Luminous disk.
I was sure I would never got a proper sample of promethium.
Fortunately, Max Whitby of The Red Green & Blue Company in England located a Swiss company that still had a supply of promethium paint, originally made by a Japanese company. Max sent them some glass buttons to be painted and then sealed shut.
I chose this sample to represent its element in my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.
