The Color Purple: Dating Solarized Amethyst Container Glass

Abstract

From the late-19th century on, there was an increased production of colorless bottles for a wide variety of products. Producing colorless glass is not difficult if pure sand with a very low iron content is available. Iron in sand gives the glass a range of colors from light green to dark amber, depending on the amount of iron in the sand. To overcome this problem, some factories that used iron-bearing sands added manganese to their batch as a decolorizer. While this produces colorless glass, that glass will turn a light purple or amethyst color when it is exposed to sunlight. Dating of solarized glass by archaeologists has relied on information from a variety of sources, including books produced by bottle collectors. Some of this information is good and some of it, erroneous. The objective here is to provide a useful chronology of the development and use of manganese as a decolorizer and to dispel some of the myths that have crept into the literature.