glass ripples?

Question

in old glass window there are some times distortions due to the glass running what is the teachnical term for that?

i stuck this qeustion here because i assumed glass blowers would know this and its an "other-visual art"

Answer 1

I also thought that glass would "flow" after a significant amount of time, resulting in the distortions to which you refer in your question. It turns out that's not true - there's no technical term for it, because the glass started out distorted (it was more difficult to make windows uniformly flat back in the day!).

Answer 2

Before float glass and drawn glass, window glass was made by blowing fairly big cylinders and before that disks of glass about 5' across. When the cylinders were cut down the side and flattened in a kiln, the outside was slightly bigger than the inside and ripples tended to show up. Glass sold today made on purpose to look like that is called rippled glass. There is a similar effect where the glass actually has varying density and the lines of distortion are called chords and sometimes all distortion is called that.
The glass made by the disk method, called crown glass, was blown into a peculiar bowl shape then reheated and spun to form a big flat disk. Small panes of glass were cut from the disk, not using the center, which is why colonial windows have all those small panes. Also the glass is somewhat thicker nearer the center and sometimes this shows up as distortions.
http://users.ticnet.com/mikefirth/winlight.htm

Answer 3

Let me start by reitterating - glass is a solid and does not flow with time. Now for the answer:

It might be better to ask why modern windows DO NOT have ripples. The common method for flat glass production is called the float process. Liquid glass is floated and cooled on a bath of molten tin. This forms a truely flat surface on one side and surface tension creates a flat surface on the opposite side. Before this process there were various methods for producing "flat glass" but they generally were some form of centrifical spinning of molten glass. This caused fluctuations in thickness across the glass and as a result of refraction (based on the thickness of glass) it appears wavy and distroted. The term used in modern manufacturing for thickness variation is waviness - it's not too scientific but hey, it gets to the point!