What can I do to clean oxidized copper?

Buy some bottom-feeding fish that feed on algae and other contaminants to do the cleaning for you.

Yes, marble countertops can also be cleaned with the same solution. It is also good way to disinfect your surface.

Just use a clean, dry microfiber cloth to wipe down your violin.

There is no real way to get a 'copper' color, but a bronze/brown is 20 volts.

Copper turns a bluish-green color (called patina) when in salt water. This is due to saltwater corrosion.

Copper sheets previously used in solar cells can be melted down and stripped of impurities, to form high-grade copper as a product, but this would require lab equipment.

আয়তাকার

You should try soaking it in lemon juice as the acid can remove copper rust.

Acid neutralizer added to your water system. I have also added a water softener to my system.