Despite the popular myth that strings for instruments are made from cat gut, gut strings are made from sheep intestines. According to Walter Kolneder, books from the fourteenth century describe the methods used, which have changed only a little since. Although many players today prefer synthetic strings, some do continue to use gut strings wound with silver or nickel, and players of the Renaissance viola da gamba use gut strings extensively. Making gut strings is a time-intensive process that requires special tools and machinery--often custom-made equipment that is protected as trade secret.
Obtain the raw materials---sheep intestines, called a "set" by abattoir workers and butchers. According to Walter Kolneder, the author of "The Amadeus Book of the Violin," domestic sheep have intestines that are too rough for strings; young mountain or prairie sheep, about 8 months old, are the preferred source for gut for strings. Gut must be taken from the animal's body while the fibers and muscles are still warm and then stripped of fat and fecal matter and put into cold water. As abattoir workers process animals, the sets are tied together, creating a long rope. They are sometimes deep-frozen for shipment to the next step in their journey to becoming strings.
Clean the sets. In the string-making industry, the sets are shipped to a dresser, who prepares the intestines for further use. The dresser soaks the sets in cold water for up to 2 days, then gives them a hot-water bath and soaks them in a chemical solution (sometimes lye-based) to remove any last bits of membranes. The dresser then sends the sets through a machine that scrapes them down to the muscle, leaving only a long tube of casing about 30 feet long. The casings are bleached for aesthetic reasons and cut into strips of the same length and thickness using either a splitting horn to separate the casings into strips or a machine designed to cut them equally, and they are stored in salt and other preservatives until the string maker is ready to use them.
Soak the strips in an alkaline solution---traditionally wine or lye---to remove the salt until thoroughly wet and then attach at one end to a turning machine or rack. Strips of the same length are tightly twisted together. The more strips twisted together, the thicker the string will be. For example, it takes only three strips to make a violin E string, but 64 strips to make a low A string for a bass. The strings are twisted almost continually until they are dry and have "glued" themselves together.
Polish and round the strings using a centerless grinder, rubbed with almond or other natural oils to prevent them from becoming brittle. Dry under low, warm air heat. Once dried, low strings are wrapped with alloys---usually silver, but also copper, aluminum, and bronze---and are tuned and checked for pitch trueness, often using specialized software. The strings are now ready to be used, although players must keep them supple with regular applications of oil.