I've seen these clay bowls with lines before and I knew they were some kind of mortar. In Hidalgo and Oaxaca, they seem to be called molcajetes, just like their volcanic rock cousins. I've also seen them called chilmoleras, which makes more sense (chiles and molinar).
In Muñoz' Diccionario Enciclopedico de Gastronomia Mexicana, he suggests it's especially useful for the classic Yucatan dish chilmole.
The issue is what to use for the pestle! I have a wooden pestle from a Spanish mortar and pestle set that I'll try out.

Visually beautiful.
Robert H.
(ceramic maker)
Posted by: Robert Hamilton | December 12, 2011 at 07:16 PM
Interesting. It reminds me of the Japanese suribachi, a grinding bowl that has lines scored into the ceramic to assist with grinding. I don't recall ever seeing a volcanic grinder in Japan, however, which you might expect to see since there is plenty of volcanic stone around.
Posted by: Marc | December 25, 2011 at 08:42 AM