Treatment for "Bronze Form"

The corrosion of the surface pointed us in the direction of repolishing the surface overall to regain a uniform luster. In the decades of display in Southern California, the polished bronze has proven difficult to maintain. Exposure to the beautiful reflecting pool, and the heat and UV radiation that break down protective coatings have made it a challenge to keep the surface evenly golden. But a regime of periodic re-polishing and lacquering would be risky, because metal is lost and the reduction of wall thickness may pose structural problems in the long run.

It is clear from archival research that the foundry polished the Bronze Form series, may have applied a thin golden patina, and added coating with wax or lacquer. However, there is reason to believe that the polished appearance was never actually intended by the artist. In fact, one of Moore’s small-scale maquettes of the sculpture has a brown patina. Further research suggests the polish was an aesthetic choice made by the studio assistant managing the project for him posthumously. Therefore, Getty conservators decided to explore applied patinas to stabilize and unify the surface of Bronze Form.

The Getty collaborated with experts in Moore’s patination, the firm Bronze et al, Ltd., who came to the Getty for treatment planning between September of 2021 and May of 2022. Several areas on Bronze Form were chosen to polish and test chemical patinas typically used by Moore on his later works, including translucent golden and medium brown colors. To obtain a golden color, a mixture of ferric nitrate brushed onto the surface after heating with a propane torch provided an appealing translucent layer.

Difficult to control, however, the light-colored blotches rich in tin and lead continuously produced an unacceptable black chemical reaction.

To address this unusual reaction, Bronze et al developed a range of chemical alternatives, such as bismuth nitrate, or added sodium thiosulfate to the ferric for enhanced etching power. The blackening of the tin/lead splotches continued. A final mixture was tested to create a medium, reddish-brown patina involving a layering of cupric nitrate and ferric nitrate to conceal the lead-rich areas.

All patinas were additionally done on cast bronze coupons that were coated with either wax or lacquers to age alongside the sculpture for eight months of natural weathering. The final patina using cupric and ferric nitrate was chosen for many reasons: it was the most stable and reliable, was easier to maintain, and would have been aesthetically acceptable to Moore. The final treatment proposal was then approved by the Museum’s director of collections and the curatorial department.

The research took years, but the treatment took two weeks. The first week involved erecting scaffolding to apply the chemical patina, while in the second week, an enclosure was built to protect the surface during the application of a spray lacquer.

First steps involved wiping down the sculpture with mineral spirits to remove grease and polishing it with 180 grit abrasive discs on an orbital sander. The first patina was formulated with 18 grams copper nitrate (CAS # 19004 19 4), 3 grams ferric nitrate (CAS # 7782 61 8), and 16 ounces distilled water.

The chemical solution was initially applied to the heated metal with a swirling flowing brush pattern. The second and final layer of the patina consisted of 10 grams ferric nitrate, 16 ounces distilled water.

The solutions were applied to the heated metal surface with a repeated stipple pattern to create the final transparent golden-brown patina. They were applied as thinly as possible to allow the underlying golden bronze metallic qualities to emanate through the patina, but also applied thickly enough to diminish the appearance of the welded seams and darkened alloy variances.

The benefit of the final chemical patina is twofold: it provides the uniform color, and an added stabilization of the bronze surface. The application of a clear coating can greatly extend a sculpture’s lifetime. Wax and lacquer were tested on both the coupons and the sculpture during the testing phase before treatment to evaluate suitability. The acrylic lacquer was superior in protection. A clear lacquer manufactured by G. J. Nikolas & Co., Inc. was chosen since it provided a formulation compliant with California VOC regulations (the #12666 2.3 California Coat RFU) and it produced a nice semi-gloss sheen with good adherence and levelling. The lacquer was sprayed onto Bronze Form in several applications to build a film thickness of 45 microns on average. After several weeks, a final coating of paste wax was brushed on top of the lacquer and buffed for an added layer of protection.

The final appearance was unveiled at the end of July to the delight of our colleagues. The new patina provided a subtle warm shift from a gleaming polished bronze (with corrosion) to a reddish, light brown having flecks of bronze metal still catching the sunlight. The restoration of Bronze Form was a rewarding and enlightening project for Museum conservators and will continue to be cared for as the new patina matures over the years.