Quartz bulb: Galss glows blue after turning off.

Hi Folks,
This may not be the right place but it looks the best...
I have a torch which uses a 130W rally bulb (12V car headlight
effectively) and is ridiculously bright. I noticed though when you switch it off, for about 10-15 seconds, the glass glows this electric blue colour. Not enough to illuminate anything but just a nice blue colour.
Why would this be? Is it a coating in the glass or the glass itself that is glowing? It happens when the torch has been on for a few minutes.
TIA
Cheers, Crispin
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