Oriental rugs have traditionally been surrounded by a mystique, and buying one today is still not a clear-cut proposition. ''There are costume rugs as well as costume jewelry,'' says Tom Chatalbash of Fine Arts Rug Inc. in Brookline, Mass. People get taken in by the generic name 'Oriental,' and they think they are covered by a blanket policy. But not every rug by that name has the qualities that make a great Oriental rug great.''
There are not only many grades of authentic Orientals on the market, but a multitude of copies as well. Rugs marketed as ''Oriental design,'' for example, are not the genuine article but a machine-made domestic version. A glowing advertisement for ''imported rugs'' is also not a guarantee that they are handmade.
Another factor to consider is whom you buy from. Harold Keshishian, an Oriental rug expert and member of the board of trustees of the Textile Museum in Washington, D.C., who participates in two family-owned Oriental rug shops, says there has been a ''dramatic increase'' in the number of Oriental rug dealers in the United States, possibly three times as many now as there were 10 years ago.
With so many newcomers to the trade, it is especially important to seek out a knowledgeable and reputable dealer. In the Oriental rug business, Mr. Keshishian says, the old phrase often holds true: ''The item you buy is no better than the person you buy it from.''
Although the market is currently down for Oriental rugs, top quality new rugs are in demand, and collectors are paying stiff prices for rare antique rugs in good condition.
At a recent auction at Sotheby Parke Bernet, a small pentagonal Turkoman rug, originally used as a decorative trapping to go on the side of a camel, sold for dated before 1800, and was in excellent condition. At Skinner's in Boston, a pictorial Dagestan rug from the Caucausus, dated 1795, sold for $28,000.
''Because the market is weak,'' one dealer says, ''only the very best are making it.''
The value of an Oriental rug, whether it is new, used, or antique, depends on several factors, including the quality of the wool, the tightness of the weave, the color, and the artistry of the design.
''The most important element in buying a new rug is the quality of the wool, '' says Tom Chatalbash. In judging wool, a rule of thumb is, ''The higher the sheen, the nicer the nap.'' A high-grade wool rug results from long hairs twisted together to create smooth, strong yarn. Good wool is also resilient and will bounce back when you crush it in your hand. Any fuzz on the surface of a carpet indicates inferior wool with yarns made from short, dry hairs that pull apart easily.
Chinese rugs are known for the superior quality of their wool, which makes them ''virtually indestructible,'' as Albert Mahfuz, owner of Araby Rugs in Boston, describes them. Chinese factory methods have not changed significantly from the last century to modern times, and the product, dealers say, has remained consistently excellent.
Chinese rugs are a very good buy on today's market. Duty dropped from 45 percent to 8 percent on top quality rugs and from 30 percent to under 8 percent for middle-range rugs imported to the US when China was granted favored-nation status in January 1980. A new 9-by-12-foot Chinese rug usually runs between $1, 200 and $2,500; a comparable fine-quality Persian starts around $8,000 and can easily reach five figures.
In addition to the wool, another gauge of quality in a rug is the tightness of the weave, which is measured in the number of knots per square inch. Today, 400 knots per square inch is considered fine knotting, whereas antique rugs may have 600 or more. Dense knotting not only extends the lifetime of the rug, but is also a tangible measure of workmanship.
Compare rugs next to each other, Mr. Chatalbash says. Often differences in workmanship are not obvious until they are seen side by side.
While handling a rug is crucial to judging the wool and the weave, it's a good idea to step back to get the full impact of a rug's coloring. Most antique rugs, which were made with vegetable dyes, have a lustrous patina developed over time that cannot be duplicated with the newer chemical dyes.
''You get a very nice ambiance from an old rug with wonderful color,'' Mr. Chatalbash says. In home decorating, the glow of an antique rug not only works well in a period room, but can take the edge off a pristine contemporary setting.
New rugs offer a wider latitude in color than the traditional crimson and indigo backgrounds of Persian rugs, although Mr. Chatalbash points out that some turn-of-the-century rugs were made in ''unheard-of colors.''
Most new rugs are made with chemical dyes, which are generally more fast and do not mellow the same way as a vegetable dye. Cheap, unstable dyes, however, may fade before a rug wears out or, at worst, bleed the first time the rug hits water. Analine dyes seem to be the sturdiest, says Harold Keshishian, but do not ''have the intangible quality that old vegetable dyes enjoy.''
People shopping for an Oriental rug are often looking for a certain color, line, and feeling. In broad geographical terms, Caucasian rugs, made in the region between the Black and Caspian Seas, and Turkoman rugs favor bold, rectilinear designs and elaborate borders. Persian rugs (or Iranian if made after 1937) are known for curvilinear, intricate floral patterns. Turkish rugs often feature a pointed prayer arch. Buddhist and Tao symbols are the primary Chinese rug motifs.
A less expensive alternative to a knotted Oriental rug is an Indian dhurry or Romanian kilim - flat-woven rugs in which the design appears on both sides. These rugs feature animals, birds, trees, and flowers, and are popular for their contemporary colors.
Specific rugs are named after the city or district where they were woven and can be identified by their design and color. The value of a rug goes up when its design or color is unusual for the town or area in which it was made. A black Chinese rug, for example, is rare and will cost more than a comparable rug dyed in the characteristic blues, yellows, or apricots.
Whether to buy an old or a new rug depends on its purpose. Antique rugs have been dignified by museum exhibitions and have many desirable qualities, but they may not always be the best choice.
A young family with children that wants to buy rugs instead of carpeting would probably be better off buying a new rug, says Mr. Keshishian. Unlike worn antique rugs, which tend to be thin, new Oriental rugs are in good condition, have a thick nap, and have many years of wear ahead of them.
On the other hand, as Albert Mahfuz points out, antique rugs have stood the test of time. They have been washed many times, and you don't have to worry about shrinking or color bleeding. An old rug with worn areas can be restored by a rug weaver.
On a more esoteric level, one advantage of an antique rug is its individuality. Although classical Persian designs have been repeated since the time of Shah Abbas the Great in the 16th century, each antique rug is different, if only by small nuances in the design. New rugs are more apt to be one of a programmed genre.
''The essential difference between a new rug and an old one is its character, '' says Mr. Chatalbash. ''But if you don't know how to read it, it's useless.''
That is where research comes in. Books such as ''The Persian Carpets,'' by A. Cecil Edwards, and other sources available at a public library give good background information. But dealers agree the best way to learn about Oriental rugs is to visit shops, museums, and galleries to train your eye and become more knowledgeable. Find dealers who will spend some time to answer your questions. Most towns also have a rug society you can contact; the Textile Museum (2320 S Street, NW, Washington D.C., 20008) can give you the address of a rug society in your area.
If all else fails, as one dealer put it, ''You lose a lot of money, like I did, and you learn very fast.''