JOAN MONDALE this week relinquishes her office. The wives of our vice-presidents do not wield power, but she did. It was not her chairmanship of the Federal Council on the Arts and Humanities, an honorary job, that brought her into prominence. Nor was her energy, her friendliness -- and the undeniable authenticity of her love for art and artists -- that earned her the position of the Carter administration's ambassador to the arts.

It is hard to cite a precedent for her peculiar role. True, one name springs to mind -- that of Vice President Nelson A. Rockefeller, an omnivorous art buyer. His bed was made by Max Earnst. But there his Washington art role stopped. He was almost never seen in this city's galleries. He did not search out painters. As far as one could tell he rarely looked for art.

Joan Mondale looked at everything. While most touring politicians yearn to meet the mighty -- union leaders, bankers, other politicians -- the traveling Joan Mondale always searched out art. Returning to this city from Sherevport, Dayton or a hundred other towns -- where she had looked at pictures and congratulated atrists -- Mondale would refresh herself by going to the galleries to look at still more art.

The artists, of course, loved it. They knew she sometimes gushed, that her speeches did not sparkle, but they did not doubt her knowledge. They thought her advocacy important, and they viewed her as a friend. Some reporters called her "Joan of Art." Friends called her Joan.

"She was our most important government ally," says Al Nidal, director of the Washington Project for the Arts. "She gave us credibility, she shared with us her power. She lent us her support. That's what we needed most."

"If you speak about Joan Mondale, you have to speak in generalities," says Frank Stella, the painter, "but generalities count a lot. She is generous and straightforward. She relates well to artists and makes everybody feel good. She has the ability to be interested wherever she is, which is no small gift. Most of us think: If it is in Des Moines, it can't be any good. She's different. She has this naive freshness. There is no doubt she helped artists. It sounds fatuous coming from me -- I make out all right -- but support and sympathy really help. Her goodness makes you feel the government isn't against you. After that, after the goodness, it's all just political. Everybody knows who gets the government's favors: a favored few. That's never good. Joan made friends. I'm one of them. What she brought to Washington was friendship for the arts."

She took an hour the other day to speak with a reporter. The following is excerpted from that interview:

You are about to leave the government. What's next?

I'm going to make a lot of pots.

You used to live on Lowell Street. Are you going back there?

Yes.

Have you thought of staying in power? How about running the National Endowment for the Reagan administration?

No, I'm not going to do that.

Are you going to take a job?

Nope. I've had a job for the last four years, a total job, total everything, and now I'm going to relax a bit and do something I've never done before. I don't know what it is yet because I Haven't done it and I haven't figured it out.

In four years, what have you accomplished?

I tried to point out what was valuable. The public isn't necessarily aware of the contributions that artists, performing artists, musicians, dancers, etc. make to our society. I'll never forget the very first White House dinner, in February 1977, when President Lopez Portillo came from Mexico to visit the new president. I was seated next to Rudolph Serkin. I was new, trying to figure out what I was going to do with this fantastic opportunity, so I asked him, "What do young musicians need most?" He said one word: "Audiences." I felt what I could do was encourage audiences, appreciate audiences. I was an appreciator. I was a consumer. I'm not a critic. I'm not an art historian. But what I could do was sort of say thank you to the arts community.

You're being modest. If someone who knew nothing about art had tried to do what you have done, they wouldn't have pulled it off.

Oh, but art has been my whole life. I mean my uncle is Phillip Adams who's been director of the Cincinnati Art Museum. I worked in the Boston Museum of Fine Arts. I worked at the Minneapolis Institute of Art. That's my area, that's what I love.I don't think people realize that whenever I went to a concert or a dance performance, or met a potter in the studio, I was inspired, I was thrilled. You see, I learned so much.Last summer, when Bob Wade made those wonderful Texas boots ["The Biggest Cowboy Boots in the World" for the WPA], I thought, oh, this is fantastic. So I went down to see the boots and met Bob Wade and we sat in that little shed with the canvas over it and we talked. "Why are you spending your life with these crazy boots and doing whatever you're doing?"

Artists have taught me so much. They gave me encouragement. They liked what I was doing and I liked what they were doing, and the more I learned about what they were doing, the better it was. I remember during the Democratic National Convention, when we went to Wave Hill, that fantastic place in the Bronx where they have that new outdoor sculpture . . .

Wave Hill?

Wave Hill. Oh, you've got to go there. You can take the bus, get right off at 125th Street, and walk right to it. But take comfortable shoes. There was a Jackie Ferrara there, and a Mary Miss -- but I couldn't have appreciated Wave Hill if I hadn't seen the exhibit at Lake Placid or the Mary Miss at Dayton. I mean, it grows.

But there comes a point where praise turns into boosterism, and what flows from boosterism is a sort of self-aggrandizement. You do have strong opinions, but you've never used your public position to club lesser art.

