Flowing Window Glass - A Myth

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Prior to the 1959 float glass process, plate glass was manufactured by the Crown process. A `bubble' of glass was first blown, then cut open and spun so that it opened out into a circular disc. Window panes were cut from the circular disc. These window panes were seldom flat and, consequently, many old windows made by the Crown process show variations in thickness. These are attributable to the manufacturing process and are not the result of viscous flow of the glass (see below) as is commonly reported. The thick centre portion in the centre of the disc was known as the "bull's-eye". This church window in Florence has been made from these bull's-eyes.

Photograph courtesy of J. Denier and J. Stott.

Flowing windowpanes - fact or fiction

Royal Society Proceedings A, 455, 1999, 2753-2758.

Y.M. Stokes
Department of Applied Mathematics
The University of Adelaide, South Australia 5005, Australia

Abstract

Glass has properties of a liquid. But do glass windows really flow over centuries becoming thicker at the bottom, as is commonly reported? Simple calculations show that the time $t$ taken for a windowpane of height $L_0$ to increase in thickness by $q$\% due to gravity $g$ is given by

$$ t=\frac{4\mu}{\rho g L_0}\frac{q}{100}, $$

where the glass has viscosity $\mu$ and density $\rho$. For the small windowpanes common in medieval times this amounts to some millions of years! Thus, window glass behaves as a solid.