[Klaus]

Yes, UV light seems to work. Some are pointing to a specific Ikea UV lamp for fixing the yellow cast. ;-)

FWIW, I found this here:

Over extended time periods, thoriated glass may develop significant discoloration. This is due to induced F-centers forming in the glass as the radioactive decay of the thorium progresses. The formation of F-centers is due to the ionizing effect of the high energy thorium decay products. This process can potentially be reversed by annealing the glass or exposing it to light.

Ref.: https://en.wikipedia.org/wiki/Thoriated_glass