I have been using Natural pigment powers as a basis for coloring etching inks, adding it to my ink "base" (essentially ink without the color). I have had mixed success, so pigments working wonderfully for this, others not so much (see below).
I'm wondering about Gilsonite (Asphaltum). In the printmaking world a product called Asphaltum is used in a ground to control the etching process.
So, I'm wondering if Gilsonite (Asphaltum) will also work as a pigment powder to create a colored etching ink? (I'm thinking that the pigment sold here is different than the product used as a ground in etching.)
As an aside, regarding my experiments in making etching inks: Vincenza White gives a wonderful warm toned color, but has some larger particles in it, so the ink is a bit gritty and can scratch plates; the raw green umber also gives a wonderful color but is really hard to wipe, almost acting like a clay. I've had good success with the earth greens (Nicosia, Cypress, etc) and also some of the reds, like Ercolano.)