Thread - A Primer
There are many different types of thread available so how do you know which to use? What is "mercerized"? What is the difference between 30 weight, 40 weight and 50 weight thread? Why cotton thread for quilts?
Mercerized cotton thread has been put through a series of processes that cause the thread to swell, become round and straighten out. Any fuzz is singed off. This process increases the luster of the thread as well as improves water and dye absorption. Fine, long stapled fibers respond best to mercerization.
A spool marked 50/3 means it is a three-ply 50 weight thread. This is generally the weight used for quilt piecing. A 60/2 means it is a two-ply 60 weight thread which is thinner and finer than a 50/3. 60/2 cotton thread is used most often for machine appliqué, machine embroidery and machine quilting. The higher the weight number, the finer the thread.
When choosing a thread, you want one that is the same or weaker than the fabric. Stronger threads such a polyester, can "slice" the weaker fibers of cotton. Hence, the strong recommendation to use 100% cotton thread when piecing.
Don't use old thread that you find laying around just for the sake of using it up. Give it a "yank" test. If it can break by yanking it off the spool, it will break in your machine. Throw it out. Don't save it for a craft project or to donate to Goodwill - just throw it away.
Also, check the thread type carefully. Just because a thread is marked "Quilting Thread", does not necessarily mean that it is 100% cotton.