Page 49 - Curriculum Visions Dynamic Book

2. Flat glass
Plate glass
The earliest flat glass was made by casting glass (see page 30). It was poured out into a shallow tray. Roman buildings had windows made in this way. However, the surface was very rough, and it was poorly transparent.
Blowing glass into a disk (crown glass, see page 39) cannot produce glass of even thickness. Eventually, however, machines were developed that produced a ribbon of glass of even thickness. This kind of glass became known as plate glass, and it was developed at the start of the 20th century. It was first cast and then drawn through rollers.
The scale of this process could be huge, with a tonne of glass being cast at a time.
The rollers determined the thickness of the glass. The process of rolling glass took away some of the clarity of the glass, so after rolling, it had to be ground and polished while it was moved along a conveyor. The grinding and polishing were an unwelcome stage in the process, using up huge amounts of energy and making about as much waste glass as finished product.
Float glass
To get over the problem, the float glass method was developed by Alastair Pilkington in 1959. In this case the glass is allowed to flow out onto the surface of a bath of molten tin. Surface problems do not occur in this process, and so no grinding and polishing are needed.
Tin was chosen because of its properties.
It hardly reacts with the glass (and even when it does, tin oxide on the surface adds to its strength), it has a low melting point (232°C) but a high boiling point, and therefore it remains liquid over a broad temperature range.
Other metal alloys can be used instead of tin to widen the range of types of glass that are produced this way.
(Above) Plate glass used in store windows.
(Below) Float glass in windows.
(Above) Specialized high
lead content glass in windows for laboratories handling nuclear materials.
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