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Purple Copper


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I distilled some pear yesterday, and when I came in this morning, the still was purple! It's gorgeous!

Anybody know chemically why that happened? I've half a mind to leave it like this. (See attachment below)



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I think the purple stain is a little more complex than the suggestion of merely an oxide of Copper.

All stills which are made out of Copper quickly form a protective layer of Oxide which is just the dull layer which is normal if the copper has not been treated to keep it bright.

It seems form the picture that the coloration is strongest at the front of the still, around and above the entry port.

Pears have seeds or pits which contain amygdalin which under acidic conditions will be converted to cyanide, th "OCN" chemical group.

In the presence of moist conditions this will form cyanuric acid.

Cyanuric acid on leaving the still body as a vapor and meeting the copper oxide on the outside of the still will form copper cyanurate which is noted for its bright purple or amethyst hue.

That's my theory. anyhow!

Alan Wolstenholme, BSc Chemistry, Edinburgh, Scotland

Caledonian Solutions Limited

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Alan! Thanks so much! This makes a lot lot of sense. When I was done with the pear, I opened the still and the steam flowed up all around the front and top of the still.

If I polish the still, I'm guessing it will remove the purple layer.

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The other thing to note is that the formation of copper cyanurate got the copper from someplace. Most likely, your still Scott!

Maybe it would be good to buffer the wash a bit bringing the pH up. That will reduce the acidic conditions which lead to the purple color. Not being a chemist I can't recommend one carbonate over another.

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