fray and eventually break. This is not as much of a problem with wound gut strings, in which the gut core, being protected from contact with perspiration by the metal winding (and underlayer, if there is one), lasts a much longer time. Nonetheless, as such a gut string ages and continually responds to cyclic changes in temperature and humidity, the core becomes weak and brittle, and eventually breaks. Furthermore, all gut strings are vulnerable to going out of tune due to changes in atmospheric humidity. However, even after the introduction of metal and synthetic core materials, many musicians still prefer to use gut strings, believing that they provide a superior tone. Players associated with the period performance movement use wound and unwound gut strings as part of an effort to recreate the sound of music of the Classical, Baroque, and Renaissance periods, as listeners would have heard it at the time of composition. For players of plucked instruments, Nylgut strings are a recently developed alternative to gut strings. They are made from a specialty nylon and purport to offer the same acoustic properties as gut strings without the tuning problems. Fluoropolymers (aka "Carbon") Fluoropolymer strings are available for classical guitar, harp, and ukulele. These strings are usually traded under descriptions like fluorocarbon, carbon fiber, or carbon, which is scientifically incorrect. The so-called Carbon material has a higher density than nylon, so that a nylon string can be replaced by a carbon string of smaller diameter. This improves the precision of higher fretted notes, and the resulting vibrational behaviour leads to a more brilliant sound with improved harmonics. In particular, classical guitarists who feel that a nylon G string sounds too dull can use strings that include a carbon G string. Other polymers Other polymers, including polyetheretherketone and polybutylene terephthalate, have also been used. Silk Silk was extensively used in China for traditional Chinese musical instruments until replaced by metal and nylon strings in the 1950s. Only purely silk strings used for the guqin are still produced, while some silver-wound silk strings are still available for classical guitars and ukuleles. The quality in ancient times was high enough that one brand was praised as 'ice strings' for their smoothness and translucent appearance. Winding materials Aluminum, silver, and chrome steel are common windings for bowed instruments like violin and viola, whereas acoustic guitar strings are usually wound with bronze and piano strings