4 ms·Lots in London, and many very old - but I've never spotted purple ones.by Angostura 7y agoLots in London, and many very old - but I've never spotted purple ones.lgeorget 7y agoWell, the color being due to exposition of the prisms to UV radiation, it will take a lot longer to reach the purple tint in London than in California.jacobush 7y agoAbout a million yearsSymbiote 7y agoIt's more likely that British glass manufacturers used a different chemical.