Copper oxidises in air. It happens much faster with heat.
For different specialist materials, I've used borax for silver soldering and 6% phosphoric acid (diluted rust remover) for low melting point bismuth solder on brass and white metal. I'd expect they would both work for conventional solder as well.
Reputedly, saliva also works on some metals.
If you are adding any substance to the metal, you cannot say for certain without knowing the exact chemical makeup that there is not something that functions as flux. Many polishes leave a protective layer to prevent further tarnishing.
For different specialist materials, I've used borax for silver soldering and 6% phosphoric acid (diluted rust remover) for low melting point bismuth solder on brass and white metal. I'd expect they would both work for conventional solder as well.
Reputedly, saliva also works on some metals.
If you are adding any substance to the metal, you cannot say for certain without knowing the exact chemical makeup that there is not something that functions as flux. Many polishes leave a protective layer to prevent further tarnishing.