Last Updated: Aug 06, 2018 Views: 460
Hello! Thank you for your question. In historical container glass research, as you noted, the late 19th century until right before WWI is generally the time frame you see cited for the use of manganese as a decolorizer. However, there really isn’t any hard and fast date range, and which decolorizers were used by various companies depended on many factors.
In addition, manganese has been used outside of the container glass industry as a decolorizer for much, much longer. In fact, it is generally accepted as the oldest known decolorizer. According to the article "Solarized Glass" in All About Glass on the Museum's website, "Manganese dioxide is believed to have been first used as a decolorizer as early as about the second century B.C." Glass technology books from the 17th century refer to it as the "glassmakers’ soap" (they believed it scrubbed the batch clean of color). By the 18th century, glass technologists began to speculate that manganese didn’t so much “scrub” the color as balance the green tint caused by impurities in the batch.
By 1913, Eberhard Zschimmer (Jena Glassworks) wrote a thorough description of the most common decolorizers of the day and how they work---specifically manganese and selenium. He touched on a few others (less common) as well.
With improvements in industrial processes, of course, the volume of glass products increased substantially. Automatic bottle machines were developed and their use in the early 20th century began to make manganese a less cost-effective decolorizer. Cost factors tend to drive many changes in glass formulas, with factories always looking for a less expensive alternative to increase profit margins. By the 1920s, selenium was the primary decolorizer, but manganese was still used for some glasses, according to primary sources from the time.
I looked in glass technology books published in the 1940s as well as industry publications such as Glass Industry and the Journal of the Society of Glass Technologists and I don’t see any reference to a resurgence of manganese use. I do still see articles debating the “best” decolorizer to use during that time but most discuss selenium.
More recent work by Bill Lockhart published in Historical Archeology attempts to correct some of the looseness around dates for this topic. [See Bill Lockhart, "The Color Purple: Dating Solarized Amethyst Container Glass," Historical Archeology 40, no. 2 (2006): 45-56.]
You may find additional resources on this topic on the Rakow Research LIbrary research guide on Color.
Feel free to contact us with your glass-related questions in the future!
- Found a bottle west of Lamy, NM some years ago with aluminum, screw-on cap that was beginning to turn purple. A soda company still decolorizing with manganese. This was in the mid-1990s.