Brass polishes

I would like to add a few thoughts to the thread on the use of brass
polishes and how this information could be conveyed to the general
public. I know Clint Fountain, and I'm sure that he is well
acquainted (along with every furniture conservator) with the damages
caused by the use of commercial polishes on furniture brasses.

Clint Fountain's and George Bailey's observations on avoiding brass
polishes which contain ammonia are well taken. Ammonia is
incorporated into many commercial cleaning products (including glass
cleaners) because it is a powerful degreaser.  However, ammonia has
a deleterious effect on copper-alloys, particularly brasses which
contain more than 15% zinc and other minor alloying elements (such
as lead, tin,and aluminum).

Very small percentages (ppm) of moist ammonia in the presence of air
forms soluble copper complexes and attacks the residual stresses
left in the microstructure from cold-working - hence it's name:
stress corrosion cracking. This form of corrosion is essentially a
form of dealloying. When this process is carried to it's extreme the
copper is redeposited onto the surface of the metal in a porous form
and the zinc may either be left in place as an insoluble compound
or effloresced as as soluble salt. A series of cracks radiate out
from the locus of the corrosion leaving the metal in brittle
condition. Jim Moss's poster with SEM photomicrographs at the last
AIC meeting in San Diego demonstrated the phenomenon perfectly.

The trouble with using such ammoniated polishes is that the residues
are almost never completely removed (unless ultrasonic cleaning is
employed), and crevice areas and pits become microscopic collection
areas and the locii for corrosion cells to form.  Swab cleaning just
will not do it, if you don't believe me then examine what you think
is clean under the microscope.

Whenever I discuss cleaning options with the general public I always
make my first point a discussion on whether one should polish or
not--in many ways an aesthetic question, but becoming increasingly
important in terms of the issues of patina, attribution, and object
value (something the general public can always relate to!).
Polishing is essentially the refinishing of an object's surface and
should never be pursued without careful thought.  If polishing will
be done, then I would only recommend the use of precipitated chalk
as a first step, and perhaps either a gamma or alpha alumina (mixed
with distilled water) as an alternative.  I always warn the public
about the fact that such "safe" polishes will take more time and
elbow grease in use when compared to those polishes and dips which
harbor late-night television and seem to work instantaneously.
Still, all polish residues and all of the water must be removed too
(even inert particles can initiate microscopic corrosion cells). For
this I recommend a soft natural bristle brush used in hot distilled
water with a mild detergent and the use of a blow-dryer to evaporate
off the remaining moisture.

If you need either test or demonstrate if a polish is too abrasive then I'd
suggest the plexiglass test as recommended by Lyndsie Selwyn, et. al.  This
really makes the point to the public (and administrators) in a dramatic and
understandable way.

The main point in communicating with the public is to help them understand the
possible effects of what they are doing when they clean objects and artworks
and to encourage them to have a more critical understanding of why it is
important to know what they are using.  This is why we all give out the AIC
phone number to anyone who is interested! Cheers,

David Harvey
Associate Conservator,
Metals & Arms
The Colonial Williamsburg Foundation
P.O. Box 1776
Williamsburg, Virginia  23187-1776  USA
757-220-7039

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                  Conservation DistList Instance 11:60
                 Distributed: Tuesday, January 13, 1998
                       Message Id: cdl-11-60-001
                                  ***