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Technical Analysis

 

Stephanie Oman
Polarized light microscopy.
To learn more about the materials that were used to create the graining, Museum conservators and scientists analyzed samples from the finish using instrumental and microscopic methods. The technical analysis of finish layers often provides more information about how the decorative surface would have originally appeared. The analysis can also provide information about later layers, including pigment identification to help date successive decorative campaigns.

Cross-section samples were analyzed using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) to provide elemental information about the pigments within the paint layers, as shown in the elemental map of a cross-section from the wainscot. The false-color map shows the distribution of lead, zinc, and barium/titanium in the sample. The presence of zinc in a paint layer is important for dating purposes when studying architectural paint samples because it suggests the presence of zinc white pigment, which was not commercially available in paints until the 1830s; similarly, titanium white pigment was not used in paints until the 1910s. 1 Regarding the earliest graining layers, lead was found to be the predominant element, suggesting the use of red lead [lead tetroxide, Pb3O4].

The identity of the pigment was confirmed by polarized light microscopy (PLM), a technique in which the morphology, size, birefringence, and refractive index of particles can help to characterize pigments. The red particles exhibited anomalous green polarization colors when viewed in crossed polars, consistent with red lead. In addition, white particles were observed that were highly birefringent with a refractive index higher than 1.66, indicative of lead white pigment. The presence of lead white (basic and neutral lead carbonates, 2 PbCO3•Pb(OH)2 + PbCO3) was confirmed by Fourier transform infrared microspectroscopy (MFTIR).

Ken Sutherland
Conservation scientist Ken Sutherland analyzing a paint sample using GC-MS.
Additional samples from the graining layers were analyzed using gas chromatography-mass spectrometry (GC-MS), a technique that allows the specific characterization of the organic components of a sample. The paint layers were found to be composed predominantly of drying oil. Later graining techniques of the 19th century used mostly spirit (natural resin) varnishes and distemper (waterborne) paints that were applied quickly and relied on blending to achieve the desired effect. The use of oil paint in the Mount Pleasant graining would have been a much slower-drying technique which would have allowed the artisan to work wet-into-wet for a longer period of time.

1. Gettens, R. and G. Stout. [1942]1966. Painting materials: a short encyclopaedia. Reprint. New York, New York: Dover Publications, Inc. p. 161,177.

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