CAN a plaster cast be art? Especially if it is brightly colored and hung in a public place? John Ahearn thinks it can - and he's working very hard to prove it. Of course , he's not the first. George Segal has had a very successful career making plaster casts of people, then grouping them in socially and psychologically thought-provoking situations. And a number of younger artists have recently used casts as components of complex, allegorical sculptures.
Ahearn, however, has carved a very special niche for himself. He does colorful and very lively portraits that begin as casts taken directly from his subjects and end up as reliefs on both interior and exterior walls. Most of his models are his Hispanic and black neighbors in New York City's South Bronx, where he moved a few years ago to be at ''the farthest reaches from the center of the art world.'' They sit for him as much out of friendship as for the little bit of local fame the finished work will bring them, and generally don't seem to mind the casting process.
That can usually be done rather quickly, thanks to Ahearn's discovery that alginate, a soft, rubbery substance used by dentists to make molds, sets fast and doesn't stick to his subjects' faces.
Once the plaster cast has hardened, it is painted for both ''realistic'' and expressive purposes. He pays very special attention to all details of clothing and jewelry, makes certain he has caught the individual's expression and personality, and does whatever else he feels is necessary to make the piece as truthful and effective as possible.
His intention throughout is to capture and to project the individuality of his subject, the special qualities and characteristics that set him or her apart from all others. At the same time, he is careful not to remove the individual from his or her social and ethnic context, or to push characterization to the point of caricature.
Color helps greatly in this. Since he accepts his subjects' tastes and preferences in dress, makeup, and accessories, he is not at all intimidated by what some might view as outlandish or garish color combinations. If that is what they like and wear, then that is part of their individual and social identity - and he would no sooner change that than he would the color of their eyes or skin.
Here again, he is primarily concerned about remaining true to the actual person from whom the cast was made. In this respect, he comes close to Norman Rockwell's practice of painting his neighbors' faces and figures. Just as Rockwell's acquaintances delighted in finding themselves on the cover of The Saturday Evening Post, so do the people of Ahearn's community enjoy seeing themselves on gallery or museum walls.
In some instances, they can also find themselves in fiber-glass groupings attached to the outside of buildings in their own neighborhoods. Some of these groupings are merely informally arranged individual portraits (''We Are Family''), while others, such as ''Banana Kelly Double Dutch,'' depict several people engaged in a very specific activity. In the latter, four young girls are shown skipping rope. It is an extremely handsome composition that in no way detracts from the unique characteristics of each girl. We even know their names: Freida, Jenette, Towana, and Stacy.
Ahearn's populist leanings make themselves known not only through his choice of subjects and his work's occasional final disposition on the building exteriors of the South Bronx, but also through his insistence that art should extol human values rather than strictly formal ones. To that end, empathy becomes his most effective tool, and he will sacrifice whatever he must in order to achieve it.
The first reaction to his reliefs, as a result, is certainly that they derive from and represent actual human beings. They are too authentic and precisely defined, too lively and alive, to be imaginary or composites. Unlike Segal's anonymous and static figures which never look like anything but the plaster casts they are, Ahearn's subjects appear real and caught in the act of smiling, skipping rope, hugging, running, or any of the many other things actual people do.
All that, of course, presents problems of creative tact and sensibility. How, for instance, does one prevent such works from resembling nothing but frozen, three-dimensional colored ''snapshots'' or illustrations? Or how does one give them sculptural substance when their identities are largely established by empathy and painterly illusion? There is also the matter of mindless mimicry, the slavish duplication of nature which must concern any sculptor working exclusively with plaster casts.
By and large, Ahearn has managed to avoid these pitfalls, and has transformed the raw material of life into rich human images that can sustain constant critical attention. Still, some of his reliefs do fall short, and remain a bit too sympathy-provoking, or never make it beyond being painted plaster casts.
The vast majority do enter the realm of art, however - even if still only on a somewhat minor level. I suspect that that minor status will soon change. His large groupings, after all, already project an aura of authenticity and ''inevitability'' seldom encountered in public sculpture today. And some of his most recent figures reveal an increased ability to make plaster casting do his bidding.