Page 379 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

importance. Linda Merk-Gould used the MHP method at a pressure of 1000 - 4000 psi for clean­
ing prior to repatination (Harris 1994). She used this method several times to clean outdoor
bronze sculpture and obtained good results without undue damage to the surfaces.
I LASER CLEANING Laser cleaning of metals is a recent devel­
opment. A neodymium (Nd) laser has been successfully used to clean surface concretions from
stone objects. The use of a controlled burst of high-energy photons produced by a laser can also
be used to clean metallic surfaces. With this method, the minerals are ablated, or vaporized,
from the surface, removing the corrosion without the need for chemical reagents. The principal
problem with this technique is controllability of the results.
A number of different approaches to the use of lasers for the cleaning of metallic surfaces
have been published. For example, Asmus (i987) suggests the use of carbon dioxide lasers for
metal cleaning, while Watkins (1997) reviews the application of Nd : Yag lasers to the cleaning of
bronzes. Removal of tarnish from the surface of silver-coated copper daguerreotypes with an
excimer laser has been investigated by Turovets, Maggen, and Lewis (i998).
Cottam and coworkers (1997) discuss cleaning metallic surfaces using a TEA-C0 2 laser-
ablation technique at 10.6 μιη in the far infrared. Copper and bronze with both naturally and
artificially grown corrosion layers were treated. The inorganic minerals comprising the corro­
sion crust showed a varied response to the laser radiation. In general, it was found that the more
complex copper-corrosion products such as brochantite were more susceptible to ablation than
simple compounds such as cuprite.

Unanswered questions Difficult aesthetic and scientific questions remain unanswered
regarding the cleaning of outdoor sculpture, especially since
many objects will require re-treatment in years to come, and some may have to be repatinated
or recoated yet again. All outdoor sculpture requires maintenance, such as washing and rewax-
ing every two years or so, and this is frequently overlooked in treatment proposals. Naude and
Wharton (1995) have prepared a useful handbook, Guide to the Maintenance of Outdoor Sculpture,
that discusses the issues involved in establishing a maintenance program for such works.

C H E M I C A L C L E A N I N G T R E A T M E N T S

Chemical methods of cleaning have a long history and traditionally include the use of domes­
tic materials, such as lemon juice, with which corroded objects could be scrubbed. Chemical
cleaning seeks to remove some or all of the patina, or corrosion, on the surface of the object. In
conservation practice, chemical cleaning usually attempts selective removal of certain types of
corrosion layers or chemical substances, leaving others unaffected. For example, in the cleaning
of bronzes it is common to leave the cuprite layer adjacent to the metal intact, since removal of
this layer usually reveals bare metal, which amounts to complete stripping of the surface, which,
in most cases, is undesirable.

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