Page 361 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

in which corrosion had selectively replaced the copper-rich alpha phase, leaving the tin-rich
delta phase intact. X-ray diffraction data by these authors show that the alpha-phase replace­
ment product was poorly crystallized or nanocrystalline cassiterite. It can be very difficult to
completely characterize fine, dark patinas of this type. Transmission electron microscopy sug­
gests that this alpha-phase replacement has a composition of Sn 1_^.(Cu,Fe,Pb,Si) x0 2.
Another example of subtle identification problems concerns the very dark green patinas
examined on a number of bronzes from the sixth century B.C.E. that were excavated from the
religious sanctuary site of Francavilla Marittima in southern Italy (Scott and Taniguchi 1999).
The greenish black patina suggested that a copper sulfide might be present, but detailed studies
by X-ray diffraction found that the only possible analogies were to chvilevaite, Na (Cu,Fe,Zn) 2S ,
2
or frerbergite, (Cu,Ag,Fe) 12 Sb 44 S 126 (iCDD 27-190). Partially substituted lattices, in which cop­
per can be replaced with other metallic ions, may explain why these oxide or sulfide patinas can
be so analytically intractable. This consideration of the corrosion of Chinese bronze mirrors
leads to a wider discussion concerning various attempts at imitating or replicating patinas, a
subject of considerable interest from ancient times.
I HISTORICAL ATTEMPTS AT REPLICATING PATINAS In
seventeenth-century Europe, Etruscan mirrors were eagerly collected objects, as were other
Greek and Etruscan bronzes, in parallel to the revival of interest in everything antique. Several
centuries earlier in the Far East, Chinese connoisseurs were fascinated by the appearance of
excavated bronzes with their varied colors and surface finishes. During the Southern Song
dynasty (1127-79) and Ming dynasty (i368-i644), many attempts were made to imitate both the
style and patination of these treasured artifacts, which often displayed black, tin-enriched pati­
nas or smooth and subtle light greenish blue surfaces, incorporating a substantial proportion of
tin compounds along with copper corrosion products. Many techniques were developed to try
to replicate these finishes; they ranged from the simple adhesion of ground-up malachite with
glue binder to the thinly patinated surface, often with a cuprite crust a few micrometers thick, 2 1
to highly complex chemical treatments. Kerr (1990) suggests that the deliberate forging of Chi­
nese bronzes was already prevalent by the time of the Song dynasty.
Several recipes have survived for producing the greens and reds of the patinas that were
much admired. Gao Lian, a collector living during the Ming period, records a complex treat­
ment to produce an artificial patina that begins with applying a mixture of sal ammoniac, alum,
borax, and sulfuric acid to the surface of a bronze object and baking it. Next the object is placed
in a pit lined with red-hot charcoal that has been splashed with vinegar. A variety of substances
are added to the surface of the object to encourage salt efflorescence, such as pigment, piles
of salt placed on certain areas of the bronze surface, metal filings, or cinnabar. The treatment
ends with burial of the bronze in acidic soil for an extended period of time. The initial mixture

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