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The painter builds up the paint layers to develop subtle effects of tone, color, and surface texture, resulting in a complex three-dimensional structure - which can be sleuthed out by an art conservator with a microscope.
The simplest microscopy method is optical microscopy (OM), which allows conservators to detect the dimension and size of the pigments’ grains, as well as the sequence of the pictorial layers.
Using a beam of electrons to form the image (instead of light), scanning electron microscopes can produce images at extremely high magnification (up to 100,000 times).
In addition, they can be used to identify the chemical elements present in each layer, or even individual pigment particles, thus allowing the pigments comprising each layer to be inferred.
These are then sectioned with a microtome to reveal a cross-section of the paint layers.
The exposed sections are examined by optical microscopy under both incident visible and ultraviolet illumination.
Microscopy reveals an amazing amount of information about a painting’s structure, based on just a tiny sample.
A sample of just tens of millimeters is enough to yield substantial new insights into a work of art.
This fragment is mounted and the layers are exposed through sanding and polishing.A scanning electron microscope is also used to collect a so-called backscattered electron image of a cross-section.This image shows differences in elemental composition: areas with high average atomic number (such as the lead-containing ground layer) appear light in the images, whereas areas with low average atomic number (such as the organic layers surrounding the bright red layer) appear dark. You can see that Degas reworked this painting many times, because the composition differs between the two images.While microscopy is more invasive than photography or radiography, once a sample of the painting is embedded in a resin it can be analyzed with a number of instruments.The cross-sectional analysis of paint layers displays a chronology of the artist’s working methods, from the initial preparatory layers through the paint and varnish layers.