Authors: Philip Gibbs
It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom. This is not true. In Mediaeval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass. Nevertheless, the frequently asked question “Is glass liquid or solid?” is not so straightforward to answer. To do so we have to understand its thermodynamic and material properties.
Comments: 8 Pages. originally published in the Physics FAQ 1997, also in Glass Worldwide, 2007
[v1] 2019-01-04 07:39:51
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