LENOX -- Yo-Yo Ma and Emanuel Ax had a surprise up their sleeves Tuesday for their legions of fans at Tanglewood: their first public appearance playing period instruments.
Both the cellist and the pianist have performed and recorded on early instruments before, but never together. Recently, though, they have been rehearsing on a 1712 Stradivarius cello, retrofitted as a Baroque instrument, and a fortepiano built in Freeport, Maine, by Rod Regier, based on instruments built from 1826 to 1827 by Konrad Graf and Ignatz Boesendorfer. Monday afternoon they made the decision to play the first half of their all-Beethoven program on the early instruments.
It was an enthralling experience, but getting there was not half the fun. The concert was sold out, with 1,200 in Ozawa Hall and a huge lawn crowd -- 2,693 lawn tickets were sold. There was also a flash thunderstorm about half an hour before the scheduled starting time, leading to gridlock between those trying to get out and those trying to get in, and many crossed signals from parking attendants. The concert began late.
Both Ax and Ma joked with the audience about the weather and about the change of instruments. ''The Steinway is at a wedding reception," Ax explained. The players said that Tanglewood was an ideal place for this experiment. But a rainy night wasn't ideal for intonation, and at one point when Ma was retuning his cello, Ax quipped, ''I'm going to get him some pliers."
Still, the experiment was worth making, continuously interesting, and often convincing. The sound awakened by Ma's period bow on the gut strings made a good match for the light, transparent, and infinitely articulate sound Ax was producing from the fortepiano. Balance problems simply evaporated, and the constant interplay of timbres, musical ideas, and the personalities of the players in the First and Fourth Sonatas was delightful.
The audience burst into applause after the first movement of the First Sonata, which may have offended a few snobs but didn't seem to bother Ma, who praised the public for being historically authentic and offered the rondo-finale ''as an encore." That so many people were hearing this wonderful music for the first time is a good thing.
The concert began with Beethoven's variations on an aria from Mozart's ''The Magic Flute," and the second half began with the variations on a duet from the same opera. This made odious comparisons inevitable -- the modern instruments are louder, more reliably responsive, more consistently in tune, and, by comparison, a bit blunt, especially in fast music. But the magnificent Third Sonata -- the cellist's equivalent to the violinist's perennially popular ''Kreutzer" Sonata -- found the famous partners in fabulous form, taking tremendous musical, technical, and interpretive risks, and carrying away the victory.
Editor's note: Because of a copy-editing error, the headline on yesterday's review closed Tanglewood prematurely. As the review noted, the Tanglewood Music Center fellows have completed their season, but Tanglewood remains open through Labor Day weekend.