The ores of metals that have low boiling points such as copper, gold, silver, lead and tin (1085, 1064, 962, 328 and 232°C respectively) can be heated in a furnace until the metal melts and drips through the porous solid mass of impurities (silicon rock for example melts at 1200°C). Iron melts at 1538°C, and Iron Age furnaces (‘bloomeries’) could only reach temperatures of around 1300°C.
Iron ore consists of iron oxides and quantities of impurities known as stony waste (‘gangue’). If the furnace is charged with iron ore and charcoal then lit and heated up with air blown in using bellows, the carbon monoxide separates from the charcoal and reduces the iron oxides to iron, which then together with the stony waste (‘slag’) forms a brittle hardened mass (‘bloom’) as it cools in the bottom of the furnace.
Metals have a regular lattice structure and when a force is applied it is relatively easy for the layers of atoms to move past each other, i.e. easier to bend. In brass, the tin distorts the copper regular lattice structure and makes it less easy for the atoms to move past each other, i.e. harder to bend.
To make wrought (‘worked’) iron the bloom is hammered with great force while extremely hot, forcing the molten slag to squirt out of the porous mass of solid iron, leaving behind the iron that can then be hammered into an object.
Wrought iron swords bent and so steel swords were made by heating iron in a charcoal furnace, hammering and then quenching it in water. Only the outer skin absorbed carbon so the core remained iron and therefore relatively soft.
Until the Late Middle Ages in Europe, furnaces were unable to produce a temperature high enough to melt iron for casting, so all Iron Age objects were wrought by hammering.
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