Dear Michael,
I would second Tim Young's statement. Peter Crew has run a
number of experimental smelts using Northants ores but they definitely
were not haematite ores.
There are a number of possibilities, the first is that they
are bloom forging slags a process that can produce a small amount of
tap-slag like material. What was the cooling rate of the high alumina
material? If it was furnace cooled then it is possible that a lot of
hearth lining was being eroded into the slag during the high
temperature phase of bloom-forging. The morphology and weight/size
distribution of the various slag types might also help to point to the
mode of origin of the material.
The second possibility is that it is indeed a alumina-rich
bloomery slag. There is a liquidus composiiton range in the FeO,
Al2O3- SiO2 phase diagram with a minimum of 1083 C , and a quick
estimate of the composition this occurs around the proportions of
SiO2, FeO and Al2O3 given. What phases are present in
your slag? Such bloomery slags are not unknown, I have seen them a
couple of times, once at least from material from this country (but I
can not instantly remember where).
The obvious question of this material is the high alumina
content a result of some deliberate process - the use of a flux
(another poor quality ore with a high alumina content or clay), or an
accidental result of the process - massive vitrification and erosion
of the furnace/hearth lining.
You say you have two types of furnace structures, how do they
differ? This may give an indication of function of the furnace if
enough of the structure is preserved, and whether excessive linig
erosion was occuring.
Chris Salter
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