2 MEASUREMENTS OF CANVAS TENSION

CHANGES OF up to 1500% in the tension of a stretched fabric in response to environmental fluctuations have been recorded by our stress tester. These changes have occurred in a sample drawn from a rather light canvas (linen, basket weave, commercially sized, and primed with acrylic paint, ca. 210 g/m2, 15 threads/cm in the warp (MD) and 18 threads/cm in the weft (CD) (fig. 1). It is likely that even stronger stress changes will occur in heavier canvas. Such changes take place in response to environmental fluctuations caused by daily or seasonal changes or by cycling environmental devices. The responses of stretched canvas to environmental changes vary greatly. In the canvas described in figure 1, the tension rises with a rise in relative humidity (RH). Another canvas (linen, basket weave, commercially glue-sized, and primed with oil paint), shown in figure 2, has its peak tensions at periods of low RH, although it looks very similar to the one in figure 1. In fact, every canvas measured in our tests responded differently to changes in RH and temperature, although all the canvases were linen, and these responses often changed gradually during testing.

Fig. 1. “Shrinking Canvas”∗ Changes in tension in a light canvas that was commercially sized and primed with acrylic resin and primer. The sample turned out to be a typical “shrinking canvas.” The horizontal axis (X) shows the hours of the test, here between 100 and 300.

∗ See General Remarks at bottom of page 48

Fig. 2. Well-Sized Canvas∗, Light canvas, similar in weight and weave to the one used in figure, but differently sized and primed with oil paint.

∗General Remarks about Figures 1, 2, 11, and 18. The top diagram (a) shows changes in temperature and RH. The second diagram (b) shows the changes in tension in response to the environmental fluctuations recorded in (a). Wherever there is a third diagram (c), it shows the values in a different vertical scale (y) in order to facilitate comparison with other diagrams in the same scale or to provide a clearer picture of smaller changes.

In view of these findings, it is impossible to assign a single value of tension even for a specific canvas. Instead, we found that each stretched canvas has a wide range of values in response to combinations of temperature and RH. When attempting to assign a measure of tension to a certain stretched canvas it is therefore necessary to use one of the environmental conditions as a benchmark. The generally accepted “normal” conditions of 21�C and 60% RH were chosen arbitrarily, since for most sized and painted canvases the tension at these environmental conditions is near their lowest point. Henceforth, when referring to canvas tension without specifying environmental conditions, we mean the tension at 21�C and 60% RH.

Tension stress is given in Newtons per meter; 100 N/m corresponds roughly to 0.5 lb of tension per inch, or 98.1 g/cm (ca. 10 kg/m). Most conservators would consider this a fairly strong tension and, indeed, this is about the maximum tension an average canvas can sustain for any length of time.