Page 142 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The X-ray diffraction data for clinoatacamite are slighdy more complex than the data for
paratacamite. One of the distinguishing features for clinoatacamite is the clustering of four lines
at 2.713,2.339,2.266, and 2.243; only two lines appear in this range for paratacamite (see APPEN­
DIX D, TABLE 2). Because of the extremely close similarity in the X-ray diffraction data for
paratacamite and clinoatacamite, diligent attention will need to be paid in future research to the
assignment of d-spacings and intensities to distinguish between these two mineral phases. This
is important because with much archaeological material it is difficult to obtain a clear enough
powder X-ray diffraction result to be able to determine exacdy what phase is present. The analy­
sis included in APPENDIX D, TABLE 2, of a light green, powdery corrosion product is from a
totally mineralized bronze rod from the Middle Bronze Age site of Tell Fara, Jordan. This table
gives comparative data for anarakite, zincian paratacamite, and clinoatacamite. The data appear
to be slighdy more in keeping with ICDD file entry 25-1427 for paratacamite, but the corrosion
material is probably clinoatacamite, especially since zinc and nickel are absent.
Since many outdoor bronzes contain appreciable amounts of zinc, it is quite possible that
some of the reported occurrences of paratacamite are correct, or that this mineral can coexist
with clinoatacamite as a corrosion product, as it can in geological formation. The whole story,
however, will not be known until further research is carried out on this aspect of the problem.

Botallackite Of the four copper trihydroxychloride isomers, botallackite is
the least stable. This accounts for the rare instances in which it
has been identified as a component of copper corrosion products on archaeological material or
as a pigment in wall paintings. Botallackite, which is monoclinic, was first identified, charac­
terized, and named by Church (i865) for the Botallack mine at St. Just, Cornwall, England. No
occurrences of this mineral on antiquities were reported for nearly one hundred years, until
Frondel (1950) found it on the interior of an Egyptian bronze figurine of the deity Bastet in the
Fogg Museum of Art, Boston. Gettens (i964) reported an occurrence of botallackite on an Egyp­
tian bronze censer in the Walters Art Gallery, Baltimore; and Schnorrer-Kohler, Standfuss, and
Standfuss (1982) identified the mineral in lead slags from ancient mines in Lávrion, Greece (see
pages 142-43). The slags were, significandy, in contact with seawater.

C O P P E R C H L O R I D E S AND BRONZE D I S E A S E

Bronze disease is a progressive deterioration of ancient copper alloys caused by the existence
of cuprous chloride (nantokite) in close proximity to whatever metallic surface may remain. 4
Cuprous chloride may lie dormant until reaction with moisture and oxygen causes this un­
stable compound to expand in volume on conversion to one of the copper trihydroxychlorides.
This creates physical stress within the object affected, resulting in cracking or fragmentation.

C H L O R I D E S AN D BASI C C H L O R I D E S
Ί25