This paper explores the rise of “industrial heritage” and the forms of memorialization proliferating around it. The site is Sesto San Giovanni, Italy's “City of Factories,” which was also a bastion of communist mobilization and which is now bidding to be recognized on UNESCO's world heritage list. Sesto's bid is an attempt not just to recuperate and reinvigorate the landscape of Sesto's ruined factories and its massive, crumbling machinery, but also to capture and render visible and graspable the traces of what this built environment expressed and left behind—the sentiment of solidarity. I thus argue for an understanding of solidarity not just as an emotion or value, but as a structure of feeling mediated by specific material and corporeal forms, in bodies collectively inhabiting a built environment and rhythmically moving within and out of infrastructures and lived landscapes. Such a materialist conception of solidarity must account for bodies and embodiment, rhythm and refrain, as well as for how certain material forms allow for the generation of proximities, coordination, and likeness across difference. It means thinking of solidarity as an arrangement and assembly of bodies in time and space, and of these bodies and their movement as generative of political feeling and action. Based on ethnographic and archival research in Sesto San Giovanni between 2011 and 2013, I tell the story of the afterlife of a twentieth-century sentiment and its fate in an era that has rendered solidarity precarious.