While the materials used to construct an oriental rug may vary, there are only a few general materials used which account for 99% of all hand- knotted rugs. Because of this, generalizations can be made which allow one to learn much about their rug solely through knowledge of what composes their rug.
Most hand-knotted rugs have a woolen pile, but this woolen pile can consist of any number of combinations of animal and hair fibers. Aside from basic sheep’s wool, goat hair, camel hair, mohair, silk, and metal fibers are commonly found in many oriental rugs. Silk rugs also have another subcategory because of the use of synthetic false silk fibers and mercerized cottons as substitutes. Blending of different materials to create finer grades of wools, cottons, and silks, is also common, but sometimes reserved for finer pieces. Wool, for example, can be blended in order to create softer, more lustrous pile materials.
The reason for the use of wool as the pile of the carpet has to do with its excellent physical properties. It is exceptionally durable. The fiber lends well to being drawn and spun into yarn. The structure and natural oils of the fiber make it easy to clean and care for. The fibers are able to be dyed. Wool has the desired amount of tensile strength. (Imagine pulling a cotton string and a wool piece of yarn. The cotton string is very strong, but eventually snaps. The wool tends to stretch before breaking.) Wool does not shrink when washed. And finally wool is breathable.
Silk may also be used in the pile of the carpet. The advantage of using silk is that silk is the most durable of the three fibers, does not require a mordant to be dyed, and allows you to place a very high knot count into the carpet. The disadvantages often outweigh the benefits. Silk is very difficult to clean and easy to stain, and the biggest downfall… silk is expensive. Because of this expense, some manufacturers often use artificial silk (art silk) which looks and feels like the real thing, but is actually mercerized cotton.
The warps and wefts of oriental rugs are almost always cotton, with finer pieces using a silk foundation, and many antique pieces using a wool or silk foundation. Again, how the foundation is constructed and what material is used will give many clues as to the age, origin, and value of the rug. Cotton is arguably the best base fiber for the average rug because of the low cost and durability of the fiber.