Treatment of the frame for Benjamin West’s Devout Men Taking the Body of Saint Stephen:
After investigation of the layers to be saved, the focus now shifts to the layer that needs to be removed—the uneven, uppermost layer of bronze powder paint. In the following image, the sparkly pile on top is bronze paint, obscuring the gold leaf beneath it.
Similarly, in the cross section below, the bronze paint appears on the top as thick black lumps. The metal is dark because it is oxidized and has lost all its shine. It expands in size as it corrodes, so the paint is now also more coarse than it was when first applied.
In order to find a solvent that softens or dissolves the binder of the bronze powder paint but does not damage the gilding underneath, a comprehensive series of tests is carried out on more than 20 solvents and solvent blends. Most solvents either act too rapidly, risking damage to the gold surface, or they work too slowly or with no effect at all.
A solution is eventually found when a few recipes for chelating gels are tested on the bronze powder paint. In this process, a water-based gel is created to hold organic compounds, which can change the chemical structure of metals. On a molecular scale, the ions of the corroded bronze particles are moved around, forming new bonds with the chemicals in the gel. As illustrated below, the main chelating agent, EDTA (ethylenediaminetetraacetic acid), in black, is bonded with the “M” metal ion. In effect, the metal dissolves into the water-based gel, but since gold is an inert element, it is undisturbed by the chelator and only the bronze alloy is affected.
Based on the results of these initial tests, conservators modify the formula, adjusting variables, such as pH, to form the perfect cleaning solution before starting the slow and steady work to eliminate the bronze paint with the chelating gel.
During removal of the obscuring paint layer, it becomes clear why the paint was applied in the first place. In some areas the gilding remains intact, while in others it is scratched and worn away. At some point, the gold surface was damaged, perhaps by over cleaning. Below is a detail of the frame mid-treatment, with its beautiful gilded surface apparent on the right side, in sharp contrast to the dull bronze that still remains on the left side. After removal is complete, much work will still lie ahead to inpaint the damaged areas of the gold surface.