I use to do quite a bit of work on scratchboard--a layer of clay is laid down on paper or board, and then a layer of ink is laid over that. You use an exacto knife or other pointed instrument to scratch lines in the ink.
It's fairly spendy, hard to find, and if you aren't careful (which I never am, lol) you get ink dust in your eyes, leaving them gritty. But I love the look you get from the medium.
Lately, I've been reading "The Last Apprentice" series. It's a young adult fantasy series (a genre that I enjoy), but it's the illustrations that attracted me. Patrick Arrasmith works on scratchboard, and I just love his art. It's been an influence of late. Even though it isn't zentangle, the very nature of medium gives it a similar feel, so I thought a lot of you might enjoy checking out his site.
I couldn't stand it and dug up one of the claybord (the clay and ink are laid over board, rather than paper) panels I had, and tangled it. Hadn't done a humgriffin for a while so I decided to add one of those. *sigh* You do lose the technique if you don't use it. For one thing, you have to remember to draw the highlights rather than the shadows. I'd also lost the ability to carve a good curve or circle. And my poor wrists ached afterward, as well as my gritty eyes. You aren't supposed to blow away the ink dust but I always forget, and then regret it.
The panel was too big to fit flat on my scanner so this jpg is a bit blurry, but you get the idea.