If you have compared three different brands of 50 wt. cotton thread, you have most likely noticed that they are all different sizes. The differences could be due to inaccurate labeling, differences in thread processing, or a combination of both.
Regardless what the label says, a high quality non-glazed cotton thread will have very low lint, a smooth, tight twist, no slubs (clumps of lint wound into the thread), and a nice smooth appearance. A low quality cotton thread might start out with similar fibers but due to budget processing, will have more lint, a looser twist, and will not be consistently smooth. The biggest clue to the quality of the thread is the price. It just isn't possible to have a high quality product at a super budget price.
Twenty years ago, the term Egyptian cotton was commonly used as a generic term (similar to how we use the term Kleenex for tissue) for quality cotton fibers, regardless of origin. This generic term is no longer legally allowed because the Egyptian cotton growers protect their name. Unfortunately, many ignore that law and continue to use the term Egyptian cotton although the origin is not Egypt. One brand of cotton thread proudly boasts on their labels, "Egyptian cotton. Made in the China." Another says, "Egyptian Cotton - Made in India." How do they do that?
The best way to distinguish quality is not by the label, but by using the product. There is more to a product than the fiber type. Processing techniques add as much or more to the finished product quality as does the raw material. Following is a list of processing terms which affect the quality of cotton thread.
Mercerized. Today, nearly all cotton thread is mercerized. If a label says only mercerized cotton, it is because there is nothing else to brag about (such as long staple or extra-long staple). Mercerizing is a process of treating cotton thread with a solution, causing the fibers to swell. This process allows the dye to better penetrate the fibers, thereby increasing the luster. Today, all quality cotton thread is mercerized even if the label does not say so.
Staple. The length of the raw material fiber. The longer the staple, the stronger the thread. If there is no mention of the staple length, assume it is a regular (or short) staple thread. If it is long staple or extra long staple, it will proudly state that fact.
Gassed. Gassing refers to passing a cotton thread at high speed through a flame, burning off the excess fuzz in order to create a higher sheen. Gassed thread does not sound very appealing so other terms have been created such as "Polished Cotton" and "Silk Finish Cotton."
Glazed. Glazing involves heating the thread and then coating it with waxes, starches, and other chemicals. Glazing results in a glossy thread with a hard finish. Glazed thread is stiffer than unglazed thread and has a wire-like look and feel. Most professionals do not recommend glazed threads for machine work as the glaze rubs off and gums up the machine. Although usually not labeled as such, glazed cottons are recommended for hand quilting only.
Silk-finish. This is not a silk-wrapped cotton. This is a nice sounding term for gassed cotton. See above.
Polished. Another term for gassed cotton. See above.