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> -----Original Message----- > From: mining-history [mailto:[log in to unmask]]On Behalf Of > John L. Berry > Sent: Tuesday, January 28, 2003 3:15 PM > To: [log in to unmask] > Subject: Re: mining-history Digest - 26 Jan 2003 to 27 Jan 2003 > (#2003-26) >> > All of which leads to MY question: does anyone on the list know (or know > someone or some book who knows) quantitatively what the inputs > (FeOx+carbon) and > outputs (Fe metal plus slag) would have been for iron age > bloomeries such as > these? In my calculations referred to above I had to make a lot > of assumptions > that I would like to refine. > The answer to this question is the usual one - it depends upon a number of factors. Archaeometallurgists like myself do these kinds of mass-balance calculations quite often to try to estimate the quantity of iron produced at an archaeological site from the volume of slag remaining. To estimate this you need (a) representative chemical analysis of the ore used; (b) representative chemical analysis of the slag; (c) surveyed volume of slag and (d) some factor to convert volume of slag to weight. In theory this sounds quite simple, but in practice there are a number of awkward complications. Getting reprentative analyses is a thorny problem in itself, since ancient metal workers didn't strive for a constant grade of ore, nor can one assume that furnace conditions were constant from run to run (so the recovery of iron may have varied greatly). There is also the problem of poor refractories - the very iron-rich slags of the bloomery furnace readily dissolve fired clay tuyeres and furnace linings, so part of the composition of the slag derives from the refractories (and also from the ash in the charcoal fuel). There is also a lot of stuff in a slag heap, typically, that isn't slag, such as broken bits of furnace, old tuyeres, rejected ore, unburned charcoal, ash, and so on. Since this is not usually evenly distributed, it's difficult to know whether the weight of slag per unit volume in one's excavated trenches into a slag heap is representative of the whole. The median figure from my own test units in slag heaps in central Africa was 1350 kg per cubic meter, which shows that slag can pack quite loosely (the specific gravity of single pieces of compact bloomery slag is usually in the range from 4 to 6 gm/cc)but the degree of packing must vary from site to site. In summary, therefore, there's a great deal of uncertainty attached to such estimates. David Killick Associate Professor Department of Anthropology University of Arizona Tucson, AZ 85705-0030 office (520)621-8685 lab (520)621-7986 fax (520)621-2088 [log in to unmask]