While today most often associated with sidewalks and classrooms, chalk has been used as an art-making medium for thousands of years in its naturally occurring colors—red, white, and black (manufactured chalks did not appear until the 18th century). Red chalk, also called sanguine, was used in Egyptian and Roman art for creating wall paintings; Leonardo da Vinci's use of red chalk for drawings on paper proved highly influential, and the medium would continue to be particularly valued for portrait studies because of its closeness to European skin tones.