Yellowing of ivory is a natural process of age with which one should not interfere. A little methylated spirits mixed with water gently applied and quickly dried will sometimes remove stains from ivory, but first ascertain that original staining is not being removed from a valuable old piece. Ancient ivories were often burnt-stained or artificially coloured. 
Ivory is particularly prone to cracking in heat, sunlight, wet or dry atmosphere.
Not everything that looks like ivory is in fact ivory. There are many man made substitutes and it can be very difficult to determine whether an item is made from ivory.
To test if it is ivory, wrap a long piece of hair around the ivory. Wrap it tightly so that it is secure around the material. Light a match and put it closely to the hair in contact with the ivory. If the hair burns, then you do not have ivory, probably bone, as bone is porous allowing oxygen to seep through thus burning the hair. If the hair does not burn, then you have ivory, because ivory has no pores and does not allow oxygen through.
Alternatively place a hot needle on an unobtrusive part of the piece, usually the underside of the base, and see if the material burns or melts. If it does, it is not ivory. If the needle point makes no impression, then it is ivory.
Water should be used sparingly when cleaning old ivory, which under adverse conditions warps and cracks. Heat must especially be guarded against, therefore only lukewarm water with a small amount of detergent should be applied to ivory and each piece needs to be dried immediately. 
Clean with 1 part Vulpex in 20 parts distilled water. Do not soak the ivory but just wipe it over to clean off dirt. If the ivory is dry in appearance paint it with almond oil and after 8 hrs, wipe it absolutely dry. Finish with one or more layers of Renaissance micro-crystalline wax.
Care Tips
Particularly prone to cracking in heat, sunlight or a dry atmosphere or if it gets wet.
  • Keep away from sunlight and radiators since these will cause the ivory to crack
  • Ivory likes some humidity and is best stored in a closed cabinet with a glass of water. Open the cabinet from time to time to prevent mould from forming
  • Dust with a brush
  • Do not wet ivory with anything but alcohol or methylated spirits on a cotton bud. The resultant white surface can be restored to its former state by rubbing with your hands
  • Store away in acid free tissue, and not cotton wool
See also Jewellery & Gemstones