"Dave Liquorice" wrote in message
idual.net...
On Mon, 23 Dec 2019 09:53:14 +0000, Brian Reay wrote:

I recall an old church somewhere with a glorious green copper roof on
its spire. We stopped and admired it. Another tourist commented that
they should polish it.

Why do copper roofs go green?

Copper sulphate from acid rain.

Copper pipe or a hot water cyclinder
left outside just goes a dark brown.

Not always. I have got some with
green marks, likely from brick acid.

However the bits of overhead mains incomer open wire copper have gone
greenish. The engineers replacing the open wire incomer with coaxial
witter something about chrome. Cooper Chromate is green so presumably
"roofing copper" is some copper/chrome alloy that oxidises to green.

Its much more complicated than that and I was a chemist once.