Forming Processes Early methods

Early crown glass was formed by spinning a 4 kg cylindrical gob of molten glass on the end of a blow pipe. The solid glass was blown, flattened out and then transferred to a solid iron rod or punty. After reheating it was spun until it opened out into a 1.5 m diameter disc. The process involved considerable wastage including the bullion in the centre, which nowadays is the prized piece. An alternative process involved the blowing of a glass cylinder which was then split open and flattened out in a kiln. This process was used for the manufacture of the glass for the Crystal Palace in 1851.

Subsequently in a major development, a circular metal bait was lowered into a pot of molten glass and withdrawn slowly, dragging up a cylindrical ribbon of glass 13 m high, the diameter of the cylinder being maintained with compressed air. The completed cylinder was then detached, opened up and flattened out to produce flat window glass.

It was only by the early twentieth century with the development of the Fourcault process in Belgium and the Colburn process in America that it became possible to produce flat glass directly. A straight bait was drawn vertically out of the molten glass to produce a ribbon of glass, which was then drawn directly up a tower, or in the Colburn process turned horizontally, through a series of rollers; finally, appropriate lengths were cut off. However, such drawn sheet glass suffered from manufacturing distortions. This problem was overcome by the production of plate glass, which involved horizontal casting and rolling, followed by grinding to remove distortions and polishing to give a clear, transparent but expensive product. The process was ultimately fully automated into a production line in which the glass was simultaneously ground down on both faces. The plate glass manufacturing process is now virtually obsolete having been replaced by the float process, which was invented in 1952 by Pilkington and developed into commercial production by 1959. Drawn glass is only manufactured for conservation work, where gaseous and solid inclusions in the glass are required to emulate the historic material.

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