Sculptor Robert Berks, known for his "chewing gum" style and his bronze heads of the Kennedys, is building yet another monument in Washington. This one - to Albert Einstein - will be one of the most expensive statues of our time.

Berks will be paid more than $1 million for his three-times life-size bronze.

His fee is $350,000. In addition he will get $667,760 for his staff, his equipment and materials. The money will be raised by the National Academy of Sciences, which projects a total cost of $1,664,405 for the Einstein Monument, which will be unveiled here next April on a site beside the Mall.

The statue of the scientist, seated on a granite bench, pondering the stars, will be placed at 22nd Street and academy's landscaped grounds.

Berks says he has employed 25 assistants, reconstructed much of his Orient, N.Y., studio, purchased a 58-foot "cherry picker" ("so I don't have to build scaffolding"), and built a dacron tent to protect the work - now awaiting bronzing - from wind and rain. He also has acquired a mechanism for manufacturing the globs of plastic clay he works with. Yesterday he called it his "bubble gum machine."

Berks, 56, did the posthumous heads of John F. Kennedy, at the Kennedy Center, and Robert F. Kennedy, at the Kennedy Center, and Robert F. Kennedy, at RFK Stadium.Both appear to have been cast from wads of clay the sculptor did not bother to smooth down.

Though neither Kennedy sat for him, Berks - who also has sculpted Harry Truman, Lyndon Johnson, Abraham Lincoln, Frank Sinatra and Jane Fonda - did have a sitting with Albert Einstein in 1953.

"We met at Princeton, at the Institute for Advanced Study. We spent more than five hours together," Berks said. "It was a turning point in my life.

"I'll never forget it. He appeared at the head of a staircase with the setting sun to his left, his hair glowing like a haeo. He wasn't a small man, he must have been 5-feet-10, but his head was so large his body seemed to hang from it - like a siral nebula."

Berks had gone to Princeton to do an Einstein head for the Chaim Weizmann Institute in Israel. "But the head was not enough. I just had to do a monument," he said.

Berks says he "begged, borrowed and stoke enough money" to spend the next year - 1954 - designing a full-length statue of the scientist. The little rough-hewn figure he came up with - he had it with him yesterday - sits in an eight-inch cube of space.

That little model has been expanded. Were his little Einstein to get up from his three-tiered bench, he would be 21 feet tall.

"I have worked since 1954 to place the monument," said Berks.

He offered it to the Weizmann Institute. They didn't take it. He offered it to Princeton. They didn't take it either. With the help of Sen. Jacob Javits, he even offered it, in 1960, to the Smithsonian Institution for placement in front of the new Museum of History and Technology. Though nothing came of that, Berks did not stop trying. Finally. through the intervention of Sen. Harrison Williams of New Jersey, Berks was put in contact with the National Academy of Sciences. His 23-year wait was over. At last Berks had his site.

But because the academy has not yet raised the money for the monument, it has borrowed $1 million from a local bank to help pay the sculptor for his work to date.

Academy president Philip Handler yesterday said a committee of "noted public figures" has been formed to raise the needed funds. Its members include Paul Mellon, president of the National Gallery of Art; John D. deButts, chairman of the board, American Telephone and Telegraph; Marian S. Heiskell, director of the New York Times Co.; Sol Linowitz, a Washington attorney; David Packard, board chairman of the Hewlett-Packard co.; Sen. Williams; Thomas J. Watson Jr., chairman of the board of IBM; David Rockefeller, director of the Chase Manhattan Bank, and others.

Some 150,000 letters asking for donations also are being sent to members of the National Academies of Sciences and Engineering, the American Physical Society, and the American Association for the Advancement of Science.

Berks, at a press conference yesterday, explained how he had transformed a tiny model into a giant sculpture. He showed color slides of the 60-feet-square concrete pad, the steel turntable, the dacron tent and steel armatures he'd built.

Berks' Einstein, dressed in sweatshirt, baggy pants and sandals, has a notebook on his knee. The open page bears three equations, one for the photoelectric effect, which he discovered; one for general relativity," and, of course. Equal c square of.The scientist is peering down at a round slab of a dark Norwegian granite which, Berks said, will be inset with more than 3,000 brass and stainless steel pins.

The pins will be arranged to indicate the positions of the planets and the stars as they appeared from earth on March 4, 1879, the day Einstein was born.

The bronze figure of the scientist will weigh 8,000 pounds. "At first, the thought of its imopsing size gave me pause, I worried that it might be excessive," wrote president Handler to the members of his academy. "For whose gratification . . . is it being built, I wondered. The answer was simple . . . It will be the multitudes of visitors who travel along Constitution Avenue . . . Up close, the seated Einstein figure will seem very, very large indeed, the physical representation of the fact that he was 'a giant among man.'" tr for add six-style berks

Berks pointed out that the curving furrows of Einsteinls brow "echo" the curving folds of his belly and the circular steps on which he sits."This is the biggest thing I've ever done," he said. CAPTION:

Picture 1, Robert Berks with a model of his sculpture of Albert Einstein, By Robert Naltchayan - The Washington Post; Picture 2, Robert Berks with models, by Richard Meek.