Lead

Alan J. Hawk <hawk [at] afip__osd__mil> writes

>I have a model of a ship made in 1875 for the Centennial Exposition.
>Some of the parts (such as the steering wheel and pullies) were cast
>from lead. However, the lead has developed a heavy white powdery
>coating (which I assume is oxidation) that is falling to the floor
>of the case.  What is the best treatment to stabilize the lead?  I
>was considering gently brushing off the powder and then painting the
>lead with Arcoloid B-72 liquid.  Would that be a good approach? What
>are your suggestions?

Lead tends to be rather stable when displayed in a controlled
environment. However when in contact with organic acids and humidity
it quickly develops soft white powdery corrosion products, such as
lead carbonates, acetates and formates. These corrosion products
have no protective function, but can quickly damage and destroy the
object if not removed.

Such acids often originate from the display material itself, such as
wood (esp. oak and plywood), wool, textiles, paints, lacquers,
certain waxes, cleaning chemicals, non deacidified paper or card, or
some adhesives.

You should carefully check your display environment for such
possible sources of contamination. Before using such materials you
should always perform an Oddy test to check for their compatibility
with the displayed material. There also exist suppliers that have a
range of tested materials. Active charcoal cloth and a ventilated,
but filtered showcase system, as suggested by George Bailey would be
another thing to look for.

Your object being not very old, I suppose that the developed
corrosion is rather superficial. You should remove as much of it by
brushing. A thorough rinsing with tap water (not distilled, is too
aggressive) and later alcohol should follow. Then you should
carefully monitor the object for further efflorescence. Hopefully it
will remain stable in an emission free display case.

In case it continues to corrode due to remaining acids, a chemical
or electrolytical treatment may be necessary. EDTA has been used in
the past to remove lead carbonates. It can be quite aggressive on the
exposed metal and tricky if not thoroughly rinsed after treatment.
Lead acetates have successfully been removed with electrolysis in
sodium hydroxide or dilute sulphuric acid with low current. However
this is a very specialized treatment that requires quite some
experience and infrastructure. The size of your object might also be
a bit of a problem for such treatment that requires a complete
immersion of the artefact in the electrolyte. Otherwise you could
try a reduction locally as David Cottier-Angeli suggests.

After treatment you could think of a protective coating like
microcrystalline wax or Paraloid B 72. Coatings however are a tricky
subject, they may even increase the corrosion rate, especially when
the object has not been thoroughly rinsed. Due to the complex shape
of your object, a complete even coating would probably not be
possible, and such a partial coating could introduce enhanced
corrosion on the unprotected areas.

I add some references that will discuss these problems and
treatments in more details:

    Green, L.R., and D. Thickett 1995
    Testing Materials for Use in the Storage and Display of
    Antiquities--A Revised Methodology. In: Studies in Conservation
    40, p. 145-152

    Titreault, J. 1992
    Matiriaux de Construction, Matiriaux de Diconstruction. Dans: 3.
    Colloque International de lmARAAFU, Paris 1992, p. 163-176

    Werner, G. 1987
    Corrosion of Metal Caused by Wood in Closed Spaces. In: Recent
    Advances in the Conservation and Analysis of Artifacts. London:
    Institute of Archaeology (Summer Schools Press) p. 185-187

    Degrigny, C., R. Le Gall 1999
    Conservation of Ancient Lead Artefacts Corroded in Organic Acid
    Environments: Electrolytic Stabilization/Consolidation. Studies
    in Conservation 44, No. 3, p. 157-169

    Green, L.  1990
    A re-evaluation of lead conservation techniques at the British
    Museum. In: Conservation of Metals. International Restorer
    Seminar Veszprem, Hungary (Jaro, M. ed.), p. 121-130

    Lane, H.  1979
    Some comparisons of lead conservation methods including
    consolidative reduction. In: The Conservation and Restoration of
    Metals, Edinburgh: Scottish Society for Conservation and
    Restoration, University of Edinburgh, p. 50-60

    Lane, H.  1987
    The conservation and storage of lead coins in the Department of
    Coins and Medals, British Museum. In: Recent Advances in the
    Conservation and Analysis of Artifacts.  London: Institute of
    Archaeology Press, p. 149-153

    Watson, J. 1985
    Conservation of Lead and Lead Alloys using EDTA solutions.  In:
    Lead and Tin Studies Conservation and Technology, UKIC
    Occasional Paper No. 3, p. 44-45

Valentin Boissonnas
Lecturer in conservation
Haute Ecole d'Art Appliqui
Conservation d'objets archiologiques and ethnographiques
Rue de la Paix 60
2300 La Chaux-de-Fonds - CH



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                  Conservation DistList Instance 14:35
                Distributed: Thursday, December 21, 2000
                       Message Id: cdl-14-35-002
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