Ton Koopman and Tini Mathot's performance at the City of London festival of Bach's final work, The Art of Fugue, made this exercise in contrapuntal complexity come fiercely alive. Playing two harpsichords, they turned this arcane music into a dizzying intellectual and emotional experience.
Each of the 19 separate canons and fugues is based on the apparently infinite potential of a single, simple theme. Every piece is in the same key, D minor, and each one is an object lesson in compositional technique. But the range of mood and style encompassed by Bach's music - and by Koopman and Mathot's interpretation - was thrillingly wide, from energetic dances to archaic, melancholic laments.
Bach left no instructions as to how The Art of Fugue should be played, or even on which instruments, so every performance is really an arrangement of the piece. Using the tonal differences between their instruments, Koopman and Mathot negotiated a clear path through Bach's labyrinth of musical lines. In the big numbers, such as the elaborate double fugues that closed the first half, they created a monumental structural energy. However, there was fantasy as well as architectural strength in their playing, especially in their delicate but decorative ornamentation, and the way they highlighted Bach's endless transformations of his original theme.
But it was the inexhaustible invention of Bach's music that was the real revelation here. Nowhere was the music more extreme than in the two-part canons in the middle of the piece: Bach contorts his theme in different keys and speeds at the same time, and the players caught the severity and strangeness of this music, the way the theme is projected into different dimensions of musical time and space.
Koopman and Mathot did not play the last number of The Art of Fugue, which was unfinished at Bach's death; instead, the virtuosity and brilliance of their final performance of Contrapunctus XI, a four-voice fugue, gave the suite of pieces a mysterious sense of completion.
· The City of London festival continues until July 13. Box office: 0845 120 7502.
