This was the first piece I started in the disused carpark. (Charcoal, chalk and emulsion paint on concrete.)
I’ve since added a layer of UV sensitive paint, so if you shine a UV light on it, or even just “charge” the paint with a torch for a few minutes, it glows.
Kind of like this.
Of course, almost nobody knows this even exists, much less that it’s UV sensitive, but I’m pretty sure it’s safe to tell your secrets to the internet. Yeah? Alright.