Page 378 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Veloz, Ruff, and Chase (i987) performed tests to evaluate the effectiveness of walnut shells
and glass-bead peening to clean both cast and polished surfaces. The glass beads, as well as other
hard abrasives, severely deformed the surface on a microscopic level. This finding was con­
firmed by Barbour and Lie (i987), who evaluated the effectiveness of three different plastic blast­
ing media compared with walnut shells, powdered sodium bicarbonate, and glass beads. They
found that using glass beads at both 25 psig and 80 psig left substantial imprints, cratering, and
deformation on polished, work-hardened bronze coupons and on corroded copper sheeting. No
alteration was found with two of the three plastic media or with walnut shells. Ground wal­
nut shells and sodium bicarbonate had no effect on the surface finish of the bronze, and the mix­
ture removed virtually all accumulated dirt, grime, and loosely attached corrosion products. A
weight loss of 2 mg/cm occurred following a moderate use of glass-bead peening, but no dis­
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cernible weight loss or surface change followed the use of walnut-shell peening. Although Weil
and colleagues claimed that glass-bead peening reduced the corrosion rate of outdoor bronze
sculpture (Weil et al. 1982), laboratory evaluation by Barbour and Lie indicated that the rate of
atmospheric corrosion may actually be increased after this treatment.
Barbour and Lie also found that a number of variables control the effectiveness of cleaning
corroded bronzes with crushed walnut shells. For example, smaller particles were more efficient
than larger particles — 60/200 mesh walnut shells gave good results — and the ratio of total mass
of air to abrasive is very significant. In practical application, the effectiveness of the cleaning
was improved by using a larger nozzle with decreased pressure. Veloz, Ruff, and Chase (i987)
used a 0.8 cm ( /i6 in.) nozzle at 20 psi in contrast to a 0.4 ( %i in.) nozzle at 40 psi. The most effec­
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tive angle of impact was within 20 degrees of perpendicular to the surface being cleaned, and
the results obtained were judged to be quite satisfactory.
I WATER BLASTING Another recent development in mechani­
cal cleaning, especially for outdoor bronzes, zinc sculpture, and marine finds, is water blasting,
using either medium-high-pressure (MPH) water at about 1000-7000 psi or ultra-high-pressure
(UHP) water at around 30,000 psi. The UHP method was used by Lins (1992) in 1987 for the con­
servation treatment of the 10.67 m (35 ft.) tall bronze sculpture of William Penn by Alexander
Milne Calder that is on the top of City Hall Tower in Philadelphia. The equipment for UHP
spraying is capable of directing rotating jets of water through a series of nozzles about 0.5 mm
in diameter at velocities of up to 670.56 m (2,220 ft.) per second, more than twice the speed
of sound. The apparatus used to clean the Penn monument could produce a water pressure of
32,000 psi, but because of the height of the sculpture, a pressure of about 29,000 psi was more
practical. Lins (1992) concluded that this water-blasting method was more effective in remov­
ing corrosion products from a pitted surface than other methods, and it did so without any
significant damage to the metallic bronze substrate. Considerable safety precautions must be
taken when using the UHP procedure because of the very high water velocities used. As with
any mechanical cleaning method, the skill and experience of the operator is of paramount

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