The accepted technique was actually to lay a resist on the ivory, then use and acid or some similar 'biting' fluid to eat away the exposed parts. The resist was then removed, and a layer of black enamel - or even various colours - were rubbed into the etched lines to enhance them.
But I digress....
Regards,
Neil
PS: nearly forgot to congratulate Tom on the aquisition of another splendid oldie and a stunning resto job - getting used to that with Tom, though!
WOW, Neil! Great description of a seemingly tedious process! Thank You, Sir!