Playing loud involves not just air but letting the air pass through the lips freely. As with the topic of soft playing, there are a combination of factors to balance, and you want it all to happen naturally. And as with the advice seen on soft playing, some of it below is helpful and some of it is more obscure in meaning.
The secret: relax the lips
Farkas in The Art of French Horn Playing gave this advice.
The secret of fortissimo is in relaxing the lips so that for any given note, they are doing much less work than ordinarily. This comparative relaxation will permit the formation of a large lip opening when the huge amounts of air necessary for a fortissimo are forced through…. This carries out to the extreme the theory advanced earlier in this book that any note equals the total of embouchure tension plus air pressure, and that when one is increased the other must be lessened.
Visualize that you are in a large space
In The Art of Brass Playing Farkas in the following gets at the topic of playing louder, with orchestral dynamics. He suggests visualizing that you are playing in a larger space.
There is a certain spirit to correct blowing, as I have tried to indicate, and sometimes a bit of imagination helps to capture the spirit. The average player spends many hours a week practicing in some little, low ceiling, ten-foot square studio. No wonder he forgets to play in a full, expansive manner! I often shut my eyes while practicing in such a room and imagine that I am playing solo in a big auditorium … accompanied by an orchestra comprising one hundred fine musicians, each producing a big, beautiful tone. Then the need for a projecting, resonant tone is the most graphically realized, and, as a consequence, I play my instrument in a much more singing manner. It might seem much too loud in my little studio, but I realize that performing in the studio is not my ultimate goal and so continue to imagine my concert-hall surroundings and play accordingly.
Practicing at a “blasting” volume is helpful
Milan Yancich includes several loud exercises in his section of warm-up routines in A Practical Guide to French Horn Playing. In one he notes that warming up at full volume
…is intended to develop strength and power of tone. I suggest almost blasting volume with attention to bigness and fullness of tone, each day playing louder and fuller than the previous day. What eventually happens is that the performer loses his propensity to blast harshly and begins to think in terms of a big, broad tone.
Aperture control, again
In A Creative Approach to the French Horn Harry Berv points to the need for to aperture control when playing at volume, when he notes “In very loud passages the aperture is large and air is blown through the lips with great intensity.”
Re-evaluate your dynamics in relation to your performing situation
Barry Tuckwell in Horn has an interesting tip on dynamics and the soloist, noting “A soloist must re-evaluate his conception of dynamics and must adopt a new musical approach, for now he is leading not only the conductor but the entire orchestra.”
Practice playing fortissimo every day
Frøydis Ree Wekre in Thoughts on Playing the Horn Well notes that
Practising fortissimo is hard on your environment and on yourself. Find times and places where you do not bother too many people, but do get some real loud playing in daily.
She lists the purposes of loud practice, which besides dynamic control include building strength and endurance, accuracy, support, and “Explosiveness of breath” and “Explosiveness of lip-action.”
Keep the lips relaxed
Relaxation is a key concept for loud playing for Douglas Hill in Collected Thoughts on Teaching and Learning, Creativity, and Horn Performance.
To play loudly you must allow the aperture to relax as it becomes larger, while the airflow increases in both size and speed. Though the sound of a loud horn is very powerful and bold, you must not get misled into believing that this muscular sound requires a great deal of body tension. It only requires a greater volume of air and an aperture that is relaxed and open enough to respond. (Let the horn be the loudspeaker; that is why it is attached to your face.)
What I suggest is…
I feel a key to loud and soft playing is learning to control perfect long tones (roughly 8 counts long) with a perfect cresc. and dim. from nothing to very loud and back to nothing, keeping exactly on pitch, the dim. being a perfect mirror of the cresc. I have done this in a very easy middle range every day for years and years (two versions of this are in my warmup book), it is central to how I begin my warmup. If you can do this, it opens a musical world up to you.