In a Nutshell
It’s often said the glass in old windows is thicker at the bottom of a pane because over time the glass has flowed downwards. It’s true that glass is an amorphous solid, and so can be described (technically) as a fluid, but it moves so slowly at room temperature that changes would never be noticeable. Most metals are far more fluid than glass. Old glass appears thicker in some parts because it was blown and stretched rather than poured like modern glass.
The Whole Bushel
It is a common myth that glass, such as that found in old windows, is thicker at the bottom than at the top because it flows over time. It is even said that glass is a ‘super cooled liquid.’ The suggestion is that over time gravity has made the ‘liquid’ glass run.
Glass is not a liquid, but neither is it a solid. The molecular arrangement within glass is not as rigidly ordered as in a crystal nor as loosely associated as in a liquid. Glass is an amorphous solid. This means that it displays a certain amount of fluidity. So doesn’t that mean it really is runny?
Not to any real extent. The ability of a substance to flow is called viscosity. The higher the viscosity the less runny a substance is; honey is more viscous than water. Viscosity is measured in units called poises. Water at room temperature has a viscosity of 0.01 poises, olive oil 1 poise. The viscosity of glass? Approximately 100,000,000,000,000,000,000 poises. In other words you could leave a pane of glass for a million years and never notice it flowing.
Any difference in the thickness of old windows is due to imperfections in the manufacturing process. Flat glass is actually made by cutting blown cylinders of molten glass which is then pulled flat. Do this incorrectly and you will end up with an uneven pane.