I was recently weaving an overshot pattern that used 10/2 cotton for the warp/ground weft and 5/2 cotton for the pattern weft. I also have a few cones of 3/2 cotton, but I didn’t know if they’d be too big compared with the 10/2. I have a large collection of 8/2 yarn as well, would the 5/2 yarn be better suited as a pattern weft for a project that uses 8/2 for the warp/ground instead? Are there any guidelines about what size yarns work best as the pattern weft for overshot versus the warp/ground weft?
Too many variables (the yarns, the specific overshot draft, and the desired hand of the fabric) are involved to give a single rule of thumb for pattern-weft size vs ground warp and weft size in overshot. Probably the most common yarns/setts for contemporary overshot fabrics are 10/2 cotton for warp and tabby weft at 24 epi and either 5/2 pearl cotton or 3/2 pearl cotton for the pattern weft. The fabrics woven with these yarns/setts are usually sturdy fabrics in a weight suitable for placemats and towels. 3/2 pearl cotton would also work (and not be too heavy) for the draft you’re using with 5/2 cotton, unless the pattern-weft floats are very short (this would be for a delicate design, usually looking very twill-like). In that case, the 3/2 pearl cotton weft would not pack in well enough and you’d see streaks of the tabby weft between pattern picks. By the same token, if your overshot design has long pattern-weft floats with large blocks of pattern, a 5/2 pearl cotton pattern weft is likely to be too thin to cover the blocks; in that case, you’d also see streaks of the tabby weft between pattern picks.
Wool pattern wefts have the capacity to full to cover the blocks with wet-finishing, so their size can vary depending on the nature of the wool. With 10/2 pearl cotton warp and tabby weft, I like using Harrisville Shetland (its heathered colors add to the effectiveness of an overshot design) or other 8/2 wools. These fabrics (cotton ground cloth, wool pattern weft) are also usually sturdy, with a hand similar to colonial coverlets. If a soft fabric is desired, as for a shawl or scarf, wool, wool/silk, or silk would be good choices for warp and tabby weft. For a soft overshot fabric in all wool, the sett should be as for plain weave, but open enough that the wool threads have room to swell with fulling. For a wool pattern weft to show well on a wool ground cloth, it should be two to three times as heavy as the ground warp and weft. I’d follow that principle for silk, too: Sett the warp as for plain weave and choose a weft two to three times as heavy as the the ground yarns.
8/2 cotton is usually sett at 20 ends per inch for plain weave. 3/2 pearl cotton would be a good size for the patten weft, but it is mercerized whereas the 8/2 cotton is not. The contrast between the sheen of the pearl cotton and the matte finish of the 8/2 cotton might work well, or might not. You’d have to sample to see. Another option is to use the 8/2 cotton doubled for a pattern weft.