Inside of a pinch, I'd do this with saliva and my finger! Softer tones together with ink traces develop a great distinction and is likely to make your drawings a little more lifelike. With this in your mind, I hope you get pleasure from drawing with pen & ink https://ink-pen-drawing-technique28427.amoblog.com/the-smart-trick-of-ink-pen-drawing-techniques-that-no-one-is-discussing-58788676