Copper plates

Rab Jackson <pr258rj [at] admin__nls__uk> writes

>Recently we had some water ingress in our copper plate store where
>some of the copper plates were in contact with water.  I would like
>to ask colleagues on this list for information/advice on the storage
>of copper plates and what type of damage can occur when copper
>plates are exposed to water.  The copper plates have a coating of
>wax and currently stored upright in zinc shelving.  My main
>questions are:
>
>    1.  What is the optimum storage conditions for copper plates?
>    2.  What effect in terms of chemical reaction can be expected if
>        copper plates are subjected to water/moisture?
>    3.  Why store copper plates in zinc shelving?
>    4.  Why have a wax coating?

    1.  Research showed that corrosion symptoms appear (for most
        metals) at 60% RH . These reactions are intensified at 80%
        RH. The safe optimum storage condition for most metals is
        set at ca. 40% (relative humidity). At this RH, there is not
        enough water present for the corrosion mechanisms to occur.
        Temperature should be around 18 degrees celsius.

    2.  Copper that is exposed to high RH (as is the case with water
        ingress) can form different corrosion products: it more or
        less depends on the other present chemical compounds. These
        reactions are very complex and not easy to summarize, but
        some possibilities can be given: often as a first corrosion
        layer tenorite is formed, a black copperoxide. The reactions
        will continue when the RH stays high, and in rural areas
        (unpolluted air) malachite cam be formed, a green basic
        copper carbonate. The presence of SO2 (common in industrial
        areas where the air is polluted) can give the green
        brochantite (basic copper sulfate, often the main component
        on green copper roofs). H2S, also often present in polluted
        air, is very reactive to copper and will tarnish it black
        (copper sulfide). When chlorides are present, a very
        aggressive type of corrosion can occur, which can damage the
        copper very badly because of the rapid propagation of this
        corrosion type.

    3.  I think that the idea of the zinc shelving is that the zinc
        protects the copper chemically. Zinc is a less noble metal
        then copper, which means that it has a greater affinity then
        copper to react with other chemicals (and thus corrode more
        easily). Zinc hence works (theoretically) as a sacrificial
        material and 'catches' corrosive compounds from the
        environment before copper does.

    4.  The wax coating is probably applied as a barrier to
        moisture. However, it is still not yet certain whether wax
        really acts as a watertight barrier. It is important that
        the right kind of wax is used. Beeswax should not be used,
        because this wax can be acid and react with the copper to
        form copper oleates. It is better to use a non-acid
        microcrystalline wax, and apply two or three coats to
        minimize the possibility of pores in the wax film.

I hope these answers will help you. A book on metal corrosion and
conservation that can be of use and also contains the information
given above is that of  T. Stambolov  (1985) 'The Corrosion and
Conservation of Metallic Antiquities and Works of Arts' Central
Research Laboratory for Objects of Art and Science, Amsterdam (it
can be ordered by The Netherlands Institute of Cultural Heritage,
+31 20 3054545, Fax: +31 3054600).

Joosje van Bennekom
Metalconservator at the Netherlands Institute of Cultural Heritage


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                  Conservation DistList Instance 14:28
                 Distributed: Monday, November 13, 2000
                       Message Id: cdl-14-28-003
                                  ***