Burnt clay is sometimes used as a substitute for sand in mortar. It La separed by piling moistened clay over a bonfire of coals and wood. As the day comes burnt and the fire breaks through, fresh layers of clay and coal, " breeze," or ashes, are piled on, and the heap may be kept burning until a sufficient supply has been obtained. The clay should be stiff. Care must be taken that it is thoroughly burnt. Raw or half-burnt pieces would seriously injure mortar. Sand is sometimes very economically obtained by grinding the refuse "spalls" left after working stones for walling. It is generally clean if carefully collected, but the sharpness of its grit depends upon the nature of the stone from which it is procured. Scoriae from ironworks, slag from furnaces, clinker from brick kilns, and cinders from coal, make capital substitutes for sand when they are quite clean and properly used. Wood cinders are too alkaline.