Last Updated: Apr 17, 2017 Views: 45
Our research scientists passed your question about manganese dioxide along to the Rakow Library. 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---glass technology books from the 1600s refer to it as the glassmakers’ soap, (they believed t 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---apparently, it works better in a crucible type furnace used with mouth-blown containers. (Don’t ask me to explain---I’m a librarian!---but an article I’ve attached touches on it briefly). 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 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. Have you see the more recent work by Bill Lockhart from Historical Archeology? In it, he has attempted to correct some of the looseness around dates for this topic. (The Color Purple: Dating Solarized Amethyst Container Glass. Bill Lockhart. Historical Archeology, Vol. 40, No. 2 (2006), pp. 45-56). I am attaching it to this email. I’d be happy to send you articles from glass industry publications in the 1940s if that would help. Let me know! I am also sending a list of sources about manganese and decolorizing. Again, if you are interested in any in particular, let me know and we can figure out how to get the information to you.
Attached Color Purple/Bill Lockhart/Historical Archeology/Vol. 40, no. 2