Cotton has been collected and spun into string or thread as far back as 10,000 years ago in Peru, when it was a labor-intensive process done by hand. Cotton has since been domesticated, and though some hobbyists still spin cotton by hand, commercial harvesting, processing and spinning of cotton fibers is now done by machines.
Cotton is a subtropical, shrub-like plant with seeds that grow enclosed in capsules, called bolls. Each seed is surrounded by the downy fiber that can be spun into cotton products.
Before mature cotton bolls are harvested, the plants are sprayed with a chemical defoliant. This chemical removes all the leaves from the plant, which makes harvesting by machine possible. Before this process became common, cotton was laboriously harvested by hand.
The cotton fibers must be separated from the seeds to which they are attached. A machine called a gin performs this task. A roller covered with spikes pulls the cotton fibers through a wire mesh and the fibers, or lint, are then collected and baled. The seeds are too large to pass through the mesh and are dropped into a collecting tray. The seeds can be used for livestock feed or to make cooking oil.
Carding is the process by which cotton fibers are separated from each other and fluffed up, making them available for further processing. Carding is done by machine.
Just as the combing of a person's hair makes their hair lie in the same direction, the combing of cotton fibers makes each individual fiber line up with the others. This prepares the fiber for being spun into a long, continuous, smooth thread.
The previous steps are applicable to the production of many cotton products. In spinning, the nature of the finished product comes into consideration. In order to turn bales of cotton into long strands, they are spun together. Fibers are twisted together to make a strong, thin thread. The finished strand can be thin or thick, tightly or loosely spun, depending on the desired finished product.
Strands of spun cotton are then twisted together in a process called "plying." Cotton string usually has at least two plies, as this increases the strength of the string and reduces its tendency to twist back on itself; but it can be composed of many more than two. Some strings are made of plies that are woven or chained together instead of twisted. Finished cotton string can be bleached or dyed to adjust the color of the finished product. Some strings are further treated, such as those that are mercerized with warm soda lye to make the string shiny, stronger and shrink-proof.