The late John Higginson referred to arsenic, cadmium, mercury, and thallium as the big bad four when it comes to closing the loop on processing nonferrous concentrates and secondary materials from urban mining. As nonferrous smelting companies move to become more sustainable by closing their flowsheets so that they do not generate “residues” for “landfilling” and societal demand is declining, or has declined, these elements need to be managed for “permanent” storage. Understanding the proposed chemical and physical forms and storage conditions, with their associated failure mechanisms that could lead to the release of the elements to the environment, is required to determine appropriate tests to assess and compare the long-term performance and sustainability of proposed “permanent” homes for these elements.