Then I had to test the ink with watercolor, to make sure it really was waterproof. This resulted.
Good ink. Very good ink! Loving it. Perfect for line and wash. And the pen is pretty good, too.
The ink? de Atramentis Archive Ink. The pen? Pilot Custom 823 with a modified nib (added flexibility for varying line widths).
7″ square Stillman & Birn Beta notebook (cold press 180lb) – I highly recommend the Beta and Zeta notebooks for both watercolor and pen sketching.