Mercury amalgam mirror

Linda Strauss <lstrauss<-a t->earthlink< . >net> writes

>Recently I accepted an 18th century Venetian mirror for treatment
>and I find that I have now become very interested in the subject of
>mercury amalgam.

re: tin amalgam mirrors

As a graduate student at the Buffalo program I researched and
treated a 17th century Venetian shrine composed largely of amalgam
mirrors on a wooden structure.

The mirrored surfaces on the shrine were highly deteriorated with a
small percentage of originally silvered areas remaining firmly
attached to their glass substrate and gray/black corroded surfaces
either loosely adhered or flaking off.  Elemental analysis using SEM
indicated that the black corrosion product was mostly tin with
insignificant amounts of mercury and sulfur and remaining silvered
surfaces contained both tin and mercury. In fact, the remaining
silver surfaces also exhibited beads of pure mercury, similar to
what Linda's seeing.

Familiarity with the techniques used in creating tin amalgam mirrors
indicates the use of excess mercury and this may be one of the
reasons you're finding beads of mercury throughout.  Additionally,
the tin constituent of the amalgam will oxidize first, leaving
metallic mercury behind to sublime or be abraded away leaving only
the blackened tin layer behind.  The cause of this corrosion is
probably related both to deterioration of the glass and to close
contact with the acidic wooden support causing the tin with its more
negative reduction potential to be oxidized more readily than the
mercury. Isolating or sealing the wooden support or inserting a
separating material may help inhibit the corrosion process.

We didn't do much research into consolidating the remaining silvered
surfaces but instead focused on re-silvering options since most of
the silvered surfaces were gone.  These processes included tin
amalgam inserts, releafing using silver, aluminum or palladium leaf,
reduction of silver nitrate onto exposed glass surfaces and backing
with 3M aluminized Mylar film.  The amalgam inserts were eliminated
as a possibility since they would be difficult to distinguish as
non-original, they're toxic and they're really hard to achieve.  The
other options may or may not be appropriate to specific situations,
often depending on the demands of the owner/curator. The one's that
looked the best, i.e. closest to an aged tin amalgam mirror, were
the palladium leaf and the silver nitrate method.

The results of the project were published in the student conference
papers from 1992.  I'd be happy to send off a copy of my paper if
you're interested.  Good luck,

Lisa Kronthal
Associate Conservator
American Museum of Natural History
Anthropology Department
Central Park West at 79th Street
New York, New York  10024
212-769-5446
Fax: 212-769-5334

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                  Conservation DistList Instance 11:20
                 Distributed: Thursday, August 28, 1997
                       Message Id: cdl-11-20-006
                                  ***