I seem to struggle with calculating the proper warp width in order to wind up with the width I want after weaving and finishing and how to weave under tension on the loom to achieve the finished length I want. Even when I think I’ve allowed enough width and length, the piece always seems to be narrower and shorter than expected. Is there a formula to determine take-up on the loom or is it just guesswork and practice?
The difficulty with determining warp width and woven length to achieve a specific finished width and length is due the number of factors involved.
The materials. Cotton, linen, silk, wool, etc., all shrink differently when wet-finished. Yarns with more plies and tighter construction will often shrink less (unless over twisted); other variations in yarn construction can also affect shrinkage.
The finishing method. Different methods of wet-finishing will affect yarns differently (heat shrinks silk; agitation shrinks/fulls wool, etc.).
The interlacement. The denser the interlacement, the less difference there will be between the width and length of the fabric measured on the loom and the width and length of the fabric when it relaxes off the loom.
The weaver/weaving methods. Using the same materials and interlacements, woven widths can vary from weaver to weaver depending on the amount of weft slack allowed in each shed. The straighter across the weft is placed, the narrower the cloth will become; the greater the weft angle the wider the cloth (using a temple also usually slightly increases warp width). For woven lengths, the tighter the tension on the loom, the more the cloth will draw up from its measured woven width when it is removed from the loom.
Even though all these variables make predicting difficult, you can come close by taking them all into account—and sampling. First, review past issues of Handwoven and notice the differences between woven widths and lengths and finished widths and lengths for projects that use the same materials, interlacements, and finishing methods you plan to use. This should give you a ball-park estimate. Then, always put on a longer warp than you will need. Weave a piece that is at least as long as it is wide. Remove it and finish it the way you plan to finish the piece. At this time, you can always add or remove threads in the width (if you absolutely need the width to be exact) and adjust woven length based on the percentages of weft and warp-wise shrinkage you calculated from the sample.
Another way to get a sense for how materials and interlacement impact the final size of your piece is to check out a related Best of Handwoven eBook. For example, if you’re designing a wool piece, you might compare the projects in the Weaving with Wool eBook from the Yarn Series for tips as to how to weave it. ~Andrea