This image taken from Raw to Ready: Mack Truck demonstrates how remaining impurities are removed from the copper during a process called electrorefining. Workers submerge the copper slabs in an electrified acid solution for ten days. When an electric current is run through, the copper atoms come off the positive side and attach to the negative side, growing a very pure copper block. All of the impurities fall out to the bottom. The copper is now nearly one hundred percent pure.