Not stained glass related, but Seattle glass related...

Here's an article on the future of Seattle's Pioneer Square prism glass. Interestingly, they weren't originally purple but have turned that color due to the manganese in the glass interacting with ultraviolet light. This is very common in windows from the 1800's and early 1900's. Selenium replaced manganese as the preferred clarifying agent in American made glass around 1915, but it wasn't immediately apparent that the glass would age differently. 

https://savingplaces.org/stories/illuminating-the-path-forward-for-seattles-pioneer-square-prisms