How was wrought iron made from a spongy mass of iron and slag?

Wrought iron was produced by consolidating a spongy, plastic aggregate of iron and slag formed during the smelting process at temperatures below the melting point of pure iron (1,535 °C, or 2,795 °F).1

Because early furnaces could not reach the melting temperature of iron, the resulting product was a bloom—a porous mass of pasty metal globules intermingled with semiliquid slag. To transform this into wrought iron, the hot, spongy lump was removed from the furnace and subjected to repeated hammering. This mechanical working served two primary purposes: it squeezed out the majority of the excess slag and welded the remaining iron particles into a compact, ductile, and coherent mass.213

Later, indirect methods like the puddling process were developed to produce wrought iron from pig iron by removing carbon through oxidation and agitation, followed by similar mechanical squeezing to expel slag.43