Brass

Carolyn Fuss <csf2 [at] is2__nyu__edu> writes

>I'm submitting this inquiry as a "layperson" who hopes to obtain
>some helpful advice from the professionals on this list. I've just
>recently bought a Victorian full-size brass bed (made of brass
>wrapped around iron) and had it polished but not lacquered.  Now it
>is bright and shiny, but I would like it to mellow in color over
>time and regain some of the patina it has lost.  What do you
>recommend as the best way for me to achieve this?

Most likely the brass that is wrapped around an iron core will  have
stresses in it as a result of producing the sheet material or in the
process of rolling it around the core. Stressed brass is very
susceptible to Stress Corrosion Cracking (SCC) when it is exposed to
ammonia and less so when exposed to various amines (mono, di, or tri
ethanolamines).

One needs to be very careful when using commercial polishing
mixtures because many of them contain ammonia ( Brasso and Noxon to
name a couple) and their use can cause SCC. Never Dull and Met-Pol
polishes are ammonia free. You should check the MSDS sheet on a
product before using it.

In addition, polishing mixtures contain abrasives (usually very
aggressive because the general public wants instant gratification).
Because the brass sheeting is very thin, repeated polishing will
eventually abrade away the brass until there is nothing left. You
should check the coarseness and aggressiveness before using a
polishing compound and use the one that is least aggressive.

Coating the brass after polishing is problematic in and of itself:
no coating is perfect thus the oxidation (patina) or lack thereof
will be inconsistent. My experience in developing an even overall
oxidation on brass objects is not to apply any coating at all and
not to handle the object except when wearing nitrile gloves or
equivalent (do not use cotton gloves as the cotton fibers will wick
the chemicals exuded from your hands onto the metal surface).
Oxidation will be noticeable in about 6 months and after a year or
two will have developed a delightful appearance. Oxidation can be
accelerated by exposing the object to low heat and/or sunlight but
do not expect a rapid visual change. After the object has reached
the desired level of patina, it can be cleaned of surface accretions
and then an appropriate coating applied. Wax is nice in that it can
be removed relatively easily but there are other coatings that are
more protective especially if the object is to be handled but they
are generally harder to remove. Remember also that coatings will
need to be renewed occasionally and that in the case of metals, it
is impossible (or more than difficult) to just remove a section of
failed coating and renew because the failed area will have oxidized
more than the surrounding areas causing a visual blemish: you
generally need to redo the entire object including polishing!

A final note: it would be wise to check with a trained metals
conservator for more detailed guidance. Regards,

Jim Moss
Horological Conservator
James Moss Clockmakers, Inc.
95 Russell St.
Littleton, MA 01460
978-952-0070


                                  ***
                  Conservation DistList Instance 14:45
                 Distributed: Friday, February 16, 2001
                       Message Id: cdl-14-45-004
                                  ***