Performing music from long ago

Question

Let's say a modern orchestra wants to perform a piece of music composed by Mozart. Which of the following is true?

1. Mozart wrote the score for every instrument in the orchestra. Those instruments remain as "modern" today as they were in the 1700s. If any new instruments have been invented since Mozart wrote the score, they cannot be used in the performance.
2. Mozart wrote the score for every instrument, but some of them have become obsolete. Someone (who?) translated Mozart's score to modern instruments that the orchestra uses. That someone also created appropriate scores for instruments that have been invented since the 1700s.

I genuinely don't know the answer. Option 1 seems more natural since in option 2, each of Mozart's compositions would have at least one extra author. It's also difficult to believe that everyone will agree on one authoritative person to do the "translation". However it's also hard to believe that there's been no technological progress since the 1700s. This would imply that engineers in the 300 years since have failed to create better instruments - if they managed, then even if the names of the instruments are the same, the music would sound different from what Mozart intended. I also remember reading that harpsichords used to be quite common, but have since been replaced by the piano.

Answer 1

Both are right and both are wrong. And more in addition. Mozart made 600 or so known compositions. Using a lot of different instrumentations. Some of these compositions have survived intact from his time, others have been added to or reconstructed by quite a few different persons. One example of many, the Requiem was left unfinished by Mozart and completed by Süssmayer

Mainly, though, we know which instruments Mozart wrote for and the instrument names have survived into modern versions (some of his own instruments are in museums ). But the modern versions of the instruments are quite different from 18 century versions. As example, if you look up the Wikipedia article on clarinet, you will see pictures of modern and 18 century instruments. The modern instrument has gone through quite a few iterations and changes. https://en.wikipedia.org/wiki/Clarinet

So what should an orchestra do? I have seen two different strategies. One of them is to play the music on modern instruments. Violins, Cellos, flutes, oboes and so on are still around and quite a few of the compositions use instruments we still recognize. There has been some changes in how compositions are notated, one example is that the so called figured bass was common in older times, but less known today, so it is normally written out today which you might put in point 2 of your question. Sometimes we will elect to change some uncommon instruments. In a way, I believe this to be in the spirit of the times -- you used the instruments available.

The other strategy is to play as close as possible to the original. There are replica instruments available from various times. It can be a quite interesting experience to hear music this way. The instruments do sound different from their modern versions. There are a number of ensembles doing this, one example might be Drottningholms Barockensemble.