Carbon coating objects for SEM/EDS

Suzanne Davis <davissl [at] umich__edu> writes

>The geologists usually coat their samples for SEM with carbon. As I
>understand it, this helps prevent stray X-rays from nearby areas
>from interfering with analysis of the particular spot you wish to
>analyze. We are placing the entire seal (they are quite small) in
>the SEM chamber and our analyses would be more reliable if we could
>coat the seals themselves with the carbon. Does anyone have
>experience doing this?
>...
>... We coated a glass slide this way, and I find that the
>graphite is very easy to remove with ethanol:de-ionized water in a
>1:1 ratio. I feel fairly confident that the coating can be applied
>and removed without difficulty, but I would be interested to hear
>from the conservation-science community any opinions/experiences in
>using a coating on small objects for SEM.

The purpose of the carbon coating is in fact to obtain an
electrically conductive surface which is necessary with traditional
scanning electron microscopes to avoid charging of the surface. If
an electrical charge builds up on the surface, the incident electron
beam is deflected. Image quality deteriorates very significantly as
a result, and analysis may be affected, too.

A glass slide, having a vitrified smooth surface, is indeed easy to
clean. You'll find the situation very different with porous stones,
as your seals are likely to be. Stone conservators often find it
rather difficult to clean soot deposits from stone objects which
have been rescued from a building on fire, which is essentially the
same problem. I would strongly recommend not to carbon coat your
seals.

Today, low vacuum scanning electron microscopes become increasingly
available at many institutions. These instruments perform imaging
and qualitative analysis (which should be sufficient for a general
identification of the type of stone in your seals) without requiring
a conductive surface. The object is simply put into the SEM chamber
as it is. My recommendation is that you find a low vacuum SEM which
should surely be available in one of the scientific departments of
your University. They are not that rare any more.

Dr Kilian Anheuser
Conservateur responsable du laboratoire, des ateliers de
    restauration et de la conservation preventive
Musee d'art et d'histoire
Rue Charles-Galland 2
Case postale 3432
1211 Geneve 3
Suisse
+41 22 418 2520
Fax: +41 22 418 2601


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                  Conservation DistList Instance 17:53
                Distributed: Wednesday, February 4, 2004
                       Message Id: cdl-17-53-002
                                  ***