There you're hitting the basic argument -- and your're going to hear it in the future -- about elitism versus populism. If the forecast of what President Reagan is going to do is right, the National Endowment for the Arts' Expansion Arts program which has expanded under the Democrats, very consciously, will disappear or wither or die. I agree with the whole idea of Expansion Arts.They'll fund a little teeny ballet company in the ghetto some place, and the little teeny ballet company will use the money, and raise some more, and get better and better. And then the question comes. They don't want to get their money from Expansion Arts anymore. They're too good for that. They want to move into the disciplines, and get money from the dance section. They want to compete with Cunningham and Joffrey and all those people and they can't. Because they're not that good, and you don't get to be that good in four years.

And you think it is the government's job to fund them?

Yes, and I'll tell you why. Private industry ain't going to take the chance. Pardon me. Isn't.

Some of us suspect your job was not a totally honorary one. You were in the trenches, too. You had something to do with White House policy. Didn't you?

I had a behind-the-scenes function.You know who my greatest ally was? Walter F. Mondale, who sat in on all the budget hearings. And I prepared Walter F. Mondale for the numbers. So I had help.

Who helped?

John Brademas was always helpful. And Claiborne Pell. But I didn't have any legislation. I didn't say. "Now I'm going to have a bill that promotes the arts and I'll pass it through the committees and I'll pass it through the House and the Senate." I didn't do it that way.

How did the NEA budget grow?

We went from $123 million to $155 million. I'm particularly proud of the Small Business Administration's and the NEA's artists workshops.

Were those the programs that taught artists to be business people?

That's exactly right. To me that's the most personally satisfying thing that I was involved in. It gives people who want to make their living in the arts another tool to use so that they don't have to leave whatever they're doing and sell umbrellas at Macy's or drive a cab. That's lost talent.

How much traveling did you do?

At 120,000 miles we stopped counting. The volunteer who was keeping track got a real job, a paid job, so we stopped.

Did you give too much attention to painting and not enough, say, to music? You sometimes seemed to feel that the visual arts were prime.

Well, that's my home. But I did a whole lot of things in the other areas.

Before we took office, [former NEA head] Nancy Hanks came over to our house on Lowell Street and she was very generous with her ideas and advice. I mentioned that the NEA had been criticized for giving too much to the performing arts. I wondered why. I needed to know those things. She said you have to lead from strength. And that is what she did all those years. She made the Endowment strong and glamorous and acceptable and desirable. What Livingston Biddle has been able to do is fill in the other areas that aren't necessarily glamorous. Dance is booming. The theater is strong. Those are glamorous people. They're celebrities, superstars. But how many people know the names of our foremost American painters? Not many. And crafts? Forget it. So I've had a little bit of bias in that direction.

So you invited them home?

They loved it. And I loved it. The best party we gave each year was the artists' party when we invited all the artists whose work we had hanging in the house.

Had Carter won, what would you work on next? What's your unfinished business?

Taxes. We couldn't budge anybody [in Congress]. When artists give their work to museums, they canonly deduct the cost of brushes and materials. Studio allowances are a problem, too. Artists have a hard time drawing a line between living space and working space. Dentists and certified public accoutants and people who do a lot of business from their homes were deducting the cost of the whole home, so they had to change the law. And it really hurt the artist, because he couldn't draw a little string between where he lived and where he worked.

And then there is the problem of estate taxes. That's bad too. If an artist who wants to give away his paintings dies, then the work is taxed at its full market value. And originally, his heirs had only one year in which to pay the taxes. You remember the Rothko case.If you sell 20 Rothkos all at once, the price goes way down because of supply and demand. Lately, the IRS has stretched it. I talked to Fritz about that so that now it isn't in the first year after somebody dies that every asset has to be sold in order to pay the taxes on the inheritance. That's softened. It's three years now, I think. Taxes. That's a failure. That's what we'd be working on.

When you first came to Washington there still were traces of know-nothingism about. You know: "A chimp could paint a Pollock," that sort of thing.

That's gone, totally gone. When I came to Washington I didn't know anyone in the arts. People I met at dinner parties didn't talk about the arts. Now they do. A lot.

Your own knowledge must have helped you. If you go to a museum and say, "What a wonderful Stella," and it's really a wonderful Lichtenstein, nothing you say thereafter has any clout at all.

But I would never say that.

But still there is some danger. People lie in wait for a faux pas. Have you been hurt in any way?

The most difficult thing I've ever done is lobby for specific pieces of legislation on the phone to members of the House . . . I'll tell you the worst question the press ever asked me. I was in Shreveport, La., at the Red River Festival in August. Carter's approval [rate in opinion polls] was at 19 percent, and I went into a library to have a press conference. I had press conferences everywhere I'd go.

"Well, Mrs. Mondale," one reporter asked, "how's it gonna feel next year when your husband's out of a job?" And I said, "President Carter is going to be renominated and he's going to be reelected." Well, I was wrong and the reporter was right. But, oh, that was the toughest question."