Cleaning the Keys

For those of you visiting from the Organ Forum or DIYapason list serve, here is the picture of my yellowing keys.  Sorry for the slightly blurred picture.  The lighting in my workshop is not great for taking photos.  However, you can still see the discoloration around the edges of the keys, and the relatively original color at the back of the key surface where it was covered by the felt.  Click on the image to see it in its original size.  I doubt my key tops are ivory, but possibly Ivorene??  What is the best and safest way to clean and brighten them?  Please feel free to comment here or on the respective groups if your advice would be helpful to others.  Thanks!!