Luster is an appearance retention issue that can apply for both synthetic and woolen fibers. Some fiber start out with luster and then degrade. This degradation of appearance my look like fading, but it is hard to tell what color is being affected.
For synthetics, delustering is due to ultraviolet
disintegration. Olefin is most susceptible to this and polyester has the least
problem. The dye on nylon with fade before the fiber. When nylon degrades, it
turns into a powder that looks like this when rubbed against a black cloth:
The cause for this could be either too much ultraviolet rays coming from either natural or artificial light source or it could be because ultraviolet inhibitors were not added or got washed out of the yarn during manufacturing. To determine who is to blame, run 200 to 300 hours of AATCC 16 E. Each cycle of AATCC 16 E is 20 hours, so that will be 10 to 15 cycles. (Some mills set an in-house standard of 200 hours while 300 hours is what most others test for). If the test sample does not delustered or turn into powder, then there is too much ultraviolet light; if it does, then the fiber is defective.
Luster in woolen fibers is likely due to medulated fibers. Medulated fibers are ones that have hollow regions inside the filaments. These hollow regions increase the resiliency of the yarn and are more prone to become brittle. They also look lighter in shade or almost white due to a lesser ability to hold dye. Berber is carpet is an example of a woolen style that has medulated fibers. Thus, Berber yarn is not twisted. In addition, true Berbers are made from naturally pigmented light brown, dark brown and black hues.
The FLOOR COVERING
DICTIONARY has the following definition:
HOW TO TEST FOR A MEDULATED WOOLEN FIBER
A test for medulated woolen fibers is to soak them in a solution of 69% oil of aniseed and 31% benzyl alcohol. This solution has a light refractive of 1.576; wool is 1.548. Thus, non-medulated fiber will become invisible; medulated ones will become white. The degree of medullation will be proportional to its degree of whiteness. The container needs to be black to show the contrast.