TO MAKE an impeller—the fan-like part of the pumps that push ore-slurry down a pipeline—the folk at Weir’s foundry in Todmorden, West Yorkshire, begin with a polystyrene cast coated in ceramic paint. They place it in a flask tightly packed with sand, into which a molten alloy is poured. The polystyrene burns away, leaving a metal copy. A 15-tonne impeller takes just a minute to cast but needs 11 days to cool. Metal ores are hard on machinery, so every three weeks one of these giant metal parts must be shipped from Britain to a copper mine in Chile. The worn impeller is sold for scrap.