Wool is the chief material used in carpets, besides cotton and silk. The wool is mainly sheep's wool, but camel wool is also used occasionally, for very fine carpets, goat's hair. Camel wool is always found undyed since it is naturally a strong golden buff colour which, while beautiful in itself, will not take a dye well. Used in it's natural state, a well-knotted camel wool rug with light golden brown a its basic colour can look like a sheet of gold in a favourable light; against this the coloured design may appear like contrasting enamels. Such pieces are found among Persian Hamadan carpets which use camel hair as a favourite ground and border colour. There is also another group of north-west Persian carpets from Azerbaijan which is characterised by its basic ground colour of natural wool.
With regard to sheep's wool, the most important material for carpets, it is the quality of the wool which is one of the most important considerations in a carpet; this quality depends on several conditions. An important part is played by the climate in which the sheep have been raised. The finest wool comes from the flocks reared at high altitudes in the Caucasian mountains and in mountainous part of Persia and Turkistan. Sheep from low-lying lands yield a coarser and less good quality of wool. Grazing is also important and, it is believed, the chemical composition of the water. Equally important is the part of the sheep from which the wool is taken. The finest quality of wool comes from the animal's shoulder, that from the legs and belly being somewhat inferior. Sheep's wool of the finest quality can appear as glossy as silk, and even be mistaken for it. Angora goat's wool excels in it's fineness and is wonderfully glossy, but it breaks easily. Where wool is of poorer quality it tends to look dry and lusterless in a carpet. Wool which has come from the carcase of a dead animal is definitely inferior, and its durability is greatly impaired.
Cotton is much used for the warp and weft of carpets made in certain areas. Persia, a country which grows a great deal of cotton, tends to use cotton for the foundation of most of its carpets. In Anatolia and the Caucasus cotton is hardly found at all, the warp and weft like the pile being made of wool. The same is true of the nomad Turkoman tribes of Central Asia. Kilims and Soumaks are always made of wool throughout.
A simple test for distinguishing between wool and cotton is burning, if the end a thread is pulled from a carpet and held against a lighted match one can tell from the way it burns whether it is made from an animal fibre such as wool, or a vegetable fibre such as cotton. Wool when it burns curls up and leaves a residue, due to its fatty substance, whereas cotton burns to a white ash without losing its shape. This simple experiment will show how to distinguish the difference between the two. The fat content of wool is always present in a carpet made under oriental conditions and even normal washing of the carpet will not completely remove it. After handling a number of woolen rugs for some time, one can often detect the greasiness on one's hands. This fat content is undoubtedly appreciated by dogs, who not only like to lick woolen rugs, but for this reason enjoy chewing them.
Silk being an expensive fibre, is much less frequently found in carpets, but high quality carpets of court manufacture are occasionally woven with silk warps and also wefts. Its fineness and strength make it an excellent for material for high-grade carpets, and silk pile carpets have a special lustre of their own, the best of them being of superb quality; but many third-rate silk carpets have been made for both the contemporary oriental and European markets. Particularly famous are the silk carpets made under Shah Abbas I (1589 - 1628) and the celebrated sixteenth century Hunting Carpet * from the Habsburg collections in Vienna.
Oriental Rugs, Page 27-28, Herman Haack, 1960