Wednesday, February 20, 2008

Glass - featured in the latest SalvoNEWS

The manufacture of glass in this country can be traced back to 1200AD, but it did not become readily available until the 1600's, and by the 1900's its usage was widespread. Glass is an entirely man made product being an amalgam of natural silica in the form of flint or fine sand, potash, sodium and lime, also recycled glass. All is then heated to melting point and then formed in the required shape.

Early glass was manufactured in the form of either "spun" or "crown" glass. In this process, a blob of molten glass was spun on the end of a shaft, due to centrifugal force the blob gradually took the form of a disc increasing in diameter as the shaft was rotated until as near a constant thickness as possible was achieved. Once cool the disc was then cut into the required shape. The central bullion was rarely used due to its irregular shape and obscured vision. none the less they can sometimes be found as decorative small panes in external doors for instance. crown glass can be recognised fairly easily as it frequently has curved sweeps formed during the spinning process which tended to develop as the disc's rotation ceased to be constant. The effect can be likened to the ripples on a pond after a stone has been dropped onto the surface.

Cylinder glass production was more complex. The glass blower would place a lump of molten glass on the end of his blow-pipe and gradually blow producing a long elongated balloon. When satisfied with the length and that the diameter was roughly constant it would be allowed to cool. the ends of the balloon were cut off leaving a cylinder, this was then cut along its length and relaxed in an annealing chamber until flat.

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