Portrait of William Spratling, Taxco

Of the major Mexican muralists, David Alfaro Siqueiros was perhaps the most militant. His idealism led to action inside and outside the studio: He organized miners, aided strikers and was sometimes jailed. His murals, meanwhile, had titles like Death to the Invader and Burial of a Worker. In 1936 he founded the Experimental Workshop in Manhattan, where Jackson Pollock was a pupil. His portraits-arguably the best Mexico has ever produced-have astounding visual power and monumentality. Based on pre-Columbian Olmec heads, this rare example demonstrates what writer Rita Eder calls Siqueiros's brutal expressionism.