Originally published in Installation & Cleaning Speacialist magazine.

A previous column dealt with inspections of rugs prior to cleaning. As part of this inspection, the cleaner should also understand the construction of an oriental rug, because this will help in detailing the pre-cleaning report and identifying a rug. Let's look at the manner in which genuine hand-tied oriental rugs are constructed.

The process begins with cotton cords (wool or silk are also used) stretched vertically between two beams that constitute the ends of the loom. These cords are the warps (see Figure 1) and they run the length of the loom. Warp yarns are usually the heaviest yarns in the rug. The warp yarns at the end of the rug create the fringe. Naturally colored fibers are often used for warps.

Wefts are horizontal yarns woven through the warps. Usually, these will be of the same fiber type as the warps. They may be white, the natural colors of wool or dyed hues. Wefts cannot be seen from the front of an unworn pile rug. The colors of wefts are sometimes characteristic of certain rug types. Blue wefts in Sarouks, or red or pink wefts in Afshars, are examples. The number of wefts between each row of knots is also characteristic of certain types of rugs. A single weft between each row of knots is typical on Hamadans. Variation of two to eight wefts between rows of knots within the same rug is typical of Kazaks.

The warps and wefts form a grid on to which the knots are tied. This combination of warp and weft is referred to as the foundation. A row of knots is tied on the warps and then more wefts are woven over and under the warp yarns.

The two basic knots used in oriental rugs are Turkish (called symmetric) and Persian (called asymmetric). These are shown in figure 2. The names "Turkish" and "Persian" do not indicate that the knots are used exclusively by craftsmen of these respective ethnic origins. The knots are tied around two warps and then the row is completely pounded down with a tool called a comb. This pounding of the knots adds density to the rug. It's impossible to identify the type of knot from the back of the rug. Only close inspection of the front of the rug will reveal the type of knot used.

The bottom of the rug is the end along which the first row of knots is tied. The two loose ends of each knot make up the pile of the rug. When the knots are tightened, the weaver pulls the ends downward. This produces a consistant pile direction. Incidentally, when a rug is returned to the customer's home after cleaning, he/she may feel the rug has changed color. This could be the result of reversing the rug pile direction in the room. Simply turn the rug to face in the opposite direction and the rug will have a different look.

The knot count of a rug is determined largely when the warps are set up on a loom. This is expressed as knots per inch. Coarsely woven rugs can have as few as 16 knots per square inch or as many as 3,000 per square inch. Among those less familiar with oriental rugs, knot count is often understood to mean, "the more knots, the greater the value of the rug." This is not necessarily true. For instance, a Nain rug has a high knot count but a moderate value, whereas a Heriz rug has a somewhat low knot count and a high value.

A wide variety of natural fibers are used in the construction of oriental rugs. By far the most common are wool, cotton, and silk. Although goat hair, jute and other natural fibers are used, they are not as important as the first three I cited.

With experience wool, cotton, and silk can be identified by appearance and texture. When in doubt about the fiber you are preparing to clean (or if you want to be certain that you are buying an authentic silk rug), a very small quantity can be burned. See figure 3 for burn test indicators.

Wool is the fiber most commonly encountered in rugs. The foundation can be wool, which is common in tribal rugs, with the pile predominantly of this fiber. (In a subsequent article, I will discuss specialty rugs that make use of synthetic fibers, including everyone's favorite plastic - olefin.) More than 300 breeds of sheep exist, but wool from only three or four of these are suitable for carpet. Wool from the sheep of New Zealand is considered the best for carpet manufacture. When cleaning wool, the pH of the solution should be slightly acid (no higher than 9.0). High alkalinity can cause dye bleed and fiber damage.

The use of cotton in the foundation of pile rugs is a very old practice that goes back to at least the 17th century Persia. Most town or factory rugs have warps of cotton. Generally speaking, cotton is not used as an all over pile fiber because of its tendency to mat. However, it is used in Turkish Kayseri rugs. Cotton is sometimes treated with caustic alkali while the yarn is under tension. This process is called mercerization, and it increases the luster of the yarn to the point where it can be mistaken for silk. (Remember to perform the burn test when in doubt.)

Silk is used in the pile and foundation of some of the costliest oriental rugs. It is also used to outline designs on some wool pile rugs. When wet cleaning silk, the same guidelines for wool fibers apply. However, wet cleaning of silk is best left to the more experienced professional cleaner. Dry cleaning, the safest approach, is effective if the rug is not too soiled.

Figure 1.
A. Warp: The parallel yarns running through the entire length of the rug onto which the knots are tied.
B. Weft: The yarns running across the width of the rug inserted between all the rows of knots. These yarns pass through alternate warp yarns. Their function is to secure the knots in parallel lines and to strengthen the fabric.
C. Knot: The term used for a strand of wool yarn which is looped around two adjacent warp yarns and then cut to form the pile (surface of the carpet).
D. Overcasting: A simple wrapping of dyed yarn along the entire length of both sides of the handmade rug.
E. Fringe: The visible continuation of the warp yarns at both ends of the carpet.
F. Kilim: The pileless web of warp and weft between the rug's pile and the knotted fringe. This is also the name for a rug without pile.