Digital BW, The Print

When possible, pencil SHOULD be used to sign a print, but when I can't
use it, what I use is a calligraphy pen set (you have several tip
widths available) filled with the lightest hue of my inkset. If
needed, you can dilute it to your own taste/needs. It works with
practically any kind of paper, and their advantage is they are soft
and have a nice flow on almost anything. Rotring has some very nice
ones but if you'd rather get more classy, Mont Blanc has an amazing
variety of tips that slide beautifully on papers.

Regards,

Pablo