The black oxide is prevented from forming on the copper by dipping it afterwards in the salammoniac solution, and then washing it in pure water. This process is entirely different from that of Mr ...

In the video embedded below, [nylesteiner] explains that copper oxide can be formed when heating a copper wire using a propane flame. When two oxidized wires are placed in contact with each other ...

Stop adding the copper oxide when it's in excess - in other words when the black powder doesn't disappear anymore. This means there is no remaining sulfuric acid to react. The next step is ...

It looks more like a charcoal briquette than anything, but the black brittle thing at the ... here is a mix of yttrium, barium, and copper oxide that goes by the merciful acronym YBCO.