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Tin-mercury inlay of a cabinet by Hendrik van Soest


#1

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 busy 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 on

http://morana-rtd.com/e-preservationscience/2004/De_Ryck_24_2_2004.pdf

The inlays at this cabinet are made of an alloy containing tin,
mercury and small amounts of copper and lead. Nowadays this material
suffers under a big variety of corrosion-forms which is very
untypical for tin objects since tin is normally a very inert material
where the surface gets passive by forming of oxidation products (SnO,
SnO2). In the case of this cabinet, under a layer formed by
romarchite and cassiterite (tinoxides) 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 chance for
contact-corrosion where the tin as less noble metal corrodes further.
So the main reason for this remarkable kind of tin-corrosion refers
probably 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 is not found out yet. The
possibility that the inlay panels where made by using tin amalgam is
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 immigrated
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
    and there are also recipes which work additional with mercury(salts).
    This treatments where also quite famous in the 18th century. (Silver
    was not found on the samples which were investigated, but those where
    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 than 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 test where the surface got bright and shiny at
first, but seemed to older by getting a white fog on the surface
which is not esthetical.

The treatment using tin amalgam was in this tests most successful.
The polishing effect came already 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 kept the same also
after aging.

It would be nice to contact me 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 taken on
amalgams. In the case of tin amalgam inlays, I found nowhere a
treatment to preserve the panels. Doe to the heavy kind of corrosion
they where mostly changed against new one without mercury.

Thanks for your interest,
Sophie Weichhart


#2

Well, Sophie, you have a real toughie, there. You’re dealing with
more knowledgeable people than I, so I won’t pretend to know. It’s
fascinating, though. Some thoughts: Tin Pest should only apply if
the piece has been frozen - so they say in research. I think the
golden bullet is the question of where the chlorine came from. To my
knowlege, that’s pretty unusual to have in the process, though the
book I show below has a couple of formulas with it. And chlorine
could cause that sort of rot. Unfortunately, it looks like your
amalgam is rotten, for whatever reason. If that’s so, you can polish
it, seal it, all manner of things, but you can never get the amalgam
or alloy back together again. That is, without destroying the art.
You could rebuild it, but that’s different from restoring it. I don’t
have the answers, but it’s a very interesting problem, I think. I
looked into a few places I have. One that has many formulas of the
sort you refer to is: “The Standard Encyclopedia of American
Formulas” Grosset & Dunlap, Edited by Albert A. Hopkins 1st copyright
1909, mine is 1953. OOP. One excerpt: One of the older formulas for
cold plating gives the following mixture: Silver Chloride, 3 parts;
salt, 3 parts; washed chalk, 2 parts; potash 6 parts. There’s also
mention of an amalgam of silver and tin. This pages 482-3. The book
is an interesting curiosity, but it has formulas dating back to who
knows when - you might take a look, if you can find a copy. I could
send you some pertinent parts, if not. Good Luck!!

http://www.donivanandmaggiora.com


#3

“Henley’s Formulas for Home and Workshop” has 10 pgs on plating and
covers some esoteric info. I believe this book, originally publ 1907,
is still available Avenel Books, NYC. Some of the material covered in
plating may be pertinent to this question; but you’ll have to check
it out on your own. Much of what’s covered is still pertinent.

KPK