Process for iron smelting .
In ancient times, smelting involved creating a bed of red-hot charcoal in a furnace to which iron ore mixed with more charcoal was added. The ore was chemically reduced (see oxidation-reduction ), but, because primitive furnaces could not reach the melting temperature of iron, the product was a spongy mass of pasty globules of metal intermingled with a semiliquid slag . This hardly usable product, known as a bloom, may have weighed up to 10 lbs (5 kg). Repeated reheating and hot hammering eliminated much of the slag, creating wrought iron , a much better product. By the 15th century, many bloomeries used low shaft furnaces with waterpower to drive the bellows, and the bloom, which might weigh over 200 lbs (100 kg), was extracted through the top of the shaft. The final version of this kind of bloomery hearth survived in Spain until the 19th century. Another design, the high bloomery furnace, had a taller shaft and evolved into the Stückofen, which produced blooms so large they had to be removed through a front opening.