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.

In Florence around 1700, the Italian instrument maker Bartolomeo Cristofori made a breakthrough now credited with paving the way towards the modern piano: for plucked strings, he substituted a system of hammers which gave the player greatly increased control of tone and attack through the amount of force used to depress the keys. The addition of extra strings for each note brought new richness and depth to the sound.

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.

For example, the Italian word 'cembalo' is a shortened form of 'clavicembalo' which means 'harpsichord'; but when Mozart writes 'cembalo' in a concerto score, he means 'fortepiano' - the latest stage in the evolution of the hammered as opposed to plucked keyboard instrument.

In the Mozart Museum in Salzburg, there's a piano by Anton Walter of Vienna. It has two fewer octaves than a modern piano, a device operated by the player's knee to 'dampen' the sound, and a knob on the fascia which acts on the damper between hammer and string.

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.

As well as the Walter instrument, Mozart used a piano by Franz Jakob Späth, but we know from a well-known letter to his father Leopold, of 17 October 1777, that Mozart's favourite instrument at the time was made in Augsburg by Johann Andreas Stein. In the letter, Mozart explains the musical qualities which he's looking for in the piano (and it's worth remembering here that when Mozart writes 'pianoforte' he means what we understand to be a 'fortepiano'!):

This time I shall begin at once with Stein's pianofortes. Before I had seen any of this make, Späth's claviers had always been my favourites. But now I much prefer Stein's, for they damp ever so much better than the Regensburg instruments. When I strike hard, I can keep my finger on the note or raise it, but the sound ceases the moment I have produced it; in whatever way I touch the keys, the tone is always even. It never jars, it is never stronger or weaker or entirely absent; in a word, it is always even.

It is true that he does not sell a pianoforte of this kind for less than 300 gulden, but the trouble and the labour that Stein puts into the making of it cannot be paid for. His instruments have this splendid advantage over others, that they are made with escape action. Only one maker in a hundred bothers about this. But without an escapement it is impossible to avoid jangling and vibration after the note is struck. When you touch the keys, the hammers fall back again the moment after they have struck the strings, whether you hold down the keys or release them.

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.

'As a born pianist,' she writes, 'Mozart understandably wanted to own the very best concert grand available. His instrument, still extant and now exhibited in Salzburg in the house in which he was born, remains the best fortepiano of the period, an excellent concert grand, precious not only because Mozart gave his many subscription concerts on it, but also because of its quality. Anton Walter's best instruments were indeed the most expensive in Vienna ... but as concert instruments they were also apparently superior to all the others.'

© Graeme Kay

Recommended reading:
Music in 18th century Austria, ed. David Wyn Jones
Cambridge University Press, 1966.

The Letters of Mozart and his Family, Translated by Emily Anderson
Third Edition: Macmillan, 1988