Continuing to produce several works throughout the late 1930s - such as "Echo of a Scream" (1937) and "The Sob" (1939), both now at the Museum of Modern Art in New York – Siqueiros also led a number of experimental art workshops for American students. He spent the better part of 1938 with the Spanish Republican Army fighting Francisco Franco's fascist coup before returning to Mexico City. After his return, in a stairwell of the "Sindicato Mexicano de Electricistas", Siqueiros collaborated with Spanish refugee Josep Renau and the International Team of Plastic Artists to develop one of his most famous works, "Portrait of the Bourgeoisie", warning against the dual foes of capitalism and fascism. The original mural shows a giant generator using the opposition of fascist and capitalist democracies to generate imperialism and war. An armed, brave-faced revolutionary, of unnamable class or ethnicity, confronts the machine, and a blue sky on the ceiling flanked by electrical towers displays hope for the proletariat in technological and industrial advances.