If ... repeat IF ... they turn out to be not valuable and you really do want them shiny, then soaking in vinegar or brown sauce for a couple of hours should bring them up.
But I repeat what Vagrant says - get them valued first. I'm in touch with a fair few archaeologists and metal-detectorists, and they all say that cleaning coins beyond 'finger clean' (ie rubbing off the worst of the muck in warm water with your fingers) can corrode and devalue them. Same goes for any other metal 'finds'.