At the forge, porous and brittle cast iron was changed into tough, strong, flexible wrought iron. To the sounds of four water wheels turning, the bellows flapping, and the ground-shaking thump of the power hammer, men toiled to make wrought-iron bars.
In the heat and noise of the smoke-filled interior, the ironworkers dragged the heavy glowing shapes. Under the blows of the 500 lb. hammer, the liquid slag burst from the glowing iron shape called a "bloom."
The glowing shape was dragged back and forth from fineries to power hammer, and was transformed from a large rectangular bar into a dumbbell shape. The reheating continued in the chafery hearth, and with repeated hammering the iron became a "merchant bar," the major product of Hammersmith.
A wrought-iron bar has a fibrous structure that makes it strong and bendable. These bars can be forged into reliable tools and hardware that will resist fracture while bending. Cast iron on the other hand has a more granular structure and therefore is quite brittle. Note the structural differences in the illustrations below.
The building sheltered three distinct work stations for finers, hammer men and helpers. In two finery hearths, cast-iron "sows" slowly remelted and decarbonized into semi-hardened iron masses. Kneading, reheating and stir-cooking condensed each mass into a pliable shape called a "loop." The sledge hammer blows of the finer splattered out molten slag and charcoal in a process called "shingling."