It's not easy assembling a great Old Master collection today, and it's almost as difficult assembling one of modern masters. Almost all the great works are safely and permanently tucked away in museums or other private collections, and when one does come onto the market, its price is apt to be prohibitive.
Gone are the days when a J.P. Morgan or Henry Clay Frick, an Andrew Mellon or H.O. Havemeyer could pick and choose from among the finest Holbeins, Rembrandts, Vermeers, or El Grecos, or could donate a major Raphael or Titian to a museum. And gone also are the days when major Picassos and Matisses could be bought in groups of five or ten - and for a few thousand dollars each.
There still are those who try to build great collections, however, and who do so with considerable determination and intelligence. Outstanding among these is Baron H.H. Thyssen-Bornemisza, whose holdings of Old Master paintings constitute the best private collection of older masterpieces today, and whose collecton of modern works is also highly regarded.
The Metropolitan Museum here is playing host to 76 of the baron's modern paintings. All but one were personally chosen by William S. Lieberman, the Metropolitan's chairman of 20th-century art, who also was responsible for the exhibition catalog's crisply written and informative text.
The paintings are informally divided into six categories, Expressionism, Abstraction, The City and Still Life, The Surrealist Generation, The Figure, and Some American Painting, and include examples by many major figures of our period.
It's difficult to know what to say about this selection. It tells us a great deal about certain aspects of 20th-century modernism, but relatively little about how good it was - and is. For every superb painting, we are confronted by six or seven that are merely representative of their style or period, or of the artists who painted them.
Most of this tendency, I'm certain, can be traced to the limited availability of important modern paintings. The baron, after all, didn't start his collection until the 1960s when it already was difficult to find major early 20th-century works.
Even so, he scored dramatically with the two Mondrians and with Klucis's ''Axiometric Painting,'' Kupka's ''The Machine Drill,'' Braque's ''The Pink Tablecloth,'' Chagall's ''The Grey House,'' Picasso's ''Man With a Clarinet,'' and Feininger's ''Lady in Mauve.''
Also outstanding are Balthus's ''The Card Game'' and Tobey's ''Earth Rhythm.'' On the other hand, Miro, Tanguy, Bacon, and Still are poorly represented - and Estes must compete with one of his very weakest works.
At the Metropolitan Museum through Nov. 27. Museum art on display
The 1983-84 art season promises some excellent museum exhibitions. The National Gallery of Art in Washington, D.C., opens the first American retrospective in 25 years of the work of Juan Gris on Oct. 16. Over 90 paintings , drawings, and collages by this very important early 20th-century modernist will remain on view through Dec. 31. Also on display in this museum will be a large exhibition of Gainsborough drawings (Oct. 2-Dec. 4), and ''Art of Aztec Mexico - Treasures of Tenochtitlan'' (Sept. 28-Jan. 8, 1984).
The Museum of Fine Arts in Boston hosts an important exhibition of American art, ''A New World: Masterpieces of American Painting: 1760-1910.'' It runs to Nov. 13 and then travels to the Corcoran Gallery of Art in Washington, where it can be seen from Dec. 7 to Feb. 12.
The Cleveland Museum of Art will exhibit over 90 of Durer's prints from Sept. 27 through Jan. 8 and as many Rembrandt etchings from Feb. 21 to May 20. It also plans an exhibition depicting the manner in which various contemporary artists utilize the human figure in their work (Nov.1-Jan. 8), and another show on ''Portraiture: East and West'' (Nov. 22-Jan. 22).
The Philadelphia Museum of Art has scheduled the first comprehensive exhibition of mass-produced decorative arts and designs made since World War II. ''Design Since 1945'' will include 450 items and will run from Oct. 16 through Jan. 8. Also on view from Nov. 19 through Jan. 29 will be 69 paintings and drawings by Dadaists Jean Crotti and Suzanne Duchamp.
In New York, the Metropolitan Museum plans to show ''Dutch Painting of the Golden Age'' (Jan. 11-April 15); ''Leonardo da Vinci: Anatomical Drawings'' (Jan. 18-April 15); and paintings of Charles Burchfield (Feb. 1-March 25) and Balthus (Feb. 29-May 13).
The Museum of Modern Art, still severely curtailed by its building program, will present ''Video Art: A History,'' an overview of video by artists from the mid-1960s to the present (Oct. 3-Jan. 3), and a selection of 100 of the museum's most important works on paper (Oct. 29-Jan. 3).
The Guggenheim Museum starts the season off with an exhibition of Charles Simonds sculpture (through Oct. 30). Next on its schedule are the 1983 Exxon National Exhibition of emerging American artists (Sept. 30-Nov. 27) and the second of the three major shows devoted to Kandinsky, ''Kandinsky: Russian and Bauhaus Years'' (Dec. 9-Feb. 12).
The Whitney Museum in New York is the host for a Morris Graves retrospective (see Monitor review of April 26, 1983) that closes Nov. 27; ''An Aesthetic in the Making: American Art, 1958 - l963'' (Sept. 29-Dec. 4); and a major de Kooning retrospective that opens on Dec. 15 and runs through Feb. 26.