West African copper alloy objects

Susan White <smwhitewhite [at] aim__com> writes

>I am in the process of examining a large group of West African
>copper alloy objects (mostly brasses) which date from the 9th-10th
>century.  Most of the pieces are quite high in iron (some are
>actually magnetic), and I've observed some fantastic and unfamiliar
>corrosion products on the surface of a number of the objects.  Most
>I've been able to identify: pure iron oxide brown umber identical to
>pigment which forms as pufflike balls on the surface, zinc oxide
>islands surrounded by tiny cubic pyrite crystals, those then
>surrounded by a corrosion product I've never seen before.  This
>mineral appears as deep Prussian blue flakes, almost feather-like.
>For the life of me, I don't know what the mineral is, however,
>surrounding this ring of Prussian blue crystals is a thicker, more
>extensive ring of black corrosion, which when cut with a scalpel is
>highly reflective and iridescent, similar to hematite.

Metals tend to turn back to their original ore condition while they
age. Blue is a quite common colour when it comes to copper objects.
Azurite Cu3(CO3)2(OH)2, a blue copper mineral is very common in
South Africa. Copper gives a deep blue coloration in the presence of
ammonia. Aqueous sodium hydroxide causes the precipitation of light
blue solid copper hydroxide. Cupric oxide gives a distinctive series
of blues.

There's a fantastic book by David A. Scott, called "Copper and
Bronze in Art: Corrosion, Colorants, Conservation" that I highly
recommend you to check. It has a really long list of copper
corrosion products.

Evangelia Kyriazi
BA Hons Conservation and Restoration
MSc Geography and Applied Geoinformatics


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                  Conservation DistList Instance 25:2
                  Distributed: Thursday, June 9, 2011
                        Message Id: cdl-25-2-004
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