For years it has been considered a truism that Verdi orchestrated his operas at the opera house during piano rehearsals. I discovered in my studies of all the operas, that the belief is a half-truth (true for only about half the operas).
Verdi did take considerable care in orchestration during a century of change. During the nineteenth century composers were presented with new challenges in assessing the role of the orchestra in the setting of drama to music. In Italy the "starting point of opera was the voice; and his early scoring shows Verdi was often unable to think of the orchestra, except in relation to it" (Budden, vol.1, 31).
By the time of Aida, considerable time and research went into selecting just the right instrumental color and the perfect combinations of instruments and instrumental special effects. The copious correspondence concerning the production of Aida is invaluable in discerning Verdi's authority in his craft and his careful attention to orchestral detail. One such detail actually involved the manufacturing of special flutes and trumpets for the first performance of Aida, which is documented in the following correspondence:Giulio Ricordi to Verdi: Milan, 13 October 1871
Verdi expended much effort in finding favorable flutes and the best trumpets for Aida. The flutes were needed for the Act I Danza Sacra delle Sacerdotesse and the return of the motive of that dance as repeated in G flat in the last scene of the opera; however, after much experimentation and consideration the regular flutes proved to be the most useful instruments for the dance motive.
Verdi only needed the tones of the overtone series to compose fanfares of originality that possess the grand dramatic impact necessary for operas such as Aida and Otello. In the "grand march" from the second act of Aida, Verdi used specially designed trumpets in two keys, one group in A flat and the other in B, permitting modulation while maintaining the desired trumpet effect. These so-called "Aida" trumpets were designed for purposes of appearance as well as sound.
In Aida Verdi used several instruments that were not part of his more neutral or standard orchestra. In all of his operas he used the following instruments: flute; ottavino (piccolo); 2 oboes; 2 clarinets; 2 bassoons; 4 French horns; 2 trumpets; 3 trombones; Cimbasso (bass of the trombones); timpani; gran cassa (bass drum); and strings.
In Aida you can see that there are a few more instruments, including 2 harps, bass clarinet and the English horn. The English horn was used in 8 of his operas and the bass clarinet in 7. To Verdi, these instruments had a meaning relative to the drama, and in Aida he waited to use them in Act IV when the dramatic situation demanded a dark and hollow sound.
The following is a selected list of some of the special features of the Aida orchestration.