3.3 Thread density
After the first study on painter’s canvas by Van Schendel, which dealt with just a few paintings by a single artist, the second study on this topic discussed a much larger group of paintings from a variety of painters, all in the collection of the Centraal Museum in Utrecht. The results of this study appeared in a 1967 publication on X-radiographical research of 404 of their paintings of old masters before 1700 by radiologist Meier-Siem and art historian Stark.1 After introductory remarks, the characteristics of paintings as visible in radiographs are discussed in detail. The first features relate to the support of the painting, beginning with panels and subsequently canvas. With only one exception, all the canvases in the Centraal Museum have been painted on a fabric with a plain weave. The next section, on the glue and ground preparation layers, also contains an interesting observation regarding canvas. Before the ground layer was applied, the canvas was coated with glue. Meier-Siem rightly observes that the glue closes off the surface of the canvas and thus determines to what extent the radio-absorbent ground could penetrate between the threads of the fabric. This influences the visibility of the canvas in the radiograph because it is after all the imprint of the canvas in the ground layer which we see in the radiograph, not the canvas itself. Another observation can be added here which was not mentioned in 1967. The composition of the ground also affects the radiographic image. In an X-radiograph of a painting with a quartz ground, the canvas is significantly less visible than in the case of a painting with a ground containing more radio-absorbent material, such as lead white.
One of the features of the fabric discussed in the Centraal Museum publication is the thread density, the number of vertical and horizontal threads per centimeter. Unfortunately, no explication is provided of how and where the thread density was measured. The thread density of every canvas is indicated by a whole number of vertical threads and a whole number of horizontal threads per square centimeter. It is not clear whether these are measured only once in each direction or the whole number is the result of rounding off the average of several measurements. The thread densities of all the canvases are presented in a graph  which plots the number of threads/cm on the vertical axis and the date of the painting on the horizontal axis. Each canvas is represented as a vertical line, whose ends indicate the number of vertical and horizontal threads/cm. Figure 1 shows that the fabric became coarser between the late sixteenth century and 1700.
Illustration from the Centraal Museum publication showing the thread density of paintings as a function of the year in which the paintings were made (1967).
1 M.E. Houtzager et al., Röntgenonderzoek van de oude schilderijen in het Centraal Museum te Utrecht, Utrecht 1967.