Piano keys eventually become soiled with accumulated oil and dirt from fingers. To clean your white keys, use a soft cloth dampened with water and a small amount of mild soap. Avoid solvents. Make sure the cloth is thoroughly wrung out, and wipe the keys back-to-front rather than side-to-side, so excess moisture and dirt will not seep down the sides of the keys. Clean only a few keys at a time drying immediately with a clean cloth.
Ivory keys are porous, and excessive moisture can penetrate and loosen their glue joints. Also, a dirty or brightly colored cleaning cloth can transfer stains into the ivory.