CORROSION AND PATINATION

2.2 CORROSION AND PATINATION

Low-power binocular microscopy (10–50x) was used to examine the bronzes, which preserve a number of different corrosion products. These products were characterized by Debye-Scherrer powder x-ray diffraction (XRD) and have been previously reported (Scott and Podany 1991) . On the Venus, for example, blue crystals were identified as azurite; off-white as a mixture of quartz, calcite, and cuprite; black as romarchite and cassiterite; green as cassiterite; and gray as cuprite and quartz. Analysis of samples from the Togati identified pale blue as azurite; light green as malachite; and a second sample of light green as cassiterite and malachite. The corrosion products on both bronzes are tin oxides and basic copper carbonates. There is nothing unusual about the identity of these corrosion Low-power binocular microscopy (10–50x) was used to examine the bronzes, which preserve a number of different corrosion products. These products were characterized by Debye-Scherrer powder x-ray diffraction (XRD) and have been previously reported (Scott and Podany 1991) . On the Venus, for example, blue crystals were identified as azurite; off-white as a mixture of quartz, calcite, and cuprite; black as romarchite and cassiterite; green as cassiterite; and gray as cuprite and quartz. Analysis of samples from the Togati identified pale blue as azurite; light green as malachite; and a second sample of light green as cassiterite and malachite. The corrosion products on both bronzes are tin oxides and basic copper carbonates. There is nothing unusual about the identity of these corrosion

Fig. 8. Fibrous malachite crystals growing from surface on the uncleaned back of the Togati. 30x

Fig. 9. Photomicrograph under crossed polars of fibrous malachite crystal fragments removed from the crystals on the Togati shown in figure 8 . Each curled malachite fiber is comprised of a

bundle of fine individual fibrous crystals, which under a quarter-wave plate show different

orientations. Mounted in meltmount, refractive index 1.659. 45x

Fig. 10. Cleaned patina on the lower front of the figures of the Togati, illustrating surface mottling. The patina is principally composed of tin oxide with darker regions of higher iron

content. 11x.

Fig. 11. (above). Partially cleaned patina close to folds of the clothing in upper right of the Togati . Dark green corrosion products cleave easily from the smooth patina. Some pseudomorphic preservation of structure is still apparent in the green stratum, suggesting

interdendritic space of a dendritic structural origin. 13x

Fig. 12. (left). Tin oxide–rich patina with isolated pustules of corrosion products. partially

cleaned surface of front of the Togati. 8x

Fig. 13. Tin oxide–rich patina with view of surface below a pustule on the front of the Togati

showing cuprite zone immediately below the pustule. 10x

Figure 8 shows a surface view at low magnification of fibrous malachite growing on the reverse of the Togati. Malachite, Cu(OH)2. CuCO3, is unusual because it

(Palache et al. 1951 ; Gettens and FitzHugh 1974) . Individual fragments may be acicular, short, or long prismatic crystals with wedge-shaped terminations, sometimes twinned. Malachite corrosion crusts are usually compact, sometimes botryoidal, mammillary, fibrous, or silky. Even similar malachite fragments on the same object may have different optical properties. For example, on a Roman mirror in the Getty collection, some crystals were found to have negative elongation with oblique extinction, while others gave a positive elongation with oblique extinction. The subtlety of crystalline variations in malachite formed on ancient bronzes, or indeed in mineral specimens, has not received any attention that the author is aware of. Each malachite fiber on the Togati is made up of an aggregate of fiber bundles, as shown under crossed polars (fig. 9) . Under the quarter-wave plate, these composite bundles give a variety of colors arising from the different orientation of the individual crystals. Fibrous malachite is not common as a corrosion product on ancient bronzes and has rarely been reported (Fabrizi and Scott 1987 ; Chase 1993) , but it is probably seen more often by conservators than the literature would suggest.

In areas where the surface has been cleaned, the Togati has a smooth, sometimes mottled patina (fig. 10) . Not only does this patina preserve pseudomorphic remnants of dendritic structure, but it also has a mottled color ranging from pale brown to umber and a smooth, matte appearance. Overlying this layer are disseminated fragments of a copper-rich phase whose globular shapes are oriented in a pattern suggesting interdendritic regions (fig. 11) . These globular particles can easily be cleaved from the patina using a gentle scalpel movement parallel to the patinated surface. Figure 12 illustrates the occasional interruption to this smooth patina on the front surface by warty corrosion. These pustules have an apparently layered structure, judging from the stepwise appearance of the contours visible in figure 12 . Beneath each pustule, at the base, the smooth patina is interrupted by a zone of cuprite, evident in figure 13 .

A detail of the reverse surface of the Roma near a large opening in the back (fig.

14) reveals an unusual hexagonal network. Each hexagon is about 1 mm across and can clearly be seen in many areas of this surface. This hexagonal network structure is rarely seen in corrosion layers, and it is difficult to explain its mode of occurrence.

Fig. 14. Reverse of the Roma showing hexagonal network structure preserved in the patina. The

boundaries surrounding each hexagon are depressed relative to the patina. 5x The patina of the Roma is enriched in tin (as tin oxides), compared with the

composition of the metal, and has a similar matte lustre with surface mottling in the patina as seen in the Togati. Tin-enriched patinas have been discussed frequently: a good general account is given by Gettens (1969) .

Dendritic pseudomorphosis on the surface of the Venus is clearly seen in figure

15 , and a complex association of crystalline corrosion products, including fibrous malachite and small cuprite octahedra, is illustrated in figure 16 . In other areas of the surface, the malachite is curved and banded in the same fibrous growth formation but has assumed a botryoidal appearance (fig. 17) .

Fig. 15. Cleaned patina on the Venus showing pseudomorphic preservation of dendritic structure

in a tin–rich surface on the lower front. 8x

Fig. 16. Reverse of the Venus (Demeter) showing complex array of minerals, mostly cuprite, melachite, and azurite. Fibrous malachite crystals are present, as are occasional cuprite octahedra.

8x

Fig. 17. Reverse of the Venus showing uncleaned surface deposits of malachite, some almost

botryoidal. The malachite crystals are also curved and fibrous. 11x

The Cleveland Nike also has areas on the reverse preserving fibrous malachite crystals, azurite, and the same patina as that shown by the Togati. A photomicrograph of a dispersion of malachite fibers from the Nike (fig. 18) displays exactly the same features seen on the Togati (fig. 9) .

Fig. 18. Photomicrograph (under crossed polars, with a quarter-wave plate) of fibrous malachite crystals from reverse of the Nike (Cleveland Museum of Art). Mounted in Meltmount, refractive

index 1.659. The same crystalline bundles are found on the Togati shown in figure 9 . 40x One other very significant remnant of associated material links these four

bronzes: small fragments of red brick or tile are preserved on the back surfaces of the Roma, Venus, and Togati, and the Cleveland Nike had such fragments, too, before they were mechanically cleaned (Christman 1992) . Microsamples of these brick or tile fragments were mounted and polished for technical examination and analysis.