Preparing cross-sections

Jennifer Barnett <reginatextilia [at] ision__nl> writes

>Is there anyone with experience of making cellulose acetate or
>nitrate sheets or films who can advise me about the procedure and
>quantities? Is there anything special that I would need to know?

I have never used Cellulose Acetate Sheets myself, but I can suggest
two other possibilities: Take a piece of PE sheet (e.g. the
transparent pockets you keep your notes in). Put one piece directly
onto the microscope slide, then the fibre, then a second layer on
top. Cover with either a cover glass or another microscope slide and
place onto a hotplate set at ca. 1200C. If necessary you can apply
some pressure with a non-pointed preparation needle. Wait a short
time until the slightly milky PE turns transparent: this means you
have reached the temperature to weld the two layers of PE. If you
have used pressure, keep the pressure upon removal until the PE has
recrystallized.

You can now cut the "sandwich" with a scalpel into extremely fine
layers (needs some practice). Use glass as working surface and work
under the microscope. Transfer the thin slices with a brush onto a
microscope slide and cover with glycerine or a embedding media with
lower refractive index. The disadvantage of this method is, that
through temperature and pressure, you might deform voids within your
fibres, which for some man-made fibres might be an issue. Of course
the method will only be of any help, if your fibre will not have a
lower melting point than PE....

The other method, with which I work most of the time and have good
results also for manmade fibres, is to take a black cross-section
sheet (similar to the cross section metal plates, but thinner and
made from plastic. Obtainable at McCrone Accessories). To fill the
relatively large hole, a bundle of known, colored fibre (e.g.
mercerized embroidery cotton thread) is pulled into the hole. The
fibre sample is placed in the middle of the protruding fibre ends
and--together with the bundle--pulled further until it is situated
within the thickness of the plastic plate. The fibre ends are cut on
both sides with a new(!) razor blade or scalpel. This technique,
too, takes some experience, but yields very nice results in very
short time. The color of the new threads used for stuffing can be
used for contrasting or enhancing light transmission through your
sample. If you want to keep your sample, just cut it out from the
plate and mount it on a permanent slide (e.g. with drops of
thermoplastic mounting media, of adhesive or of nail polish between
microscope slide and cover glass (do not stain your  cross section).
The disadvantage of this method is, that you cannot choose the
thickness of your cross section yourself. If your fibre sample is
really vertical, this should not be a problem for man-made fibres
(it is, however, sometimes for darkly died or "burned" vegetable
fibres). If your fibre, however is on a slight angle (by movement
while cutting or while pulling the threads into the hole), the light
might be transmitted inefficiently. Also make sure not to add even
more thickness when cutting: cut exactly with the slide surface.

Both these techniques have not been invented by myself. They have
been published in various places. I only have a German publication
at hand : Stefan Wulfert: "Der Blick ins Bild--Lichtmikroskopische
Methoden zur Untersuchung von Bildaufbau, Fasern und Pigmenten",
Ravensburger Buchverlag 1999, B|cherei des Restaurators Band 4,
Herausgeber Ulrich Schiessl. (p.269-270).

Hope this helps,
Karin von Lerber
Prevart GmbH
Oberseenerstr. 93 CH
8405 Winterthur
+41 0 52 233 12 54
Fax: +41 0 52 233 12 57


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                  Conservation DistList Instance 17:54
               Distributed: Wednesday, February 18, 2004
                       Message Id: cdl-17-54-003
                                  ***