Since iron is found in a combined state and mixed with either sand, lime, or clay and a small percentage of manganese, phosphorus, and sulphur, it is necessary to free it of the elements with which it is combined. This process is performed in a blast furnace (Fig. 185). A blast furnace in a modern plant consists of a tower of masonry from 50 to 100 ft. high, lined with firebrick and cooled by means of water flowing through the pipes surrounding it. Boiler plates are used as a reinforcement. The furnace is charged from the top through a hopper, first with kindling wood, followed later by alternate layers of coke, iron ore, and limestone.
Fig. 184. - Inspection of Pig Iron. The amount and kind of pig iron is carefully determined before it is placed in the cupola. This illustration shows the inspection of the pig iron before it is dumped into the furnace. Notice the shape of the bars.
Fig. 185. - Blast Furnace.