Rossini is the first Italian opera composer, looking back, who seriously interests us. Rossini in his youth copied the music of Mozart and incorporated into his otherwise purely Italian style Mozart's ideas of orchestration. We want more from an orchestra than prior Italians usually provided. Rossini used his orchestra in ways that Mozart didn't--to give the music a lively, driving rhythm with the unique Rossini accelerando heard nowhere else.
But is there any reason to believe that styles of singing were radically changed by Rossini from those of his Italian contemporaries? I would propose not. He is still light in weight and more leggiero than Mozart. He adores the complete facility of Italian coloratura. Toward the end of his opera composing period he began increasingly to compose the coloratura, but we should presume this represented the prevailing Italian style. The Italians retained throughout the bel canto the feeling of extemporization in the coloratura even after it was no longer extemporized.
By the time of Rossini the importance of the castrato was far less than with Handel. I was able to find evidence of only one very early role written by Rossini for a castrato. There are many heroic contraltos, including Tancredi, Falliero, Arsace and Neocle, but all were created by women. The female coloratura contralto was prominent in Rossini, and the operas were written for many companies and several different contraltos, not just one. Maria Malibran is known to have sung in Tancredi, Otello, and Semiramide. The concept of a countertenor singing in falsetto simply did not exist in Italy. [Footnote: The mezzo role in Otello is Desdemona. Otello is a tenor.]
We may say with assurance that the twentieth century revival of serious Rossini was the result of the appearance of Marilyn Horne to take on these heroic roles. What was the sound of a coloratura contralto in the time of Rossini? I prefer to imagine that they sang with the same dark color and medium to light weight as Horne, Bartoli or Genaux, and not the much heavier weight associated with the Verdi mezzo-soprano of Zajick or Cossotto.
The problem with reviving Rossini lies in the fact that a singer must specialize in this repertoire to do it justice, especially mezzo-sopranos and tenors. The standard technique of a late romantic mezzo-soprano or tenor will simply not do. Technically Rossini is closer to Handel than to late Verdi.
But by the time we come to Bellini's Norma in 1831 all has changed. Rossini has written his last opera and feels no inclination to compose in the new style. What happened?
For one thing Beethoven happened. Beethoven's big three mature vocal works--the Ninth Symphony (1827), the Missa Solemnis (1823) and Fidelio (usual version 1814)--all employ a new style of singing, a style not even hinted at in Mozart and Haydn.
Beethoven wasn't content with the serene entertainments found in those around him and sought to shake people up. There is no apparent Italian connection in his music, least of all in his vocal music. He wrote music even for chorus where the part is more shouted than sung. He sought to frighten or elate you beyond your every day experiences and cared nothing for mere entertainment.
Beethoven invented the Heldentenor and Heldensopran, possibly without ever actually hearing them. In the 1820's Fidelio was sung around Europe starring Wilhelmine Schröder-Devrient in the title role, introducing people to something completely new. The most significant change is that a heavy tenor might be the heroic lead in serious opera.
This is all done with a held down larynx. I don't have to explain that, do I? If you hold your larynx down with sufficient force, you lose a lot of flexibility.
I only write about this because Beethoven's place in the history of singing is entirely overlooked. It is not to be supposed that the Italians payed any attention to a mere German. Weber followed Beethoven's lead, as did Wagner. The rise of German serious opera comes from this.
The other thing that happened was Gilbert Duprez. Here is this nice paragraph about Duprez from an article in the Times by Anthony Tommasini Published: February 16, 2003.
"This celebrated French tenor began his career in 1825 as a 19-year-old agile lyric tenor, a 'tenore di grazia,' to use the traditional Italian terminology. In 1831, at the Italian premiere of Rossini's 'Guillaume Tell,' Duprez became the first tenor known to take his husky chest voice up to a high C. Rossini likened the sound to 'the squawk of a capon with its throat cut.' But the pragmatic Rossini soon got used to it as he watched the increasingly frenzied reactions to Duprez's singing. The 'tenore di forza' was born, and the public has never stopped loving the voice."
I would like to question the sentence that "the pragmatic Rossini soon got used to it." From that day Rossini never wrote another opera. What evidence is there that he simply got used to it? He stopped writing opera and disappeared into retirement. He was 39.
For more information about the progress of the tenor in Italian opera see here.
How people sang can probably be deduced from the music that was written for them. With the exception of tenors, there is no reason to suppose that styles of singing suddenly shifted from light coloratura to the heaviness of late Verdi. Donizetti continued to write for a lighter tenor, as in La fille du régiment (1840), an opera comique. Change in Italian singing was undoubtedly gradual.