I am currently using Walnut Drawing Ink, from Enkaustikos, for my under drawings on encaustic portraits on wooden panels. It is a permanent water soluble, acid free ink and doesn't fade over time like natural walnut ink.
It works beautifully on wood and gives a wonderful depth under transparent and semi transparent hot encaustic wax.
This ink can also be thinned with water for lighter washes.
Enkaustikos also offer a walnut ink darkening medium for even darker images. It appears almost like wood burning when used on a wooden surface.