THEY passed the sponge cake around at Jacob Kainen's 80th birthday party, because the painter whose work hangs in 29 major museums around he world is fond of it. Kainen beamed through the surprise birthday party, held at the National Gallery in the Widener Porcelain hall full of Ch'ing dynasty and other priceless vases. Smiling and silver-haired, the Connecticut-born artist looked dapper in a gray suit, pink shirt, and crimson tie. He said that he'd felt downhearted at some of the other milestones in his life - 30, 40, 50. ``But today I'm not downhearted, I'm indignant!'' over being an octagenarian. He continues to work actively, painting from l0 a.m. to 6 p.m. daily. A show of his color woodcuts was held in l987, and one of his recent paintings in l988.
The party which the Trustees of the National Gallery threw for him also marked half a century of Kainen's art, and icing on the cake was a surprise exhibit of some Kainen works selected from the Gallery's 40 Kainen drawings, prints, and monotypes.
A brief accompanying biogaphy by Andrew Robison, the National Gallery senior curator and curator of prints and drawings who staged the surprise party and show, noted that ``Kainen has frequently been seen as an originating force behind the abstract paintings of the Washington Color School, but has worked in various styles at different times for different purposes.
``Constant in his mature works are a classic sense of forms combined with an extremely refined sense of tone, expressed through a graphic range from delicate, whispery washes of color to intense depths of black.''
The son of a Russian immigrant, Kainen studied at the Art Students League, graduated from Pratt Institute, and haunted museums and galleries with fellow artists Arshile Gorky and John Graham. Among his other friends and fellow artists: Stuart Davis, David Smith, Willem de Kooning.
In l935 he joined the Workers Progress Administration (WPA) as a graphic artist and held that job until his move to Washington in 1942 to join the Smithsonian, where he later became director of what is now the National Museum of American Art.
Among the national and international museums which have Kainen works in their collections are the Metropolitan Museum of Art, the Museum of Modern Art, and the Whitney Museum of Art in New York; the British Museum; Kunsthalle, in Hamburg, West Germany; and the Australian National Gallery.
As National Gallery director J. Carter Brown said in tribute to Kainen, ``Long may he wave.''
The Kainen birthday luncheon, with the help of his wife Ruth, was planned to include his favorite foods, among them a seafood quiche and chocolate baskets filled with ice cream and chopped nuts. Plus, of course, the sponge cake.