On April 15, Sheldon and Jill Bonovitz will celebrate 46 years of marriage.

Who knows where they might be or what they might be doing, but chances are, if past is prologue, it will involve art.

Jill Bonovitz is a ceramic artist and a founder of the Clay Studio in Philadelphia. Sheldon Bonovitz is better known for his legal artistry, although he says he has "an artistic side."

"I have," he says, "a very strong sense of what works and what doesn't work."

What works is art. The two share a passion, and for the most part a sensibility, that has found mutual expression in a decades-long collecting collaboration. They may not agree on everything. They may not want to acquire everything. They may not seek to dominate a field.

But in the end, their collection, particularly of what is known as outsider art - works crafted by self-taught artists - has grown into one of the finest in private hands. And they have now bequeathed most of it to the Philadelphia Museum of Art, which is celebrating the gift with "Great and Mighty Things: Outsider Art From the Jill and Sheldon Bonovitz Collection," which opens Sunday and runs through June 9.

The gift immediately makes the museum, which has been building its outsider art collection for several years, one of the most important U.S. centers for such work.

"The gift will join a collection rich in mainstream early modern and contemporary art, a context in which the Bonovitz work will eventually be shown, and in which, it is hoped, we can better understand the different but parallel operations of the outsider field and contemporaneous mainstream art," Ann Percy, exhibition curator and the museum's curator of prints, writes in an essay in the show's catalog.

In other words, the museum's position is that outsider art should be viewed within the context of art as a whole, a view not universally held in the art world.

The Bonovitzes agree completely.

Dressed in a fog-gray shirt and matching bow tie, shocks of hair falling across his forehead, Sheldon Bonovitz exudes a youthful exuberance for the art in his life. Across the table on the 12th floor of the Duane Morris law firm's offices, his wife, dressed in black shirt and pants and flaming orange sneakers, displays a quieter enthusiasm.

She is the daughter of the legendary Janet Fleisher, whose gallery largely introduced the city to outsider art; when John Ollman joined Fleisher in 1970, the field had two of the country's most knowledgeable proponents. Fleisher died in 2010; Ollman now runs Fleisher/Ollman Gallery and has had a strong influence on the Bonovitzes.

"He's been very important," says Jill Bonovitz. "He's really often brought pieces to our awareness. He'll call us from an art fair or something and say there's a [Bill] Traylor that you absolutely have to have! He's been very influential. Very helpful."

It was Ollman who introduced the Bonovitzes to the work of James Castle, amply represented in the museum show. "I remember when we went to the outsider art fair when James Castle was shown there for the first time . . . . As soon as we walked in the door, John took us and said, 'You have to come see this artist.' "

Perhaps not surprisingly, the couple, who live near Rittenhouse Square, feel outsider art is as much an expression of aesthetic sensibility, done with as much acuity, as any mainstream modernist or contemporary painting, assemblage, construction, sculpture, drawing or installation.

"There's no fence around genius," says Sheldon Bonovitz, 75. "It's just available. People are creative given the opportunity. Given the opportunity, they can succeed."

"Yes," agrees Jill Bonovitz, 72. "I work in clay, and I also work in wire. There's been forever this thing about craft and art. . . . I think it's all art, but there's good art and bad art. When they start saying, 'This is clay, it's craft,' I just think that's the same thing [as dismissing] outsider art."

The major artists in their collection - about 200 works will be shown - have been acquired over three decades.

Castle, deaf from birth, was the subject of an art museum exhibition in 2008-2009, the first such museum retrospective for the Idaho artist, who died in 1977. Sculptor William Edmondson has been likened to Brancusi. Howard Finster is often characterized as a religious visionary. Martín Ramirez spent half his life in a psychiatric hospital. Bill Traylor, born a slave, was discovered drawing on the sidewalk in Montgomery, Ala., by artist Charles Shannon.

There are 27 unique artists in the show, comparable only in their uniqueness. Each demonstrates a singular vision, style, medium, and directness of purpose. None was formally trained. The work of each is instantly recognizable. All but one are dead.

For 30 years, the Bonovitzes have added here and added there. But Sheldon Bonovitz once bought every ceramic piece by Eugene Von Bruenchenhein held by a Chicago gallery. Von Bruenchenhein never sold a piece before his death in 1983, leaving a house full of paintings, photos, chicken-bone sculptures, and the ceramics the Bonovitzes wanted.

"Jill and I were the only ones buying it," says Bonovitz. "It culminated in our buying 250 pieces . . . and we've given it to some museums, and we've helped make popular his ceramics."

His wife laughs. "The difference between us is that I would have kept buying a piece here and a piece there. It never would have crossed my mind to buy the whole collection!"

Sheldon Bonovitz thinks for a moment.

"We have this collection, it's at the museum, but the really great experience is for Jill and me to have done this over a period of 30 years. I mean our whole lives are so much entwined in art we see art every weekend.

"So its like a journey for us. It's like the means don't justify the ends. Here the means for us was really the thing, the whole trip, 30 years, really since the time we were married, our trip through the art world together, and doing this together."

Jill Bonovitz agrees. "I think we're really lucky to have this thing we both love and love to do because that doesn't always happen between couples."

Yes, says her husband, "This is probably our deepest mutual interest, common interest. It's been fabulous for us. And we get along doing it."

Contact culture writer Stephan Salisbury at 215-854-5594, ssalisbury@phillynews.com, or @SPSalisbury on Twitter