Cathy,

It all depends on whose source you read with regards to manganese. The
general hypothesis is that it was used to flux molten glass between 1885 and 1914
and that war in Europe terminated the supply. Other writers will speak with
great authority that manganese supplies lasted through 1920. Still others will
claim manganese is a myth. Based on physical fact, amethyst colored glass is less
common on sites dating after 1915. Then there is the straw-colored glass
hypothesis that a replacement flux during World War I existed from 1914 to 1920,
but there are detractors who declare that as a myth. Whatever the truth, clear
non-tinted glass is most common after 1920.

Ron May
Legacy 106, Inc.