Flowing Windowpanes: Fact or Fiction?

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 LQ to increase in thickness by q% due to gravity g is given by t= 4μ/pgL0 q/100 where the glass has viscosity μ and density p. For the small windowpanes common in medieval times this amounts to some millions of years! Thus, window glass behaves as a solid. © 1999 The Royal Society.