An excellently pieced seam requires skill as well as good components. One of the major components of a good seam is the thread. Most quilters prefer cotton thread for piecing because cotton has a high heat tolerance and is therefore iron-safe. Cotton is also softer and more pliable than most other fibers.
Here is what to look for in a quality cotton piecing thread:
1. Twist. The twist should be consistently smooth and quite tight. Hold the thread up to the light. If you see bumps in the thread or inconsistent twisting, it is not high quality. These bumps, or slubs, will get caught in the tension disc and in the needle which will result in lint buildup and a weakened thread as they are snagged. They will also add unwanted bulk to the pieced portions, resulting in not-so-flat seams.
2. Fuzz factor. If you hold the thread up to the light and see no fuzz at all, that's bad. Either the thread isn't cotton or it is a glazed cotton. Glazed threads are for hand quilting, not machine piecing or quilting because the glaze rubs off in the tension disc and other areas and gums up the machine. If you see a lot of fuzz and uneven amounts of fuzz along the strand, it is not a quality thread. If you see a very small amount of fuzz and the thread is consistently smooth, that's a good one.
3. Thread thickness. Cotton thread is almost always either a 2-ply or 3-ply thread. A high quality 2-ply thread can be stronger than a lower quality 3-ply thread due to the fibers and the processing. A thin, smooth thread will make the best seam because it lies flat in the fabric. Therefore, assuming the quality is the same, a 50 wt. cotton thread will make a better seam than a heavier 40 wt. cotton thread. When pressed, the finer thread will make a better seam and the points will match up better.
4. Strength. A non-glazed, fine cotton thread will not win any strength contests by itself. However, because the average stitch length for piecing is 12 stitches per inch, the strength is in the stitching. A high-quality 50 wt. cotton is the perfect piecing thread.
5. Piecing with matching colors. I saw a beautiful quilt that had what looked like pencil marks next to all the seams. I wondered why the quilter didn't erase them. Upon closer examination, what I was seeing was a gray thread showing through a yellow fabric. Had the quilter pieced with a matching yellow thread, it would have not been noticeable. Although many think that white, cream, and gray threads are the only necessary colors for piecing, matching the piecing thread color to the fabric really does make a big difference.