The Care of Your Scrimshaw: 

Try not to touch the engraved surface too much (or let others run their fingers over it...),as the oils in your skin can cause the colors to turn yellow or tan. If the piece gets wet, just allow it to air dry without wiping and it should be fine.

I also recommend to keep it out of direct sunlight. The pigment I use were chosen for their permanence, but sunlight will degrade all pigments eventually.