The last step in the extraction of pure copper was refining. This made the brittle coarse copper fine and malleable. Refining was a process of oxidation which removed the majority of the remaining alloy components. These were primarily zinc, tin and cobalt, which would otherwise have hampered the further processing of the copper.
The refiner was constantly taking samples to check the quality of the tough-pitch copper. It was kept in molten form until the surface took on a brassy colour and it stuck to a slag hook.