At Fitchburg Furnace, a wooden casting house covered the sand moulds where iron was run, and a depression in the stone above the archway probably supported the shed’s roof. Long trenches sloping from the front of the furnace were formed from moulding sand mixed with enough clay to hold the desired shape. When the iron was ready to be drawn, a worker used long-handled tongs to grasp and remove the clay plug in the damstone, sending molten metal through the opening into the main feeder trench. As that trench filled, guttermen directed the iron into side trenches, creating bars 4 to 6 feet long and weighing 60 to 250 pounds each. Bars from the main trench were called sows, and bars from the side trenches were called pigs, giving rise to the term pig iron.