How string quartets engage with the musical challenges and opportunities of performing ‘old’ four-part music – whether Purcell’s Fantasias or The Art of Fugue – depends largely on how a perception of period sound and gesture can help to modify the richness of a modern string group. Cuarteto Casals’s approach starts with a discriminating use of vibrato and textural lucidity. As Contrapunctus II reveals early on, the phrasing here always seeks to identify Bach’s control of tension and release in carefully calculated units.

How the narrative of The Art of Fugue evolves from the inventio of the first piece is a hallmark of success in this medium (one thinks here of the arc that the Keller Quartet create, and Phantasm on viols), and this is where Cuarteto Casals are less persuasive. The repository of articulation tends to be tiringly staccato, as the triptych of Contrapunti V to VII conveys. V is wearingly perky and VI fails quite to fly in its détaché French overture. VII is impressively poignant in its magical opening but the imploring augmentations are too consciously busy and not sufficiently embedded in the fabric, and the bass tends to anticipate the beat rather than rest on it. I long for incipient yearning and struggle in the restless chromatics of VIII and a contemplative gravitas in XI. Instead, the quartet concentrate on exemplary ensemble and a similarly placed (and paced) animato, often at the expense of poetic exploration.

Still, the playing of Cuarteto Casals is nothing if not impressive. They hardly drop a stitch but they are afraid to seek out the characterful conceits that lie within the technical tour de force of Bach’s compositional ambitions. The last Contrapunctus is tantalisingly well judged in its variety of colour and articulation (although the cadencing at the point Bach ‘left it hanging’ is perhaps not such a good call). One wishes that such all-enveloping engagement might have occurred earlier.