Conservation of archaeological composite object

Karin Abelskamp <k.abelskamp [at] archeologie__nl> writes

>... On one
>of our larger projects, the Roman military site Vleuten de Meern
>Wachttoren, the following composite object was found: a piece of
>wood in an iron "shoe". We think that it might be the lower end of a
>lance/spear or even a Roman "signum". The wooden part, approximately
>35 cm long and broken in 5 pieces, is attached to the iron "shoe"
>with 2 crossed nails (visible with X- radiography).
>...
>My question is: would any one of you know of a method to desalinate
>a wooden/iron composite object without having to take it a part?
>What objections might there be or not be in separating the wood from
>the iron for the treatment? What would be a suitable solution for
>this conservation problem? ...

>From your description it is not very clear if your composite object
has been dried before or during the treatment, or if it is still
humid. I would assume that you did a controlled drying for the wood
and then applied a wax coating to the iron once dry. To do any
further treatment you will have to remove your wax layer, which can
be quite messy. I would recommend you to use a more reversible
protective coating such as Paraloid B44 or 72.

Concerning the high temperature treatment I would strongly recommend
you not to use any of these methods, even though they are still in
use in some places. According to the investigations of
Arnoult-Pernot, insoluble chlorides, such trapped within
akaganeite,will only be removed at temperatures around 400 deg.C and
treatment times of up to 200 hours. The problem you face with such
high temperatures and long exposure is that your remaining
metallurgical structure can be altered. Important manufacturing
related structures, such as martensite from quenching, will be
altered or removed. Remaining wood fragments would also be damaged
by these temperatures. From an ethical point of view such treatments
remain highly questionable.

A better choice would be to desalinate all of your objects in
alcaline sulfite baths. Your wood however, which is probably
partially or totally mineralized, needs a protective coating not to
be affected by the alcaline sulfite. In the past microcrystalline
wax has been used as a protective coating, but the results are not
always satisfactory. A good coat of Paraloid B 44 (not 72) however
will not be affected by the solution and the temperature of the
baths (which is around 50 deg.C). The Paraloid can be easily removed
after the treatment. Your objects will then need a protective
coating, for that Paraloid B44 could also be used.

I would recommend you to remove as much wood as you can from the
iron before desalination. Since the wood is broken into 5 pieces,
you will be able to remove some without introducing new breaks. The
pieces that are nailed to the iron should be left in their original
context and treated with the iron with the appropriate protective
measures. To be on the safe side, do take all samples needed for
wood identification before using any coating before or after the
treatment. Good luck!

Valentin Boissonnas
Lecturer in metals conservation
Haute ecole d'art applique
Rue de la Paix 60
2300 La Chaux-de-Fonds
Switzerland


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                  Conservation DistList Instance 15:33
                 Distributed: Friday, October 19, 2001
                       Message Id: cdl-15-33-001
                                  ***