AMERICAN PORCELAIN: NEW EXPRESSIONS IN AN ANCIENT ART -- At the Renwick Gallery through August 16, 1981.; CERAMICS AS HISTORICAL EVIDENCE -- At the National Museum of American History through December 31.

"American Porcelain: New Expressions in an Ancient Art" was conceived as a small showing of contemporary ceramics. But when 350 artists asked to be included, says Renwick Gallery director Lloyd Herman, "We decided to make it a national survey."

About a hundred samples of work by as many artisans were eventually selected for the show. Most of the pieces are functional designs: bowls, vases, bottles, lamps, cups and saucers and plates; there are also several sculptural objects: wall hangings, busts and assorted constructions among them.

"Porcelain is the artistocrat of clays," Herman says, explaining that many potters turn to the hard, handsome substance for its durability as well as its esthetic qualities.

One of the most interesting pieces is an intricate, complex construction by Susan Beckman depicting a child's torso in 16 symbolic stages; decidely mystical, it's clearly the work of a talented artist. Another fine sculptural work is a graceful, flowing

rompe l'veil[ tie by Adele Y. Schonbrun.]

On the (almost) functional side is a whimsical "Tea Party Teaset" by Diane Flyr that might be too fragile to actually use, though I'd use it if it were mine. And there's an elegant set of children's dinnerware by Charlotte Potok that could mollify even the pickiest eater.

Unfortunately, many of the pieces are displayed in Plexiglass cases that reflect light badly and give the show a boxy, rectilinear feeling that conflicts with the curving, flowing lines of the porcelain. There's obviously a desire to preserve the better pieces for posterity, but plastic display cases aren't the solution.

In contrast to the Renwick's contemporary focus, the National Museum of American History looks back at "Ceramics as Historical Evidence" with a fine, small exhibit primarily designed to show how scientists and historians are pooling their knowledge to learn more about ancient ceramics and the people who made them.

Colorful photographic enlargements of various ceramic samples reveal where certain objects were made, as well as when and how; these "petrogrphic" photomicrogrpahs" have become invaluable aids to archeologists. For historical reference points, the display also includes a selection of pre-Columbian and Indian ceramics as well as a lustrous 18th-century Chinese teapot and a 20th-century cup and saucer designed by Frank Lloyd Wright.