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A brush with most of the ink worked out—"dry brush" —is effective in rendering the general growth direction of "twiggery", but avoid a too mechanical look. See cover Field Maples. Less hygienic but more convenient and effective is the "spit-and-ink" method, or, more politely, ink and smudge, in which the pen line is softened or a tone developed by the use of a little saliva on the finger tip. This is particularly effective with non-waterproof Indian ink; use the side of your drawing for trial effects, where also a blob of ink can be left to act as a kind of palette.
Amateur Artist, June 1967, page 22