The museum has become the architect's canvas. No building form brings a client of more discrimination, nor attracts an audience of more discernment. The spate of museum additions has attracted more attention than the art within, it sometimes seems.

Museum directors now pick architects to build the same way curators pick artists to exhibit. Choosing a Bruce Goff building (winging, erratic, quirky), or an I.M. Pei structure (sleek, refined, technocratic), guarantees what will appear almost as much as picking a Richard Estes superrealist work or a Morris Louis stained canvas.

When the Chicago Museum of Contemporary Art asked Melvin Charney, an architect and conceptual artist, to build a new facade in front of its 1978 structure, it played on a phenomenon of the '80s:

Deliberately tentative, the large-scale installation of plywood and wood went up May 5, only to be dismantled Aug. 29.

Painted to look both spontaneous and suggestive of the real museum behind, the short-lived facade suggests the disposable quality of old museums and the exhibitionist character of the new ones.

A century or less ago, when America built the bulk of such cultural strongholds, it was otherwise. Museums were awesome - masonry tombs positioned with Beaux Arts symmetry and a heft that would inspire if it didn't daunt.

Sometimes they intimidated, sometimes they invited. Either way, it was clear that the mission was to preserve and protect the artifacts within. Until the last generation, the design was almost predetermined, no matter where.

Today, as the Whitney Museum of American Art's current exhibition substantiates, the aim is otherwise.

In seven new museums shown at length there, 12 others exhibited sketchily, and in the Whitney's own addition to its Marcel Breuer building (forthcoming from architect Michael Graves), the architecture is totally unpredictable, the architect chosen for his capacity to make an artistic statement.

''New American Art Museums,'' on view through Oct. 10, underscores this building boom and, as organizer Helen Searing suggests, presents the mixed approaches or styles that ''reflect the diversity of American architecture today and offer examples of every major architectural trend.''

It is not the only place to see that mix, of course.

In fact, the American wing of the Metropolitan Museum of Art, by Kevin Roche, manages to suggest the range within a single courtyard.

The addition, opened in 1980, is a symbol. The room sits under a glass roof - Louis Tiffany stained-glass windows to one side, mosaic to another, the US Custom House facade installed on a third, a glass wall on a fourth, Rodin's ''Gates of Hell,'' temporarily, in between.

More transition zone than room, the courtyard is at once surreal and overwhelming, inside and out, a structure that, like so many in this mode, unsettles.

Its three-ring-circus architectural approach to art as public entertainment summarizes the varied styles gathered at the Whitney exhibition and, more pointedly, across the country.

Beyond chaos of form, commercial considerations have become the most ''readable'' message and trend of new museum buildings. The gift shop, the boutique, the restaurant, the profitmaking apartment building, loom large and appear in the buildings.

Thus, though the Whitney catalog insists that the seven-plus assembled architects try to honor their old environments with restrained design, the buildings indicate otherwise.

''Whether the designs are for wholly new structures or additions to existing ones, the architects have taken pains to respect the existing physical context and to reinforce the museum's role as an active participant in its community,'' the catalog says.

Nonetheless, the loftiest entry, Cesar Pelli's gallery expansion and Edward Durrell Stone Associates' tower for the Museum of Modern Art, now rising above midtown Manhattan, is scarcely typical. At 53 stories, it is scarcely self-effacing.

The L-shaped building, scheduled for completion in a year, may have positive charms, but deference to its low-scale site is hardly one.

If the Museum of Modern Art of the 1930s was ''an aggressive tastemaker,'' and the Philip Johnson remake in the '50s a standard-setter, one can only hope that this moneymaking successor will have less influence.

Similarly, a second of the seven museums on view renders more lip service than genuine compassion to the landscape of Portland, Maine.

The Portland Museum of Art, a reinforced-concrete structure with red brick trimmed in gray granite, replicates the materials of main street, Maine, but the whopping circular and half-circle windows and the monolithic facade scarcely conform or conciliate the broken-up small-scale streetscape.

Of course, it is difficult, if not impossible, to render judgments from mere sketches and models spread out as they are at the Whitney. The problem of dissecting designs with too little information makes the show as tantalizing as it is informative.

You can predict Richard Meier's High Museum of Art for Atlanta by knowing Meier's white cubist work. Or you can project the forthcoming mix of wit and vivacity in Charles Moore's museum for Dartmouth College in Hanover, N.H., trusting that its meandering in and out of the college will have more order and definition than the model indicates.

But, despite the samples of material and the dollhouse details of some models , it is hard to offer a view on the Dallas Museum of Fine Arts, by Edward Larabee Barnes; to comprehend how Hardy Holzman Pfeiffer Associates will double space at the Virginia Museum of Art; or to assess the Shin'enKan in Japan, by the late Bruce Goff, which looks like a launch pad modeled after a Japanese tearoom.

Certainly this last is ''freed from the dictates of outside interest,'' as the museum states; but its extreme is typical of the freewheeling elements of the museum-making trend.

Clearly, though, if the medium of marble and masonry was the museum's old message of permanence and stability, of confidence and civilized agreement, then today's word from the Whitney, like modern notions of art and culture elsewhere, is ambiguous, spontaneous, and indeed uncertain.

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