PARTICLE CHARACTERISTICS OF PRUSSIAN BLUE IN AN HISTORICAL OIL PAINTFrank S. Welsh
ABSTRACT—This paper outlines current research to identify the pigments, specifically the blue pigment, used to make the original 18th-century finish paints in the Long Gallery and in the Tower Stair Hall at Independence Hall in Philadelphia. The restoration of these rooms' blue and blue-green painted finishes was carried out from the mid-1950s to the early 1970s under the guidance of the National Park Service Historical Architect, Penelope Hartshorne Batcheler. The recent analyses confirmed that a blue pigment in these paints was one very commonly used in the 18th-century, Prussian blue. This study and confirmation not only establishes one of the very earliest uses of the pigment in this country in that century, but it brings to light the obvious differences in physical appearance between 18th-century Prussian blue agglomerates (large and angular)1 and the small, amorphous 20th-century Prussian blue agglomerates. The research also establishes light microscopy as the method of choice for analyzing architectural paint pigments.
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