Early music played on historically authentic instruments has had such a fitful history in Philadelphia that the baroque orchestra Tempesta di Mare perhaps understandably celebrated its 15th anniversary as though  it were its 25th -- with a retrospective program, thank-you speeches to those who made it possible, and a reception with cake.

The downside of the group's annual visit to the Kimmel Center's Perelman Theater was that the concert Saturday was presented as a cross section of the group's identity. And for all its distinctive contributions to the Philadelphia music landscape, Tempesta displayed as many quirks as  strengths.

Cofounders Richard Stone and Gwyn Roberts have made a special cause of German baroque composers, many of whose manuscripts disappeared into the then-Soviet Union after World War II and have been rediscovered only in recent decades. Though second- and third-tier composers sometimes write first-rate music, you would have to combine the best elements of the 18th-century trio of pieces that occupied the concert's first half to come up with something worth more than a momentary resurrection.

Johann Gottlieb Janitsch's Overture Grosso in G for double orchestra was the strongest -- a virtuosic piece of composing, employing more antiphonal interplay effects than you ever thought existed. But however serviceable the thematic bones of the piece, you can't say it displays an eloquent gift for melody. That quality wasn't lacking in Johann Sigismund Kusser's Overture 2 in F from Apollon enjoue, which was also full of novel orchestral effects, but it seemed unable to sustain any idea for very long.

Johann Friedrich Fasch's Concerto in D was so colorfully orchestrated it could be called a concerto for orchestra, with its unusually numerous solo opportunities. But if a more melodically simplistic piece has ever been written, I haven't heard it (and hope never to). Tempesta proudly admits to having played 21 Fasch pieces, but only a few of them are of any consequence.

The second half was on more solid ground, with Vivaldi and Rameau. My protective instincts for the "Spring" concerto of Vivaldi's The Four Seasons kicked in during Stone's pre-performance explication/demonstration that highlighted the music's incredibly clear descriptive effects, circumventing one's own rediscovery of them during the performance. Violin soloist Emlyn Ngai, a key member of the group, showed how much expression can be projected within the slender lyricism of this music.

The opening moments of the Rameau suite to his short opera Pygmalion -- reprised from past seasons -- finally put you in the presence of a great composer. The music has everything, including profound rhythmic invention and orchestral layers that create attractive friction for not quite fitting together. The performance was beautifully rehearsed, but like what came before, tempo choices had a moderate sameness. Curiously, faster-tempo passages didn't sparkle or crackle. Is it possible that Tempesta has hit some glass ceiling in the quality of performances?