Tin mercury inlay

I am studying at the Royal Academy for Fine Arts in Antwerp,
Conservation Department of Metals, Prof. P. Storme. In one of my
projects I am working with a very special case of tin corrosion. The
object is a cabinet manufactured by Hendrik van Soest in the first
decade of the 18th century in Antwerp.

An article on the investigation and photographs of this object are
published under the title:

    "The tin mercury inlay of a cabinet manufactured by Hendrik van
        Soest: A case study"
    <URL:http://www.morana-rtd.com/e-preservationscience/>

The inlays at this cabinet are made of an alloy containing tin,
mercury and small amounts of copper and lead. Nowadays this material
suffers a big variety of corrosion-forms. This is very untypical for
tin objects, since tin is normally a very inert material, where the
surface becomes passive by forming of oxidation products (SnO, SnO2).

In the case of this cabinet, under a layer formed by romarchite and
cassiterite (tin oxides), is a layer of tin oxide chloride
hydroxide. The chlorides in the corrosion layer can be a result of
the mercury content in the alloy since Hg attracts chlorides from
the surrounding. When the tin corrodes the liquid mercury comes in
drops out of the alloy and there is a chance for contact-corrosion
where the tin, as the less noble metal, corrodes further. So, the
main reason for this remarkable kind of tin-corrosion probably
relates to the mercury in the alloy (there is also a possibility
that the glue used to attach the panels works as an electrolyte and
therefore also induces corrosion, but this is sure not the main
reason for the degradation).

Why there is mercury in the alloy has not been found out yet. The
possibility that the inlay panels were made by using tin amalgam has
been eliminated since the Hg amount is much to low for this kind of
manufacturing technique and the appearance of the panels (visual and
microstructure) would be different in this case. The conclusion that
there was no reason to work with an amalgam in the beginning, led to
the belief that there was a finishing technique used on the surface
of the tin panels which contained Hg. The mercury migrated
afterwards in the panels where the alloy was formed. Possibilities
for such finishing treatments could be:

    1.  Silvering of the surface by using pastes or liquids which
        work by depositing silver on the surface through a chemical
        reaction. Therefore silver salts, for example, silver
        nitrate, silver chloride, etc.--if silver chloride was used,
        it would be also an additional reason for the presence of
        chlorides in the corrosion layer--are used. There are also
        recipes which work with mercury (salts).

        These treatments were also quite famous in the 18th century.
        (Silver was not found on the samples which were
        investigated, but those were all taken from a crack in one
        panel where it is possible that the thin layer of silver
        already disappeared).

    2.  Amalgamating the surface before silvering it, to get a
        better attachment of the silver layer.

    3.  Fake silvering, where the surface is amalgamated to get a
        silver-like appearance.  These recipes work like those for
        silvering with the difference that instead of silver,
        mercury is used.

    4.  Tinning and polishing with amalgam. These finishing
        techniques were used for example on ancient Chinese bronze
        mirrors to get the surface bright and shiny. These
        treatments all work by rubbing a paste containing tin
        amalgam on a surface.

In practical tests where these historical techniques where used it
seemed that it is very difficult to silver tin with these kinds of
pastes. If they worked, then you had to rub the surface for a long
time with the paste, which would be a lot of work on the engraved
panels on the cabinet. Amalgamating the surface also did not seem to
be a very useful pre-treatment in these tests.

A fake silvering was tested, where the surface got bright and shiny
at first, but later developed a white fog on the surface which is
unaesthetic.

The treatment using tin amalgam was in this tests most successful.
The polishing effect came after rubbing the paste for a very short
time on the surface of the samples. The appearance afterwards was
better than on all the other samples and remained after aging.

It would be nice know if somebody has additional information or
experience with related objects. My main interest is now in

    *   Recipes which are meant for silvering tin. Most of the
        recipes are meant for silvering copper (alloys). Only one
        for silvering tin was found till now.

    *   Information about cases where similar finishing techniques
        or recipes were used in Central Europe than in ancient
        China. (Polishing with tin-amalgam)

    *   Information on objects where the surface was only
        amalgamated to get a polished effect.

    *   Restoration and conservation treatments which were done on
        amalgams. In the case of tin amalgam inlays, I found nowhere
        a treatment to preserve the panels. Due to the heavy kind of
        corrosion, they were mostly changed [compared to the] new
        one without mercury.

Sophie Weichhart
Klapdorp 73
2000 Antwerp


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                  Conservation DistList Instance 20:43
                  Distributed: Tuesday, March 6, 2007
                       Message Id: cdl-20-43-020
                                  ***