When the fuel is burning, a strong blast of hot air of a temperature of 1200° F. is forced through the tuyeres (pronounced tweers) at the bottom of the furnace. The blast of air acts on the burning fuel causing an intense heat which gradually melts the iron. In order to make the iron ore melt easily, limestone - called a basic flux - or sand, an acid flux, etc., is added to the coke fuel. The flux combines with the earthy materials of the ore and causes them to melt at a lower temperature. When the iron ore contains a great deal of limestone it is said to be basic in character and sand is used as a flux; when the ore contains clay, the flux must be of a limestone nature. In each case, the sand, clay, and limestone unite at a red heat to form a salt called slag.

The space inside the blast furnace below the tuyeres is called the crucible. Here the metal separates from the slag. Immediately above the crucible, the diameter of the furnace is made wider to provide for the contraction in the volume of the charge before melting. The sides of the furnace slope gradually from the top to allow for the expansion of the charge as it heats up before fusing. When the flux unites with the earthy material (called gangue) of the ore it forms a glassy matter (called slag), setting free the iron. As the iron melts or fuses, its high specific gravity causes it to fall to the bottom of the furnace. When a suitable quantity has accumulated it is allowed to flow out of a tap-hole onto a sand bed along a large groove called a sow, from which at right angles it enters smaller grooves or hollows, forming the molds for the pigs. When the iron cools it is broken up into lengths suitable for shipping to foundries and is known as pig iron.