Metal polish

Tom Dixon <tom.dixon [at] ngv__vic__gov__au> writes

>I have used Nevr Dull <URL:http://www.eagleone.com/> occasionally
>over many years, but not often or regularly.  Recently I was
>contemplating using it on a fairly continuous basis to polish a
>large gunmetal bronze sculpture over perhaps many years. This raised
>the issue of what was actually in the product.  The manufacturer
>provided a MSDS but this lists the components as "trade secret".
>Further inquiry with the manufacturer revealed only the solvent
>involved was similar to Stoddard solvent and the other chemical
>components were secret.

In 2001 while at Queen's University, I conducted a study on
suspected residues on metals from commercially available,
all-purpose metal polishes. Four commonly used polishes were chosen
for testing after surveying approximately twenty objects
conservators from both regional and museum conservation labs.  The
selected products were Nevr Dull, Flitz, Autosol and Brasso.  As the
study progressed, it became clear that the problematic ingredients
were oily non-volatile materials added to prolong the polishing
action of the products.  As an aside, the latter three have the odor
of ammonia.

To identify the non-volatiles, the polishes were first diluted and
centrifuged to remove the abrasive, and then extracted with solvent
and allowed to evaporate on KBr discs.  FTIR was performed, and the
non-volatile materials were likely identified as oleic acid and
hexadecanoic (palmitic) acid, both are common ingredients in
lubricating oils.  GC-MS of the extracts suggested the same results,
indicating molecular chains with the number of carbons ranging
around oleic and palmitic.  The Merck Index mentions that oleic acid
is mildly irritating to the skin and mucous membranes.  All four
polishes were found to contain similar ingredients in their organic
phases.

The approximate relative amount of non-volatile material in each
polish was measured by extracting the polishes again, and weighing
the residual material left after the extracts evaporated.  Nevr Dull
was found to have the least amount of non-volatile material, but
this is likely because it was the only polish in wadding form out of
the four products, and not in a cream or lotion form like Flitz,
Autosol and Brasso.  Nevr Dull was also found to have the coarsest
abrasive particles out of the four, and could be the most
aggressive.

When one considers the action of a polishing compound, even when
wiped and buffed until invisible, a residue could still be present
as a slurry of fine abrasive and metal particles in oleic and
hexadecanoic acid. This residue may become visible over time as the
metal particles and lubricants oxidize.  Generous rinsing with
Stoddard's solvent was found to remove residues on systematically
polished and rinsed metal coupons. Ethanol did not seem to work as
well.

All of this underscores the importance of rinsing an object after
any polishing mixture is used, which is generally something that is
already part of a conservator's standard working process.  Feel free
to contact me for more information.

Michael Belman
Mellon Fellow in Object Conservation
National Gallery of Art
6th and Constitution Ave, NW
Washington, DC  20565
202-842-6770
Fax: 202-842-6886


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                  Conservation DistList Instance 17:54
               Distributed: Wednesday, February 18, 2004
                       Message Id: cdl-17-54-012
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