Fine yarns: from fiber to yarn
Fine yarns: from fiber to yarn
If you pull a thread from a cotton fabric and examine it carefully, you will see that it is made up of a number of thin fibers (ranging from a few millimeters to about 5 centimeters long) twisted together. The same applies to a wool thread, but in this case the fibers are longer (5 to 15 centimeters). In a silk thread, the fibers are even longer (even several meters). In any case, the strength of the thread depends on the number of twists given to the fibers.
Until two hundred years ago, fibers were laboriously spun by hand, using spindles and distaffs. In 1764, James Hargreaves invented a machine for spinning cotton that operated eight spindles at once. Hargreaves' invention was called the jenny (after Jenng, his wife) and was the first in a series of machines that would revolutionize spinning technology. In 1769 came Sir Richard Arkwright's spinning frame, and in 1779, Samuel Crompton's intermittent spinning frame appeared, which, unlike the previous ones, produced very fine yarns.
Before being spun, textile fibers must undergo a cleaning process. Raw cotton must be extracted from bales weighing approximately 250 kilograms that are tightly compressed, and then passed through a series of machines that loosen, mix, and beat it, thus freeing it from large impurities and traces of soil. Once cleaned, the cotton is compressed into laps (sheets) and is ready for carding. This machine consists of a horizontal cylinder, covered with a large number of iron teeth, which rotates rapidly. These teeth pass (comb) the cotton fibers from the lap to the cylinder, which quickly becomes covered with a thin layer of fibers arranged parallel to each other. The cylinder rotates directly under a series of so-called "flats," also equipped with iron teeth. The flats comb the fibers off the cylinder in such a way that, upon exiting the carding machine, they are reduced to a loose ribbon about the thickness of a finger.
These cotton slivers are very different from the raw material tightly compressed in heavy bales as it arrived at the factory: carding not only untangles the fibers and removes any residual impurities, but also eliminates weaker fibers that would reduce the quality of the yarn. The fibers are now packaged without compressing them, free to slide over each other. This thick sliver, passed through special machines called draw frames, is then stretched until it becomes progressively thinner. To ensure a uniform union of fibers throughout its length, several slivers are sometimes arranged in parallel to be stretched together. To obtain a particularly regular yarn, before stretching, the slivers are passed through combing machines that make the fibers even more parallel. Once they have been stretched to the desired thickness, the slivers are twisted into yarn.
Raw wool needs to be washed to remove natural grease and accumulated sweat. Wool is spun roughly like cotton, with some variations due to the fact that wool fibers are longer and more wavy. During the carding operation, the processes differ depending on whether one wants to obtain a woolen carded yarn (in which case the fibers are carded so as not to keep them parallel, but to let them go in all directions) or a worsted yarn (in this second case, the fibers are carded and combed to arrange them perfectly parallel).
Silk does not need to be carded. Silk cocoons are immersed in hot water to dissolve the gummy substance that holds the fibers together; once this operation is completed, the fibers are reeled from the cocoons and twisted to form the yarn.
Artificial fibers are reduced to yarns directly from the cold-drawn filaments that constitute them, but if other natural fibers, which are shorter, are to be joined to artificial fibers, it is necessary to cut them beforehand, so as to bring them to approximately the same length as the natural fibers.
Yarns can be strengthened by further twisting, which, however, makes them harder to the touch. This hardness is suitable for worsted fabrics used for men's clothing, but not for wool yarns used in knitting, which should only be twisted enough to give them the necessary strength, which can also be increased (without reducing softness) by twisting two or three thin yarns together.
Artificial fibers become "elastic" yarns through the so-called false-twist process, which causes the fibers to take on the shape of long, thin spiral springs that, however, after being stretched, more or less return to their original length.
