West African copper alloy objects

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.

If anyone has seen this sort of corrosion pattern before, I'd love
to hear from you or anyone who has discovered this feathery deep
Prussian blue corrosion product on a copper alloy which is high in
iron content.

Susan White
White Conservation Services


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                  Conservation DistList Instance 24:53
                   Distributed: Monday, May 30, 2011
                       Message Id: cdl-24-53-015
                                  ***