I imagine this question has been asked before, but I couldn't find the thread. Traveling in Mexico I found an exposed cliff wall full of earth pigments - purples, reds, yellows, greens. I've taken some home and played with them and they make beautiful paint. But I recognize that they should be processed before being used in an actual painting (i.e. to be archival). What are the various steps necessary to make an earth pigment suitable for painting? What are the drawbacks of working with pigments from the backyard that have not been washed, etc?