Page 337 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

through the copper sheet. A new imaging attempt was made with an exposure of 190 kV at 5 mA
for two minutes with a tube distance of 1.3 m. The resulting radiograph revealed motding, which
showed that the copper sheet had been hammered into shape rather than rolled. Electron-
emission radiography using the method of Bridgman was also tried with this painting, but it did
not reveal any detail of an underdrawing.
An alternative approach to examining paintings on copper supports is scanning X-ray fluo­
rescence analysis. This is more strongly penetrating than scanning electron microscopy - energy
dispersive X-ray analysis and provides more information about paint layers than conventional
X-ray radiography; in addition, it can reveal the composition of the copper substrate below the
paint layers. This technique was used to examine Pictura {An Allegory of Painting), a work by
Dutch artist Frans van Mieris the Elder (i635-8i) that is in the collections of the J. Paul Getty
Museum. This small painting, 12 by 9 cm, on copper sheet is shown in PLATE 67A. 2
PLATE 67 Β clearly shows roughening of the surface of the copper sheet under X-ray fluo­
rescence. This is evident in the elevated concentrations of lead white (revealed in the lead map),
which was used as the ground; the lead white is thickened in the grooves that were cut into the
sheet to give the surface better adhesion.
Elemental scans of the face, shown in PLATE 67C, reveal the presence of iron, manganese,
calcium, and mercury in the painted image. For example, two small white dots can be seen in
the center of the calcium map; these correspond to the bone black used for the pupils. The
higher concentration on the left side of the calcium map shows that bone black was also used
for the background. The iron map shows that earth colors were used for the shadows in the flesh
tones; there is a faint echo of this pattern in the manganese map, which shows that some umber
was used. The mercury map highlights the red blush on cheeks and lips, indicating that cinna­
bar was used for these parts of the face. The small area of tin barely visible on the far right side
of the scan for tin shows that van Mieris made use of lead-tin yellow in the painting.
The elemental distribution maps for calcium, iron, mercury, and tin in the palette are illus­
trated in PLATE 67D. The calcium content of the black pigment is strongly indicative of ivory or
bone black, which contains calcium hydroxyapatite, Ca 5 (OH)(P0 4 ) 3 . Many areas of the paint­
ing show both iron and a small amount of manganese, attesting to the presence of umbers. Since
the only common red pigment containing mercury is cinnabar, the presence of this pigment can
be inferred from the elemental maps.
Not only do the maps illustrate the presence of clearly different pigment areas, they also
reveal that van Mieris slighdy shifted the position of the painted palette from its original out­
line, as indicated by red arrows on the elemental scans in PLATE 67D; this shift is especially
visible in the maps for iron and mercury. The discovery of these pentimenti by scanning X-ray
fluorescence analysis indicates the potential of the technique.

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