THERE COMES a time. WHen nancy Hemenway and husband Robert Barton came back to the United States in 1972, she said, "You've chosen the first 30 years, I'd like to choose the next."

Hemenway was lucky. Barton didn't leave for Timbuktu or ask the questions some husbands might. He resigned from the Foreign Service and, as it turned out, went on to perhaps a better job with the Senate Foreign Relations Committee.

And Nancy Hemenway, at last - with the three children grown, her husband settled, no representational breakfasts, coffees, luncheons, cocktails, dinners to give - could go about becoming famous. In six years, she's gone far toward that goal.

On Friday, the Textile Museum will open an exhibit of tapestries by Nancy Hemenway called "Texture of Our Earth." It will run through Oct. 28.

The tapestries are remarkable for their design and for their innovative technique. The designs are based on many painstaking sketches and photographs of nature. They are all worked on either handmade wool ro organdy. Insofar as Hemenway knows, no other tapestry woker uses such materials. She calls her tapestries "bayetage," combing the Spanish "bayeta" (handwoven wool) and the English "collage."

The exhibit was organized by Bowdoin College Museum of Art and has traveled to the Virginia Museum of Fine Arts and the Seattle Art Museum. The tapestries are large, important works. Several have been purchased ($250-$15,000) by museums, including the Seattle, and collectors like David Rockefeller, former Pennsylvania senator Hugh Scott and Sen. Dewey F. Bartlett (R-Okla.).

The Wool Bureau Inc. has rented 52 of her works to show in department stores around the country - though the complexity and artistic genius of her work may discourage more needle-workers than it inspires. She won't always sell her work. One tapestry she sold to Texas gubernatorial candidate William Clements only after he promised to leave it to a museum in his will.

Some 14 years ago when the Bartons were posted to the U.S. Consulate in Bolivia, Hemenway, as an experienced Foreign Service wife, knew it would be months, perhaps even a year before, as they say, their "effects" arrived by boat. All Foreign Service wives know about fitting basic black (short and long), the family silver, two years of 6A shoes and the allergy prescriptions into airlines overseas baggage allowances.

"My easel and paints would be long coming, I knew. I could hardly imagine doing without," she said. In her show's catalog she expalins, "From the time I can remember I had colored crayons and paints . . . My father was a painter but became a designer because oil paints gave him terrible headaches. My mother did large glowing watercolors when she wasn't tending five energetic children and a farm. Color and texture in nature were my earliest knowledge."

She studied art at the Art Students League, Wheaton College, and in Europe; music composition with Walter Piston at Radcliffe; and Spanish lyric poetry at Columbia. But flying into the high Andes of Bolivia, she needed the quintessential Foreign Service avocation - something "light, unbreakable that would fold into a small space."

Almost as an afterthought, she tucked away into a small bag, yarns and odd bits of material. Hemenway, like many artists, had always used her artist's dexterity to keep the house together. She mended and sewed for her family with the skills she had begun to learn at 6. At 12, she said, she made her own clothes and even hats.

But the needle as a replacement for a brush, ah, that was something new - Bolivian born. There is a saying in the Foreign Service that the first three months as a post are the most important. Then you see it all with bright fresh eyes. After three years at a post, the exotic becomes everyday and you lose that excitement of discovery.

Right away, Hemenway discovered the rough wools, handloomed by the country people with the warp threads stretched along the ground and the yarn dyed with wild flowers. She worked with handicapped girls at a school doing designs for their craft. She studied pre-Columbian tapestries in museums and private collections.

She used her needles (all sorts, upholstery of silk) in hard and soft lines as though they were pencils. "And I tried above all to give the work rhythm." Her years of listening for the right note to complete the fugue, the right word to complete the stanza, the next gust of win to ruffle the waves - all came together in the tapestries.

Since then, she has searched for wools, especially natural-toned yarns in all sorts of far places, including Ireland, Indonesia and Mexico. In Namibia (Southwest Africa) she found a strong piece of wool woven by a woman outside a hut. She buys alpaca and karackul from Bolivia, South African mohair and the finest of organdies from Mexico and Ireland.

(During their Mexican post, Hemenway started another cooperative for young girls near Guadalajara. They make pillows and clothes to her designs which are sold in boutiques in this country.)

Hemenway is a sturdy, wiry woman with shoulder-length, sun-tanned hair and a few freckles. One day recently, while hanging her show at the Textile Museum, she wore blue and white cullotes and a sport shirt. She walks with a no-nonsense stride, as though she were on her way somewhere. Though she's small, she lifts the heavy tapestries with an easy grace. She speaks the way she walks - briskly, but with humor and the taste for a word. You can see that she would be much at home in Boothbay Harbor with the sand, the waves and the rocks.

The Edwardian wood-fame, Detached house in Georgetown Hemenway shares with her thoughful husband is like a diary is like a diary of their lives. The pottery from her ceramic period, the ancient chest and cupboard from Bolivia, the paintings, the pillows from her Mexican school - and hanging over it all, the tapestries.

In winter she works in a third-floor studio, atop their home. The studio seems to shrink as her work grows and she wonders if they'll have to move again.

The newest tapestries are from her recent two months in Maine, working in a studio almost at the world's jumping off place. She begins with an observation: waves excited by the wind, lifelines engraved on an oyster shell, tree roots grapsing the earth. She makes sketch after sketch, or even photographs. Sometimes she makes a poem about the design while she's worrying about how it will all come together.

She picks out a piece of fabric. The best ones are the heavily textured wools in strong natural colors. The wools with the assertive designs woven in are not as successful as backgrounds for her work.

She hangs all her wools on a pole above a handsome Bolivian baroque settee. She chooses her skeins as she works, drawing with her needle, painting with the yarns.

"I used to work on the floor, but it gave me such a crick in my neck and back, I had to give it up," she said. Now she works on several tables, pushed together. The tapestries takes a long time.

"Thirty years," she sometimes says.