Copper plates

I just want to correct some of the myths regarding copper corrosion
which are based on some "schoolbook errors" and are following the
conservation literature.

Joosje van Bennekom <metaalrestauratie [at] xs4all__nl>writes

>    2.  ..... often as a first corrosion
>        layer tenorite is formed, a black copper oxide. 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).

The first corrosion layer on copper exposed to room temperature is
the copper oxide cuprite (Cu2O). This corrosion product may appear
brown or black when a thin film is formed on copper. It may also
appear red or orange.

Tenorite (CuO) is only formed at high temperature or sometimes in
small amounts when pH is high on the surface.

Green copper in open air is NOT Malachite. It is very rarely found
on copper exposed to air due to low concentrations of CO2. However,
the green objects found in archaeological environments do often have
malachite patina due to large amounts of carbonates in the soil.
Instead, the green corrosion on copper in air depends of pollutants
(as SO2, chlorides etc.) forming green copper hydroxy compounds,
like the copper hydroxy sulfate *brochantite* (in outdoor air), the
copper hydroxy chlorides *atacamite* and *clinoatacamite*
(previously named paratacamite), and even soluble Cu(II)salts.

>    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.

Zinc corrodes, i.e. *oxidizes* and is the anodic surface, while the
copper surface serves as a cathode (oxygen is reduce on the surface)
an is protected from corrosion.

>    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.

The question of beeswax or micro waxes for the protection of copper
and bronze are not unambiguous. Beeswax may sometimes have some
advantages to microwaxes.

Helena Strandberg
conservator/scientist
Slottskogsgatan 55 B
414 70 Goteborg
Sweden
+46 31 149399 (phone/fax)


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                  Conservation DistList Instance 14:29
                 Distributed: Monday, November 20, 2000
                       Message Id: cdl-14-29-007
                                  ***