In the climax to The Protestant Ethic, Max Weber writes of the stahlhartes Gehäuse that modern capitalism has created, a concept that Talcott Parsons famously rendered as the "iron cage." This article examines the status of Parsons's canonical translation; the putative sources of its imagery (in Bunyan's Pilgrim's Progress); and the more complex idea that Weber himself sought to evoke with the "shell as hard as steel": a reconstitution of the human subject under bureaucratic capitalism in which "steel" becomes emblematic of modernity. Steel, unlike the "element" iron, is a product of human fabrication. It is both hard and potentially flexible. Further, whereas a cage confines human agents, but leaves their powers otherwise intact, a "shell" suggests that modern capitalism has created a new kind of being. After examining objections to this interpretation, I argue that whatever the problems with Parsons's "iron cage" as a rendition of Weber's own metaphor, it has become a "traveling idea," a fertile coinage in its own right, an intriguing example of how the translator's imagination can impose itself influentially on the text and its readers.