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Red copper


Ako Shirotani was asking about red copper. The redness he
produces is due to a surface film of copper oxide, of which there
are two: CuO, which is black - the dreaded firescale - and Cu2O
which is red, and a chemically reduced CuO which has lost an atom
of oxygen. One way of producing the red oxide is to give the
perfectly cleaned copper an even coating of copper nitrate, then
carefully heat it in a reducing atmosphere. A soft, large
yellowish flame on charcoal as he says would be best, but the
copper nitrate film must be even - you’d probably need to spray
it. There are ways of getting a reducing atmosphere, but I
reckon they are (a) too dangerous and (b) too complicated to talk
about here. If other items are to be soldered to the copper, then
that operation should be done first. After all, the purpose of
flux is to dissolve and thus REMOVE oxides from metals. If a
hard grade of silver solder is used - even enamelling grade
solder - it should take and flow easily on to carefully cleaned
copper with Easiflo or borax flux - I’ve done it many times: it’s
stainless steel which doesn’t like silver solder. But as I
said, you’d have to do all the soldering operations first.

   / /    John Burgess, 
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