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).
Polarized light microscopy.View Presentation >>
Conservation scientist Ken Sutherland analyzing a paint sample using GC-MS.
1. Gettens, R. and G. Stout. 1966. Painting materials: a short encyclopaedia. Reprint. New York, New York: Dover Publications, Inc. p. 161,177.