What is glass?

Glass is essentially sand (silica) which is melted and then allowed to cool rapidly.

Silica sand has a crystalline structure: it is many-faced or faceted, somewhat like a diamond. Because of its structure it reflects light and is opaque. When it is melted, the crystalline structure is broken down and the rapid cooling freezes it in this non-structured or disordered state. Because of this it is called a supercooled liquid.

This process of melting and losing a crystalline structure is called ‘vitrification’; the material is said to have become ‘amorphous’, which means ‘without order'. In this state the particles are so small that light rays can “thread their way between them” and the glass is transparent.

Don’t be fazed by the fancy words. There’s lots more to come, but they’re all in the glossary if you forget what they mean.

Other materials are added to lower the melting point (silica on its own melts at about 1710°C), to make it easier to work, to make it clearer and more transparent, to prevent it dissolving in water.

The most commonly encountered glass objects, such as bottles and jars and window panes, are made of a mixture of sand, soda and lime; plus a few extra ‘herbs and spices’. For convenience, this type of glass is known as ‘soda lime’ glass. The silica isn’t mentioned, as it’s the basis of almost all types of glass. 

Another type of glass well known to most is ‘Pyrex’. It’s able to withstand rapid changes of temperature, is made of boron oxide (boric oxide, borax, B₂O₃) and silica, so it’s called a ‘borosilicate’ glass.

Still other glass has substantial amounts of lead added. This adds brilliance and clarity to the glass, making it ideal for cutting and polishing into sculptural and tableware products such as ‘Waterford’ crystal. As lead presents a significant health hazard it is being superceded by products from firms such as ‘Orrefors’ who have developed superior soda lime glasses free of lead.

However, for most kilnforming applications the glass used will be ‘soda lime glass’ and includes clear glass and all of the vast range of coloured glass offered through art and craft outlets for kilnforming and leadlighting.

Modern clear glass is made by floating the melt on a bed of tin and is referred to as ‘float’ glass. This gives it very true and even surfaces and uniform thickness. Most coloured and all patterned glass is made by passing it through rollers, so the surfaces are not smooth nor true and the thickness can vary across a sheet. This is referred to as rolled glass.