>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/
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