The Impromptu is an elusive and irreducible genre not practiced by many of the great composers--Schubert and Chopin, primarily; Skryabin and Fauré, secondarily; (we may perhaps consider Beethoven's late set of Bagatelles [Op. 126] as Impromptus in all but name).
A very Romantic genre, the Impromptu is generally characterized by a certain spontaneity, atmosphere, mood of fantasy, and poetic economy of expression. In some cases the Impromptu may exhibit étude- or toccata-like technical fireworks.
Though they are fairly substantial pieces together forming a special set within Chopin's oeuvre, it is regrettable he didn't conceive more of these magical works with their exquisite champagne-sprays of notes and délicieuse jeu perlé figures:
No. 1 - Ab, Op. 29 (1837);
No. 2 - F#, Op. 36 (1839);
No. 3 - Gb, Op. 51 (1843);
No. 4 - c#, Op. 66 (1834).
While Deyanova, Vásáry, Ohlsson, and Anievas too are immensely satisfying, especially admirable is Kempff's charming, light and stylish way with these pieces both in his 1958 London studio sessions and his 1960 Cologne `live' realizations.
And yet, for performance, programme, sound and price, Arrau's 1980 complete Impromptu set remains a favourite.
Arrau's muse is characteristically weightier as he subdivides the internal episodes with surgical skill.