Can I warp with cotton and use acrylics as the weft? Or, conversely, can I warp with acrylics and weave with cotton? Would one be better than the other?
I have only one cotton yarn and many acrylic colors and would like to use them together.
When you mix fibers in a project, you generally have to take into account their relative degree of shrinkage. In the case of cotton and acrylics, cotton is likely to shrink more than acrylic, but not by very much. If you use cotton in the warp and an acrylic in the weft, the fabric may shrink more in the warp direction than in the weft direction. The degree of difference will depend also on the density of the weft: The more firmly the fabric is beaten (the greater the number of picks per inch), the less room the warp has in which to move during wet-finishing. If you use acrylic in the warp and cotton in the weft, the cloth will shrink slightly more in the weft direction (I'd guess about 10%, again depending on the specific yarn and the density of the warp).
One way to gain an idea of relative shrinkage (which is more important to know with yarns that are more different from each other than these) is to bundle some lengths of each yarn and tie each bundle with knots at both ends (I usually also make a tie in the middle). Then I measure the lengths of the bundles and wash them the way I would like to wash the final product. I measure both when they are dry and compare the wet-finished length with the original length to determine the percent of shrinkage to expect.
One way to minimize problems with two different yarns is to mix them in both warp and weft, and mix them frequently. If you alternate an acrylic with a cotton, one-and-one, for example, the yarns help each other reach a kind of compromise in the amount of shrinkage. Sometimes, if one shrinks a lot more than the other, the resulting effect is an added, but pleasing, texture.
In any case, I think you can mix acrylic and cotton without much worry.
PS. Several readers wrote in response to my advice to Jack, who wondered why one or two warp threads became loose on the edges as he was weaving. They pointed out that he might have beamed the warp with sticks or paper that did not extend far enough beyond warp width on the beam so that the outside threads fell off the edge of the warp on each side. That is absolutely a possibility. Since he was a new weaver, he might not have noticed this. I usually suggest that the paper or warping sticks extend 1 or 2 inches on each side of the warp as it is wound on the beam.