Use of Ultraviolet Light as an Aid to Pigment Identification

Abstract

lution of an historic structure. The use of ultraviolet light has long been included in the standard methodology for performing paint analysis at the Building Conservation Branch (formerly the North Atlantic Historic Preservation Center), North Atlantic Region of the National Park Service, and has been described as part of the Center's standard practices and procedures in a 1978 article in the APT Bulletin by conservator Carole L. Perrault.' Although chemical tests take precedence (primarily to identify lead paint and to differentiate between oil and water base paints), the ultraviolet technique has been employed to a greater or lesser degree, depending on the preference of the individual researcher and the nature of the specific project at hand. The methodology in the abovecited article explains that placing paint samples under long-wave ultraviolet light in a darkened room to induce fluorescence reactions during microscopic examination can provide clues to the composition of paint layers. The fluorescence of certain paint layers may stand out consistently in a population of samples and thus provide a reference point for comparative analysis. Based on the knowledge that zinc oxide pigments exhibit a bright yellow fluorescence and that they began to be economically competitive with lead paint around 1850, it was possible to place a mid-nineteenth century date on paint layers that fluoresced yellow. However, it was recognized that the fluorescence test needed refinement before it could be readily and reliably employed as a tool for pigment identification. At the same time, the established methodology at the Building Conservation Branch (BCB) was based on the expectation that every test that becomes a part of the laboratory procedure be adaptable to the equipment at the BCB and to the specific training of the professionals who are conducting the tests.2 While the BCB draws from other fields such as fine arts conservation, the methodology of architectural conservation has its own particular approach. Consequently, the ultraviolet light test has been used primarily to determine the presence of zinc oxide pigments in paint samples. In 1986, during the analysis of the interior finishes of Arlington House (the Robert E. Lee Memorial, ca. 1802-50), in Arlington, Virginia, it was discovered that a number of unexpected reactions were occurring among the paint samples.3 In many of the key samples, the sodium sulfide (Na2S) test showed lead paint almost continuously throughout the layers, and many of these layers showed fluorescence as well. It was apparent that relying on chemical tests alone or on the ultraviolet light test to identify only zinc oxide pigments would not provide enough evidence for the establishment of an accurate chromochronology. This awareness led to an investigation of