Defining Critical Materials

Should we worry about the sustainability of supply of non-renewable resources? To study these questions, flows of metals into and from human use (the anthropogenic metal cycles) have recently been quantified for several of the common engineering metals. Still more recently, the stocks of some of those metals in active use have also been determined. Do the resource availability issues play out differently for the metals used in large quantity (e.g., copper, zinc) and those which see low-volume specialty uses (e.g., indium, dysprosium)? These issues will be discussed from the perspective of results from the Yale Stocks and Flows Project, and a recent National Research Council study on the criticality of metals.