Dear Chris
Thanks for your comments. To start with the last first (!) I would
agree that this is nothing to do with smelting - this is a
blacksmithing slag. Several other respondents have suggested
these might be sulphides "inherited" from a sulphide-rich ore - but I
doubt such minerals would survive the smelting process, although
it is just conceivable that the sulphur could be thus derived and has
subsequently formed new sulphide minerals. Later incorporation
during contact with coal in the smithy seems much more likely -
but the mechanism is less clear to me - and I'll come back to that
below).
The only analyses I have at present are rather poor (don't ask...),
but give S:Fe (atomic proportions) of about 1.3, but what this really
means I wouldn't like to say.
I interpret the slag/iron contact and the slag "inclusions" texture as
a product of the freezing of the system with the iron fragment in the
process of dissolving in the slag. The marginal texture certainly
appears this way - and the internal "slag inclusions" also appear to
me to be over enlarged - they probably are closely connected with
the surface out of the plane of the section. The slag composition in
the pores and immediately outside is very similar in terms of major
elements:
pore outside metal typical
Al2O3 10.0 10.6 14.6
SiO2 50.5 50.1 56.0
FeO 38.9 38.9 17.9
MnO 0.24 < <
MgO 1.9 2.9 6.2
TiO2 0.49 0.50 0.77
CaO 5.65 5.43 3.1
K2O 2.77 2.53 3.7
S 0.73 0.38 0.1
P 1.73 0.38 <
(where typical = glass composition about 1cm away, where iron
fragments are absent)
So there is a strong compostional gradient in the glass
composition across the specimen (from lining dominated material
at one extreme) to iron-rich where the metal is dissolving. But, the
glass close to the iron is the same as that inside (more-or-less).
The slag within the metal represents penetration of the melt outside
exploiting perhaps the original slag inclusions of the wrought iron -
or maybe even exploiting the sulphide inclusions. It is intriguing,
however, why Ca, K, P, S show such a strong association here
with the iron.
So - back to the sulphides and their origin. It seems to me that
there are two possiblities:
1. as you suggest, that the sulphides have been incorporated either
from surface, or through incorporation in a weld, BEFORE this
fragment was lost from the workpiece. As you mention, this
material would be very hot short (perhaps explaining why it
detached from the workpiece?).
2. alternatively, a piece of conventional iron was lost to the hearth
where it has scavenged sulphur (perhaps preferentially within the
carious texture developed during its dissolution into the relatively
silicic slag) AFTER being lost. The presently enclosing slag is
relatively low S however.
The fine debris from the smithing operations includes both coal and
charcoal residues and it seems both were being used in the
hearths. I haven't analysed any coal to see if it is particularly
sulphurus.
I'm rather pleased to hear that you have seen similar material in
modern smithing debris - do you know what the fuel was in those
instances (coke, coal...?).
Best wishes
Tim
Dr Tim Young
Email: [log in to unmask]
Web: http://www.geoarch.demon.co.uk/
Phone: 02920 747480
Fax: 08700 547366
Mobile: 0802 413704
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