Quality is determined by two factors:
1. the quality of the cotton fibers, and
Cotton is classified by the length of the staple, or fibers. We often hear the term 'long-staple cotton' but we never hear the term 'regular-staple cotton' although the majority of cotton thread is regular staple.
There are three classifications of cotton:
A long-staple cotton thread may cost 50% more than a regular-staple thread. Likewise, an ELS cotton thread may cost 50% more than a long-staple thread. With a difference of only one-eighth inch between the classifications, is it worth the extra cost to use the higher grade cotton? Yes, yes, yes! With each upgrade, you will have a much stronger thread and a lot less lint. The value of a long staple thread over a regular staple thread is ten times. Similarly, the value of an extra-long staple thread over a long staple thread is ten times. The increased value is evident in strength, meaning less breakage and less down time, and also in the reduced amount of lint, which means less wear on your machine and less cleaning time. If you have never tried an extra-long staple cotton, you're in for a very pleasant surprise. It is like going from a Model A Ford to a brand new Cadillac.
Although the main difference in quality is determined by the staple length, processing also contributes to the quality of the thread. Whereas some inexpensive regular-staple cotton is not mercerized, almost all long staple and extra-long staple thread is mercerized. Mercerizing is a process of treating cotton thread, causing the fibers to swell. This process allows the dye to better penetrate the fibers, thereby increasing the luster and strengthening the thread. Even if a thread is not labeled 'mercerized cotton,' if it is a long or extra-long staple, it probably is mercerized.
If the thread has a hard and shiny coating or wire-like stiffness, it is probably a glazed thread. Glazing involves heating the thread and then coating it with waxes, starches, and other chemicals, resulting in a glossy thread with a hard finish. Most professionals do not recommend glazed threads for machine work because the glaze rubs off and gums up the machine.