This is a really interesting question and it brings to mind a video I made years ago demonstrating how the pedals interact with tone and techniques that add sustain to the tone of the piano by utilizing the pedals. If you push down the sustain pedal after you’ve played a note, you can enhance the tone after the initial attack. You can also depress the una corda pedal (soft pedal) which shifts the action so only two of the three strings are struck directly by the hammer for each note which makes the attack softer and creates a more sustained tone. By using these two techniques in conjunction with one another, you can achieve a very sustained tone. But what about just using the soft pedal alone? Is that ever done?
Here’s an interesting fact for you. Way back when the first piano was developed by Bartolomeo Cristofori around 1600, he had sort of a una corda pedal. It wasn’t like a modern type, but indeed he had a mechanism on his early pianos that could provide a softer tone. However, that piano didn’t have a sustain pedal! Later on in Mozart’s era, the early pianos had a lever that could be operated with the knee that did the same thing as the sustain pedal on modern pianos. You could combine changes of registration achieved by engaging felt on the strings, along with the sustain lever, thereby softening and sustaining the tone. This is like on a modern piano combining the sustain pedal along with the soft pedal. For example, in the second movement of the Mozart sonata K330 in C major; I always love to take the una corda pedal in the minor section. I use the sustain pedal along with the una corda pedal to achieve a soft, singing sound.
If you’re playing Baroque music which predated the invention of the piano, composers wrote for various keyboard instruments including the harpsichord, clavichord, virginal as well as others. These instruments had no sustain pedal. Yet there were changes of sound with registrations which engaged different sets of strings. This is why many people believe that you should not use the sustain pedal in Baroque music for the reason that it wasn’t on any of the keyboard instruments of the time that composers like Bach and Scarlatti were writing music for. So this is one instance where there is a good case for using the una corda pedal without using the sustain pedal. You can hear the change of color on the accompanying video which demonstrates engaging the una corda for a change of tone. Indeed it is possible to use just the una corda pedal without necessarily combining it with the sustain pedal as is usually the case.
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