On Thu, 11 Nov 1999, Stefanie Berg-Hobohm wrote:

>  I have heard that nozzle-saving (in german: Düsenschutzstücke) pieces
>  can have the same shape than loom-weights (pyramidal). So it might be
>  difficult to differ such objects from one and another.

Dear Stefanie,

depends on whether you mean pyramidal (or rather conical?) in longitudinal
or cross section! Indeed, conical tuyeres are relatively common (though
not always in the archaeological record) as they much help channel the
blast into the furnace and one reference I've got handy is Tylecote 1986,
142-3 for the UK. This is rather commonplace, though, and I suspect what
you really want to know is whether there exist block tuyeres that are
trapezoidal in front aspect rather than the usual cubical. 

I cannot think of a reference for those at the moment (though I'd love to
hear about this if there are any) but assume that, due to the generally
crude nature of furnace lining as well as the conical shape of the shaft
block tuyeres could easily take a slightly trapezoidal form.
 
>  But I could not found yet such nozzle-savings in the literature. Are
>  they round-shaped?

Tuyeres are wide-spread throughout Western and Central European Celtic and
Roman iron smelting contexts (the non-slag pit domain) as well as in
sub-Saharan Africa, and the air inlet as well as the surrounding clay is
circular in cross section. The rest is subject to furnace construction. 

As regards the question of distinguishing (block) tuyeres from clay
loom-weights - and this should answer the question perhaps more precisely
than any pondering about shape - is that

a) loom-weights are fired at low temperatures under oxidising conditions
(if in doubt, make a section) or not at all. I've seen many clay
loom-weights that were simply air dried during my studies in North Africa.
Clay tuyeres were initially fired the same way but then were exposed to
very high temperatures and this may show through any transformation of the
material, such as molten clay or general deterioration, particularly where
the tuyere meets the furnace interior.

b) then there is of course the tell-tale sign of slag adhering to the
tuyere. If you are dealing with slag on a more general basis, one
indication of tuyeres having been used on site may be the existence of
Schlackenzapfen (after G. Sperl; we call them slag-rods here).  

c) generally, the internal diameter of tuyere holes is about 3cm, much
smaller than the one for loom-weight holes. 

d) the clay material used for either purpose can vary considerably from
one another. Whereas tuyeres essentially need refractory clay the choice
of clay is not that important for loom-weights as long as they weigh the
frame down! Incidentally, you may find similar clays used for hand-made
coarse pottery and loom-weights as both domestic activities are assumed
to have been relegated to one and the same sphere of responsibility. 

Hope this helps.
Beste Gruesse,

Irene

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Irene E Schrufer-Kolb                                                    
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