Recent discoveries of Patent yellow (also known as Turner’s yellow) a brilliant yellow lead-based pigment, in collection objects from The Colonial Williamsburg Foundation initiated research into the history and use of this under-researched colorant and an exploration of the most suitable analytical methods for its identification. Patent yellow’s precise introduction date and narrow window of use (1781 – ca. 1830) make it an important benchmark for dating and contextualizing objects, while recent documentary research shows it was an important and widely used inorganic yellow that may have been produced in the United States as early as 1783. However, it is little-known and rarely reported in conservation or art historical literature, possibly because lead (in the form of lead white) is ubiquitous on most historical painted surfaces, and chlorine, especially in the presence of lead, can be challenging to detect with techniques common to most conservation science laboratories such as XRF and SEM-EDS. Efforts to find and obtain reference samples of this pigment were fruitless, and attempts to synthesize it have, to date, been unsuccessful. These and other conditions can make this yellow frustratingly elusive to confirm.
Collaborative analyses carried out at Colonial Williamsburg using cross-section and polarized light microscopy, XRF, and SEM-EDS, with further analysis using XRD and Raman spectroscopy at the Smithsonian Natural Museum of Natural History and the University of Delaware Microscopy and Microanalysis Laboratory have contributed to a better understanding of this pigment and challenges to its identification. Findings indicate its chief component is lead oxychloride (Pb7O6Cl2), consistent with Lorettoite, a (now-discredited) lead mineral, although other lead-oxide-chloride phases may be present. Raman and new XRD data for Patent Yellow have been obtained through this research, which has not previously been reported elsewhere in heritage science literature. Photomicrographs of Patent yellow paint dispersions collected from case studies illustrate some previously unreported optical and morphological properties and demonstrate the effectiveness of polarized light microscopy in identifying this pigment, as it exhibits unique microscopic characteristics compared to other yellows, making optical microscopy a critical, simple, and effective first step in identification. Patent yellow case studies include varied decorative and fine art objects such as a painted coffeepot, a drum, a chair fragment, an easel painting by a Baltimore portraitist, and, most recently, a period room at the Museum of Early Southern Decorative Arts. This research suggests that Patent yellow/lead oxychlorides may be more common in painted surfaces than previously documented. It is hoped these findings can facilitate the identification of this pigment in other collections to better understand its broader use, properties, and role in late-eighteenth and early-nineteenth-century painted cultural heritage.
