BBC: A toxic spill of red sludge has cascaded through dozens of villages in Hungary, killing four and injuring scores. The sludge, which contains a mix of metal oxides, is now making its way towards the Danube, Europe’s second-longest river. But what exactly is the sludge, and how hazardous is it to humans? The sludge is a by-product from the early stage of aluminium production, which leaked from storage reservoirs. As part of the process, bauxite, the raw material, is …