Float glass

[edit] Introduction

Float glass (sometimes referred to as clear float glass) is glass that that has been manufactured from molten glass floated on the surface of a heated liquid of greater density such as tin (or in the past, lead) and then cooled. The result is a smooth glass of uniform thickness that gives undistorted vision, high light transmission and optical clarity.

Float glass is the most widely used form of glass for consumer applications. While in a semi-molten, waxy state, it can be easily shaped and formed to suit a variety of uses. Once made, it can be further processed to create other types of specialist glass, such as toughened glass, laminated glass and acoustic glass.

[edit] Manufacture

The first commercial application of the float glass process (the ‘Pilkington process’) of forming a continuous ribbon of glass floated on a molten tin surface was devised in the mid-1950s by Sir Alastair Pilkington and Kenneth Bickerstaff at Pilkington Brothers in the UK. The glass was flattened by its self-weight and flowed through the process under the influence of gravity. However, it was not until 1960 that the bulk process was made a commercial success.

The raw materials that go into float glass mainly include silica sand, soda ash, limestone and other ingredients. Once mixed, the materials are fed into a furnace with any recycled glass that may be included and heated to as high as 1,500°C to become a molten mass. It is then cooled before being poured into a tin bath.

Smoothness on the metal side is imparted by contact with the molten tin and on the upper surface (the air side) by polishing with fire. No additional polishing is required. Poured onto the molten tin in ribbons at an initial temperature of around 1,200°C, the speed of the glass through the process can be left to gravity or by top roller gears on the glass surface propelling it forward.

At around 1,100°C, the glass enters a lehr oven to be annealed i.e allowed to cool very gradually. This controlled cooling allows any internal stresses in the glass to dissipate. When it emerges from the oven, the glass is further cooled by jets of air after which scanners seek any imperfections which, if found, will result in the glass being discarded or recycled. Once at room temperature, the glass is trimmed and cut to suit customer requirements, after which the sheets are packed for shipping.

[edit] Applications

Because it results in a perfectly flat surface, float glass can be used in most flat-glass applications. It is available in a wide range of sizes and thicknesses for use in mirrors, automobile applications, windows, doors, insulated glass, light fittings and so on.

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• Overheating.
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• Secondary glazing.
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