Used mostly in Asian ink painting and calligraphy I guess. Sumi = Charcoal. So it's made up of soot, water and some kind of binding agent...like shellac. Can be bought from Jet, Dick Blick, Jerry's Art-a-Rama and maybe Amazon in pre-mixed or ink stone and stick form. Much used in inking comics and Anime I think. I have a bottle of one of the Kuretake varieties made for dip pen inking, but have used it very little. NOT for fountain pens...dip and brush only thanks...but Iroshizuku has a black that's an homage...their Take-Sumi (Bamboo Charcoal). That's all I know, short form.