Page 117 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Basic Copper Carbonates

CHAPTER

The blue pigment is a sand. In old days there were three varieties: the
Egyptian is thought most highly of; next the Scythian mixes easily with
water and changes into four colours when ground, lighter or darker
and coarser or finer; to this blue the Cyprian is now preferred. To these
were added the Pozzuoli blue, and the Spanish blue, when blue sand-
deposits began to be worked in those places.—PLINY THE ELDER 1

o

are important both
nly two of the copper carbonate minerals
as corrosion products and as pigments; these are malachite, CuC0 3 -Cu(OH) 2 , and azurite,
2CuC0 3 -Cu(OH) 2 . Both minerals have been known since antiquity, although the ICDD files
credit Beudant n 1824 with the first formal description of the properties of azurite. The two
i
minerals can be principal components of bronze patinas formed during land burials; minor
phases in corrosion products formed during outdoor exposure or sea burial; and postexcava-
tion alteration products of other minerals. Typical crystal forms for azurite and malachite are
shown in FIGURE 3.1. The exotic and unstable mineral georgeite, CuC0 3 -Cu(OH) 2 , has
recently been shown to be an isomer of malachite. Chalconatronite, Na 2 Cu(C0 3 ) 2 -3H 2 0, is a
mixed sodium-copper carbonate often associated with Egyptian bronzes and is more common
than the remaining three mixed copper-zinc basic carbonates: rosasite, (Cu,Zn) 2 C0 3 (OH) 2 ;
aurichalcite, (Cu,Zn) 5 (C0 3 ) 2 (OH) 6 ; and claraite, (Cu,Zn) 3 (C0 3 )(OH) 4 -4H 2 0. TABLE 3.1
summarizes the physical properties of these basic copper carbonate minerals.