A monochrome oil painting in gray. I got tired of drawing still lifes, and there were no naked people around. A charcoal "study" was done first on paper, then a charcoal underdrawing was rendered on the canvas before painting in oils.
The technical art term for a monochrome gray painting is "grisaille". Renaissance painters often did a grisaille rendering or other monochrome rendering (also usually called grisaille even if it isn't gray) before adding the colors.