The wearing of the.... 540 nanometres?

For the day that's in it, some thoughts on the colour green...

Firstly, exactly what colour is 'green'. A few sources say that any wavelength of electromagnetic radiation between 520 to 560 nanometres is considered green. Others go from 492 to 577 nm, but I think that that range stretches right into the colours most of would call yellow and blue-green (or cyan). Either way the middle of range is 540n, so I'm taking that to be green-green.

Whatever its limits, most would agree that copper sulphate burns with a green flame - as does sodium borate and barium. Less widely known as green flame sources are compounds of thallium, antimony and manganese.

When did it become associated with Ireland? Going backwards, it seems to have become established as a link in the 1800's, particularly in America. That may have something to do with popular ballads like 'the wearing of the green,' which may in turn come from the wearing of green by the 1798 rebels - which may have been an echo of the use of a green flag by Owen Roe O'Neill in 1641, which may have been a conscious rejection of the blue flags previously used in Ireland.

The association can only have been helped by the fact that green dyes of natural origin are reasonably easy to find. But this led to problems in Victorian England when the Wilhelm White Lead Company began selling a dye that was notable for its vivid and strong colour. It became known as Emerald Green and was immensely fashionable - not just in clothes but in carpets and wallpaper. Which was problematic because the contained more than a trace of arsenic. Many deaths followed before it was eventually banned.

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