A problem that sometimes occurs with older rugs is the appearance of white tufts or flecks or small 1/8″ round, hard, white nubs in an area on the face of the rug. This problem only occurs in rugs woven on cotton warp.
The white flecks that show on the face of the rug are actually the untrimed ends of knots in the warp strings. Weavers don't always use continuous lengths of cotton string as the warps–sometimes they tie together shorter lengths to make a warp long enough to string from loom beam to loom beam. Often the weaver doesn't trim the ends of the knot in the warp string closely, relying instead on the eventual thickness of the rug's pile to hide the knot and the ends of the knot. When the pile of the rug wears (to perhaps half its original thickness or less), the end of the knot in the warp becomes visible as a white tuft on the face of the rug.