I studied on the Black India ink and read a few accounts of persons simply rubbing dirt into the etchings. I used calligraphy pigment ink. I’m guessing anything they had available to get down into the etching would hold because they could wax over it with beeswax. I sealed my end caps with beeswax and coated the entire horn in it to try to prevent heavy staining from use. Not sure if that’s a good train of thought or not, but so far it works on mine. I gave the scrimshawed one to my uncle. Makes me want to do more research. I’ve got two more on the backburner.