Mercerized cotton yarn is a type of cotton yarn that has been produced in a certain way that makes it stronger, more lustrous and less likely to shrink. It also makes the cotton mildew- and lint-resistant and helps it accept dye more readily, allowing for stronger, more brilliant colors. The mercerizing process was developed by John Mercer, a British chemist who patented the process in 1851, but the process was popularized by Horace Lowe in 1890 when he altered the process slightly in order to confer a high luster to the resulting fibers. This type of processed cotton yarn also is referred to as pearl yarn or pearle yarn.
Producing mercerized cotton yarn involves exposing the cotton fibers to a caustic solution such as lye or sulfuric acid. This causes the fibers to swell, straighten and become rounder, as well as making them absorb liquids, including dye, more readily. In order to increase the luster of the fibers, the most desirable quality in mercerized cotton yarn, the caustic solution must be applied to the cotton while the cotton fibers are held under tension. Tension is held for about 10 minutes while a 21-23 percent sodium hydroxide solution is applied at room temperature. This process not only causes the cotton fibers to swell and straighten, it also makes the fiber surface much smoother, causing the luster for which mercerized cotton yarn is known.
Certain types of cotton fibers work better to make mercerized cotton yarn than others because of the way the sodium hydroxide is absorbed and the way the cotton fibers respond to it. In general, mercerized cotton yarn is made from cotton that produces longer fibers, such as Egyptian and Pima cotton, because they do not need to be twisted as much as cotton with shorter fiber lengths. The reduced twist in these fibers also makes them more absorbent.
For knitters, crocheters and other crafters, mercerized cotton yarn comes in a wide variety of sizes and colors. Yarn gauges and yarn weights from very fine, lace-weight yarn to heavier worsted weight can be found easily in any craft store, making mercerized cotton yarn a convenient choice for any project. Cotton yarn in general, though, has a rougher texture and a less delicate drape than other yarns, especially fine wool yarn, alpaca yarn and various handspun and novelty types of yarn. When choosing a type of yarn, it is important to consider gauge, weight and drape.