weeks before adding the next lift. As they dried, the walls would be trimmed back with a paring iron, to between 20 and 36 inches (500 and 900 mm) in thickness, leaving them plumb and straight. Today cob builders make their walls about 2 feet (600 mm) thick at the base; they can use a tractor rather than oxen for mixing; and they often add a fine gravel of crushed shale to reduce shrinkage and cracking. A number of different techniques are being used to build the walls, most a variation on either the mud loaves or the shovelling method.