It's would be cheaper to buy new copper sheet, and leave the copper
pieces for alloying with other elements, or for sculpture.
When you re-melt copper, you need to add flux (borax will do), and
you need to skim the dross off the top of the melt. Your copper will
Brass... I hate melting brass, the zinc fumes can make you very sick
and can make you very dead. If you have to melt your brass do it
outside or with very effective powered ventilation.
If you have large quantities, of these metals, I'd advise making a
furnace to melt them.
Melting large quantities of the metals you've listed above is
relatively easy to do.
Regards Charles A.