Use a damp cloth with some non-abrasive cleaner (Eg. Windex, Vinegar and Water). Avoid using one with ammonia as this may harm the casework or dissolve the blackness of the sharps. Don’t allow any moisture to get down between the keys, otherwise they may swell up and stick/warp.
Once ivory becomes yellowed it is very difficult to bring it up to white again, as the discolouration goes through the grain.
Yellowing is caused from ivory being in the shade for long periods. Makkreel Piano, provides ivory pieces for piano restoring at $5.00 each, some cheaper based on colour and size. We also provide and instruction sheet on what glue should be used and for cleaning ivories. Please contact us, and we will be happy to email this to you.
Take a look at our Ebay deals for ivory pieces if you are looking to buy new keys.