The mineral witherite (barium carbonate) was discovered in spoil from the mines in the 18th century. It was distinct from other sources as it contained above two per cent of carbonate of strontites. Up to this time witherite was considered worthless and used as rat poison by locals. Josiah Wedgwood used it to manufacture Jasper ware and tried to keep the source secret, but after a visit in 1782 by two Frenchmen, a local farmer, James Smithels, exported the mineral to Germany, charging five guineas per ton.