Purple haze

If you’ve ever strolled around the Beacon Hill or Back Bay area of Boston, Massachusetts, you may have come across windows with panes of distinctly purple glass. These panes date from the mid 1800s when manganese dioxide was commonly added to glass as a decolouriser. Glass made from pure silicon dioxide is naturally colourless, but any iron present (which there usually is in sand) will give the glass a greenish hue – hence the green colour of many old bottles. But we’d rather not look through green-tinted windows, and adding manganese dioxide to oxidise the iron is a cheap way of decolorising the glass, at least temporarily. However, on exposure to sunlight over a number of years, this glass turns gently lilac or purple as the manganese itself is oxidised. The original residents were perhaps not too pleased at having their view turn purple, but with the passage of time these panes have become something of a status symbol.

Until recently, if you’d asked me where to find purple glass I’d have directed you to Boston, but on a stroll around a local village just outside Cambridge (UK) last autumn, I found several beautiful panes staring me in the face, one of which is shown in the picture below. I’m not quite sure how I missed them before (perhaps the blinds are new) but now I can’t help pausing to admire them whenever I pass.

Manganese glass

Incidentally, the second Windows 10 update of 2020 was code-named “Manganese”, but this is nothing to do with its use in panes of glass. Microsoft have been working their way along the Periodic Table; 2019 ended with Vanadium, 2020 began with Vibranium and then came Manganese. That might not be the first row of transition elements you are familiar with, but Chrome(ium) was already taken so they had to improvise.

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