Walnut ink is a beautiful, multipurpose ink. I absolutely LOVE to practice with it- it is easy to clean up, has a remarkably clean consistency which results in gorgeous transitions, hairlines and swells, and is inexpensive, especially when made with crystals.
But, like ink will sometimes do, walnut ink can bleed or feather on certain papers, which is not suitable when practicing or producing finished works. If you experience this, try adding a few drops of gum arabic to your ink (the same can be done with Higgins Eternal and some other inks). I've found about 4 drops will fix a 1/2 oz. jar right up. Because gum arabic is a binding agent, it'll reduce or prevent the feathering of the ink as well as smudging. Be careful not to add too much of it, since it'll change the intensity and consistency of your ink.
Ink on, B