Catenary arch. The salt chamber's shape was determined by hanging a chain, tracing the curve and inverting it. A wooden arch support was built to hold the weight of the brick during construction. When all the brick was in place, we removed the form and the arch was complete, able to stand on it's own. The three exit flues leading into the chimney are visible on the left. The firebox area is on the right, and cement blocks show the position of the bagwall.