Hi all,
I would like to add some commentaries to the (excellent) email of Chris.
To my opinion the most important point is: "what is the scientific question when you want to determine the carbon (or the P) content (or whatever). Actually, because the ancient iron based artifact are highly heterogeneous(particularly the one obtained by the bloomer process), the expression of a very local carbon content, has no sense. Moreover express an something as an average representative content is also very difficult. An archaeological ferrous artifact obtained by the bloomery process can only be understood by looking the most complete transverse section as possible.
To make a long story short, a single carbon content data is past useless. We must take into account:
- the heterogeneity of the sample
- the presence of P, Ni, As
- the grain size, hammering deformation, presence of inclusion etc
If the question is "was the alloy of good quality", or "was is resistant to traction, shock etc", "was it expensive", "was it rare", the answer is not the same and the analytical method is not the same.
An example: we made several tensile test on iron bars reinforcing the Vincennes Dungeon (XIVth c.), mainly constituted of ferrite but with various P and slag inclusion contents and, on the same bar (10 samples) we observed about 10 different behaviors !, from very brittle to very ductile...
I also totally agree with the remark on EDS (despite I like this technique because of its flexibility). False measurements are very easy to do, especially with porous samples. This point will be even more crucial in the future because of the appearance of new systems that allow by combining SDD detector and image analyses, to analyze automatically about 100- 1000 points in a very short time (I remember the very nice examples showed by Chris in Plas Tan Bwlch during the "Pater Crew" congress).
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 Chris Salter
Envoyé : jeudi 27 mai 2010 16:42
À : [log in to unmask]
Objet : Re: requesting information on iron analysis
Dear Amalia,
The reason why your SEM-EDS analysis will give a high value is that
it analyzing the signal from the carbon laid down on the surface by the
electron beam cracking the vacuum gas. It is possible to perform carbon
analysis in a EPMA - a specialized SEM using wavelength dispersive
spectrometers and running a better vacuum and anti-contamination systems
than in a standard SEM but even then it is difficult. I have done it but
normal would send samples off to LECO if destructive analysis was
possible. However, if you are considering conservation this method
looses a lot of information that you can obtain using metallography and
specially when combined with EPMA or LA-ICMS.
The metallography will give a fairly good estimate of the carbon
content - a standard undergraduate metallurgy practical as outline by
other than have replied and it is quick. When combined with hardness or
micro-hardness testing it will also give an indication of the mechanical
history of the metal from data such as grain shape and size (but
phosphorus will have a big effect on this so has to be considered). The
levels of phosphorus will be barely detectable in a normal SEM can be
significant, both on expression of the iron-carbon microstructures and
the hardness of the metal. Phosphorus will alter the corrosion
properties of the metal especially where there are large changes in
chemistry with respect to the adjacent metal. This effect on corrosion
can be seen in the lower part of the images in the below URLwhere
corrosion has gone in along a zone of high phosphorus material.
http://users.ox.ac.uk/~salter2/Lowbury/Sword_Section.html
I would beware of using EDX analyses from SEMs in general, the technique
has a great tendency to be misused. The results should only be treated
as semi-quantitative unless you have gone through the full
standardization procedure, which is rarely the case from my experience,
and run a secondary standard to check that there are no problems with
the analysis. The analysis has to be run under exactly the same
conditions as the standards. These conditions include obvious things
such as accelerating voltage and processing time, but also using the
correct working distance and the correct beam, sample, detector
geometry. A few degrees tilt on the sample will throw the results off
several percent. That is before thinking about problems caused by the
nature of the electron-beam sample interaction and and the X-ray sample
interaction. Therefore, unless all these variables are controlled or
fulfil the assumptions made in the correction programs the result will
be wrong. In this case, the program assumes that the carbon is even
distributed through the interaction volume, whereas the contamination is
only a thin film on the surface. However, the program corrects for the
substantial absorption of the C X-rays coming from depth hence the very
high carbon content.
Chris
On 27/05/2010 10:27, Amalia Siatou wrote:
> dear all,
>
> I'm a metals conservator from Greece doing a master in materials science. Part of my thesis is the study of archaeological iron and bronze. I came across some difficutlies in measuring the carbon content of archaeological iron (roman nails). Using SEM-EDS the percentage measured was particularly high (almost 10%) which doesn't follow the dating of the objects. Furthermore, I was anable to find adequate equipment to perform chemical analysis.
> Do you know of a different method of estimating the carbon percentage in iron?
>
> thank you in advance
>
> sincerely
> Amalia Siatou
>
--
Chris Salter
Senior Analyst,
BegbrokeNano,
Department of Materials,
Begbroke Science Park,
Sandy Lane, Kidlington,
OX5 1PF
