Michael Keech wrote:
> The second type of furnace, produce results in the slags which are high in
> alumina levels (12-16%) in both furnace and tap slag, iron levels between
> 23-53% and calcium levels of 5-6%. The alumina is not provided from the
> ore, which has been analysed and is hematite.
Thanks to Tim and Chris for putting straight the geology of East
Midlands ores - indeed predominantly limonite, transformed to hematite
by roasting. However, there is one instance of hematite (from medieval
Stamford) published in the relevant literature: Tylecote 1986, 126 Table
68. Rather confusingly, the same sample is cited elsewhere as a roasted
ore nodule, which is indeed chemically hematite (Tylecote 1970, Table 1
C)! Talking back to the East Midlands geology group at Leicester
University in the course of my own research into Roman iron production
in the East Midlands resulted in that there appear to be very localised
hematite deposits north and east of Rutland Water, sources I was not
able to identify on the respective geological maps.
Among the published analytical data, there is indeed one single slag
from Roman Sacrewell with Al203 14.82, FeO 48.44 and CaO 1.85w%
(published in Dannell and Wild 1987, 107, Table III). Sacrewell is a
site with substantial evidence for (presumably late) Roman iron
smelting. It cannot be totally excluded that smithing was also carried
out at the site, although archaeological evidence for this is lacking.
All other published data of Roman smelting slags from the region, as
well as my own XRF values for a number of sites in the area, do not
usually exceed 7w% Al2O3 (only Great Casterton, Roman or medieval,
9.53%). My analyses of Roman furnace linings give Al2O3 of up to 23%,
suggesting a contribution of the aluminium in the furnace lining to the
slag composition.
One sample from Saxo-Norman Stamford (Tylecote 1986, 186, Table 91, no.
3b) gives Al2O3 7.4, FeO 52.7 and CaO 4.9w%. Calcium does not usually
exceed 2.5% in Roman smelting slags from the East Midlands, and there is
no evidence as yet to suggest that calcium was added deliberately as a
flux in Roman and early medieval times. Used in the bloomery process,
due to their comparative impurity, East Midlands ores are self-fluxing.
It will be interesting to see how potential technological changes in
medieval times affect the composition of slags from the area.
Irene
--
Irene Schrüfer-Kolb MA, PhD, AIFA
http://physics.open.ac.uk/~uckolb/isk
|