In the Slow Cloth issue (November/December 2010), Rita Hagenbruch’s coverlet on page 43 uses unmercerized 16/2 cotton and 20/2 pearl (mercerized) cotton for the tabby weft. This is unusual—I would think that the unmercerized and the mercerized cottons would shrink differently. Is there an assumption that the coverlet would be dry-cleaned rather than wet-finished? Certainly the merino pattern weft would wet-finish differently than the mercerized cotton. If one plans to wet-finish the coverlet, should the mercerized cotton not be used?
Unmercerized and mercerized cottons do have slightly different shrinkage rates. (Unmercerized cotton shrinks a bit more than mercerized. Neither shrinks a lot.) Overshot, however, especially in the coverlet you mention, is a very dense weave—the tabby-weft and pattern-weft yarns are packed in very firmly. This means that some of the usual considerations don’t apply in quite the same way. Weavers often measure and adjust their beats allowing for the warp to relax when tension is released, for example. Overshot is so dense that the difference in measurements under tension and with relaxed tension are a lot less than with more open weaves. There just isn’t much room for the fibers to move, and this is also true during wet-finishing.
The pattern-weft yarn for this coverlet is another matter, however. Even in a dense weave, merino yarns will full and shrink. If this coverlet were, say, tossed in a washing machine, the result would not be good. The merino would shrink from being wet under agitation and full/fuzz (the more and longer the agitation, the more the shrinking and fulling). So for that reason I would recommend dry cleaning for future use, or soaking and rinsing carefully in cool water with no agitation. Even more, I would recommend not getting it dirty.