I too would be inclined to wax it rather than soak it in oil. The movement of the oil through the pores of the substance could destabilize it some. From a chemistry standpoint, I'm trying to wrap my mind around "I believe that the dry climate sucked the moisture out by evaporating the oil." If we agree that moisture is H2O, I'm not sure the oil has any of that. Also don't think a cold climate can evaporate oil, which typically has a high evap rate. The dry air could have pulled moisture from the ivory, but that would have been unrelated to the oil. Waxing it in such a way that seals off some of the pores of the material could prevent some moisture from escaping. In addition, re-hydration would need to be done with water, not oil.