Here is a woman in Montana who uses them:

"Art work is painted on canvas or handmade watercolor Arches (140lb wt) paper with cattle-markers, which are oil-based paints sticks that ranchers use to mark their livestock. "I enjoy working with the markers because they allow me to draw with paint. I do not use any paintbrushes, just my hands to manipulate the image". The color is vivid and dries permanently with rich texture."

Some Interesting Art Work Here