AN ANCIENT world of dolls whose original owner's great-great-grandchildren are now too old for play . . . Of quilts too precious to warm beds, but which instead now hang in great museums . . . Of silver spoons and teapots made by Revolutionary War heroes . . . and tavern signs made by itinerant artists . . .

The Index of American Art--17,000 pictorial records, most of them exquisitely detailed watercolor renderings of early-American life and art--is the National Gallery of Art's least-known treasure. It is securely ensconced behind a door made to look like a wall in what is almost a secret suite of the Graphic Arts Department, headed by Andrew Robison.

Visitors who throng to see the Alfred Stieglitz photographs in the gallery outside the door don't realize that in the hidden rooms are a wealth of visual information about objects made, used and cherished by Americans from the 1600s until the early part of the 20th century.

Among the treasures recorded in the renderings are:

An earthenware dish with sgraffito decoration (incised into the clay), a Valentine or a love token with double doves and tulips inscribed to Katharine Raeder, probably of Montgomery County, 1786.

A maple-and-cherry rocking chair, made in the 19th-century by Shakers in New Lebanon, N.Y.

A Saint Acacius bulto, a wood votive figure made in New Mexico in the 18th century.

A figurehead of the "Lady with a Rose," made before 1814.

A maple wing chair made in New England in the early 18th century.

A silver mug made by Paul Revere in Boston, 1768.

As well as: accordions; bootees; calipers; door knockers; embroidery; flycatchers; firemen's hats; ice cream freezers; honey, slop and tobacco jars; knapsacks; lamps of these types--Argand, astral, Betty, baker's oven, bull's eye, bunker, camp, camphene, candle carriage, garden, grease, hurricane, kerosene, miner's peg, petticoat, spark, spirit, spout, street, student, wall and whale oil; mannequins; neckerchiefs; ox carts; parasols; quilt covers; rain gutter stirrups; sauerkraut stompers; teakettles; urns; violins; walking sticks; and yokes.

The Index began during the Great Depression as part of the Work Projects Administration's Federal Art Project. It ended when World War II began. Unemployed graphic artists were hired to record antique objects in 35 states. (Bureaucratic difficulties and ignorance of regional antiques kept the project from being instituted in some southern and western states.)

The antiques came not only from museums, dealers and important collectors, but also from attics, basements and parlors of people who had inherited the objects from their ancestors.

For each object, there is a questionnaire with varying amounts of information and of varying reliability.

One such form lists a Mexican blanket as "typical," "native" and finally, for verification, simply "God Knows." Another form quotes "grandmother's grandmother" and a note by the researcher says, "grandmother's grandmother was wrong."

"Whenever we have an authority come to do research in our collection," said Lina Steele, the curator of the Index, "we ask them to tell us as much as they can about each object they work with."

Steele and her associate, Lisa Fukui, have begun to research the renderings, but a vast amount is yet to be done. The renderings are in the process of being matted and encased. All are stored flat on large metal shelves in a climate-controlled room, in the newly remodeled ground floor.

"Our facilities are just wonderful now, compared to what we did have," Steele said.

The renderings--careful, stylized watercolor paintings--were made according to a formula taught by Susan Chapman, now of the Boston Museum, who studied under Egyptologist Joseph Smith.

For example, one artist, Alfred H. Smith, rendered a bombe' chest from Massachusetts, showing a streak of faded wood across the middle and the light that faded it, still falling across it.

A few artists rebelled under the meticulous method--notably Perkins Harnly, whose wonderful interiors are much admired.

By appointment, from 2 to 4 p.m. Monday through Friday, visitors may come to the Gallery, look through the Index's microfiche set that records virtually all the renderings and the accompanying catalogue, and select which renderings they want to study.

Fukui photographed the renderings for the microfiche before she became a part of the Gallery staff. The catalogue was compiled by Sandy Tinkham of Alexandria for Charles Chadwyck-Healey Ltd. of Cambridge, England. The microfiche sets, sold mostly to libraries and museums, cost about $4,000, but can be consulted free at the Gallery.

The best book about the collection is "The Index of American Design" by Erwin O. Christensen, the first curator of the Index, with a foreword by Holger Cahill, who had served as National Director of the Federal Art Project. Though out of print, the book is still available at the National Gallery shop.