The program read "performed on the harpsichord," and until just a few hours before the concert last night that's what the Library of Congress thought it was presenting--a harpsichord recital of 18th century Spanish keyboard music by Antonio Baciero. But when Baciero got to town they found out that he plays his 18th century music on the modern piano instead.

There had been a misunderstanding, so a piano had to be rolled into the Coolidge Auditorium and tuned posthaste.

The mostly baroque program consisted of small but often striking baroque works that Baciero, who is also a scholar, has uncovered in libraries around the world. He says he uses the piano because to play each work on its authentic instrument "would require a museum of instruments." He's a practical man.

The only really well-known names were Domenico Scarlatti, Antonio Soler and Haydn. Obscurity, however, does not always mean mediocrity. One of the most delightful compositions was a sonata by one Julia'n Prieto; it was full of wit, with all kinds of harmonic accidentals and melodies set in odd ranges.

Another treat was the massive A minor fugue by Sebastia'n de Albero, a bold work with great octave subjects. And there was an especially dramatic and grim C minor Recercata by the same composer.

Baciero concluded with 12 lighthearted Haydn dances that he had dug up in the archives of the Library of Congress.

He is a fluent player, though not particularly virtuosic. One problem with such a concert is that, counting the dances, it included 36 pieces, squeezed into two hours. The listener couldn't help but get a little lost. Once the audience mistakenly applauded between movements of an obscure sonata by Mariana Marti'nez. And even Baciero seemed to be consulting a crib sheet to find out what came next.