Scanning x-ray fluorescence spectroscopy and micro-x-ray absorption near-edge structure analysis as a guiding tool for the conservation treatment of two eighteenth-century Philadelphian portraits

Authors: PORELL, MINA - Winterthur Museum; CUSHMAN, MATT - Winterthur Museum; FISCHEL, JASON - Environmental Protection Agency (EPA); Fischel, Matthew; SPARKS, DONALD - University Of Delaware; GRAYBURN, ROSIE - Winterthur Museum

Submitted to: X-Ray Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2023
Publication Date: 4/3/2023
Citation: Porell, M., Cushman, M., Fischel, J.S., Fischel, M.H., Sparks, D.L., Grayburn, R. 2023. Scanning x-ray fluorescence spectroscopy and micro-x-ray absorption near-edge structure analysis as a guiding tool for the conservation treatment of two eighteenth-century Philadelphian portraits. X-Ray Spectrometry. Article e3345. https://doi.org/10.1002/xrs.3345.
DOI: https://doi.org/10.1002/xrs.3345

Interpretive Summary: Oil paintings in the 1700s relied on natural minerals as pigments, like arsenic minerals, for their vibrant yellow colors. However, natural arsenic minerals are sensitive to oxygen in the air, which causes them to lose color over time, and makes it necessary for art conservationists to restore the paintings. We determined where the arsenic minerals had lost their color within the paint layers using X-ray analysis in two 1776 portraits by William Williams. Information determining arsenic distribution and form informed the paintings' restoration without losing more of the yellow arsenic color. This research is critical for historic painting conservation, which often uses arsenic minerals for yellow pigment and where cleaning must not further destroy the yellow paint. The results are beneficial for art conservationists and scientists studying how arsenic pigments in oil paintings lose their color over time and specific methods to restore these historic pieces so the public can continue to enjoy the works in a condition similar to their original production.

Technical Abstract: An in-depth technical examination and conservation treatment of paintings by William Williams (Bristol 1727 – 1791 Bristol) has shed light on the artist’s materials and technique. This investigation centered primarily on Williams’s two 1766 portraits of William and David Hall. The paintings are considered the earliest life-sized, full-length portraits executed in the Philadelphia area. The analysis of the artist’s palette indicated deliberate choices in the use of orpiment (As2S3). The mineral’s tendency to oxidize to colorless and water-soluble arsenic oxides, likely caused color changes and degraded organic binder in the orpiment-rich areas. '-XANES revealed photodegradation of orpiment at the paint surface to arsenate (As5+) compounds, some found deep in the ground layers, likely transported in an aqueous phase via capillary action in a prior restoration. Just below the surface, arsenic remains bound primarily as arsenite (As3+), with some still complexed to sulfur as orpiment, suggesting that the paint is liable to further degradation by photooxidation and the use of moisture would be detrimental. Given this treatment-critical degradation phenomena, it was important to identify all arsenic-containing areas of both portraits. MA-XRF allowed us to collect maps from both portraits rapidly and accurately. The elemental maps of arsenic identified the orpiment-rich areas of the painting, which would be susceptible to further degradation upon exposure to water during treatment procedures such as humidification, consolidation, and surface cleaning. Since it had been determined that an aqueous adhesive was necessary for the consolidation of the cupped paint of the glue-paste lined paintings, the arsenic maps guided the use of two different consolidants–BEVA 371 for the water-sensitive orpiment-rich paint, and sturgeon glue for all other areas, striking a compromise between esthetic improvement and long term preservation.