The copper is already green. The original wire was salvaged from old
open-wire railway telegraph lines. The sulphur from years of coal-burning
steam locomotives gave it a good coating of copper sulphate. Open-wire
telephone wires that I have seen were always (very rusty) steel. I suspect
the railways used copper clad steel because it would last much longer in the
environment of coal fumes that plain or galvanised steel, and not because
copper is a better conductor than steel.
The black stuff forms right on top of the green coating. The green coating
is very hard and must be scraped off with a knife or sandpaper to expose
bare copper, but the black stuff is loose, and can be rubbed off with the
fingers, just like soot. And just like soot, it is very dry and has a soft
feeling to the touch, not gritty or oily. The green CuSO4 patina seems
unaffected by its presence. When the black is rubbed off, the copper
sulphate coating underneath appears intact. That's why I am not sure if it
is a simple a chemical reaction with the copper, or a living organism.
Normally, copper that is not exposed to coal fumes around here just turns
brown, like a used penny.
The wire in question is #10 copper-clad steel.
I have used the stuff for ground radials, and it does turn green when buried
for a long time in the local soil. However, I have pulled up copperweld
that has been buried for over 30 years, and very little, if any, of the
copper jacket appeared to be eroded away. I occasionally check my present
buried ground radials (#12 solid soft-drawn copper) and they, along with the
silver-brazed connections to the ground bus, appear intact, except for mild
surface corrosion. With buried copper wire, the green tint doesn't appear
until it is thoroughly dry. When freshly pulled out of the damp soil, it is
sort of brown. I think a copper radial system would last for decades in
this soil. As I recall the last time I used a soil analysis kit, it tested
pretty close to neutral, PH-wise.
We do have a large coal-burning electric power plant a few miles from here.
My galvanised tower has to be protected from rain runoff dripping from the
copper wire. Otherwise, enough copper leaches off the wire to eat away at
the zinc coating, causing it to develop a greenish corrosion. I made some
rubber drainage tubing out of old automotive hose stock, and that keeps the
runoff from the wires from dripping onto the surface of the tower structure.
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