Even though it's currently illegal in the United States to manufacture pianos with ivory keys, due to a wildlife conservation act that protects elephants, plenty of older pianos with ivory keys still exist. Ivory piano keys can suffer from the accumulation of dust and dirty fingerprints, and will eventually need cleaning. Yet, unlike plastic keys, ivory keys are porous, so excess moisture can result in damage.
Fill a small container with warm water. Add 4 or 5 drops of mild soap to the water. Agitate the water with your fingertips to mix the soap and water.
Dip a clean, white, soft cloth into the soapy water. Wring the cloth free of excess liquid.
Wipe two or three ivory piano keys with the damp soapy cloth from top to bottom. Wipe the damp keys with a clean, white, dry cloth immediately to remove the residue.
Rinse out the cloth and wring it as needed. If the cloth becomes soiled, retrieve a clean cloth for use. Repeat Step 3 as many times as needed to clean all of the ivory keys.