"When you get them in the air they'll lighten up and look like you just painted them in," said sculptor Mark Di Suvero, making a little brushstroke in the air with his hand.

Barechester and smiling through the sweat that covered his bearded face, he was pointing to the 35 tons of I-beams and other steel and iron scrap strewn on the northwest corner of the Hirshhorn Museum plaza, where the reconstruction of his monumental sculpture entitled "Isis" got underway yesterday.

First built at his studio-yard in Petaluma, Calif., the work is a gift to the Hirshhorn from the Institute of Scrap Iron and Steel, Inc. to mark its 50th anniversary.

Earlier in the day, two cranes had lifted the component parts from three trucks over the Hirshhorn's 10-foot high concrete wall and onto the plaza

By midafternoon, the main element of the work, a 12-ton section of a ship's bow, was on the ground and being bolted back together. It has been cut into three sections for transport. Though it was 92 degrees and sweltering on the outside, Di Suvero was on the inside of the hull bolting pieces back together.

"There, now it looks like a ship again," he said, as he emerged from the boiling innards and downed one of several Cokes consumed in an effort to keep going.

"Now let's move that other piece into place," he called to his crane operator, and the piece was lifted. "The crane is the sine qua non of this kind of art," he explained, as he trudged back off to work.

Critics have said that Di Suvero uses his crane like a paintbrush, as may be observed for the next 10 days prior to the formal dedication on July 19.

Strong but somewhat portly at 44, he still limps from a near-fatal accident in 1960 which crushed him between a load of lumber and the top of an elevator. During his convalescence, he continued to work from a wheelchair but with the crane serving as his hands, shaping the detritus of modern life into huge virgorous gestures against the sky. His sculpture bears distinct resemblance to the abstract expressionist painting of Franz Kline, but in a more lyric, upbeat mode.

His work also is closely related to the art of assemblage, but on a monumental scale, incorporating bold thrusts of line with space, light and sky.

Born in Shanghai, China, to an Italian diplomatic family that emigrated to California in 1941, he studied philosophy and sculpture at Berkeley, and subsequently moved to New York. By the time of his accident, he had received enthusiastic critical response.

Disturbed by the war in Vietnam, he moved to Europe in 1971, living on a barge at Chalon-sur-Saone, France. In 1957 he was honored with the only exhibition ever given a living sculptor in the Tuileries Gardens of Paris.

Upon his return to the United States in 1975, the Whitney Museum chimed in with a rousing show that sprawled all over the five boroughs of New York City, including an indoor retrospective at the Whitney.

Since then, his reputation has continued to soar, and as of yesterday afternoon, he was the hottest sculptor in America - literally and figuratively.

The title, an acronym for the donors, also alludes to the ancient Egyptian goddess of fertility, but what "Isis" will look like when "she's" finished he always addresses his sculpture in the feminine gender) is still a mystery. The Hirshhorn has working drawings, "but they're lousy," says the artist, waving them off. Di Suvero constructed the piece "in effect" in California, "but adjustments will be made to fit the site. This isn't a statue, you know."

Several huge pieces lying around include the cab of an old diesel engine and a submarine float pierced by an L-beam, but it is not yet clear whether they will be included in the final piece.

"All we know is that it has a 43-foot-high tripod-like framework of steel beams from which the ship's bow will be suspended with a huge link anchor chain," said Hirschl Cutler of ISIS, which provided the $85,000 for the commission, $20,000 for scrap, plus shipping and placement costs.

"That's how Mark works," he said, coughing up another $5 to buy Cokes for the crew when it was discovered that the Hirshhorn had not provided the necessary amenities. "Nobody really knows where it's going to go."

Though Cutler seems to have absolute faith in his chosen artist, Di Suvero's improvisational way with tonnage has caused at least one notable flap in the past. Two years ago, when the General Services Administration discovered that a work they had commissioned for a federal building in Grand Rapids wasn't like the maquette, they refused to accept it. The citizens of that city subsequently sent 400 letters and a petition with 600 signatures to GSA in protest. "It is clear that the people want the sculpture," conceded the then GSA commissioner, and they got it.

California filmmaker Suzanne Simpson, one of the few people who saw "Isis" in California and is willing to talk about it said, "It's absolutely beautiful, so lyrical against the sky."

Simpson is making a film about the sculpture, "the film Mark wanted made," she said, in an oblique criticism of another film on the artist, entitled "Northstar," which will be shown nationwide on Public Broadcasting System on the eve of the dedication. Written by critic Barbara Rose and filmed by Francois de Menil that film evidently did not please the artist.

Today, John Mondale will visit DI Suvero at the site at 2:30 p.m., but it won't be the first time they've met. "She invited me and my mother to her house for tea four months ago, and my mother loved it," said Di Suvero. "I though she was wonderful, too. She fills an aching gap in this country, a gap filled by ministries of culture in Europe. It's the women here who love art. Don't forget, to put that in your story."