X-Radiography
X-rays undergo absorption when they pass through an artifact such as a painting. The extent to which they are absorbed by various materials depends on an atomic property called the mass absorption coefficient which is a function of x-ray wavelength and atomic number. It is a measure of the ability of a material to stop x-rays and, at a given x-ray wavelength, it increases with the average atomic number of the chemical elements present. For a given element, or combination of elements, the mass absorption coefficient increases with x-ray wavelength because longer wave x-rays are less energetic and less penetrating.
In practice, the transmission of x-rays depends on paint layer thickness, pigment density, the ratio of pigment to vehicle, and the thickness and density of the support (canvas, panel, stretcher, etc.). Assuming equal paint thickness and ratio of pigment to vehicle, paints containing pigments composed of chemical elements of higher atomic number, such as lead white, vermilion, lead-tin yellow, and lead antimonate (Naples) yellow, absorb x-rays much more strongly than pigments of lower average atomic number, such as azurite and green earth, or umbers, ochres, and siennas. Organic materials such as the canvas itself, varnishes, organic pigments, lakes, and dyes are virtually transparent to x-rays because they are composed of carbon, hydrogen, nitrogen, and oxygen which are all elements of low atomic number.
Lead white is a common constituent of grounds, and tends to concentrate in the interstices of the canvas threads making the pattern of the canvas visible in x-radiographs. Stretcher garlands--a scallop pattern at the edge of a canvas originating at the time when it was first stretched--are revealed in this way. Their presence or absence can help determine if a canvas has been cut down in size in the past. Lead white is one of the most common paint pigments and is frequently the pigment which is most critical in the formation of radiographic images. X-radiographs can reveal wood grain, cracks, defects, voids, joins, pentimenti, additions, alterations, and composite canvases or panels. Insect damage, caused by borers such as the death watch beetle or common furniture beetle, shows up in x-radiographs if holes have been filled in with lead white containing putty. Double paintings can often be detected. Conversely, an x-radiograph can demonstrate that a painting is in excellent condition or free of alterations.
An x-ray tube is aimed at the painting and the radiographic image is recorded on film which is placed closely behind it. The spectrum of x-ray wavelengths emitted by the tube is determined by the electrical potential at which it is operated, usually in the range 10 to 25 kilovolts (kV). Better resolution can be achieved if the film can be placed adjacent to the painted surface and irradiation is from the back. An overhead suspension system allows the tube to be moved orthogonally over large paintings when it is necessary to expose a mosaic of overlapping films. The standard film size is 14" x 17".
