carpets are more likely to lie flat on the floor, whereas wool tends to shrink unevenly, and carpets with a woolen foundation may buckle when wet. Chemically treated (mercerised) cotton has been used in rugs as a silk substitute since the late nineteenth century. Silk Silk is an expensive material, and has been used for representative carpets. Its tensile strength has been used in silk warps, but silk also appears in the carpet pile. Silk pile can be used to highlight special elements of the design. High-quality carpets from Kashan, Qum, Nain, and Isfahan have all-silk piles. Silk pile carpets are often exceptionally fine, with a short pile and an elaborate design. Silk pile is less resistant to mechanical stress, thus, all-silk piles are often used as wall hangings, or pillows. Spinning The fibers of wool, cotton, and silk are spun either by hand or mechanically by using spinning wheels or industrial spinning machines to produce the yarn. The direction in which the yarn is spun is called twist. Yarns are characterized as S-twist or Z-twist according to the direction of spinning (see diagram). Two or more spun yarns may be twisted together or plied to form a thicker yarn. Generally, handspun single plies are spun with a Z-twist, and plying is done with an S-twist. Like nearly all Islamic rugs with the exception of Mamluk carpets, nearly all Persian rugs use "Z" (anti-clockwise) spun and "S" (clockwise)-plied wool. Dyeing The dyeing process involves the preparation of the yarn in order to make it susceptible for the proper dyes by immersion in a mordant. Dyestuffs are then added to the yarn which remains in the dyeing solution for a defined time. The dyed yarn is then left to dry, exposed to air and sunlight. Some colours, especially dark brown, require iron mordants, which can damage or fade the fabric. This often results in faster pile wear in areas dyed in dark brown colours, and may create a relief effect in antique oriental carpets. Plants Traditional dyes used in Persian rugs are obtained from plants and insects. In 1856, the English chemist William Henry Perkin invented the first aniline dye, mauveine. A variety of other synthetic dyes were invented thereafter. Cheap, readily prepared and easy to use as they were compared to natural dyes, their use is documented since the mid 1860s. The tradition of natural dyeing was revived in Turkey