Scale of Iron Production at Populonia

[Eleatic Guest]

The scale of the (Etruscan) iron production at the site of Populonia, on the mainland across from Elba which was rich in iron ore, seems truly staggering. According to Theodore Wertime (1983, p. 451) between 500 and 100 BC, an estimated 4 million metric tons of iron were produced. This would mean an average output of 10.000 t per year. Wertime gives the following figures for the iron-smelting process (in million t):
slag: 2-4
charcoal: 1.5-3
coppice: 1.5-3

This means that the ratio between the slag, which was recorded at the site before it was reworked in the 19th century, and the iron end product is 0.5-1 to 1.

By contrast, another estimate is offered by David Gill (1987): "The slag-heaps from iron-working are estimated as weighing 2 million tons, with an estimated annual extraction of ore at 10-12,000 tons over four centuries. If 8 kg of iron can be produced from 50 kg of ore then this represents an annual production of 1.600-2.000 tons of iron."

Even taking into account that he assumes the slag-heap to be only half as big as Wertime, Gill arrives at a much lower figure. Which estimation is more viable and what is a realistic ratio between iron ore, iron and slag for ancient iron-smelting?

[Eleatic Guest]

Henry Cleere, The Iron Industry of Roman Britain, 1981, p. 72 has this to say:

Slag production can be equated directly with iron production. Work on reconstructed furnaces and calculations based on furnace remains (eg Bielenin 1974; Cleere 1976; Gillies 1961; Tylecote et al 1971) indicate a 3:1 slag:metal ratio: ie 3 tonnes of slag were produced for every 1 tonne of iron. The weight of slag in a dump can be calculated from the volume measured by assuming a specifi c gravity of 3.0: thus, a slag volume of l00m3 is equivalent to a slag weight of 300 tonnes, which represents an iron production of 100 tonnes, which can be simplified to the equation:
Slag volume (m3) = Iron production (t)

3 to 1 realistic?

[Dan Howard]

You can't determine a ratio of slag to iron unless you know the purity of the ore that is being mined and the efficiency of the smelters.

The required amount of charcoal in the first post seems to be way too low to me.

Sim's "Iron for the Eagles" might be a better reference for this kind of study.

[Eleatic Guest]

Too bad. Now that we have for once a reasonably reliable figure in the form of the slag deposits, there is still a huge amount of disagreement among experts at deducing the iron output from it.

[Robert]

Even if you split the difference, that still gives you an annual production of 5.000 tons or 5.000.000 kilo's. Just to put that in perspective, a sword of that time weighs about 1 kilo, a utility knife 100 grams, a bodkin arrowhead 20 grams (50 to a kilo), a trilobate 15 grams. A blacksmith is very thrifty with his stock (unlike some of the forging examples in Iron for the eagles, where there is a lot of cutting as opposed to shaping) and any low carbon scrap could be forgewelded, so most of that iron did end up as a finished product. And this is just one site!!

[Dan Howard]

Rio Tinto is another good site. There is about 6.6 million tons of slag dating to the Roman period.