A lot of Sowerbys' "colourless" glass develops a distinct yellowish tint when exposed to sunlight, especially that made in the 1940s and 1950s. Arsenic was used in the mixture both as an aid to decolourising with selenium and cobalt and as an aid to speed up melting and hence to increase production. The latter effect needed many times more arsenic than the former. It was only after some years that we noticed the horrible colour effect which was put down to using far too much arsenic. To this day I cringe and feel guilty when I see this, mainly at boot sales. For the record, just in case anyone has access to a suitable kiln, if the article is re-annealed, the effect disappears. A sideline for someone, Adam A?
No doubt other manufacturers fell into the same trap.