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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. μ‐XANES revealed orpiment photodegradation to arsenate species at the paint surface, with migration to the ground layers. Just below the paint surface, arsenic remains bound primarily as arsenite, with some associated with sulfur as orpiment. This As distribution suggests that the paint is liable to further degradation by photooxidation and use of moisture would be detrimental. Given this treatment‐critical degradation phenomenon, it was important to identify all arsenic‐containing areas of both portraits. Scanning XRF allowed rapid and accurate collection of maps from both portraits. 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. An aqueous adhesive was necessary to consolidate 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. [ABSTRACT FROM AUTHOR]