By the way, we got for several years a µRaman in our lab and have a relatively nice database on iron corrosion products and an some phases present in slag from iron metallurgy.
If some other "Raman owner" are interested by exchanging references, we would be glad to do that.
About the issue chemical composition vs mineralogy, once again its depends on the question you ask. In the beginning of my work I was very "mineralogy" oriented but now, for provenancing or reduction process studies, I look mainly at composition. Nevertheless I try not to forgot to look at mineralogy times to times, just not to miss important data. Indeed both are obviously linked. Maybe composition analyses allow to screen an important set of samples more easily and to reach statistical significance. But if you want to better understand processes and thermodynamic, the best is to have a look on phases.
Philippe Dillmann
responsable du Laboratoire Archéomatériaux et Prévision de l'Altération
IRAMAT LMC CNRS UMR5060 et SI2SM LAPA CEA/CNRS UMR3299
00 33 1 69 08 14 69
Adresse postale:
LAPA/SIS2M
Bat 637
CEA Saclay
91191 Gif sur Yvette Cedex
-----Message d'origine-----
De : Arch-Metals Group [mailto:[log in to unmask]] De la part de Thilo Rehren
Envoyé : jeudi 2 septembre 2010 21:57
À : [log in to unmask]
Objet : Re: iron silicates in slags - some additonal ramblings
This is an interesting observation, and one that I share to some
considerable extent. In my opinion, one typically does what one has learned
in an early / formative stage of one's career, and few people are
intellectually mobile enough to embrace different approaches later in life,
unless they offer significant advantages. For historical reasons, lots of
the German school of '-men' (mainly Bachmann, Hauptmann, Keesmann; more
recently with the French addition of Dillmann) have this strong mineralogy
background (Begemann is an exception, as a chemist); and I still admire the
aesthetically most beautiful thin section micrographs that Ingo Keesmann
used to present at conferences. It is no surprise then that they set a path
that others in Germany followed. In the UK, scholars such as Gowland,
Tylecote, Northover and Salter have been the formative ones: all with hard
core metallurgy backgrounds and much focussing on the metal and artefacts,
while Morton and Wingrove with their slag studies never made much of an
impact beyond their few but important papers they published on iron slags.
(Ian Freestone, a mineralogist by training, is an exception, but he quickly
moved on to ceramics first and then glass, so is more or less 'lost' for
archaeometallurgy; Paul Craddock is a ?physicist? by training? and appears
to have pretty much stayed clear of slags).
My own experience - and one which I am guilty of passing on to my students -
is a certain frustration with XRD, since too often I have seen people
happily interpreting the spectra 'as read', and finding quartz next to
fayalite or magnetite next to metallic iron: phases which should not coexist
in equilibrium and which are either mis-identified, or present due to
corrosion and dirt inclusions. A microscope-based method is therefore always
my preference, since I see what I analyse, and whether it is a primary or
secondary (=weathering) phase.
Since thin sections rarely give good information for the opaque phases, and
in metal slags it is the opaques which carry at least half of the important
information, I decided early on to work with polished blocks rather than
thin sections (of course there are polished thin sections, and my former
colleague Andreas Ludwig at the Bergbau-Museum in Bochum is a true artist in
preparing those). Clearly, this is a direct reflection of my background in
ore microscopy... But microscopy is an art more than a science, and it takes
some years to get the level of expertise necessary to cover at least the
main ore minerals AND metals AND metallurgical byproducts, and the modern UK
university system simply doesn't allow this luxury of a sound in-depth
training within an MSc or PhD programme. (This is an observation, not an
endorsement of the practice.)
So what I try to teach our students is the very rudimentary basics of
optical reflected light microscopy of the main phases relevant for their
particular research, and the judicious use of SEM-EDS analysis as a suitable
(!) and relatively commonly available analytical tool. After all, an
ordinary SEM-EDS spectrum gives more than just peaks for iron and silica; it
does give quantifications both as weight percent (commonly reported in
archaeometallurgy, for historical reasons), and as atom percent. Running
known stoichiometric compounds as test samples routinely gives result such
as 48/52 at% S and Pb for galena (despite the peak overlap problem of S and
Pb); surely this is close enough to recognize it as PbS (14 wt% S / 86 wt%
Pb). For fayalite and other olivines, we teach that the sum of Me2+ ions
should be twice the sum of Me4+ (basically just Si4+) ions, and for a member
of the pyroxene family, the ratio Me2+ to Si4+ (in atom percent, not weight
percent!) should be (and typically is) close to 1. I am sure that I have
seen various pyroxenes in European slags, but mostly in copper smelting slag
(often more lime-rich than iron smelting slag). Of the top of my head I
think that some of our Chinese students have pyroxens in their iron smelting
slags, but again those are (very) lime-rich.
