Trevor,
The combination of bone ash and galena is a bit puzzling and does not have
a straightforward explanation. Bone ash residues could be expected to be
found either in the context of cupellation or afterwards when the test
hearth was broken up and smelted for its lead content. The presence of
galena does not fit well with either of these. If a reverberatory furnace
were used for smelting then the whole charge would be roasted together as a
first stage before the furnace was closed up and the temperature increased.
Galena, bone ash, coal and coal ash could well be found in this context.
However, your chronology of 16/17th century is a bit early as this process
is generally accepted as appearing in the UK around 1700. I suppose that a
cupellation furnace could eaily be used for roasting and smelting galena
when not being used to extract silver but I have never heard of this being
done.
If the chronology is correct then there would be two possible processes
which could be used:
1. Roasting of galena in a heap followed by smelting in a small blast
furnace. This is described in Agricola etc. . Broken up cupels could
have been used to line the floor of the heap to prevent escape of
material into the ground. Roasting is not a 'clean' process and even
under modern conditions can produce lead, lead sulphate, litharge and
basic sulphate as well as unreacted galena, so a rudimentary lining of
the heap would be a fairly obvious thing to do if suitable material were
available.
2. Roast/reduction of galena in an ore hearth. Broken up cupels could
have been added to stop the charge going sticky but the presence of coal
in this context is most unusual as coal was not normally used in ore
hearth smelting until C17-18th, although this aspect is not very well
researched.
SEM/EDX examination of the bone ash residues might give some insight into
the process being used.
Regards
Richard
Richard Smith
3M UK Bracknell,
Environment, Safety and Security [log in to unmask]
Tel: 01344-858154 (Trim. 8-230-2154)
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