It is very interesting about Beethoven Ian. I think he used more legato than Mozart (Mozart's technique was much like harpsichord).
Most definitely, and he pioneered new conventions of technique in this respect, also with respect to notation - at least according to Czerny (though it's not out of the question that this might be Czerny's agenda as much as Beethoven's), when a series of notes have no slurs or phrasing marks, the default touch is legato, whereas in Haydn and Mozart scholars have pretty definitively concluded that the convention then would be non legato.
What did he say about playing.
There's lots of stuff in three books in particular, Sandra Rosenbloom's
Performance Practices in Classical Piano Music: Their Principles and Applications, William S. Newman's
Beethoven on Beethoven: Playing His Piano Music His Way (awful title, but wonderful book), and George Barth's
The Pianist as Orator, the latter looking in particular at the conflicting accounts of Beethoven's playing by Czerny and Schindler. I have all of these, and will try to dig some stuff out from them later.
I think he said to Czerny to not stop his nephew at every little mistake he plays, but let him play on. He was also baffled at huge tractats that piano theoreticians wrote. He thought it was much easier than that.
Yes, indeed.
Yes, the concept of weight is very misleading. In fact one doesn't feel weight when things go well. Was it Deppe who said that the arm should be like a feather.
Deppe pioneered the 'free fall' technique (about which one can read quite a bit in the book by my own teacher, György Sándor,
On Piano Playing: Motion, Sound and Expression, though Sándor doesn't specifically refer to Deppe. This technique is sometimes mistakenly taken to represent 'weight technique', when really it is 'gravity technique', quite a different thing. 'Weight technique' was pioneered in particular by Rudolf Breithaupt in his book
Die Grundlage der natürliche Klaviertechnik (1905), and was influential on a number of pianists including Teresa Carreńo and later Claudio Arrau.
I also like it when they talk about turning the arm little at a time so by the time one is on say E (playing C major scale with the right hand) the thumb is resting on the key already.
Couldn't agree more, a far more economical approach than the 'thumb under' technique.
In arpeggios it is even more important to turn hand out without turning elbows. It sounds very cumbersome.
That occurs when pianists move the elbow without also moving the wrist, so they have to make vastly over-extravagant motions. A composite lateral motion of the wrist and elbow enables the hand to turn outwards in the manner you describe.
I think arm has connecting use to bring fingers to the right places.
Often at a first lesson I ask a student where they think their finger muscles lead to. Some point at the knuckles, others at the wrist, few seem to know that they extend right along the forearm towards the elbow. A finger is an extension of its forearm muscle, and the position of the wrist, as affects whether such a muscle is bent or not (thus causing friction) significantly affects the freedom and flexibility of each finger. The fingers can never be separated entirely from the arm, whatever generations of coins on wrist, books under elbows, teachers will say.
Many teachers talked to me about sensitivity in the finger tip. Now a friend tells me about harpsichord playing and that the touch is even lighter. (I was always afraid of superficial touch).
I'd say that it's the sensitivity in the joints (those between the phalanxes of the fingers, the knuckle joints, the wrist, even the elbow joint) that counts in terms the way in which the pressure on the key at the moment of impact is affected.
I like it what you write about slow damping mechanism in Chopin's pianos. I did not hear about that. This is a good reason to use finger legato and not pedal. A lot of the time one can not use pedal and has to use fingers.
I was tought not to rely on pedal too much.
Well, do bear in mind that Chopin's pedalling indications are very extravagant indeed (though the somewhat lesser sustaining power of the instruments he knew should be taken into account). As well as the common 'use the pedal most of the time, change with every harmony' approach that has been drilled into many students the world round, there were a wide range of different approaches to pedalling in the 19th century (and afterwards), some employing more pedal than is now common, some employing less.