What one wants to know is when singing began to break out of the Italian Baroque vocal ideal, where new styles of singing arose and why. The French ideal is different but somehow smaller, less serious.
Mozart was carefully trained to compose in the Italian style, including composing in Milan for the Italian theater. Mitridate re di Ponto falls in this category. These operas show no particular surprises, no departures from earlier composers.
When we move into the mature part of his career he becomes more German, which means that the orchestral texture acquires density and the coloratura is consistently composed. These qualities of Germanness also distinguished Handel from his Italian contemporaries. In Mozart's time the harmonic rhythm is slower than Handel's, and the tempo is somewhat freer. For my ears I hear in mature Mozart an increased emphasis on legato which is his distinguishing technical characteristic. From the modern perspective mature Mozart represents the vocal ideal toward which all strive. Mozart is the place to start.
Mozart composed opera seria in his mature years. Both Idomeneo and La Clemenza di Tito contain roles for castrati, but there are none in his far more famous comedies. Opera buffa never used them. Cherubino was always intended for a woman to sing. The central hero for both of Mozart's mature opera seria is a lyric tenor. The castrato is no longer the primo uomo.
The Mozart tenor still never goes beyond the lyric category, but his sopranos are all over the place. Barbarina, Zerlina, Susanna and Despina are soubrettes. Pamina is a lyric soprano. La Contessa is a lyric soprano with maturity, practically a spinto. The roles increase in heaviness through Fiordiligi and Donna Anna.
And what could one possibly say about the Queen of the Night? Are there Italian models for a demented character who can do a series of high F’s? I tend to feel that she is an extension of technical expectations with her much higher range. I prefer a soprano who attacks her with great intensity, making her completely mad.
Mozart conceives heavier voices for his characters, but none are for a heavy technique. The ideal for production is the same throughout, and the different voices come from the music. There is a mixture of leggiero and legato for variety, with an increased emphasis on legato even in the coloratura. He is the school for singing.
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