Scanning Electron Microscopy

While microscopic examination is commonly performed on <a href="http://www.artcons.udel.edu/about/kress/examination-techniques-and-scientific-terms/cross-sectional-microscopy" target="_blank">paint cross-sections</a> and dispersed pigment samples, the use of a scanning electron microscope that is capable of energy dispersive X-ray analysis (referred to as SEM-EDX) can be of equal importance. A paint cross-section is placed inside a vacuum chamber and an electron beam is focused on the exposed surface of the sample. Two types of images can be generated using SEM-EDX. An SEM back-scattered electron image (BSE image) provides visual information relating to pigment morphology as well as the range of atomic weights present (e.g. light areas correspond to elements with higher atomic weights such as lead while darker areas indicate lower atomic weight elements). The second type of image, referred to as an EDX map, is generated using very similar technology described in the X-Ray Fluorescence section (link to XRF site) X-rays excite electrons present on the surface of the paint sample, generating elemental information that can help scientists and conservators identify pigments (typically inorganic such as those obtained from crushed up stones and minerals) present in discrete paint/ground layers.<br/><br/>Samples collected from the "Triumph of David" were prepared by project leader Kristin deGhetaldi and examined with scientist Catherine Matsen at the Winterthur Museum Scientific Research and Analytical Laboratory. A Zeiss EVO MA15 scanning electron microscope was used with a LaB6 source at an accelerating voltage of 20kV for the electron beam, stage height of approximately 11mm, and sample tilt of 0 degrees. The EDS data was collected with the Bruker Nano X-flash(R) detector 6|30 and analyzed with Quantax 200/Esprit 1.9 software.

SEM Sample 1


This sample was collected from the shot-silk robe of the woman holding the tambourine after removal of the degraded surface coating. The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn) while the paint layers appear to contain iron oxides (particularly green earth; Fe, Si, Mn, O) and lead white (Pb).<br/<br/><i>Top Left: Visible light/200x magnification; Top Right: BSE image/259x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Ca, Al, Pb, Fe, K; Trace: Mg, Mn, Na, Ti)<i>

SEM Sample 2


This sample was collected from the blue robe worn by the kneeling woman in the lower right corner before removal of the degraded surface coating. The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn) while the dark blue paint layer appears to contain an extremely pure grade of lapis lazuli (Al, Na, Si, O) followed by a thin layer of lapis mixed with lead white (Pb). In this particular sample the upper layer of the double ground is extremely thin and can only be detected in the EDS map for iron. A large red, iron oxide particle can also be seen in the lower left.<br/><br/><i>Top Left: Visible light/200x magnification; Top Right: BSE image/264x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Ca, Al, Pb, Fe, K, Mg; Trace: Na)</i>

SEM Sample 3


This sample was collected from a heavily overpainted back of the kneeling woman in the bottom right corner. The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn) while the uppermost layer of overpaint contains Zinc White (Zn), a pigment that was not available to artists until the 19th century. Spot analysis of red and yellow pigment particles mixed into the original lead-containing paint layers confirmed the presence of vermillion (Hg) and lead-antimonate yellow, most likely Naples Yellow (Pb, Sb). Top Left: Visible light/200x magnification;<br/><br/><i>Top Right: BSE image/302x magnification; Bottom left and right: EDS elemental maps (Major Elements: Pb, O, Si, Ca, Al, Zn, Fe; Trace: K, Mg, Mn, Hg, Sb)</i>

SEM Sample 4


This sample was collected from the green-colored cushion on King Saul's throne before removal of the degraded surface coating. The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn); the upper ground layer contains more iron and manganese than the lower layer, indicating a higher concentration of earth colors/umbers in the second ground application. The presence of green earth particles was confirmed by the detection of high levels of silica (Si) and moderate to low levels of iron (Fe).<br/><br/><i>Top Left: Ultraviolet light/200x magnification; Top Right: BSE image/217x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Ca, Al, Fe; Trace: K, S, Mg, Mn)</i>

SEM Sample 5


This sample was collected from Abner's yellow ptergues (leather straps worn at the waist by Roman soldiers). The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn); the upper ground layer contains more iron and manganese than the lower layer, indicating a higher concentration of earth colors/umbers in the second ground application. The upper paint layers contain lead white (Pb) and yellow iron oxides (Fe, O). Top Left: Visible light/200x magnification;<br/><br/><i>Top Right: BSE image/399x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Ca, Pb, Al, Fe; Trace: K, S, Mg, Mn)</i>

SEM Sample 6


This sample was collected from the green-colored mountains in the distant background after removal of the overpaint and degraded surface coating. The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn). The paint layer contains lead white (Pb), earth colors (Fe, Si, O), and lead-antimonate (Pb, Sb). The presence of green earth particles was confirmed by the detection of high levels of silica (Si) and moderate to low levels of iron (Fe). A few yellow particles were found to contain lead and antimony (Pb, Sb) pointing towards the presence of Naples Yellow, however a large whitish agglomerate appeared to contain lead, antimony, and high levels of silica (Si). It may be that a range of lead-antimonate particles were used, with some possessing more of a "glassier" nature than others. Colored glass has been found in Italian Renaissance and Baroque paintings.<br/><br/><i>Top Left: Visible light/200x magnification; Top Right: BSE image/2124x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Pb, Al, Ca, Fe; Trace: K, Mg, Sb)</i>

SEM Sample 7


The red-brown ground contains iron oxides, calcium, and alumino-silicates (Fe, Ca, Al, Si, O, Mg, Mn). Multiple paint layers were found to contain lead-antimonate yellow (Pb, Sb), lead white (Pb), and earth colors (Fe, Si, O, Mn). The presence of phosphorous in the black paint layer indicates the presence of bone black (P). Images obtained using x-radiography confirmed that these colored paint layers beneath the black most likely correspond to a hidden figure that is present beneath the shield. Top Left: Visible light/200x magnification;<br/><br/><i>Top Right: BSE image/997x magnification; Bottom left and right: EDS elemental maps (Major Elements: Si, O, Pb, Al, Ca, Fe; Trace: K, Mg, Sb)</i>