Can you please explain why so many weaving patterns for towels call for unmercerized cotton, whereas other items like mats and runners usually call for mercerized? I have woven with mercerized cotton and enjoy it (for towels and other items), but haven’t found a clear explanation why one would choose one of these two types of cotton yarn over the other.
If you were to look at all the projects that have ever appeared in Handwoven, my guess is that the majority would be woven in mercerized cotton. This is at least partly because mercerized cotton yarns, usually called pearl cottons, are more available than almost any other yarn for weaving. They are made by several US manufacturers in a huge range of colors in 3/2, 5/2, 10/2, and 20/2 sizes and are available from almost all weaving retailers. The most common size of unmercerized cotton (also in a wide range of colors) is 8/2. 16/2 unmercerized cotton (manufactured in Scandinavia and Canada), also comes in many colors; both 8/2 and 16/2 are available from many weaving retailers. (10/2 unmercerized cotton is currently only available undyed, as far as I know.)
Both 10/2 pearl cotton (4,200 yd/lb) and 8/2 unmercerized cotton (3,360 yd/lb) are good yarn thicknesses for weaving placemats, runners, and towels (the items probably most frequently made by handweavers). The differences between the two types of cotton yarn (in addition to a small difference in thickness) are related to their relative absorbency. Pearl cottons absorb dyes better and therefore show more saturated hues; mercerization also provides a sheen that tends to intensify color. Unmercerized cotton yarns are duller in color and more matte in texture. They absorb water better, however, which makes them a good choice for towels. This is probably the reason that so many towel projects call for 8/2. However, I have found that repeated washings tend to increase the absorbency of towels woven in 10/2 pearl cotton.
In general, though, you’d pick mercerized for brighter colors, unmercerized for greater absorption.