I knew that glass was a very viscous liquid (at least most glass) and always wondered about how much activity really occurs in a pane over the years,
It's not only old churches that have drippy windows. Even 18C houses show evidence of the viscosity of glass. To go off on a little glassy tangent, if you know that a building's window glass is original, it can be used to (roughly) date the house. The earliest form of window glass is called crown glass. It was blown out in a large flat circle, and then rectangular pieces were cut out of it. This leaves concentric arcs of bubbles and other imperfections in each piece. The small central piece of the crown, called the bullseye (with a little nub from where it was attached to the blow pipe) was often used in transoms or front doors. Around the turn of 19C (I think -- I don't have my reference books here) a process was developed to make so-called cylinder glass. A large sphere of glass was blown, and then that sphere (still attached to the pipe) was swung back and forth in a heated trench, so the sphere would stretch into a cylinder. While still slightly molten, the ends would be cut off and the cylinder was cut down its length and unrolled to form a flat rectangular piece that could be cut to size with much less wastage. Cylinder glass, therefore, has long, stretched-out bubbles and imperfections in parallel lines. Drawn glass was developed c.1915, in which the glass was simply pressed into sheets with asbestos rollers and finally float glass (in which the glass cools on a bed of molten tin) was introduced in the 1940s. Float glass is the most optically pure, but it has none of the character of the old handmade glass.[/tangent]
Oh, and welcome back BY!
