In article ,
Tony Williams writes:
Bare copper is a reactive metal, subject to long term
oxidation, or verdigris, or other chemical attack.
To make a reliable crimp joint onto bare copper needs
something that first of all cuts through any oxidation
that may be there, and then makes a gas-tight connection.

Copper Oxide (unlike Aluminium Oxide) is a conductor though.
So providing there's only a thin layer and the copper is still
reasonably copper coloured and not dark red or black, there's
normally no problem. The deformation of the surface of the
copper in any type of terminal will break through it anyway
(again, unlike Aluminium Oxide which is extremely hard).

--
Andrew Gabriel