Materials are all around us. In many ways, they structure our world; economically, socially, and physically impacting the places they move through. In this paper, I approach material flows from a sociological perspective. I am interested in how material flows are sustained (Wallsten, 2015).

To do this, I use Cape Town’s scrap metal material flow as a boundary object to study it quantitatively, through urban metabolism’s material flow analysis, and qualitatively, through a sociological case study. I call this a social material flow analysis, which I define as an analysis of a material’s flow in a human system , delimited by space and time . I place it within sociourban metabolism instead of urban metabolism.

I have three major findings. (1) My design for a social material flow analysis works and is useful. (2) Capetonian scrap metal exports were twice as large as previously thought. (3) Describing the flow of scrap metal as a social structure highlights the systemic exploitation and extreme insecurity of Capetonian waste pickers.

I establish that the social MFA is useful for three reasons. (1) I t allows me to understand the scrap metal fl ow not in economic or environmental terms , but as part of the structure that is Cape Town. (2) It uncovers the relationships that move scrap metal through its value chain. (3) I t provides a foundation for future research.