Glass, like most other materials, contracts on cooling. However, due to its low thermal conductivity, it does not cool uniformly and the surfaces, which cool more rapidly, shrink more quickly than the centre. This prod uces uncontrolled strain in the article. If the internal surface of an unannealed container is scratched, the container will disintegrate. Badly annealed glass articles cannot withstand thermal shock and are liable to break in use. The excessive strain can be avoided by slow cooling at a controlled rate, called annealing. Annealing is done in an oven, called a lehr, through which glass articles pass on a slowly moving conveyor belt.
A container, for example, would enter a lehr at approximately 450 o C. As t he conveyor moves through the lehr, which is approximately 20m long, the temperature is at first increased to about 560 o C, at which the glass just begins to flow and is then gradually reduced to a temperature at which no further strain can be induced, and then cooled by fan air to room temperature. The time required for this process depends on the size of the article and the wall thickness but is normally completed in less than an hour.
Glass has an extremely high compressive strength and when it breaks it does so due to induced tension on the surface. Glass can be thermally strengthened by inducing invisible thin layers in compression on the outer surfaces. In order to break such toughened or tempered glass, the compression has to be neutralise d and additional tension applied. Toughening is obtained by re - heating the glass article uniformly to a temperature just above that at which deformation could take place and then rapidly cooling the surfaces by jets of air. If one can imagine a sheet of gl ass as consisting of three layers then the process becomes easier to understand. The air jets rapidly cool and freeze solid the outer layers while the inner layers continue to contract. While it is contracting it exerts compression on the outer layers whil e putting itself under tension. This method can be applied to flat glass or simple shapes like curved car windscreens or even tumblers. Glass thickness must be uniform, not too thin, and the shape of the article must be such that all surfaces can be unifor mly cooled at the same time. Bottles do not satisfy these conditions and cannot be toughened in this way. However, it is possible to toughen bottles chemically by immersing hot bottles in a molten potassium salt. Potassium ions replace sodium ions on the s urface and, being larger, create a very thin layer of compression.
Toughened glass cannot be further processed since any damage to the surface will expose the centre layer, which is in tension, and the glass will shatter. The shattering of a car windscre en is a good example of this phenomenon.
The coating of glass surfaces has been practiced for centuries. Mirrors are a good example of this art. However, this method of giving glass new physical, chemical and optical properties has made great str ides in the last few decades. Lightweight glass containers are coated with organic compounds to give the surfaces a degree of lubricity and thus preventing abrasion in handling. This adds strength to the container and has enabled glass manufacturers to mak e a lighter and better product. Coating containers with tin compounds also produces a stronger product. Coating glass containers with plastic materials for added strength and safety is a further way of lightweighting or increasing internal pressure resistance.
Other forms of decorations are etching with hydrofluoric acid, sandblasting and vitreous enamelling. In the latter, vitreous enamels, which are low melting point glasses held in an aqueous medium are deposited on the glass through very fine wire me sh screens and are then fired in an enamelling furnace. The enamel thus becomes an integral part of the glass article.
Formed and annealed glass may be further processed. This may be done by taking away from or adding to the surface of the glass. It may also be heated, manipulated, and reshaped. These methods include:
- Taking away: A disturbance of the surface of glass may result in a matt or obscured finish. Where a transparent surface is then required this is produced by polishing on felt or wood wheels of by hydrofluoric acid solution.
- Adding: Vitreous enamels, which are glasses that melt at relatively low temperature and can be coloured, may be applied to the surface of formed glass. Metal compounds can also be applied. In both these cases the article is then reheated after application of the enamel or metal coating so that it fuses permanently to the surface of the glass. Also metal films can be applied by spraying, or by chemical or vapour deposition; and
Decorating Domestic Glass
- Manipulating: Glass which has been formed and annealed may be reheated and manipulated into a new shape. It then has to be re-annealed and may be toughened.
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On average, every family in the UK uses around 330 glass bottles and jars each year.