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? ...

At our conservation lab we do some conservation of archaeological
composite objects, it could be waterlogged wood/iron, bronze/iron
and so on. Its done when there is no other alternative, for instance
when its not possible to separate the materials. If we talk about
waterlogged wood/iron(bronze/iron is another story), we would
desalinate the object in NaOH pH 11-12, until you can't detect
chloride. We change the bath every week and look for Cl with AgNO3,
when you can't detect Cl anymore, you can try to heat the object to
50 degrees C overnight and then look for Cl again. But even though
you can't detect Cl and you stop the process, you can't be sure they
are not still there to give you trouble later on. So after
conservation is done you must keep an eye on objects treated this
way. In short terms, we don't consider it a safe method of
conservation.

The waterlogged wood is part of the conservation problem and must be
dealt with. In such a case we would use the Cellosolve-Petroleum
method by Poul Jensen et al.. The national Museum of Denmark,
Section of Wet organic materials. 2800 Lyngby, Denmark.

If we have archaeological iron(not composite objects)we often use
furnace (heat) treatment with a gas chamber where there is a flow of
gas(mixture of 10% H2 and 90% N2)to prevent any oxygen being there.
The temperature is set to 800 degrees C for 20 hours. We consider
that a safe method of conservation, even though it changes the metal
structure some. It seems to me that in your case having done the
described conservation treatment,(what have you done to the
wood?)the best thing would be, if possible, to remove the wood and
remove as much of the wax as possible and then give the iron a
furnace (heat) treatment.It will be difficult to desalinate once you
have treated the iron with wax, unless you use an appropriate
solvent for a longer time.

Simon Botfeldt
Roskilde Amts Konserveringscenter
Denmark


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                  Conservation DistList Instance 15:32
                 Distributed: Tuesday, October 16, 2001
                       Message Id: cdl-15-32-003
                                  ***