- Claudio Giardino –
They were cylindrical shaft furnaces that tapered upwards, with the base partially sunk
into the ground; they appear to be approximately half a meter high with an internal
diameter of about 30 cm (Fig. 37.13). The walls, about 15 cm thick, were made of
hewn stones and slag held together by clay; inside, the air intake was through nozzles
(tuyeres) positioned toward the bottom of the furnace (Voss 1988: 92–96; Giardino 1996:
271–272). The furnaces allowed the escape of the slag during the smelting process:
the slags of Populonia are indeed fl attened and have the characteristic traces of sliding
tracks associated with their run-off from the furnace in a semi-fl uid state (“tapped slag”)
(Fig. 37.14). The analyses carried out on slags have shown that the furnaces operated
at temperatures of around 1200 – 1300°C, allowing easy transformation into metallic
iron of the ore that consisted of iron oxides: hematite and magnetite. From this was
obtained a spongy mass, the bloom, consisting of a porous mass of iron, charcoal and slag.
Therefore a subsequent forging process was necessary, to obtain the iron by means of hot
hammering (Giardino 2010: 205–206).
Figure 37.13 Remains of a furnace from Populonia.Figure 37.14 Large tapped slag for iron smelting from Populonia.