Japanese Sugito in the Philadelphia Museum of Art: Investigation of
Painted Decorations and Soluble Nylon Coatings
A technical study was carried out on twenty polychrome Japanese cedar wood sliding doors (sugito) in the East Asian Art Collection of the Philadelphia Museum of Art (PMA). Commissioned for architectural spaces during the Edo Period (1615-1868), the sugito are painted masterfully with figures and flora/fauna carrying seasonal and symbolic meanings. Scientific information on sugito paint materials is scant relative to other painted Japanese works of the period. The PMA sugito provided a unique opportunity to study the painted decorations and share information with colleagues in Japan, as these objects
are outside the National Treasure classifications in Japan. Judicious sampling based on object condition allowed for multiple analytical techniques. Our sugito study is interinstitutional and ongoing to 1) elucidate painting materials and techniques, which have not been studied extensively and 2) confirm that the polymeric consolidant is nylon-based and determine composition. Our study employed a complement of invasive and non-invasive analytical techniques, though the latter were limited by the presence of soluble nylon coatings applied to the sugito in the 1960’s.
The color palette for all sugito groups is red, blue, white, green, orange, black, & gold, except for one group with this same palette but no blue. Paint constituents were characterized by vibrational spectroscopy (FTIR and Raman), pyrolysis gas chromatography-mass spectrometry (Py-GCMS), x-ray fluorescence, and scanning electron microscopy with energy dispersive spectroscopy. The pigment palette is limited and no modern colorants were detected. The simple palette belies the skill of the painter and the complexity of compositions. Vermilion/cinnabar, lac, red iron oxide, red lead, yellow iron oxide, gamboge, azurite, indigo, calcite, carbon black, and gold (Au) colorants also were detected. The binder is proteinaceous (probably animal glue), implying use of glue bound paints (distemper). Calcium oxalate (weddellite, CaC2O4·2H2O) was found in association with the binder, likely formed via oxidative degradation of organic material & reaction with calcite pigment or other particulate.
While the color and pigment palettes are simple, the green colors are of particular interest. On some sugito, in-situ XRF analysis detected Cu,As, and Zn in green areas. XRF element maps show the distribution of these elements in foliage on Courtier on Horseback.
In an unusual finding, the basic copper sulfate brochantite was detected as a pigment. To our knowledge, this is the first published report (in English) of its use in painted Japanese works of the period. Spectroscopic investigations of the green colors containing malachite, supplemented by EDS and XRD, revealed mixtures with varying amounts of the copper arsenate minerals, cornwallite and philipsburgite. Philipsburgite can be found in association with cornwallite and both can occur with malachite. The Yamato mine in the Yamaguchi Prefecture was cited in 2011 as the only known locale with philipsburgite in Japan. These findings show that the copper green pigments of classical Japanese painting may be more varied than previously reported.
Left: Courtier on Horseback. Right top: Detail of pine tree in visible light. Right bottom: XRF false color map showing distribution of Cu (red), As (blue) & Zn (green) in green paint containing philipsburgite.
We have characterized the color palette, major pigments and proteinaceous binder of the PMA sugito by FTIR and Raman. Notably, the uncommon mineral philipsburgite was identified by XRD. We will explore other mineral occurrences to group the sugito. Of particular interest is the chemical nature of the soluble nylon and its interaction with the metal cations of the various pigments & binder now characterized.
I shared this research in a poster entitled Japanese Sugito in the Philadelphia Museum of Art: Investigation of Painted Decorations and Soluble Nylon Coatings, at the Infrared and Raman Users Group 13th Biennial Conference in Sydney, NSW.