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Courtesy of Harvard University Art Museums, 2006
© President and Fellows of Harvard College.
CROSS-SECTION ANALYSIS
Cross sections are tiny paint samples taken with a micro-scalpel.
Straus Center Paintings Intern Sandra Kelberlau prepared the samples
by embedding them in resin, then ground and polished them for
examination under a polarizing light microscope. The purple pigment in
the sample was tentatively identified by Straus Conservation Scientist
Narayan Khandekar as fluorite, a rare pigment found most often on
16th-century German paintings.
Since fluorite (CaF) is too light to be detected by x-ray fluorescence, the
sample was taken to the Museum of Fine Arts, Boston, where a scanning
electron microscope was used to positively identify the pigment as
fluorite.
A scanning electron microscope allows elemental analysis of individual
pigment particles. The image above shows how heavier pigments appear
bright white and lighter pigments are darker. Most of the bright white
particles are lead white. The ground layer (the lower half of the cross section)
contains calcium carbonate, which has a low atomic weight.
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MICROSCOPY
In order to understand the “faded” or subdued appearance of the
pattern in Saint Ivo’s robe, a more detailed study of the pigments
was made, starting with microscopy. Interestingly, in addition to
finding azurite, lead white, and a red lake, Narayan Khandekar, Senior
Conservation Scientist, also tentatively identified fluorite. Although
a commonly occurring mineral, the use of fluorite as a pigment is
uncommon in the history of art and appears to be peculiar to paintings
from central and southern Germany dating from the early sixteenth
century. The mineral fluorite comes in many colors (green, yellow,
black, etc.), but purple fluorite was identified in pigment samples. In
order to be sure that the pigment observed in the cross section was
indeed fluorite it was examined using SEM-EDX.
SEM-EDX
Four cross sections from the Saint Ivo panel were analyzed using the
Museum of Fine Arts, Boston’s Scanning Electron Microscope with
Energy Dispersive X-ray Spectroscopy (SEM-EDX). Jens Stenger
and Richard Newman, Senior MFA Boston Conservation Scientist,
identified single pigment particles in the back scatter electron image
and determined their elemental composition. By comparison with the
polarized light microscopy, fluorite, azurite, lead white, and calcium,
could be identified.
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