As a past member of a loosely organized old house homeowner association I often visited heritage homes in our city. The owners are always justifiably proud of the results of their work on restoring and preservation efforts and, as you might imagine, people who attend these kinds of events often know each other.
On one occasion, the topic turned towards the preservation of original windows. One person told us that old glass has direction. In the past, savvy window manufacturers knew to install panes of glass either vertically or horizontally. If a pane of glass was replaced, even with old glass, and the direction was wrong, it would look “off”.
I remember, when visiting one of the cathedrals in England, a tour guide saying the stained glass windows were thicker at the bottom than at the top because glass was supposed to be a liquid that settled over hundreds of years and mentioned that. The member of our group responded by saying this was a myth and the uneven thicknesses of antique glass has more to do with the way it was made at that time and for some reason, the installers placed the thicker section at the bottom. In order to look right, the restorers today need to be able to match the thickness not only on the top but at the bottom. Who knew? She sent us all an article from Scientific American magazine which I have attached a link to HERE. They can explain it much better than I ever could.
Another myth busted… Oh well.