Conservators routinely turn to a gamut of imaging techniques to provide information beyond what the eye is able to see. These methods can reveal the artist’s materials and techniques, as well as the condition changes that have happened over time. X-radiography is one of the most useful and common techniques paintings conservators use to see beneath the surface of the work.
Like a medical x-ray, this technique shows variations in the density of the materials that make up the object. The resulting image includes everything that is present—the wood, nails, paint and ground layers—all in the same image. It can be difficult to decipher, but does often provide a great deal of information.
To produce the x-ray, the painting is placed between an x-ray source and a plate that is sensitive to x-rays. The elemental and structural components of the artwork attenuate the x-rays to varying degrees as they reach the plate. The resulting exposure on the plate represents these variations in elemental and physical densities.
X-rays of the three panels of the left wing (Saints Christopher, Augustine, and Stephen) are taken first. An unusual feature observed on all three panels is the application of score lines to the wood before the gesso and paint were applied. This is presumably to texture the wood in preparation for the gesso layers, ensuring better adhesion. The gesso was polished flat, so this texture is not visible on the paint surface.
As seen above, the score lines (white lines) on the two outer-most panels of the left wing (Saint Christopher at left and Saint Augustine at right) meet at the edges and continue across both panels. This indicates that the panels were prepared together on one larger piece of wood. The image also shows little to no trimming of the inner edges (the other two edges have been trimmed on both panels), although the time at which they were cut and separated into two panels cannot be determined. Scoring of this kind would be very unusual for an Italian painter, but has been observed in works of other Cretan painters, such as Angelos Akotantos, who was active during the mid-fifteenth century.
In the x-ray of the Saint Augustine panel below (and the following four details), it is possible to see the grain of the wood, placement of metal nails and screws, evidence of insect damage and old repairs, the use of pigments containing heavier elements such as lead white and vermillion (which include lead and mercury, respectively), and tool marks on the back of the panels where the wood was planed down, amongst many other features.
For example, the gray lines seen below outline a butterfly join that was inserted into the wood from the back of the painting. The butterfly shape prevents the vertical split in the wood from opening. These joins, documented in the object’s files, were added in 1939, shortly after the altarpiece entered the collection.
The metal in the nails and other hardware utilized throughout the altarpiece are radio-opaque, and appear white in the x-ray below. Some are original to the painting, while others are modern pieces added during repairs or installations.
The subtle variations in the following detail represent the grain of the wood, with its slight deviations in density throughout the material. The species of the wood has been identified as populous alba, commonly known as poplar.
Pigments containing heavy elements, such as lead or mercury, are also radio-opaque and appear white. Below, lead white is used for highlights around the eye and for the white dots that embellish Saint Augustine’s miter.