Washington's John Van Alstine -- good sculptor getting better--is exhibiting new work at the Osuna Gallery, 406 Seventh St. NW. This is a breakthrough show.

For half a dozen years or so, the strong point of his art has been its sense of strength, of stiffness forced to limberness, of gravity defeated. Van Alstine knots and coils wrist-thick steel bars as easily as you or I might bend a length of rope. He knifes through steel I-beams. He levitates the heavy. The enormous chunks of stone, the sandstone slabs and granite blocks his art has long included do not merely squat there. When stacked they seem to float. They curtsy and they prance.

But often in the past, his intractable materials--those graceful twists of steel, those balanced blocks of stone--seemed richer, more impressive than the schemes that they obeyed. His workmanship was fine, his materials were imposing. The thinness of his sculpture was a thinness of the mind, a slightness that showed up in his sometimes safe and sometimes slick ideas.

That flaw has been corrected. Van Alstine, in his new work, has banished old timidities. He relies less than he used to on the safeties of simplicity. He has set his sculpture free.

It is suddenly far richer, stranger, more organic. A number of the sculptures here wear thorns and teeth and claws. One piece, called "In the Clear," has a flowered steel tendril. This show is filled with references--to standing stones and pyramids, to growing things and monsters. The pieces called "Stone Totems" have spirits in them. "The Fourth Beast of Daniel" is a sort of friendly dragon. Its claws are sharp, its body snakelike. Like the Statue of Liberty, it wears a jagged crown.

These found elements of steel do not try to hide their dents, scars and corrosions, and these granite blocks acknowledge the eons that produced them. Their sides have not been polished. Some wink with specks of mica, others wear old stains. All the sculptures here work well in the round. One easily forgets the torches and the hammers, the gantries and the drills that were used to make them. For they look as if they grew themselves, as if their many elements--their wedges, I-beams and pipes, their spines and crests and blossoms--somehow came together through mutual attraction. Van Alstine, 30, teaches at the University of Maryland. His memorable show closes July 4. Barbara Herzberg: Scary Fun

Washington's Barbara Herzberg, now showing at the Foundry, 641 Indiana Ave. NW, calls her exhibition "What is There to Fear?" Her answer is: a lot--spiders in the attic, alligators in the basement, gorillas in the bed. All the works on view, the life-size installations, smaller doll-house sculptures, photos and collages, summon things that frighten--snakes, syringes, guns, fire, blood and corpses. But because these monsters, these reptiles and apes, are as scared as we are, all fear soon dissolves. This show transforms the frightening into reassuring fun. It closes May 28. Etchings by B.G. Muhn

The Stoneman Gallery, 408 Eighth St. NW, is showing etchings by B.G. Muhn, a Korean-born artist who teaches at the University of Maryland. The two best prints displayed--"Night Garden" and "Memory of One Rainy Day"--are enough to justify a visit to his show. Both manage to combine the airiest of atmospheres with the sharpest of line drawings. Both mystify a bit. Why is that pregnant nude, the one with the umbrella, accompanied by foxes? What are those women thinking of there on that soft lawn? These fine etchings mix the spirit of the Orient and the Occident. They blend the observed and the dreamt. Muhn is an artist worth watching. His show runs through May. Pastel Images of Haiti

Nyangoma's Gallery, 2335 18th St. NW, is showing pastels made in Haiti by Paul Goodnight of Boston, an artist who appears to have a two-part mind. With one part he records what he has seen. With the other he discovers colors of a brightness one only finds in dreams. Goodnight draws the Haitian scene--women at the market, a priestess in a flowered dress, children on the street--with extraordinary accuracy. But the colors that he then applies--oranges and purples, magentas and strange greens--must have been imagined.

Though Haiti's hues are hot and odd, these are hotter and odder. Three friends that he encountered there--Ralph Allen, Jean-Rene' Jerome and Washington's Louis Mailou Jones--also are showing at Nyangoma's. This is the gallery's last public exhibition, at least for a year. When it closes on May 28, Lusetha Rolle and her partners, JoAnn Katabaruki and Constance Hamilton, will move to 1439 Rhode Island Ave. NW, where they will work as private dealers. The suspension of their exhibition program, though temporary, is a real loss.