From Edward Foreman:

All of our sources consider the voice to have but two Registers, variously the Natural Voice or Chest Voice, and the Head, Falsetto or Feigned Voice. According to Tosi and Mancini – who were writing about the castrato voice – it was rare to find a voice without a clear division between the Registers, although neither of them used the word “break.” (They imply the single register voices were usually female.) In most cases the Chest Voice was the stronger register, in which case the Head Register was exercised and then the range was extended upwards from the Chest, singing gently without forcing, taking the high notes with sweetness.

No one before Corri actually provides illustrative exercises for extending the Range. (The limits of the Chest Voice given by both Tosi and Mancini are much higher in pitch than those of modern “normal” voices, another indication they are speaking of the castrato voice.) In part because the very young castrati had flexible cartilages and muscles, and it was fairly easy to get the voice to “stretch.”

Corri gives the extent of the Natural Voice as “no more than one Octave and 2 3 or 4 notes beyond […]” He advises the Scholar to attain as much of the Natural Voice as possible, and likens the process to stretching leather – if done gently, it stretches, if done violently, it breaks. When the full extent of the Natural Voice has been achieved, “his great study should be to contrive to unite the Natural to the first Note of the Falsetto, to blend them with such nicety, that the union may be imperceptible.” Transposition may be necessary to accommodate the extent of the Natural Voice without breaking into the Falsetto inappropriately, although it may sometimes make a necessary effect.

Corri gives four exercises for “encreasing [sic] the compass of the Voice.”

The interesting point about these exercises is that Corri says the compass of the Natural Voice is ordinarily an octave and 2-4 notes, yet he gives a two-octave exercise. (There is corroborative evidence, however. In “The Art of Singing,” a book purportedly written by Luisa Tetrazzini in 1909, she advises that singing this kind of scale will eventually result in smoothing the registers and opening the top of the voice.) Nevertheless, if the singer will assiduously practice these exercises – transposed to a key comfortable for the Natural Voice – and not force the high notes, great benefit will accrue.

Celoni does not mention the Registers, or Range. Manfredini, quoted in the first section, says the vocal range is 12 or 13 notes in the Chest Voice, to which the Falsetto must be joined. Garcia gives an octave and a fifth as the usual range of the voice. Longer ranges were not rare among the castrati, especially in their younger years. Some of the castrato soprano voices sank to contralto, and Farinelli seems to have had at least a three octave range, from C in the bass clef to soprano C above treble clef. No female voices seem to have had as great a range as that, and two octaves was considered quite acceptable for them. Tosi and Mancini are indefinite about Range, beyond their comments on the upper limit of the chest voice. Neither of them suggests a limit on the Falsetto. [Example by Author]

Since this applies to the castrato voice, it is given here as a curiosity rather than an aid. When the singer has mastered the Messa di voce and the Portamento, extending the range with care, comfort and common sense – while it is painstaking and somewhat tedious – is not difficult. The exercises on the Scaletta, transposing it upwards, followed by Corri’s exercises is as good a way as any, if done rapidly and lightly. There is an old saying: “Quantity first, then comes Quality,” which refers to extending the range and then smoothing the sound.

Foreman, Edward. A bel canto method: or, How to Sing Italian baroque music correctly based on the primary sources. Vol. 12. Pro Musica Press, 2006.