3.9 Warp and weft
An important structural component of fabric is the direction or orientation of the weave in the loom. The threads that run lengthwise through the loom (the warp) may differ from the transverse threads (the weft). When a canvas consists of two or more strips of canvas, the direction of the warp is often the same as the seam, because the strips were usually sewn together along the selvedges. With large seamless paintings it can be assumed that the warp runs parallel to the length of the painting. Especially for small canvases without selvedge or seam, Van de Wetering formulated possible criteria to distinguish warp and weft. When a difference is detected in the quality of the yarn in the two directions of the canvas, the warp direction tends to show evenly spun threads, while irregular threads with thickenings can be found in the weft. A second criterion that can help to distinguish between warp and weft is available when there is a marked difference in the spreading of the threads between the two directions. The warp threads tends to show less spreading since they are held in place by the comb during weaving, while differences in the force applied to the beater bar explain the more irregular density of the weft threads. Recently a third phenomenon, which distinguishes warp and weft, has been explained in a publication: the ‘weft snake’.1 A weft snake occurs in hand looms when the laid-in weft thread has not been given enough length prior to beating. In such a case the weft thread is too tight. The warp, which is perpendicular to the weft, is drawn together by the tight weft thread. This happens despite the fixed spacing of the reed. This tight weft thread will cause compression waves in a few slackened weft threads in the adjacent portion of the already-formed fabric.
1 C.R. Johnson, Jr., D.H. Johnson, I. Verslype, R. Lugtigheid, and R.G. Erdmann, ‘Detecting Weft Snakes’, Art Matters 5 (2013), pp. 48-52.