The question raised by many scientists in the past was ‘Is glass a solid or a liquid?’ On its first appearance, it appears to have a definitive shape and volume, which would suggest that it is a solid. This was believed for many years, but when people examined the bottom of very old windows they found that the glass was thinner at the top than the bottom, suggesting that it has a definite volume but no fixed shape, making it a liquid. This idea was also rejected as well, because when glass is cooled to 0K, absolute zero, it remained in its so-called super cooled liquid state. This would mean it has no solid state, which is an impossibility. Glass is therefore loosely classed as an amorphous solid, but it is more accurate to say it exists in a vitreous, or glassy state in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity.
As I mentioned before, glass has an almost infinite number of uses. It is therefore adapted in many ways to its task, so that can meet the specifications needed. When glass is industrially manufactured, metal oxides are added to the silicon dioxide before it is burnt, to lower it’s melting point. This is because raising any material to 1713˚C takes sophisticated, expensive equipment and also a lot of time. Therefore by adding compounds such as calcium oxide and in the case of optical glass sodium oxide, commonly known as soda. With the addition of only 25% soda, the melting point can be lowered to 793˚C. This addition does have drawbacks though. The soda actually alters the chemical composition of the glass by breaking some of the irregular rings. This then weakens the material. As glass is amorphous, it can’t undergo slip, as no planes are present. This then means if glass is bent it cannot absorb the energy into its atomic structure, but rather breaks bonds leading to a fracture. Fractures always begin on the surface of glass, since there are always surface defects that weaken the chemical bonds holding glass together. If glass is bent, stress builds up on the surface to create a crack centred on the surface defect, which then propagates at the speed of sound throughout the glass, leading to shattering.
For an optical glass to be produced, manufacturers add manganese. This counter acts the effects of trace iron, which leads to a brown or green tinge. Also either arsenic or antimony is added to the glass. This helps with the releasing of small, microscopic bubbles that lead to cloudiness in the glass. For a very clear glass, lead is added. This makes the glass heavy and improves its ability to refract light, which also makes it suitable for lenses and prisms.
Glass used in windows has been around since the 1st century. It was originally made by casting or blowing hollow cylinders that were then slit and flattened into sheets. Today, nearly all window glass is made mechanically by drawing glass upward from a molten pool fed from a tank furnace. In the Fourcault process the glass sheet is drawn through a slotted refractory block submerged in the surface of the glass pool, into a vertical annealing furnace from which it emerges to be cut into sheets. Windows are made of glass because it is transparent, but light propagates at a slower speed in glass than in vacuum or in air and is therefore refracted, or bent at the air-glass interface. This enables lenses, which focus light, to be made of glass, with applications in spectacles, telescopes, microscopes, and cameras. The refractive index is different for different colours; by combining lenses of different glasses, achromatic compound lenses can be made with the same focal length for all colours. Glasses that are transparent to infrared radiation over distances of tens of kilometres or more are the basis of almost all telecommunications. Optical fibres work by using the total internal reflection of infrared light. The fibres are made of two types of glass; and inner core through which the light reflects, and an outer cladding layer. Where these two types of glass meet the light reflects, and therefore moves along the fibre at a very high pace. A single pair of optical fibres, only just bigger than a strand of human hair, can transmit over 50, 000 telephone conversations simultaneously with no interference and little resistance, compared to only 24 telephone conversations in metal wires.
