I just finished a huck-lace table runner (my first project on a 4-shaft loom and my first project with a pattern); and I had lots of problems with warp tension. I started getting a “smile”: the warp threads in my plain-weave borders (two inches wide on each edge) became tighter than the threads weaving lace in the center. I’ve searched the internet, but have been unable to find anything about this problem.
When you are weaving huck lace on four shafts, the threads on shafts 1 and 2 interlace more than twice as often as the threads on shafts 3 and 4. The more warp threads interlace, the more of their length is taken up by their passing over and under the weft. If some warp threads interlace less often than others, their “take-up” is less and they eventually become looser.
Therefore, when you are weaving huck lace, when these threads become noticeably looser (this will take a bit of weaving), raise shafts 3 and 4 and place a smooth rod (a 5/8” dowel will work) under the raised threads behind the shafts and slide it over the back beam and down to rest under the warp beam. For awhile, the rod alone will take up the extra slack, but eventually the threads will become loose again and you’ll need to add a little weight to both ends of the rod. If you weave huck lace on more than four shafts, raise all shafts but 1 and 2 to place the rod.