Not only is a proper temperature necessary for best annealing, but, as stated, a sufficient time must be allowed for the separation of the carbon and iron. The separation requires many hours and the cooling from the annealing temperature must be slow in order that the carbon and iron may not again unite, as they certainly would do were the castings chilled in water or otherwise cooled too fast. Most manufacturers produce tonnage enough that castings have to be annealed in quantity. As iron at red or higher heat wastes very rapidly on account of scaling (in fact would take fire in air if hot enough), and the quality of castings deteriorates somewhat if even a small amount of air is allowed to come in contact with them during the annealing process, they are generally protected by enclosing in iron containers on which tops are luted and cracks filled with stiff, fire-resisting mud which keeps out the air. These iron drums are of suitable size and shape that they can be stacked one upon the other to a height of from four to six feet. The stacks or “sets” of “saggers” as they are called, are run into large brick-lined retort chambers which are heated either with coal from a grate, by powdered coal, oil, producer gas or other fuel. The larger the furnace and the greater the tonnage of iron which must be heated, the longer will be the time necessary for bringing the furnace and castings up to the cherry-red heat which is necessary for the annealing. Therefore, the larger furnaces require a somewhat longer time than those of smaller size, though the time required for the annealing of the castings themselves is no longer. In this country with ordinary-sized furnaces the usual time for the annealing operation is approximately one week. This includes the heating of the furnace and castings and the cooling to a black heat again. [Illustration: No. 109. PHOTOMICROGRAPH OF WHITE CAST IRON ] Some manufacturers, however, anneal without pots but they aim to have the castings protected from the flame and air. [Illustration: No. 110. PHOTOMICROGRAPH OF NO. 109 AFTER SEVERAL HOURS OF ANNEALING ] Handling devices have been designed which facilitate loading and unloading the furnaces. With these the many sets of pots or saggers which the furnace holds can be very quickly charged or removed. After shaking the castings out of the cooled pots, the dark coating is removed from them by “tumbling” with iron shot, pieces of leather or other polishing material in tumbling mills after which they are ready for any machining which may be necessary.