Seamless glissandi are one of the things that due to the unique construction of the Trombones can be executed very easily on them. However, their maximum span can only be a tritone (the range the slide covers between all in and all out (or rather first and seventh position)).
Additionally, the range of the glissandi might be limited as you need to be aware that the harmonic scale of the Trombone builds on top of the Bb two octaves and a whole tone below middle C and only notes included in the natural harmonic scale based on that Bb can be produced in the first position (with the slide all in). If you want to learn more about this, here are some excellent visualisations.
For instance, if you want to glissando downwards from the B a major ninth below middle C on a regular Tenor Trombone, this will not be possible, as this B can only be produced with the slide full out (see chart linked above). For the glissando to be seemless, you also need to stay on the same harmonic. It is possible to cheat glissandi while jumping to different harmonics but it will not give you a completely seamless impression.
Effective glissandi however don’t need to cover huge spans. Most glissandi on Trombones don’t expand over a third. In fact, even semitone glissandi can be highly effective. Generally, glissandi upwards are more effective than downwards as they also crescendo on their way up making the effect more prominent.
Hello, my name is Simone Candotto and I am principal trombone at the NDR Elbphilharmonie Orchester in Hamburg (Germany). I got the link on your Web site through my best friend. I just want to thank you for explaining about Trombone glissando. I play in professional orchestra since 1993 and still now, when we play new compositions, when it goes about trombone glissando seems that lot of composers (including famous ones in the classical contemporary spectum) do not really know from wich note to wich note the glissando is possible. They probably learned that trombone can play glissato for a tritone, but the didn’t get that not every tritone is possible to gliss.
On the other hand, it is difficult to remember everything from every single instrument, but if you have a doubt just ask the person involved with that specific instrument 🙂
In my free time I also enjoy arranging, composing and do orchestrations, but just as hobby. I arranged 2 traditional songs for our orchestra for our tour to Japan and Spain (San Sebastian) and we played as encore. It was a great feeling.
Greetings from Hamburg and best of luck.
Dear Simone, thanks for your comment! It is fascinating to see how many trombone players send feedback over just this one small article alone, saying basically the same thing as you did. Seems like the understanding of trombone glissandi is really a weakspot with many composers and orchestrators. I hope that this article eventually helps to bring awareness to this issue and eventually will make yours and your colleague’s life easier in the future. All the best!