The application of Optical Coherence Tomography in a study of The Mellow Pad by American Modernist painter Stuart Davis
In 2015, the Brooklyn Museum was awarded a Bank of America Art Conservation Project Grant to study the materials and techniques used in Stuart Davis’s seminal artwork, The Mellow Pad, a highlight of the museum’s American Art collection.
Davis (1892-1964) is recognised as one of the Masters of American art. His distinct paintings bridge Synthetic Cubism and Abstract Expressionism, and even serve as a pre-cursor to Pop Art (Kelder 1991). The Mellow Pad, an oil painting executed on artist-primed canvas, is Davis’s most complex composition.
The Mellow Pad by Stuart Davis, Brooklyn Museum
Davis took six years to complete the picture (1945-51), working out in excruciating detail his theories of “colour-space logic” and finally achieving what he called “the most powerful objective Art realisation of my life”.
The technical study focused on potential causes for recurring interlayer cleavage and discoloured paint. Because of the painting’s complexity, non-invasive analytical techniques offered the best opportunity to gain comprehensive knowledge of the materials without excessive sampling.
The techniques used included optical coherence tomography (OCT), fibre optic reflectance spectroscopy (FORS), portable x-ray fluorescence spectroscopy (p-XRF), multi-band imaging (UV, colour and near infrared imaging, UV and visible induced luminescence), reflectance transformation imaging (RTI), microscopic examination, and x-radiography. The investigation yielded overall image maps for comparison and spot analysis for approximately 50 locations to clarify the artist’s materials and technique.
In conjunction with historical documents, this data elucidated The Mellow Pad’s unique creation process, and, inherently linked to this, the sources of the current condition issues. Investigation suggested that underlying chalk, zinc-containing pigments, and interlayer grime all contribute to inherent, extensive lifting paint. Areas of selectively discolored magenta appeared to be caused by a chemical reaction with the underlying layer, and loss of pink coloration in other areas was caused by fugitive organic dyes (Ford et al. 2016).
Addressing the Challenge
Optical coherence tomography (OCT) is a non-invasive imaging technique based on the Michelson interferometer. It is capable of 3D volume imaging of subsurface microstructure. Over the last 12 years, OCT has found numerous applications in the field of art conservation, history, and archaeology. In recent years, authors Liang and Cheung have developed OCT devices specifically for the analysis of cultural heritage. While OCT has been successfully applied to the study of old master paintings, this is the first time it has been applied to a modern art painting.
The instrument used in this study is an in-house built ultra-high resolution OCT at a central wavelength of 810 nm. The OCT was set to take 500 scans in succession to cover an approximate 5x5 mm square surface area.
The UHR-OCT system in action
The physical depth of penetration is determined by the scattering and absorption properties of the material. The result is a virtual cross-section cube that can be viewed from multiple angles, using any individual cross-section or any selected depth of the surface. A series of thin slices in depth parallel to the painting surface can also be visually averaged, to create highly detailed IR images.
There are two main keys to interpretation. OCT cross-section images do not show actual thickness, but rather optical thickness, and each interface in the cross-section represents a change in refractive index. Colourants also affect interpretation. Opaque pigments, such as titanium dioxide, scatter the light and obscure lower layers. Colourants that absorb IR, like charcoal, stop the light completely and also prevent viewing lower layers.
Making a Difference
The scattering effect of opaque modern pigments (titanium white, cadmium colors, etc.) was a significant limitation for examination of The Mellow Pad. However, variation in scattering and absorption helped distinguish some materials.
In general, the technique proved most useful for the examination of the stratigraphy visible within boundaries between colours; crack structure, and materials seeping forward through cracks or applied over them; and the condition of the surface coating. Observation of underdrawing and partial paint layer stratigraphy were also observable in several locations. A long wavelength OCT at a central wavelength of 1960nm, also developed by Cheung and Liang, can potentially overcome the opacity of some of these pigments (Cheung et al. 2015b).