Philip Glass performs in a benefit concert at The Phillips Collection October 2. (Courtesy of The Phillips Collection/James R. Brantley)

Composer Philip Glass told a Phillips Collection audience Sunday that he has played the piano seriously since he was about 15 years old. He is now in his mid-70s. Let’s assume, just to be conservative, that he plays the piano about three to four hours a week, and that the average Glass piano piece ticks by at about 90 beats a minute. Do the math. It’s reasonable to assume that Glass has played well over a million arpeggios during his time on this planet.

Which makes it curious that he doesn’t play them more smoothly. Glass opened the Phillips Collection’s annual concert series with an hour-long recital of his keyboard works, ranging from the clear, sweetly sad sonorities of his 1970s minimalist style to an ongoing series of etudes, begun in 1994, that are darker, more harmonically complicated and layered.

But while Glass is presumably definitive when it comes to the interpretation of his music, he isn’t particularly proficient at the keyboard techniques he uses so frequently. His tremolos, rolling chord figures and repeated scales are presumably meant to sound even and clearly articulated. They weren’t. One had the sense that Glass was improvising his music rather than performing it from a set score. Changes in texture often sounded tenuous at first, then came into focus as the composer’s fingers caught up with his mind. Several of the works he performed are unpublished — private pieces, he called them — so it’s impossible to be sure.

“I had to practice all week to do this concert,” Glass said after performing. Although his music seems simple to digest, with its long repetitive passages built around a few basic ideas, it is anything but simple to play, requiring substantial technique to produce even, unbroken accompaniment figures. Much of Glass’s piano music is built on musical filler that extends a harmonic idea through time. Bad transcriptions of orchestra pieces from the 19th century are full of these techniques. But the modern piano is a merciless machine, exposing every little flaw in the player’s execution of these simple-sounding figures, and it exposed plenty of flaws in Glass’s finger work.

The composer compensated with a lot of pedal, which smooths everything out and creates a more resonant and enveloping sound. But the pedal homogenizes the sound, and after about half an hour of music, it was clear that Glass had defined some fairly narrow limits of where his technical skills could take the audience.

When the music is largely repetitive, variety and contrast become even more essential to prevent boredom. The ear searches for subtleties and variety to compensate for the relatively predictable course of the harmonic journey. Which is why a better pianist could make this music more interesting.

But it was Glass who performed Sunday, and much of the evening seemed to be about the celebrity artist, not the music itself. One basked in his presence, his low-key personality, his genial humor, his articulate explanations of his music and artistry. During a question-and-answer session after the performance, Glass said he has a fondness for the Phillips Collection, which he visited as a teenager living in Baltimore. That alone will surely get him invited back.