FORTY-SIX YEARS ago, the frugal housewife in search of a beautiful carpet was making the investment of her lifetime - little did she know it then!

The woman who picked up an Oriental rug for a mere song can sell it today for many times its original purchase price, provided it had tender loving wear.

Some Oriental rug dealers in Washington have those old advertisements in which small throw rugs, called Eagle Kazak from the Caucasus mountains of Persia, were offered by local stores for $37.50. Today those same small rugs are selling at between $6,000 to $10,000 apiece.

In the same newspaper ads of 1932, you were offered your pick of Kabistan throw rugs for $17.50 to $22.50. Today these small occasional rugs are bringing between $3.000 to $6.000.

Even 10 years ago, you could buy a room-size, 9x12-foot Tabrix rug from Iran for $2,000. Today the price is $10,000.

What happened? Well, the experts tell us that the oil-rich sheiks of Araby have suddenly become aware of these rug treasures snapped up under their noses. So they are using their petro-dollars to buy back the art work of the Oriental rug makers.

A fines Oriental rug ages gracefully, provided it has been handled properly, cleaned and turned according to the proper cycle. They will last for 100 years with the beauty of their colors softening, but increasing with age.

People have discovered the glory of the Oriental rug.

And fine collectors are now beating down doors to find such rugs.

No wonder the rug seminars held every Saturday morning at 10:30 at the Textile Museum at 2320 S ST. NW are so jammed these days. No one at the museum will tell the rug bearers what their treasures are worth in dollars and cents. But they will give an idea of age and how to care for these handmade glories of the Oriental world of rug makers.

These Saturday seminars are conducted by Harold M. Keshishian, a member of the museum's board of trustees and one of the best known of Washington's Oriental rug dealers. Owners of rugs being them in, like a sack of potatoes over their shoulders, and spread them out in the large back garden of the museum on fine summer days.

Keshishian walks among the priceless - and not so priceless - carpets and rugs spread at his feet. He has the manner or an Oriental potentate as he surveys these offerings of hopeful owners for his advice. And he can be very touchy too.

"Take that home and soak it in the bathtub overnight," orders Keshishian. "It is filthy."

He tells the woman who had brought it wrapped up like a babe in arms that the rug is a Tekke Turkoman, "possibly 100 years old." She said she had picked it up "for a song" a few days before.

For cleaning fluid for the long soak, he suggests the detergent Tide.

Not all the experts agree on how to soak a small rug and what kind of detergent or soap to use. Louise W. Mackie, curator of Old World Textiles at the museum, does not like Tide because of the tremendous lather it creates. And she dislikes the bathtub too.

Mackie suggests laundering the rug outdoors, spread over an ordinary window screen. She prefers Orvis, a liquid made especially for rugs and sold at pharmacies.

Keshishian says all rugs should be turned at least once a year and should be professionally cleaned every three years. Both he and Mackie are in favor of frequent vacuum cleaning or even the old-fashioned carpet sweeper.

Keshishian recommends a small pamphlet on the care of fine rugs, called "Aids," published by the Association of Interior Decor Specialists at 4420 N. Fairfax Dr., Arlington, Va. 22203. It has various divisions, including the Carpet and Upholstery Cleaning Assn. and the National Institute of Rug Cleaning.

The Textile Museum has one of the finest collections of handmade rugs in the world plus a library with works on the subject. The museum is closed on Mondays, but is open, free of charge, Tuesdays through Saturdays from 10 a.m. to 5 p.m.

Officers of the museum have written extensively about the collections, including the director, Andrew Oliver Jr., Mackie and Lilo Markrich, the museum shop manager.