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?

I have to treat an Egyptian bronze collection (nearly 150 artefacts)
with BTA that I had treated recently. This case history is quite
interesting from the conservation view point rather than from that
of the collection. As a matter of fact, the collection was recovered
from a damp basement on summer 2005 and therefore treated as a
disaster emergency case.

The treatment consist in removing all past lacquers and wax
coatings, drying down to 40% RH, followed by a 1,5% BTA in 98%
ethanol / 2% acetone, drying conditioned to 40% RH and re-coated
with B72 in 2 or 3 crossed layers. In Spring 2006, the collection
was reintegrated into a newly built private Museum.

An unexpected new disaster was occurred: clouds of chlorine were
inadvertently sprayed directly in the museum room, and therefore on
the artefacts, over several weeks. I will skip over the detailed
chemistry, even though I would be quite happy to discuss this
further.

I come now to the BTA stabilisation. I was satisfied to observe that
where BTA had properly reacted with the copper, the corrosion--even
under those severe condition--had remained stable. Even more, where
previously there was a very active area, now it was rather stable.

Internal condensation seems to had the worst effect on the
artefacts. This could had been caused either by the object mass or
by previous restoration.

Now I come to your query. I would emphasise the vacuum impregnation
at a lower percentage rather than a higher concentration for a
longer time. Remember that all you need is to have enough product to
react with your surrounding copper.  Any excess has to come out, and
BTA is a really harmful substance for us.

If the previous treatment are vital to the artefact, then fast
impregnation allows you not to disturb the B72 too drastically. From
experience, the removal of all previous treatment gives decisively
the best result.

Control the internal RH before locking the surface with a lacquer or
a wax;, this allows you to minimise the internal condensation for
better long term passive stabilisation.

I would be quite happy to extend this discussion.

David Cottier-Angeli
CottierMetal.ch
5C Route des Jeunes
CH-1227 Geneva
+41 22 300 19 55


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                  Conservation DistList Instance 21:7
                   Distributed: Monday, May 28, 2007
                        Message Id: cdl-21-7-003
                                  ***