Fortepiano is a Fortepiano?
After the early music rivaval which was began by a major interest to the harpsichords and much later clavichords, baroque violin, traveres flute but maybe early on viols...carried on as late as 1990s (one really can not say before that there was no interest but mainly very few people played or made fortepianos! and in 1990s there were many good harpsichords made, of course later the quality increased a lot!) but fortepiano appeared late on recordings and concerts.
At first it was enough for many musicians of the early music field to have a ''fortepiano'' for their Beethoven, Mozart etc.
And for instance Johann Schobert (died 1767) in France could be played on a Johann Fritz grand piano of ca.1815-20 !! which is almost as far to this music as a modern grand is! or Jadin played on a Schott ca.1830-40 which is not a sin but raises the question ''what is a fortepiano'' ?
Only using a fortepiano doesn't mean that we are doing something basically historically aware!
Generally what most of us agree on, is that apart from expression, understanding of the harmony and all this, one should accept the fact that it is better and more of that ''historically informed'' to use a similar instrument used by the composer of that time or region. On this basis, we can discuss endless about greats deals of knowledge, style, tecunique etc. but the basis is having the right tool for it!
One can perform same music on a modern grand or a Roland electric harpsichord but then we can not say that we are applying the authentic elements that the composer or composers of that particular period had in mind.
We know that in early 1780s fortepianos in Vienna were made with very delicate hammers, very light stringing and had a transparent sound. Mozart's own piano and many other similar pianos of those years sound similar to each other.
When we come to the end of the 18th century, still pianos were made very delicate but very little heavier...but stil these pianos did not exist like that when Schobert died in Paris in 1767! so when we perform them on a Walter 1795 which is very much like the Weimes 1808 (because in Germany they were more conservative and old fashioned and it took a decade to copy the Viennese fast so called developements!) so, for the case of Schubert for instance, such a piano is something from a different world!
At the end, one must see what is available, but this is the duty I feel to enthusiasts of historically informed performance to give them this possibility as much as it's possible to try different pianos of each decade...for instance, almost from each 2 years of 1800-1830 there is one or more pianos of different countries that give the performer the possibility to explore through the different qualities each piano give. At the end, one experiences that the hight of the piano's body (less the length) and the weight of the hammers which also influences the string gagues all make huge differences in the charackter of the sound and therefore decision for choosing the tempo, style and touch of the whole performance, dynamics, articulation etc.
For instance only on a Viennese style (German or Italian) one can hear the difference of articulations...on a Clementi or Broadwood, the dampers come down gently from above on the strings in a way that the sound is never stopped by its own...it takes a second or even more to fade...but on a Viennese piano like Weimes or Malleck one can play with another style which has a more speaking quality and not only gentle, lovely sound! this happens through the V shaped dampers of thin soft leather which kill the sound almost immediately! gentle, kills...the English one sounds more pleasing to the reader, but for the pianist who wants to try his/her hands on a Viennese action piano for Mozart sonatas, this is more interesting to be able to have the sound when the strings are down and create an intelligent micro comma silience whenever it requires by getting the fingers off!
This kind of articulation makes the music of (for instance) Mozart much more alive because he too played on such a piano.
On the other hand, one can learn from the instrument itself to a great deal!
One will learn from the right fortepiano of the composer's style that for instance what Tessitura means practically.
In the treble one can not play louder and louder as one can do such a semi crescendo direction in the middle part of the piano which is quite natural...on the other hand, the right piano leads you to diminish the sound naturally when going higher! this is also the case for violin of the period. On a violin with gut strings one can not play louder and louder when playing high positions on the finger board. It is simply impossible because the sound will crash and make only noise.
This is the same story about the natural horn etc.
The fragile quality of these old instruments is an interesting aspect of learning from them. On the other hand, the bass notes of Mozart's Walter sound very reedy and powerful and can project very good.
About projection of the sound of such pianos Haydn writes :
So not every Fortepiano is the Fortepiano for the particular music of a period.