This is fascinating. I've walked over those countless times and always wondered what they were. I am curious about this one bit though:

> When vault lights were first installed, much of the glass was clear. But when the manganese is exposed to UV rays for long periods of time, it photo-oxidizes and turns purple or pinkish. Hence, the reason so much of the glass is now purple.

> This process can take decades. So when you see colored glass, it’s either really old or someone dyed it to mimic the old glass.

So what happens to dyed glass after decades? Does it have a deeper purple? Is it made out of materials that _won't_ change color? Something else?