Benzotriazol

Sabine Brechbuhl <sabine.brechbuehl [at] erz__be__ch> writes

>I am involved with treating three archaeological bronze bracelets
>which show signs of active corrosion. They have already been
>consolidated and glued with Paraloid B72 (Acryloid B72) in 2000, but
>no stabilisation treatment was carried out. With comparing pictures,
>substantial loss due to corrosion is obvious. As for instance we
>cannot guarantee stable climate conditions, a stabilisation seems
>necessary.
>
>Can a bath of benzotriazol, BTA 3% in ethanol during 24 hours
>(according to D. A. Scott, Copper and Bronze in Art), penetrate
>enough and if so, is this stabilisation effective even on a
>consolidated object?

It sounds like these objects are in very serious condition.  I think
the presence of B72 consolidant and adhesives will cause problems
for a benzotriazole treatment.  It is also a problem that the
corrosion was so deep and extensive as to need consolidation.  The
active corrosion is deep down in the pits of the metal and the
layers of consolidated corrosion will not allow deep penetration,
especially if no vacuum is applied.  In addition, BTA treatments
often have highly variable results under the best of conditions.

Depending on whether the objects can withstand it, I would recommend
removal of the B-72 as far as possible to allow maximum penetration.
Also, many have found that a pre-treatment in a weak sodium
carbonate solution (under vacuum) to neutralize the acidic nature of
the corrosion will help the BTA to be effective.  This can be
followed by treatment in benzotriazole, also applied in a vacuum.
After this, a lacquer coating is usually suggested.

If the B-72 cannot be removed first without causing loss, you can
try immersing in BTA in a 50:50 water/alcohol, applied under vacuum,
followed by thorough drying.

If you do not have access to a vacuum chamber, you can try a BTA
treatment, but it is unlikely the treatment will be successful.  An
alternative is to provide a totally dry, acid-free environment in
small, sealed containers, as this will at least slow down the
progression.  I have not provided details of the treatments
mentioned for the sake of brevity, but feel free to contact me for
more information, as procedures are important to prevent damage.

Linda S. Roundhill
Art and Antiquities Conservation, LLC
Woodinville, WA
USA
425-481-0720


                                  ***
                  Conservation DistList Instance 21:7
                   Distributed: Monday, May 28, 2007
                        Message Id: cdl-21-7-004
                                  ***