Self-Reduction Process: The cations of the less electropositive metals like Pb, Hg and Cu may be reduced without the use of any additional reducing agent.
Self-reduction of Cu:
$2\mathrm{C}{\mathrm{u}}_2\mathrm{O}\mathrm{ }+\mathrm{ }\mathrm{C}{\mathrm{u}}_2\mathrm{S}\to \mathrm{ }6\mathrm{C}\mathrm{u}\mathrm{ }+\mathrm{ }\mathrm{S}{\mathrm{O}}_2\left(\mathrm{g}\right)$
It is called self-reducing because copper oxide itself reduced the copper sulfide to give copper and sulfur dioxide.
Similar reactions occurs for Pb and Hg.