We proceed similarly with the free iron oxide (mostly wuestite 'FeO' and
magnetite / spinel family) as a key indicator of redox conditions, even in
heavily weathered samples full of hydroxides etc. in the cracks and pores.
Of course, the EDS system does not allow to determine oxidation states
directly; but the characteristic minor oxides such as Al3+, Cr3+ and Ti4+
are restricted to magnetite / spinels, while wuestite is typically
relatively clean (but not stoichiometric!), and has typically only has low
levels of Me2+ minor compounds, such as Ca2+, Mg2+ or Mn2+. Crystal or
aggregate shape is a further important discriminator, as is colour: back to
the optical microscope, which as a matter of principle should be done in
detail before any SEM work is undertaken. With the odd solid solutions
prevalent in many slag systems, XRD data bases are often not able to cope,
and throw up weird and wonderful phases, which are often chemically
impossible for the material in question.
In summary: I agree with Dave's concerns about inappropriate analytical
work, but this is not specific for SEM-EDS analyses, but true for any
method. Whatever you learn and apply as a method, it is crucial to have
sufficient proficiency in it, and to know one's limits and whom to ask if in
doubt: or better how to solve a questions through independent logical
thinking and literature research. For publications, it is crucial that peer
review (either formally as for journals, or informally among colleagues
prior to submission to a book chapter etc.) picks up any serious issues. A
certain trans-cultural mobility is also quite healthy, to see and learn the
approaches of those who work elsewhere. I certainly benefitted enormously
from my half year with Peter Northover and Chris Salter, but would never say
that I am as fluent in any of their skills as they are.
Thilo
-----Original Message-----
From: Arch-Metals Group [mailto:[log in to unmask]] On Behalf Of
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Sent: 01 September 2010 20:02
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Subject: iron silicates in slags
I was recently reminded, when reviewing manuscripts for journals, of the
striking differences between Anglo/American archaeometallurgists on the one
hand, and German/Swiss/French archaeometallurgists on the other, in the
matter of identification of minerals in slags and ores from archaeological
sites. (As far as archaeometallurgists of other nationalities are concerned,
I don't have enough data to offer an opinion). Although I obviously belong
to the Anglo/American group, it is clear to me that the German/Swiss/French
archaeometallurgists are the more reliable in their mineral identifications.
The reasons for this, I think, is that many of the G/S/F group come to
archaeology from mineralogy or geology, whereas in the A/A world most
archaeometallurgists come to the subject from metallurgy/ materials science,
or (increasingly commonly) have no background in science before enrolling
for training in archaeometallurgy.
One consequence of this difference is that in Anglo-American
archeometallurgy the main technique for investigation of slags and ores is
the scanning electron microscope with energy-dispersive x-ray analysis
(EDAX). Obviously this gives chemical composition, not mineralogical
identification. Sometimes this is combined with XRD for mineral
identification, but increasingly rarely. Using EDAX alone, mineral
identification is, in essence, just an informed guess.
Let me give a real example. With iron smelting slags, there is a widespread
tendency in A/A archaeometallurgy to assume that if you get X-ray peaks for
iron and silica, then the phase is fayalite. But there are other
possibilities, namely the iron-rich clinopyroxenes and orthopyroxenes. I'm
particularly puzzled by the fact that in African iron slags I quite often
see orthoferrosilite (identified in thin section by petrographic methods)
but that this has not, to my knowledge, ever been identified in British iron
smelting slags. Is this a real difference between slags in the two areas? Or
is orthopyroxene just not being identified in British slags? (I'm not
picking on the Brits - for obvious reasons I don't have much opportunity to
examine iron slags from the Americas!)
Does it matter? Even though I don't have a German gene in my body, I do
side with the continental archaeometallurgists in thinking that full and
accurate description does matter, even if it has no obvious consequences in
term of technological reconstruction. I think that students working on
extractive metallurgy should be trained in optical crystallography as well
as in SEM/EDAX.
Yet very few in A/A archaeometallurgy can use the petrographic microscope
effectively, and even fewer can identify ore minerals in reflected light.
These skills are much more often used in G/S/F archaeometallurgy, and it
shows in the quality of their publications on extractive metallurgy.
Petrographic and ore microscopy may decline in importance as Raman
microscopy becomes more developed - at the moment the limiting factor is the
availability of Raman reference spectra for minerals - and Raman microscopy
is certainly easier to learn to use. But I don't think that it will entirely
replace the optical methods, and I think that we should be pushing students
in A/A archaeometallurgy programs to learn some optical crystallography and
become at least competent, if not expert, in these methods.
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