Choosing the right thread is a major part of any quilting, embroidery, or sewing project. In addition to choosing the fiber content (rayon, poly, metallic, cotton, etc.), there are subcategories within each field which affect the quality and durability. Know and choose what is best for your project. Each step of the thread manufacturing process adds expense to the overall cost of production. Cheaper brands skip some of the processing steps in order to cut costs. This becomes obvious when comparing quality. The look, the feel, the properties, the strength, and the amount of lint all depend on the processing techniques.
Mercerized. Most quality cotton thread today is mercerized. Even though the label may not say "Mercerized Cotton," if it is a quality cotton thread, most likely it is mercerized. Mercerizing is a process of treating cotton thread, with an alkali solution, causing the fibers to swell. This process allows the dye to better penetrate the fibers, thereby increasing the luster. Mercerizing also increases the strength of the thread.
Glazed. Glazing involves heating the thread and then coating it with waxes, starches, and other chemicals. The thread is then polished to a high luster. Glazing results in a glossy thread with a hard finish. Glazed thread is stiffer than unglazed thread. Glazed thread is not recommend for any machine quilting or embroidery because the glaze will rub off and gum up the machine. It's OK for hand work, but not for machines.
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. This is sometimes called silk finish or polished cotton.
Bonded. Polyester and nylon threads are sometimes bonded, which means the thread is treated with a resin which surrounds and seals the filaments, resulting in a smooth, strong protective coating. Bonding strengthens the thread and makes it less abrasive.
Length of the fibers. Cotton is classified as regular staple, long staple, or extra-long staple (ELS).
Lubricants. Polyester threads are usually treated with a minimal amount of lubricant as they are wound. This is necessary to reduce friction and will not cause harm to the fabric or machine as long the amount used in the manufacturing process was not excessive. If the thread feels oily, it has too much lubricant on it. Cotton threads should not have any lubricant on them.