Suspect that the coins are actually copper plated steel, so it's unlikely that copper is involved.

Attached is an image of some similar artwork. Aged rust can be seen on some of the welds ...

Hi, Am intrigued as to how this artist has created his face artworks: https://shaungagg.co.uk/#/money-in-mind/ I hope he won't mind me linking to his page. Fabulous artwork. Seems to be copper (probably steel) pennies welded together. The welds look very fine, no splatter, no mess. I'm a complete no...