PARTICLE CHARACTERISTICS OF PRUSSIAN BLUE IN AN HISTORICAL OIL PAINT
Frank S. Welsh
3 TOWER STAIR HALL
FEW, IF ANY, have postulated that Prussian blue was also the pigment used in the original finish paint found in Independence Hall's grand stair area, the Tower Stair Hall (Fig.4). A broad stair in a room some 36 feet high, it was designed by Edmund Woolley and dates from the 1750s. For more than 20 years in this century, the Liberty Bell stood encased on the hall's brick floor. Growing crowds of visitors speeded deterioration of the Stair Hall's woodwork, and so the Bell was relocated to its own building in time for the nation's 1976 Bicentennial.
Tower Stair Hall, Independence Hall, Phildelphia, PA.
Over the past three decades, a number of laboratories have analyzed the Stair Hall's pigments. Samples of the original paints were submitted by Mrs. Batcheler to the National Lead Company Research Laboratory (December, 1956), to the Freer Gallery, whose laboratory was then headed by Rutherford P. Gettens (November, 1957), and to The Glidden Paint Company of Reading, Pennsylvania (May, 1958). The conclusions of all analyses were contradictory and never resolved. National Lead concluded the blue pigment in question was natural ultramarine. Glidden Labs concurred, even after conducting X-ray diffraction and spectrographic analyses. Dr. Gettens disagreed. He found no blue in his microscopical analysis-only iron oxides and charcoal black. 14
The conflicting analysis left the identity of the Stair Hall's original paint's pigments a mystery. In 1985, the files were reopened to attempt to find an answer. From the NPS architectural storage collection was retrieved a carved wooden rosette (cat. #1271) that was part of the main cornice of the Tower Stair Hall. Although the cornice was installed in the 1750s, the first finish paints may or may not have been applied at that time.
Stereo microscopical inspection indicated the first finish to be a light grayish-greenish blue oil paint over a very light oil primer. Through the stereoscope we saw large black pigment agglomerates. Agglomerates of the pigments in the paint were too small to see easily at 20–80x. The absence of large blue pigment agglomerates, similar to those found in the 1730's Long Gallery paint, suggests this Tower Stair Hall paint layer, potentially from the 1750s, may have been made with a different blue pigment or else the same one but more finely ground before dispersal into the liquid paint.15 A tiny sample (0.5mm) of the entire paint layer was removed with a tungsten needle and mounted on a glass slide in Aroclor¯ for inspection with the polarized light microscope. Easily seen were white, black, yellow and red pigment particles. The blue pigment particles were difficult to find-they were remarkably small.
The characteristics of the black pigment were those of charcoal black. The pieces were large and very angular, very unlike lamp black, which is very tiny and rounded.16
The yellow and red pigments looked like yellow and red iron earths. The white had characteristics of white lead. The blue had identical optical and shape characteristic of the blue sample from the Long Gallery's window jamb board, with one exception. This sample's agglomerates size was generally much smaller, ranging from 3–11 micrometers with 75% closer to 4 micrometers—a factor contributing to the potential for its misidentification.
These particles were examined one by one with the polarized light microscope and with microchemical testing and found to be the following: black, charcoal black; red, red iron earth and haematite; yellow, yellow ocher; white, white lead, and the blue, Prussian blue.