Page 38 - Curriculum Visions Dynamic Book

Glass
When silica is heated to very high temperatures (about 1600°C),
the natural bonds of the silica break down and the crystals change to
a noncrystalline amorphous, glassy material called fused silica. If metal compounds are added to the melt, the result is the transparent material we know as glass.
All glass contains a natural glass, for example silica in the form of sand and a flux such as sodium carbonate to make the glass melt more easily. Glass and a flux alone are not stable and, for example, such glass is soluble in water (see water glass on page 6). As a result, a stabiliser, in the form of calcium carbonate (limestone) must be added.
The sodium–calcium–silicon glass is called soda lime glass. About nine- tenths of all glass is soda lime glass. Its main use is as windows and bottles. It has about one-eighth soda, one-eighth lime and three-quarters sand (silica).
Sometimes boron oxide is added
to the melt. This changes the property of the glass so that it does not expand or contract significantly with changes in temperature. This means the glass will not crack when heated or cooled suddenly. This is the form of glass used in most cookware applications.
 Silica, soda and lime react to form soda lime glass. The addition of these materials splits up the tight structure of pure quartz, lowering its melting point. This makes it less viscous and easier to melt and shape.
The top picture shows a rod of soda lime glass bending in a Bunsen flame. Notice the flame colour has been changed because of the presence of sodium and calcium atoms, which produce orange and red colours, respectively.
The structure of glass
Glass is noncrystalline, or amorphous. The silicon atom is completely surrounded by four oxygen atoms. Each silica molecule touches those adjacent to it and shares oxygen atoms, making randomly arranged chains and rings. The sodium and calcium ions in soda lime glass fit in between the rings and help to lock them together.
38
38