Ivory, which has become friable by age, may be made to recover its original goodness by boiling it in a solution of gelatin for some hours. It may be made soft and translucent by laying it in phosphoric acid of moderate strength, drying it in pure linen, previously rinsing it in water. When dry it is translucent and hard, but will soften on being dipped in warm water and milk. The time of immersion in the acid differs in different pieces of ivory, and if certain parts are to retain their original character, they should be covered with a varnish before immersion.
This article was originally published with the title "How to make Ivory Soft" in Scientific American 13, 11, 84 (November 1857)