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Object conservation


Standing adorned Buddha successfully treated for bronze disease


image: Before treatmentBefore treatment  more detail

image: After treatment in conservation

After treatment in conservation  more detail

The sculpture dates stylistically from the twelfth century Angkor Wat period of Cambodian history. At that time the influence of Khmer culture extended well into the eastern and central provinces of Thailand. In fact, the Buddha’s symmetrical stance, hand gestures, elegant robe and skirt cloth, and elaborate jewellery all support north-eastern Thai origins. The sculpture has been in Western collections for over half a century: it was sold by Baroness Cassel van Doom through Parke Bernet Galleries in New York in 1942, and subsequently passed through a number of owners and dealers including C.T. Loo of Paris, and Spinks in London. The National Gallery of Australia purchased it at auction in 1998 at Sotheby’s in New York.

While we were aware of these recent transactions at the time of purchase, we now perhaps have a better understanding of why the sculpture may have changed hands with considerable frequency: it had bronze disease. At the time of purchase, the Gallery hired a leading American bronze conservator to report on the sculpture’s condition, which she assessed favourably. In 2002, however, unexpected evidence of what must have been a recurrence of bronze disease was detected in the Buddha during a precautionary survey carried out on all Asian bronzes in the collection. Freelance conservator Gillian Mitchell was commissioned to assess the collection and reports on her treatment of the Khmer standing Buddha here.

Robyn Maxwell

The Buddha exhibited pustular green corrosion in many areas which, after testing for the presence of chloride ions, was confirmed to be bronze disease. High concentrations of chloride ions are often the result of the prolonged burial of archaeological material.  Once bronze disease was confirmed, a treatment program needed to be developed. The first step was to understand the internal construction of the Buddha. To do this, radiation imaging techniques were to be employed, but only after thermoluminescent dating of the core. Because the original clay core of the bronze had been subject to intense heat in the lost-wax casting process, the approximate creation date of the sculpture could be confirmed by thermoluminescent testing. This dating technique is not reliable after an object has been X-rayed.  X-rays and CAT scans revealed a series ofthin wire armatures within the clay core. This was important to know because it meant that the corrosion could not be treated by full immersion in a chemical bath. The risk of completely wetting the core and the potential for inducing corrosion of the armature was considered unacceptably high.  Instead, a poultice application of a benzotriazole (BTA) solution was developed as an alternative, more controllable method. A gel of BTA, distilled water and ethanol was applied to the surface of the sculpture — not unlike a mud pack! BTA was selected for efficient chloride removal, long-term corrosion inhibition and minimal change to surface colour.

The embedded gems on the Buddha were protected from damage by the application of a layer of acrylic resin, followed by epoxy putty. The poultice was left on the bronze for twelve consecutive days. During this period the concentration of chloride ions in the gel was monitored every day to determine the success and health of the treatment. Once the gel had softened the corrosion deposits, these were removed carefully using a scalpel and toothpick.  The surface of the bronze was then rinsed and dried using applications of alcohol. The chemical stabilisation process resulted in the sharpening of the detail of many decorative areas. The final stage of treatment involved coating the surface of the Buddha with microcrystalline wax. The wax was used to hold additional BTA at the surface to inhibit future corrosion. The wax coating also sealed the metal from exposure to high humidity, which accelerates corrosion. As a result of this treatment the Standing adorned Buddha when kept in a stable environment, will be protected from the ravages of bronze disease for many years to come.

Gillian Mitchell

  image: CAT scans taken to assess internal construction of Standing adorned Buddha image: CAT scans taken to assess internal construction of Standing adorned Buddha image: CAT scans taken to assess internal construction of Standing adorned Buddha
  CAT scans taken to assess internal construction of Standing adorned Buddha  click each to enlarge.