Tavistock was one of the three stannary towns of Devon and large quantities of refined tin ore were exported through Morwellham from twelfth century until 1838, when the requirement to pay duty on the metal at one of the specified towns was relaxed. The opening of the Tavistock Canal, between Tavistock and Morwellham, in 1817 facilitated traffic. Later, the East Cornwall Mineral Railway provided an outlet through the quays of Calstock from the Cornish side of the valley. Other significant cargoes exported were quarried granite and, later, copper, lead and manganese ores, with their important by-product of arsenic. Arsenic was extracted from mispickel, once regarded as a waste product but later offering an important source of revenue as copper and tin extraction declined in profitability. The refined product was exported worldwide, in particular to the southern United States, where it was used as an insecticide in the cotton fields.