Density
An Oriental rug is a rug made of natural fibers or pile (most commonly wool or silk), entirely woven by hand on a warp and weft or foundation. Being woven by hand, Oriental rugs vary in quality, which is determined by the wool and knot density. Although the weaving techniques and designs may vary from region to region and country to country, the basic principles of rug making are very similar, and have remained unchanged for centuries. Pile refers to the material (fiber) used for weaving rugs. Only natural fibers are used in hand-woven rugs. The main pile materials are wool, silk and cotton. Sometimes, goat and camel hair are also used.
Wool
Wool is the most frequently used pile material in hand-woven rugs because it is soft, durable, easy to work with and not too expensive. Climatic conditions, the animal's breed, age and diet are all deciding factors in the quality of the wool. After shearing, the wool is washed, the fibers straightened and then spun into yarn. The wool yarn is then combined into various plies. One, two, three, and even four ply yarns are used in rug making. The ply of the yarn determines the number of warp threads and number of knots required to make a dense pile. Having been spun the wool is gathered into skeins and dipped into boiling vats of carefully prepared dye. Seeds, earth, flowers and bark are used to create the colors. Once it has been suspended in the dye for the right amount of time, the wool is hung out to dry slowly in the sun. This ensures the even coloring of the wool. The wool is now ready to be used for weaving.
Silk
Silk is an expensive fiber, and therefore, it is less frequently used in hand-woven rugs. Silk comes from the cocoon of silkworms, which thrive on mulberry leaves. The best quality silk in Iran is from the south shores of the Caspian Sea. Silk is both fine and strong. Silk can be used alone or in combination with wool. Because of their fineness, pure silk rugs due to their fineness are mostly used as decorative items and usually hung on walls.
Cotton
Cotton is used primarily in the foundation of rugs. There are some weavers who also use cotton for weaving small white details into the area rug in order to create contrast. Pile-woven or knotted rugs are created by knots. Warp threads, usually of cotton, are strung between the horizontal beams at a consistent tension. The weaver, seated before the loom, and reading the design from a detailed illustration (or cartoon), begins weaving the rug by tying a knot around two adjacent warps and sliding the knot down to the base. The yarn is then cut with a knife. The length of the yarn determines the thickness of the carpet. Once a row is completed, weft threads are shuttled through horizontally and beaten down with a comb to secure the knots tightly. Even though all pile rugs are woven with knots, different weaving groups use different types of knots. There are two main knot types.
The Persian or Asymmetrical knot
The Persian or Asymmetrical knot is formed by wrapping the yarn around one warp strand, then passed under the adjoining warp strand and then brought back to the surface. This type of knot, also known as the Senneh knot, produces a finer weave.
The Turkish or Symmetrical knot
The Turkish or Symmetrical knot is formed by wrapping yarn around two adjoining warp strands and bringing it to the surface in the middle of the two strands. Turkish and Kurdish tribes of Turkey, Iran and the Caucuses use this type of knot, also known as the Ghiordes knot.
Knot density or KPSI
Knot density or KPSI refers to the number of knots per square inch in a hand-woven rug. Knot density is measured by counting the number of knots per linear inch along the warp, and the weft. Both are visible on the backside of the rug. You take both numbers and multiply them. So a count of 13 knots along the warp and 11 knots along the weft would give a KPSI of 143.
A rug can consist of 25 to over 1000 knots per square inch. A skillful weaver is able to tie a knot in about ten seconds, meaning 6 knots per minute or 360 knots per hour. It would take a skillful weaver 4480 hours to weave an 8x10-foot rug with a density of 140 knots per square inch. If we divide this number by 8-hour working days, which means it would take one weaver 560 days (approximately one and a half years) to weave such a rug. Two or three weavers usually weave an 8x10 rug in a workshop, so the above time can be reduced by half or third. Handmade rugs are truly functional and exceptional works of art created with great patience and love.
Once the weaving is complete, the rug is cut from the loom and the final touches take place. The side edges are bound and overcast with yarn to reinforce the sides. The warp ends that were attached to the loom are braided to form the fringe. The rug is then washed to eliminate dirt accumulated during the weaving, to add sheen to the wool, and to soften the colors. The rug is then dried in the sun. Once dried the master sheerer shears off the top of the knots of the carpet to create a uniform pile.