no obvious melting point, instead its viscosity decreases so that it begins to flow and can be moulded or blown.
History
Glass has been used for thousands of years, vessels dating back to 1500BC have been found in the area which was Mesopotamia. The Egyptians made coloured glass vessels by adding transition metal ores. The Romans were able to blow glass and so made it available to the common people. Lead-crystal glass was developed in Britain in the 17th Century. It contained lead oxide which gave it a sparkle and made it easier to cut and engrave.
Pyrex
The most important recent development in the glass industry was the invention of borosilicate glass, known by the trade name “Pyrex”. This is made by adding borax or sodium borate to the initial mix. This forms boron oxide in the glass and gives important new properties to the glass. Borosilicate glass is tougher than normal glass, it withstands higher temperature and rapid temperature changes and it is more chemically resistant. This means that it is now used in oven-ware and laboratory glassware.
