Just got these as a gift and I confess I had never heard of them. I got a set of four - java, walnut, eucalyptus and terra cotta. They are all apparently made of walnut crystals and some sort of dye. They're in spray bottles. I couldn't find much about them on the Tsukineko website, oddly. This is all they had:
Walnut Inks
Anyone used these? Are they an alcohol ink? Water based? What do you do with them? The fact that Tsukineko says you can use them on wood and leather makes me think they are not water based.