It is important to note that ivory is porous. Excess moisture causes ivory keytops to warp, resulting in permanent and costly damage. Mix 3-4 drops of a liquid dish soap in a gallon of warm water. Start with two clean cloths, soft and white. Soak one in the soapy water, and ring as much excess moisture from it as possible so that it is nearly dry. Work with one white key at a time, gently scrubbing each it from front to back (not side to side), ensuring that you are not leaving any standing water and that water is not seeping between the keys. After scrubbing each key, immediately dry and buff it with the other dry cloth. As the stain is sometimes released from the black keys, it is generally a good idea to have separate cloths for the white and black keys.