Dear David,
I did have a solid cylinder of slag from a Medieval mixed metal
smithing site in the High Street, Oxford. It had the flat top surface,
and a convex lower surface - a normal smithing hearth slag form but
for cylinder of slag running out down at an angle of about 10 degrees
from deeper side of the base of the smithing hearth bottom. I am not
sure the data was published with the site, but I will check - I did ask
for it to be illustrated.
I assumed that this was formed in a waist-level hearth which was
blown from the bottom of the back or side using an upward sloping
blowing tube. Either the slag was more fluid than expected or was not
cleared from around the tuyere before blowing stopped, allowing the slag
to flow down the air tube before freezing.
I have also seen slag rods from smelting sites from the slag tapping
hole, or even the blowing hole, but not tubes that often. However, it is
possible for a slag tapping to produce slag tubes without the need for
rodding. When tapping, slag flows very much as does basaltic lava does
in the case of the Hawaii volcanoes. The exterior solidifies first, and
interior stays molten for a long time. If the lava (slag) reservoir is
exhausted before the flow to the lava front has stopped the upper part
of the system empties resulting in lava tubes - in the case of a
bloomery furnace the diameter of the tubes is in the order of a few mms
not 10s of metres. The crystal orientation will indicate whether this
has happened.
I have on one occasion produced a open tube during experimental smithing
when using a bellows protector plate. The tube consisted of soil and
iron oxide semi-fused together along the line of air blast. I would not
normally expect these to survive in the archaeological record as they
would normally accumulate to form a smithing hearth bottom, or be
destroyed on cleaning the hearth.
What were the diameter (external) of your tubes, and the date of the site?
David Starley wrote:
> Dear slag fans
> Just wondered whether anyone feels they're recognised any patterns (
> date/technology/region) in the occurance of the tube-shaped pieces of
> slag which turn up from time to time.
>
> I've just come across some nice examplef ( from a supposed smithing
> site but I'm assuming it's residual from earlier smelting activity).
>
> I've also assumed such forms occur where someone has rodded through a
> blocked tapping arch and the resultant flow solidifies in the hole
> formed.
> Any other hypotheses?
>
> Cheers,
> Dave
>
>
>
--
Chris Salter
BegbrokeNano/OMCS
Department of Materials,
Begbroke Science Park,
Sandy Lane, Kidlington
Oxon OX5 1PF
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