In past, wool had a substantial part in carpet weft and warp, but at present, this part is converted to the cotton. Even now is some regions (such as Shiraz, Qashqai tribes and Gonbad in Iran), that warp and weft are produced by wool, but wool is generally substituted by cotton, owing to dearness of wool.
Cotton is produced in many parts of Iran, and scarcely, a province cultivate no cotton. The main region of cotton cultivation are Isfahan, Kashan, Qazvin, Yazd, Meshed and Kerman. Sometimes, seeds of American cotton, with long filament are sowed, but still quality of Iranian cotton is not in demand, while its filament is short.
One of the privileges of cotton fiber to wool is unwillingness of moth to cotton; therefore, carpets with warp and weft of cotton, while moth eating, only its pile would be spoiled, but no hole or cavity perforates the carpet. The loss could be mend easily by tying new piles on the network of the safe warp and weft of carpet.
Cotton filaments are stronger than wool, and due to less tension, are better than wool, for warp and weft of carpet, but it could not be used for pile of carpet. In some instances, very scarcely, they make cotton shining and silky, to counterfeit silk carpets and sometimes, they use viscose filaments instead of cotton for weft. To distinguish this trick, one may steep the dubious filaments in water. Viscose fibers tear away when wet, but cotton filaments resist longer.
However, it is certain that cotton is the best marital for warp and weft of a hand-woven carpet.