Thread Counts

When paintings on canvas are documented with high-resolution X-radiography film, it is possible to reveal the underlying canvas in detail, making individual threads visible. X-radiographs do not detect the actual canvas because it is transparent to X-rays. They do, however, detect the ground layer (used to provide a smoother surface on which to paint), which sinks into the canvas weave pattern. Due to the absorption characteristics of this ground material, the X-radiograph can produce a clear image of the canvas.

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Infrared reflectography and underdrawings

The infrared (IR) region of the electromagnetic spectrum is located beside the visible colour red, at wavelengths just longer than visible light. IR can penetrate through paint, with the depth of penetration depending on paint thickness, paint composition, and the wavelength of IR light applied. Longer wavelength IR light can penetrate through paint layers, but will be absorbed by blacks, a quality that enables researchers to use IR to look below the paint layers.

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Non-invasive vs. micro-destructive methods of analysis

An important distinction in the technical analysis of artworks is that between invasive and noninvasive methods. When possible, technical analyses are restricted to noninvasive methods that do not require removing any material from the object. Examples of noninvasive techniques include UV fluorescence, infrared reflectography, and reflectance transformation imaging.

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Dendrochronology

Dendrochronology was used to date the supporting wood panel of the portrait of a man, in the manner of Tintoretto. It can also be used to date a wide range of other wooden objects such as musical instruments or support beams in buildings. Some wood species, such as poplar, cannot be dated with dendrochronology, but oak, spruce, beech, and fir are types of wood that can all be analyzed using this technology.

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Cradles

Many old master paintings executed on panel were later cradled, especially in the nineteenth century. Cradling is a process whereby slats of wood are affixed in a grid-like fashion to the back of the panel. Before the cradle was applied, the panel was typically shaved and thinned to create a perfectly even surface for the wood slats.

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Neutron radiography

Neutron radiography is a type of nondestructive imaging similar to X-radiography whereby neutrons are used to create an image. While the earliest examples of this technique appeared in the late 1930s, it was not until the 1950s that it began to see routine applications, typically for industrial inspections and structural quality assurance testing for metals manufacturing. More recently, the use of neutron radiography has broadened to include applications in the nondestructive imaging of objects in art and archaeology.

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Cross Sections

Cross sections yield a wealth of information as they reveal the buildup of layers in a painting including the ground layers, paint layers, glazes, and any varnish coatings. Using a scalpel or similar tool, a very small amount of material, usually no bigger than the head of a pin, is carefully removed from a painting. In some circumstances multiple samples are taken from different areas of a painting.

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Reflectance Transformation Imaging

Reflectance transformation imaging (RTI) is a highly useful imaging technique for recording the topography, surface texture, and colour of a work of art. Using a stationary, high-resolution digital camera, multiple images are taken of a painting using light from a strobe lamp that is aimed at the surface from various directions. The images, which record the shadows and highlights produced by the strobe, are then processed with sophisticated computer software to produce a model of the painting’s surface.

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