Amethyst-colored glass is caused by the reaction of manganese used in the glass batch to decolorize (colorless) with UV light, particularly sunlight aka solarized glass). Manganese started being used regularly ca. 1870 & fell out of favor for a number of reasons by ca. 1920.

Color ranges from a slight purple/pink tint so subtle that analysts have to place shards on white paper to tell, to a dark purple. This is related to the amount of manganese used & time exposed to UV rays. Collectors have been known to place bottles under UV lamps to enhance it.

Early myth in archaeology was that manganese stopped being used bc shipments of the mineral from Germany were halted bc of WW1. Lockhart (2016) says that an alternate, selinium, was found to served the same decolorizing purpose before the war. Manganese producing countries

friendly to the US still shipped manganese, but the mineral was in demand for the war effort, thus glass companies stopped decolorizing or turned to other methods, the main being selinium. Some glasshouses had piles of manganese that they still used, thus the date lag to ca 1920.

It should also be stated that there could've been a lag in generic bottles produced with manganese in warehouses too. In another case, here in SE Louisiana we have a case where a German immigrant glassblower in Covington who produced bottles for local businesses had a pile of

manganese that he used into the 1930s. How many small glasshouses across the US continued using manganese into the 30s, 40s, even 1950s is unknown, thus we have to be careful relying solely on amethyst glass as a dating tool for archaeological sites.

I should close by adding that by the time that World War 1 ended, most large glasshouses had switched to using selinium & other minerals which were seen as having more stable supply & less demand during wartime than manganese.