Early orchestras were dominated by the presence of a newly perfected instrument family: the strings. I believe this is undisputed. However, another instrument family was just starting to be improved when the strings achieved the height of their power in the orchestra.

While virtuoso violinists were popping up all over Europe, brass instruments were still only able to hit notes in their harmonic series, and were practically unable to do play melodic lines (except in the altissimo register) and were used mainly as "effects", such as the horn playing a distant horn call to give the work a particular feel.

That is, all brass instruments but one were stuck playing harmonics. The sacqueboute, the sackbut, the posuane, or as we now know it, the trombone is the one lone dissenter. This was a brass instrument that could play in any key, and could do so starting in at least the 16th century (well before the violin was perfected in around the early 18th century), and very few changes were made to that instrument in order to change it into the form we have today.

If this was the only brass instrument at the time that could take a melodic role and play more than harmonics, why then was it included in early orchestration? For example, this is the "classic" orchestra in question:

2 flutes
2 oboes
2 clarinets
2 bassoons

2 horns
2 trumpets
Tympani

1st Violins
2nd Violins
Violas
Cellos
Basses

Why would the trombone be excluded from the orchestra, but horns and trumpets allowed in? It can’t be because the trumpet and horn were easier to play, because it was impossible for them to change keys without completely changing instruments. The trombone, at that point in history, was one of two instruments that could play in both loud and soft ensembles (the shawm is the other, a predecessor to the modern oboe). Its versatility was unmatched by any other brass instrument. It could be made to play in pitch was both the trumpet and horn (soprano and alto trombone), though the soprano would have been very hard to play.

And yet, it wasn’t included in the orchestra until the Romantic period. It was never scored for as a solo instrument (do you know of any trombone concertos?) It simply was ignored by composers.

Was it a lack of talented musicians? A scoffing of an old fashioned instrument? Or some other factor I totally can’s see?

As a teacher of orchestration I will try to answer your very interesting question as fully as possible but there will be some gaps.

Sackbuts and shawms (the forerunner of the trombone and oboe respectively) were part of the pre-baroque musical scene in ducal and royal courts across Europe. unforunately they were far too loud for the new fashion and vice of the primarily French and Italian courts, the string band and there was a dynamic gap. The louder wind instruments were used for a while in churches and outdoor festival music but the strings were the first love of the monarchy and, as such, they were where the money was. Slowly wind instruments with less dymamic fortitude were developed to suppliment the String Bands and these gave rise to the baroque style orchestra from Monteverdi to Bach. The groups of instruments found in the compositions of the time always reflected the available talent when they were written, especially the solo groups and since as you not quite rightly point out the trombone (a relatively new instrument derived from the sackbut) has relativley few concertos from the baroque period we can see that there were very few good players around. The same applies to all instruments even today. In the 60s there was a shortage of good bassoonist and consequently fewer concertos were writen but now, after the 'Klaus Thuneman Explosion' there are many good players and they are comissioning works so bassoon concertos are on the rise.

It is interesting too to note that the use of horns and trumpets in the Classical era and the avoidence of Trombones and Ophilides etc. was also due to tonal balance on the one hand and harmonic nescessity on the other. Classical compostion is very 'tonally polarised' and it is actually very easy to whrite for horns and trumpets in this style using only the notes available (the harmonic series). Gradually however as compsitions became more harmonically complex and modulation to remote keys within the same movement became more and more popular developements were made to the instruments (valves added) and orchestrational tricks were developled (two pairs of horns in different keys).
The orchestra is a conservative place and changes are usually made slowly and after much deliberation sometimes taking 50 years (adding a fifth string to the double bass for instance).

I hope this goes some way to answering your question.
FC

Thanks for your expediant reply!

Also, please note that I made a mistake. When I said trombone (in this case meaning sackbut, not the modern trombone) and shawm were the two instruments that could play in both loud and soft ensembles, I actually meant trombone and cornett. My bad.

Have you ever heard of the alto trombonist Thomas Gschladt? I believe he was a trombonist known for consirable trill that was probably written for by Beethoven's teacher Albrechtsberger, and had good relations with Mozart.

A very interesting question and answer.

My own feeling, and I don't mean this entirely facetiously, is that the trombone is a very difficult instrument to take seriously. It is just funny. (And the same applies to jazz trombone.) I have just been sent for review a CD of Dominick Argento's cantata Jonah and the whale, with an orchestra of three trombones, three percussion, piano, harp and organ. And one can't quite take the trombone music seriously.

For my money Berlioz is the only composer who made trombones really effective - whether wonderfully accompanying Mephistopheles in La damnation de Faust or the fabulous pedal notes in the Requiem (copied by Havergal Brian in his Gothic symphony).

Does the trombone blend well with other instruments, or is it able to carry on an equal and clear dialogue with others? If not these might be the reason it didn't have a more prominent role.