Hi Rok - as the resident horn player, allow me to clarify Frogman's answers (which were good). The period instruments (what we now call "natural" horns), had no valves. So they could only get the notes of the natural harmonic series of the key that the instrument was pitched in. To change the key of the instrument, tubing had to be added or subtracted to it, pieces of tubing called "crooks." To fill in the notes of the scale, the players used a hand technique, which is much easier demonstrated than described - they would "close" the bell of the instrument with their hand, which bends the pitch to the correct note they want. These "stopped" notes have a much more metallic sound - we still do this today, actually, for the sound effect - Mahler in particular wrote a great deal of "stopped horn" notes. With the invention of the valves, this allowed the horn to play chromatically and get any notes needed without hand stopping them, with an even tone quality. I hope this is a decently satisfactory explanation - feel free to ask any questions this brings up, and I will try to clarify.
Most modern players do not have training in the specialized hand technique required to play the period instruments well anymore, only the people who actually play these early instruments would need to learn it. The technique can be demonstrated roughly on a modern instrument, however, which I often do at educational concerts, but I am not really a "natural horn" player.
It is no surprise at all to me, by the way, that the horn players on your Gardiner recording sound much better than the others. The guys in his group are pretty much the best in the world at it, as far as orchestral playing goes, anyway. The French groups are not nearly so well regarded as far as horn playing goes, and this actually goes for the modern instrument as well.
One thing to bear in mind is that in those Handel pieces, there aren't that many "stopped" notes anyway - hence, they will sound much more like modern instruments than they would if they were playing say Mozart or Beethoven, who wrote much more difficult and complicated parts for the horn. If you listen carefully, however, to that Gardiner recording, you will hear the difference between the "stopped" and open tones. Part of the technique is to make the "stopped" notes sound as much like the open ones as is possible, and those guys are really good at it. So they won't sound so metallic, they might sound more "covered" instead. That is a great recording, in fact the best I know of for the natural horn playing.
Most modern players do not have training in the specialized hand technique required to play the period instruments well anymore, only the people who actually play these early instruments would need to learn it. The technique can be demonstrated roughly on a modern instrument, however, which I often do at educational concerts, but I am not really a "natural horn" player.
It is no surprise at all to me, by the way, that the horn players on your Gardiner recording sound much better than the others. The guys in his group are pretty much the best in the world at it, as far as orchestral playing goes, anyway. The French groups are not nearly so well regarded as far as horn playing goes, and this actually goes for the modern instrument as well.
One thing to bear in mind is that in those Handel pieces, there aren't that many "stopped" notes anyway - hence, they will sound much more like modern instruments than they would if they were playing say Mozart or Beethoven, who wrote much more difficult and complicated parts for the horn. If you listen carefully, however, to that Gardiner recording, you will hear the difference between the "stopped" and open tones. Part of the technique is to make the "stopped" notes sound as much like the open ones as is possible, and those guys are really good at it. So they won't sound so metallic, they might sound more "covered" instead. That is a great recording, in fact the best I know of for the natural horn playing.