In the 18th century keyboard instruments were evolving at a rapid rate. Since first appearing in the middle of the 16th century, the harpsichord - with its mechanically-plucked strings held at low tension - had become ubiquitous. By adding keyboards and other devices to modify the sound, the harpsichord had become more versatile, but composers and performers were increasingly looking for ways to derive more expression in performing the music, specifically in the 'touch' of the keyboard.
During the 1700s, Cristofori's ideas were taken up especially by German organ builders such as Silbermann, and by the middle of the century, a number of hybrid instruments had appeared: 'compound' keyboards which combined plucked-string and hammer action; pianos with stops to produce a harpsichord effect.
Modern pianos produce a rich but cloudy sound in the bass region. What makes the fortepiano ideal for the performance of Mozart's music is the clarity it offers in these lower registers, where Mozart would often make use of the 'Alberti' bass figure of oscillating notes. A fortepiano is used by Ronald Brautigam in the recording of the Piano Concerto No 20 featured in this television series. The delicate fortepiano sound and feel comes from the low depth of key strike and the low pressure required to depress the key.
But it seems that when Mozart settled permanently in Vienna, his allegiance moved to the local firm of Anton Walter. Academic Eva Badura-Skoda is an expert on the history of the fortepiano; she suggests that Mozart, whose life is chronicled in minute detail in his letters to his father Leopold, would have avoided writing a letter similar to the 'Stein piano' letter because of his embarrassment about the cost of a Walter fortepiano.
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