Gerald J. is correct.............whatever you do to polish or clean the kettle is going to remove the patina, which in the antique world is a big no-no.  If it's something that has sentimental value and you intend to keep forever, do whatever pleases you to get it to look the way you want it to.  If you have intentions of ever selling the kettle, it's best to keep it old and dirty looking.  Antique-ers value patina almost more than the item itself.  If you do decide to clean it up, be very very careful using anything that is abrasive.  Copper is pretty soft, and cleaning powders, ScotchBrite, etc. will scratch the surface and not let it shine like a new copper penny.