The cause is the same -- i believe it is alpha radiation but several pseudo-experts I just checked claim it to be the ultraviolet part of the light spectrum. By both sun-exposure & irradiation, it is manganese used as a decolorizing agent that turns amethyst purple. The story has been told so many times by so many well-intentioned but un-scientific collectors that the details get blurred (I just spotted a website with one version of the story claiming Magnesium ! is the chemical. No, Manganese, not Magnesium!) But this source does point to the presumed year 1915 as the cutoff for when American clear glass chemistry changed to use a different decolorizing agent; I suggest arsenic, this source says selenium, I think both have actually been used, but not Magnesium.
Through all this discussion, I do not know for sure how much bottle glass would have been "improved" by the use of a decolorizing agent; bottles are not my thing but I expect that the manganese / arsenic / selenium used in improving crystal glass used in making goblets, bowls & vases was probably not used as much in bottle-making, --where clarifying the glass to make it less discolored was not a major concern when mass-producing cheap condiment containers. When I find a potential goblet or decanter for my collection and I detect purpling, that's the kiss of death -- it's poor glass or it has been mistreated (or both), proving it is not worthy of a fine antique collection. BobB
See Elaine Henderson's website http://www.patternglass.com/sunpurple.htm ...her home is Phoenix AZ where every antique shop has old purple glass, real-fake-reproduction.