Terra cotta, for more than half a century one of the commoner building materials in these United States, suffered a lamentable decline in the postwar years. It was not steel, not concrete, not glass -- in other words, not modern. What's worse, it came in many colors and, worse still, many forms. To modern architects, it was a symptom of a dread disease: ornament.

So it has been heartening to see that terra cotta has been making something of a comeback in recent years, along with color and ornament in architecture. It really is a fine material. It is malleable, durable, wonderfully reflective of light and -- this was somehow missed in the blanket dismissal -- it can be, and usually was, mass-produced.

The National Building Museum highlights this encouraging development with an exhibition that opened Wednesday, "Ornamental Architecture Reborn: A New Terra Cotta Vocabulary." The title claims a bit too much, but otherwise it is a solid little show. Pithy, I think, is the proper word.

One of the best things about the exhibition, as museum Director Bates Lowry was saying the other day, is that "it's infectious -- you find yourself walking down a street and saying, 'Oh, look at that, it's terra cotta.' " Washington was never called "terra cotta city" (as Tulsa was by manufacturers during its art deco days), but there are plenty of high-image old buildings out there whose terra-cotta patternings will delight the eyes of city walkers prepped to discover them.

The primary focus is not so much objects, though these are handsome and instructive enough, as it is a process: The show is the culmination of a design competition organized by the museum and supported by the Ludowici Celadon Co., one of the few still-functioning manufacturers of architectural terra cotta (but whose business in recent decades has been heavily weighted in the direction of roofing tiles).

This collaborative effort between museum, designers, artisans and industry is an example of the kind of program the fledgling institution wants to do more of, Lowry says. It is an example, indeed, of why the Building Museum, which opened last October in the restored Old Pension Building after a long and difficult birth, got its particular name: To concentrate too narrowly on architecture or design, the founders wisely realized, would have robbed the idea of much-needed scope and support.

Included in the exhibit are the winning designs selected last summer by a jury of six prominent architects (instead of operating by consensus, each juror choose a single entry); sketches made after the fact by each of the jurors, demonstrating how the designs might be used in an actual building; a sampling of tiles made by Ludowici Celadon from the first-place designs; and photographs and text panels assembled by the museum staff tracing the proud history (and sorry decline) of architectural terra cotta in the United States. Altogether, this makes the show an informative entertainment.

Louis Sullivan (1856-1924) was among the most prolific, and certainly the most creative, users of terra-cotta tiles for ornament and cladding in the history of American architecture; his Guaranty Building (1894-95) in Buffalo still stands as the supreme example of merging ornament with forthright expression of structure. Sullivan's vegetative ornament was incredibly alive and intricate, and it remains something of a miracle how he combined these qualities in patterns notable for their overall clarity.

Predictably, there were many Sullivanesque designs among the 110 submissions to the competition. Only one, however, was chosen. This was the design by Louisville architect H. Stow Chapman, selected by James Wines, president of Site Inc., a New York firm well known for the whammo legibility of its suburban showrooms for Best Products. Wines' surrealistic proposal for actual use of the Chapman module -- he drew a boxlike building in which the pieces, set between panes of glass, are progressively shattered -- cleverly vitiates the potential nostalgia of Chapman's design.

The brightest, and in a way the best, of the designs are the polychrome tiles designed by Terry Brown of Cincinnati (selected by Robert Venturi). In the highest sense, these might also be called Sullivanesque: Not at all imitative, but quite intricate and extremely flexible, they could be used to create wallpaper-like patterns as well as to make appealing points of emphasis in a monochromatic field. Brown fully exploits the colorful potential of glazed terra cotta, and Venturi, in his rendering of a house as a child would visualize it, skilfully demonstrates the possibilities Brown opens up.

Ornament lends itself to playfulness and wit, a potential that has been overexploited in its recent revival. But Louisiana architect Peter Fortier's "pigeon tile," along with juror Stanley Tigerman's renderings of its possible uses (for smokestacks, columns, gates), is funny without being cloying, appropriate without being overwrought. Fortier's versatile bird-shaped tile -- it could be used a sculptural image on a building cornice, or to create a more or less abstract screen -- might well, as he hopes, "find its way into the construction of even the most modest of buildings."

The remaining three winners are all of a piece -- a good piece, I should say, for each in its way exploits the capacity of the lightweight material to be mass-produced, to be used systematically over broad surfaces and to bring color and light to structures that might otherwise be too massive for their own good. The designers of these modular systems, all architects, are Eric Gazley of Portland, Ore., Carl Vogtmann of Chicago and Giorgio M. Zigliotto of Palo Alto, Calif. In their own presentations, each of the jurors -- Robert Frasca of Portland, Taft Architects of Houston, and Hugh Hardy of New York -- exploits the admirable qualities of these systems.

The exhibition is enhanced to no end by the presence of actual tiles, although, alas, most were modeled by hand. "We'll put them into production as soon as we get the orders," a Ludowici Celadon spokesman explained with a nice note of optimism. Although these six designs hardly constitute "a new terra-cotta vocabulary," they do suggest that the old material's time may be coming again. One hopes so, for until the orders start rolling in, terra cotta will remain a luxury item.

To celebrate the show and to emphasize the importance of craftsmanship in producing architectural ornament, the Building Museum today will host a number of crafts demonstrations -- terra-cotta tile making, of course, and plaster work, decorative wood carving, wood turning, wood joining, scroll and forge ironwork, marbleizing and sheet metal crimping. Events begin at noon. The exhibition itself continues through Oct. 13.