CNCHnet . . . The Textile Arts Webzine of the Conference of Northern California Handweavers

Ratios, Rationale, and Rate of Shrinkage

Teaser: In the process of designing cloth, determining warp length and width to allow for shrinkage is a critical piece of information; it is also one of the most misunderstood and most often miscalculated.

Many weavers routinely add extra length to their warp and extra width to their weft “just to be sure” they have enough. But to plan precisely and conserve precious materials or time, one needs to understand how to accurately determine warp and weft dimensions.

An online search for “warp calculator” reveals a number of sites that will automatically calculate the necessary warp length if one knows the rate of shrinkage. But there is no explanation of the formula used in the calculation. One site, while encouraging the use of their calculator, contains a disclaimer stating “no responsibility for projects gone awry”. Understanding the process and the formula will assure that you can plan your projects confidently.

Let’s begin by first talking about the length. Assuming you are weaving a sample, carefully measure the woven length while under tension on the loom. When the sample has been removed from the loom and wet-finished, measure again. The difference between the two measurements will give you the percentage of shrinkage. For example, if the sample measures 10″ long on the loom and 8″ long after finishing, the shrinkage rate is 20% (10″– 8″ = 2″ which is 20% of 10″).

It may seem logical, if the shrinkage rate is 20%, to simply add 20% to the desired finished length, but …it doesn’t work!  Many experienced weavers make this mistake; a recently published book by a well-known author as well as a highly regarded website include this erroneous instruction. Let’s see what happens: If I want my finished length to be 10″ and add 20% for shrinkage, I will weave 12″. If the 12″ shrinks 20%, my finished length will be only 9.6″ long. That’s not a huge difference (less than ½” short), but what happens over a longer length? If I need 3 yards (108″) of finished cloth and add 20% for shrinkage, I will weave 130″ (129.6″ rounded up). If twenty percent is lost in the wet-finishing process, the cloth will be 26″ shorter, so the finished measurement will only be 104″; four inches short! That is a huge difference …and is magnified if it means there’s not enough cloth for the intended project.

Here’s the kicker: Rather than the percentage of shrinkage, it is the ratio (a comparison of two similar quantities obtained by dividing one quantity by the other) of woven cloth to finished cloth that gives the correct number to work with.

To understand how the ratio works, let’s use the three yard example above:

Sample measured under tension on the loom = 10″
Sample measured after wet-finishing = 8″

The ratio of 10 is 1.25. (10 divided by 8)

Multiply the desired finished length by the ratio: Three yards (108″) x 1.25 = 135″.

135″ less 20% shrinkage = 108″ (Exactly the three yards needed!)

Remember when determining the ratio that the on-loom measurement is the quantity to be divided and always goes above the line. The finished length is the divisor (the number by which the on-loom measurement is to be divided) and always goes below the line.

The same formula may be used to determine the on-loom width of cloth: Establish the ratio by dividing width in the reed by the finished width of your sample. Multiply the desired finished width of cloth by the resulting ratio.

If you wish to modify a project from Handwoven you can easily determine the working ratio by checking the Dimensions listed in the Project at-a-Glance. For example, the woven length of fabric for a vest in the Jan/Feb 2013 issue is 112″. The finished length is 95″. Using the above formula, divide 112 by 95 to get a ratio of 1.1789. Round up to 1.2. To make the vest longer by adding 12″ of finished length, multiply 12 by 1.2; this equals 14.4. Round up again for an additional length of 15″ and add that amount to the warp length.

Finally, don’t forget to add 10% to the warp length for take-up and then add loom waste. Now you’re ready to weave with confidence knowing you’ll have exactly what you need.

Bio:
Sarah H. Jackson earned a Bachelor of Fine Arts, concentration in textile design, from the University of Kansas.  She owns a business dedicated to designing and marketing textiles and is the weaving editor for Handwoven magazine as well as a member of the tech editing team.  An experienced teacher, she conducts workshops for guilds and regional conferences that explore garment design and color composition.

She can be reached at:jackson.sb295(at)yahoo.com 714-832-5739

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