Dear Peter and John:
Two more case of local deposition of the majority of the metal dispersed into the atmosphere from a smelter:
1. Getchell Mine, N.C. Nevada, USA: a smelter operated at this arsenical gold mine in the early part of the 20th century, and a narrow dark streak of metal-bearing soot can still be seen on satellite images extending 5-8 miles downwind.
2. Nkana Copper Smelter, Zambia. Surface geochem samples (taken in the mid-sixties after less than 30 years of smelter operation) showed up to 5,000 ppm (0.5% Cu) at 10" (25 cm) depth in the soil over a limited area about 5 miles west of the smelter. For the times, this was a highly efficient smelter, losing only about 0.5% of the copper in the ore as copper sulfate dust in the flue gas. it also had a very tall stack. However, production was 150,000 tons of finished copper per year, so at least 750 tons of copper was escaping from the chimney each year (I may be low on the volume of production - poor memory).
The anomalous area was over Basement schists, but was on a gentle hillside facing the smelter. Pitting beneath the shallow anomalies showed typical basement values of 25 - 50 ppm Cu. The giveaway was that the sap of trees that had been axed crusted malachite green in a week or so, and there were also areas of malachitic crusts on the laterite outcrop at the edge of the dambo. I washed the dust off some leaves and had it analysed: I've forgotten the results but I think that it was ore grade (>2% Cu).
This was a very local phenomenon - covering only about 1 sq.km. on a hillside facing the smelter: elsewhere pollution was not apparent in our samples, in spite of a great deal of tailings dust blowing around - this ran 0.9% Cu but did not seem to release it readily to the soil.
If the average value of Cu in the top 25 cm of the soil in this 1 sq. km. area were 0.25%, for a soil bulk density of 1.25 we obtain a copper content of 750 tons, or 1 year's losses. If the smelter production had remained constant in the 25 years (1937-1962) between its construction and the soil sampling, 4% of its losses to the atmosphere would be on this hillside alone. Since the smelter's production rate had increased almost continuously since it was built, the actual figure would probably be closer to 10%. What proportion of the remaining 90% could possibly end up in an ice core I don't know, but I suspect very little for a source in tropical Africa, because of the very dusty atmosphere in the dry season and the strong southern hemisphere circumpolar vortex. For ancient smelters with no to low stacks, the local fall-out would be a much higher proportion than this, as has been said already.
John Berry
Date: Thu, 13 Mar 2003 11:54:56 +0000
From: Peter Claughton <[log in to unmask]>
Subject: Re: lead production in the Roman period
>However, when it comes to trying to estimate how much lead will have
been
>smelted
George,
Estimating lead/silver production in antiquity is all it can be -
estimating
- with no written records we are largely reliant on the sort of
archaeological evidence you quote from the ice and peat data. Even when
documentation is available it is patchy and we are still reliant on
estimates for overall production.
By the late medieval period we are really looking at two processes, 1)
to
recover lead, non-argentiferous lead, for construction purposes, and 2)
to
recover silver, by smelting arentiferous lead and refining it by
cupellation
to recover the silver, the resulting litharge is then resmelted to
recover a
silver free, 'sterile' lead which is then used for construction
purposes.
Each stage in these processes resulted in losses.
The first was the most efficient - at around 45 percent for the wind
blown
bole or bale hearth. Not all the lead unaccounted for was lost in fume,
the
majority remain in the residues left after smelting.
With the second process, where the aim was silver recovery, much more
lead
was lost in supplementing the bole hearth with higher temperature
bellows
blown furnaces whose efficiency was around 38 percent - with the
majority of
the losses being through volatalisation in the fume. Quantifying the
'average' losses in refining and the subsequent recovery of sterile
lead
from the litharge is difficult. Looking at production from the Devon
silver
mines in the early 14th century - over a 13 year period losses would
appear
to be around 37.38 percent - but we cannot be sure that all the sterile
lead
recovered has been accounted for in the documentation. Looking at
certain
years where we have good documentation, e.g. 1307/8, the figure is 32
percent loss, and for the following year 28 percent loss.
Not all the lead lost would end up in the upper atmosphere, and in the
ice
or peat remote from the smelting site, most would be dispersed on
adjacent
land leaving a useful marker for smelting locations - see Wild and
Eastwood
in L. Willies and D. Cranstone (eds.), Boles and Smeltmills, (Matlock
Bath,
1992).
Peter
______________________________________________
Peter Claughton, Blaenpant Morfil, Rosebush, Clynderwen,
Pembrokeshire, Wales SA66 7RE.
Tel. 01437 532578; Fax. 01437 532921; Mobile 07831 427599
University of Exeter - School of Historical, Political and Sociological
Studies
(Centre for South Western Historical Studies)
E-mail: [log in to unmask]
Co-owner - mining-history e-mail discussion list.
See http://www.jiscmail.ac.uk/files/mining-history/ for details.
Mining History Pages - http://www.exeter.ac.uk/~pfclaugh/mhinf/
_____________________________________________
------------------------------
Date: Thu, 13 Mar 2003 13:16:34 +0000
From: John Mason <[log in to unmask]>
Subject: Re: lead production in the Roman period
Hi Peter,
Your comment "Not all the lead lost would end up in the upper
atmosphere,
and in the ice or peat remote from the smelting site, most would be
dispersed on adjacent land leaving a useful marker for smelting
locations"
reminded me of some work I did at Eglwysfach between Aberystwyth and
Machynlleth years ago. Someone wanted to start an organic veg
enterprise
and they asked me to look at soil geochem at some sites. One was NE
(ie.
upwind) of the old Ynyshir Smelter. Here are the soil data from OMAC
laboratories:
Pb - 3480ppm Cu - 59ppm Zn - 290ppm Ag - 2.7ppm
Needless to say they decided to grow their carrots elsewhere!
Cheers - John
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