J.S.Bach :: Chromatic Fantasy & Fugue, BWV 903
J.S.Bach wrote the chromatic fantasy probably around 1720, the year in which his first wife died. You could assume that this is a kind of "lamento" piece, dedicated to his beloved wife.
Not too much is known about the history of this intriguing piece of music, but the fugue probably is not conceived at the same time, but still during Bach's lifetime at least in one copy (if I am right here just digging in the information in my head), that has been made before 1750.
Interesting with this fantasy is the fact that Bach prescribed dynamic marks, p and f. Often played at the harpsichord (sometimes to great effect), it is impossible to make the prescribed dynamic differences by changing from manual 1 to manual 2. I think there is no question about the fact that this fantasy is pure clavichord music, adding to the drama of the music the intense expression of this intimate instrument.
The clavichord was, as we know through Forkel, Bach's preferred keyboard instrument.
But in general, it is safe to state that most solo keyboard music was meant to be played at the clavichord. The harpsichord was used at larger occasions, where orchestra was involved (Starken Music), or keyboard concerti, not for playing the more difficult and complicated pieces, as this one, or the Well-tempered Clavier, or the Partitas, ...