As in the blast furnace, limestone is added as flux to make liquid and dispose of sand, dirt, scale, etc., which are detrimental. The liquid slag formed from union of limestone with these impurities floats upon the molten iron in the cupola hearth, as it is less than half as heavy as the iron itself. It flows almost continuously from a higher hole called the “slag hole,” in the rear of the furnace and just beneath the tuyères. The slag has little value except as material for filling purposes, etc. So-called “slag wool” can be made by blowing air through it. Sometimes the blast from the cupola blows it in such a way that this pure white “wool” is formed and blows out of the slag hole of the cupola. About Christmas time some of the workmen take quantities of it home for decoration and for fireproof whiskers for “Santa Claus.” These operations go on continuously throughout the day, each cupola making the particular grade of cast iron or “semi-steel” which is best adapted to the particular castings to be poured, size, shape and purpose of [Illustration: DETERMINING CARBON BY DIRECT COMBUSTION In an electric furnace with pure oxygen passing over them, the drillings burn as would splinters of wood. From the gas given off the total carbon is determined with great accuracy. ] [Illustration: WEIGHING PIG IRON FOR THE CUPOLA From three to six varieties of pig iron are used in each charge. ] [Illustration: THE RECEIVER BUILDING Here pig irons of different compositions are separately kept in numbered and labeled bins. The magnet which is used for unloading and handling the iron may be seen. The grab bucket for sand is at the right. ] [Illustration: WEIGHING THE CHARGE OF COKE Close weighing is necessary, for a variation of ten or fifteen pounds may affect the running of the cupola. ] the castings being the three main determining factors. [Illustration: SECTIONAL VIEW OF A CUPOLA ] When nearing the end of the day, charging ceases, the charging door is closed and the last charge, gradually descending, melts and flows into the bull ladle about an hour later. As soon as the bull ladle is emptied it is run out of the way, the cupola is drained of all iron and considerable slag through the tap hole, and the bottom doors of the cupola are dropped by pulling from under them the “props” which have held them in position. The great mass of bed coke follows with a great burst of heat, light and flame. This is quickly subdued with a stream of water and removed. When cool, the slag and accumulations are chipped from the cupola lining, burned areas are patched with bricks and stiff, fire-resisting mud, the bottom doors are raised and fastened in position, and the eight-inch sand bottom is packed in ready for the next day’s run. After building a fire and getting a good “bed” of glowing coke, the cupola is ready again for the charging of iron. [Illustration: A CUPOLA IN OPERATION