Cleaning tin discoloration on copper/tin lid
Well you can clean the copper and the tin with some tamarind, it is a trick we use in our home to clean the copper and brass vessels and they come out to be very clean. Your may notice discoloration again in some 20 or 30 days after cleaning but then you can clean it again! 
Just wet the cleaning surface with some water and apply tamarind or simple tamarind paste to it, then simply scrub it with a normal scrubber and  wash it.