In the extraction of copper from its sulphide ore, the metal is formed by the reduction of $\mathrm{Cu}_2 \mathrm{O}$ with $\mathrm{Cu}_2 \mathrm{~S}$. This reaction completes with the process of auto-reduction. Chemical reaction occuring in this reaction is as follows
$\begin{aligned} & 2 \mathrm{Cu}_2 \mathrm{~S}+3 \mathrm{O}_2 \longrightarrow 2 \mathrm{Cu}_2 \mathrm{O}+2 \mathrm{SO}_2 \uparrow \\ & \mathrm{Cu}_2 \mathrm{O}+\frac{1}{2} \mathrm{Cu}_2 \mathrm{~S} \longrightarrow 3 \mathrm{Cu}+\frac{1}{2} \mathrm{SO}_2\end{aligned}$
The molten copper is allowed to cool. During cooling, dissolved $\mathrm{SO}_2$ comes out in the form of blisters, soit is called blister copper.