Cotton thread falls under the category of a plant based fiber. One important characteristic to note about cotton thread is that it is a dyed fiber, and there is some risk of it bleeding (though I really have only found this true of red thread on white fabric).
Two other very important elements to cotton thread are its inability to stretch, and the fact that is shrinks when you wash it. These two points are key to keep in mind when deciding what fabrics to use cotton thread with. You really never want to sew a jersey knit or anything that has stretch to it with a cotton thread because the cotton will not stretch. Also, keeping in mind the cotton thread has a certain amount of shrinkage to it, you would really want to be wary of using it with a synthetic fabric that will not shrink, or you may get an unpleasant surprise the first time you wash the finished piece (blanket or garment)
There are many things cotton does extremely well. When paired with cotton fabrics, you really can't beat the feel of cotton. A cotton thread has a softer feel than a synthetic like polyester. It is great for piecing and quilting. It is very durable, can be washed, dried, ironed, dry cleaned, ect. with out the worry of it becoming damaged. Another bonus is that it is available in a variety of weights, from heavy duty to very fine.
One thing to watch out for, poor quality (read CHEAP) cotton threads will lint up your machines worse than any other type of inexpensive thread.
Also, if you want all of the benefits of cotton, combines with the wonders of polyester (which we will be going over next Thursday) consider opting for a cotton wrapped polyester. Often these are labeled all purpose, or dual duty. Again, cheap thread labeled dual duty can be a lint monster.
I know I've been yammering on about the evils of lint, but it can do some serious damage to your machine. At the very least cause skipped stitches, at the very worse, make your machine go out of alignment. And you can guarantee needing to have it in to be cleaned, oiled, and adjusted sooner (which runs around $80.00).
I'm not saying you need to break the bank of super fancy thread, but it is something to keep in mind. You'd hate to spend hours putting together something beautiful to have it be ruined by bad thread that is causing skipped stitches or is constantly breaking, or buy a nice machine to have it gunked up by linty thread.
Two of my favorite cotton threads are Aurofil and Mettler. Aurofil is a 2 ply thread great for piecing and quilting, however it is too weak for garment construction. Mettler comes in 3 ply, and is a great all around all purpose cotton thread that won't break the bank.
Next week, on to the synthetics! Polyester here we come!