In this series I have generally been starting with a quote from The Art of French Horn Playing. The discussion there of tuning is pretty familiar and I would recommend it for the purposes of setting up the slides on your horn. Rather than quote the whole thing (a good version of it may be found here in the Osmun website) I would rather move forward and begin with his later publication, The Art of Musicianship. He looks in it at the topic of intonation more from the angle of playing in ensembles.
There are two types of intonation consideration which must concern the performer: vertical and linear. Vertical intonation refers to the relationship of the various notes in a chord. Since each member of an ensemble generally plays only one note of a chord…, it is critical that each performer be sensitive to his position in that chord….
The second … is that of linear intonation. Hear the concern is that of having each note of a melodic line exactly in tune, interval wise, with the note which preceded it….
We should clearly understand that a note which we consider in tune in one circumstance might not be in tune in another circumstance—even in the performance of a single composition. A note being held as the root of a chord might not have the same intonation as it will a moment later, when it acts as a major third of another chord.
Farkas also points to the desire of many musicians to play slightly sharp. Some players seem to prefer the extra “brilliance” of the tone when it is slightly sharp.
Barry Tuckwell, the great horn virtuoso, has a humorous saying which he uses to ridicule this odd belief: “Always remember—it is better to be sharp than to play out-of-tune!” I will paraphrase this saying by stating: “Always remember—nothing, but nothing, takes the place of good intonation!”
Harry Berv in A Creative Approach to the French Horn presents a practical discussion of tuning the horn, noting that
Tuning by slides is, in fact, a careful compromise….
Since it is not possible to readjust the entire instrument for every contingency we may encounter, we must, as intelligent horn players, be flexible enough to adjust….
He suggests a method to tune the horn to get it closely in tune with itself. In conclusion he explains “Remember: It is not possible to tune every note perfectly, but the intonation of each note must be as nearly in tune as possible, so that with the aid of our other tuning devices, including the embouchure, the hand-in-the-bell, and alternate fingerings, it is possible to play the horn in tune.”
In Practical Hints on Playing the French Horn David Bushouse brings up another very practical aspect of playing the horn in tune, that of temperature.
The tuning of wind instruments is affected greatly by temperature extremes. A cold instrument contains a cold air column which has greater density than warm air. Sound waves pass from air molecule to air molecule, and there are more molecules in the cold air than in the same volume of warm air. Therefore it takes longer for sound to travel in a cold air column, resulting in a slower velocity. This means the cold air column has a lower pitch than the same air column when warm.
Thus, in a cold performance or rehearsal situation the horn will tend to play flat and will need pushed in, and in a hot performance or rehearsal situation the horn will tend to play sharp and will need to be pulled out. I have more on this in relation to orchestral playing in this post on the temperature clause.
Bushouse also addresses the topic of slide settings, noting that “it is possible to play in tune with the valve slides set in a haphazard fashion or all the way in” but the “closer the slide setting, the less correction needed and the better intonation will be.”
In the end, finding that best compromise setting to the valve slides is a key, a good mouthpiece is a key, and your getting used to how your horn plays is a key. Warming up with drones can help a great deal, there is a point where every player really needs to beat themselves up and get their horn playing to a point where you can hit equal temperament extremely well.
But … there is one more key element that classic horn methods never mention in relation to intonation. As you read on in the 2016 updates you will see references to the MRI horn studies. One major notable thing is that in the high range the vowel formed by the tongue among elite horn players is “hee,” and in the low range it is “haw.” If your vowel sound is off significantly, no matter how great your horn is and how expertly you have set up the slides, you will tend to be out of tune in certain ranges. Sharp in the high range for example could mean too much “hee” or it could mean you slides are not right. The topic of vowels will be fleshed out further as this series continues.