Today I used a 6 inch by 6 inch clayboard that was a free sample at an art supply show years ago. It's a strange surface, as it soaks up (as does clay) any moisture or oil painted on it. I had to build up the layers to get the effect anywhere near what I wanted. The colors all turned out darker than I painted them, and I had to work with the values to get them right. The subject is the town of Silverton arriving from Durango, Colorado.