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.
In the middle of the twentieth century, art historians attempted to characterize canvases by manually measuring the densities of the vertical and horizontal threads visible on X-ray films. They hoped to use this data to determine if two paintings originated from the same canvas roll, also called rollmates. During Van Gogh’s lifetime, canvases could be bought already prepared for painting. These canvases could be cut from a single bolt of canvas, making it possible to match paintings whose supports share the same bolt. Obtaining manual measurements was very time consuming and generated only average thread densities because data was collected at only a few points on the canvas surface. These averages gave only a crude assessment of rollmate status.
With the advent of computational technologies, X-ray films could be scanned and signal-processing algorithms could be used to find the vertical and horizontal thread densities across a painting’s surface. The algorithm exploits the regularity of thread positions and computes the spatial spectrum, which clearly measures thread regularity in threads per cm. After using this procedure on paintings by Van Gogh and many other artists, it was discovered that thread densities vary in distinctive ways that are specific to the weaving process. Using these patterns, we can distinguish how painting orientation relates to weaving directions. For the McMaster Van Gogh, the painting’s vertical threads correspond to the loom’s vertical threads. Thread density patterns also provide a much more precise basis for determining boltmates. Many paintings in Van Gogh’s oeuvre have boltmates, with over forty groups found to date. Some of these groups have over fifty members, indicating that Van Gogh used several canvas rolls that had been cut from the same bolt. No boltmate has yet been found for the McMaster painting.