what kind of ink does Robert Crumb use when drawing?

Since Robert Crumb uses a Rapidograph pen, when drawing, I would assume that he uses Rapidograph ink. There are many types of ink that can go into a Rapidograph pen, but the most common is Ultradraw.
Ultradraw ink was created to provide technical pen users a rich, non-smearing, black India ink with a long open time for non-clogging. It says on the bottle that it is waterproof, (but if you try coloring/painting over it, there is some smearing) dries quickly on all drawing surfaces and is also suited to airbrushes, crow quills and calligraphy pens. In a .75 oz. filler bottle.

Rotring Rapidograph Replacement Nib 0.18mm Technical $29.69

Rotring Rapidograph Techincal Pen - 0.70mm Nib Size $31.58

Rotring Rapidograph Replacement Pen Nib - 0.30mm $19.54

Rotring Rapidograph Ink Cartridges - Blue x 3 $7.80

Drafting Compass Tool Set Made In Germany & Koh-I-Noor Rapidograph $15.99

ROBERT CRUMB HRDCVR ART BOOK UNDERGROUND COMIX SKETCHES WAITING FOR FOOD OOP BBW $18.88

CRUMB, ROBERT - HOT WOMEN-WOMEN SINGERS FROM THE TORRID $11.99

The Book of Genesis Illustrated by Robert Crumb (2009 comic form Hardback) $24.99

Life and Death of Fritz the Cat NEW by Robert R Crumb $17.38

The Sweeter Side of R. Crumb by Robert Crumb (2010, Paperback, Reprint) - New! $2.50