The following excerpt is taken from Edward Foreman’s “A bel canto method, or How to Sing Italian Baroque Music Correctly Based on the Primary Sources, Pro Musical Press, 2006.” I am including his footnotes in the BODY for ease of reading in parentheses.

As a private teacher, I can attest to success with the “long tone” and messa di voce in the studio. The difference in the voice over time is significant, and in this blogger’s opinion is one of the vital exercises in the singer’s arsenal. This exercise should be mastered before any such agility exercises are attempted. From a registrational, functional perspective, a sustained tone encourages the action of the chest register, a thyro-arytenoid dominated sound, especially as more volume is added.

Theoretical Discussion: This section deals with the most fundamental moment of training, when actual vocal emission is begun. Presumably the teacher has ascertained if the student possesses the disposizione to become a singer, and may now proceed to undertake lessons.

Practical Exercises

Typically, exercises in intonation went hand-in-hand with the beginning solfeggi. Various authors gave directions for these first exercises, but in general the procedure was to begin with single pitches of a whole note or two duration, accompanied by a messa di voce. 

The first exercise is based on Tosi’s use of the Scaletta, the six-note scale which was the original basis of the modes. It seems easiest to begin with a modest challenge. The messa di voce should be made on each pair of tied whole notes, as indicated, and a breath taken at the comma between tied pairs. The tempo should be as slow as possible without running out of breath. Practice over time will permit the messa di voce to be held longer. [Author, adapted from Tosi.]

The manner of executing the messa di voce is this:

6. With a relaxed but not limp body, well-balanced on the feet, the chest comfortably open but not rigid, relax the abdominal wall and let the lungs fill with breath.

Author’s Note: Based on the information in the sources, this may be called a “natural” breath, one which is neither forced, contrived nor “managed.” When the abdominal wall is relaxed and the chest does not cave in – erect posture without rigidity – the lungs fill to their comfortable capacity automatically. Since this does not introduce any pressure into the thorax, it takes no energy to maintain a full breath.

7. The articulating of the vowel – “phonation” or “the onset” of the “tone” (Another stuffy point: the voice does not produce “tone.” It produces vowels and only vowels. The origin of the word “voice” is Latin “vox,” which means both vowel and voice. By definition then, the voice is a vowel.) should be carefully attended to. It wants to be neither nasal nor throaty, (Nasality and throatiness – which is extremely prevalent in modern “power” singing – were the two faults most often mentioned in the sources as far back as the Late Renaissance. They are to be avoided and corrected whenever heard.) but rather precise and neat.  The IPA symbol for the preferred vowel is [a], as in “father.”

8. The vowel should commence exactly on pitch – no scooping or sliding – at the softest dynamic level of which the scholar is capable. It is then swelled or crescendoed – increased in volume – through every possible gradation to the loudest comfortable volume, and then decreased by the same gradation back to the original soft volume.

Author’s Note: It is difficult to explain this verbally, but the smoothness of the increase and decrease of volume is essential, as is the “centered” quality of the vowel, which should not seem to touch the sides of the throat or require any effort in the increase or decrease of volume. Modern singers have a tendency to get caught in the throat as they get louder.

One of the principle difficulties arises from trying to describe “how to get louder.” One just does, and trial and error will make this apparently mysterious action quite clear, if the singer bears in mind that neither force nor effort is part of the process, and if the vowel is kept accurate and centered. The tendency to make the simple vowel “interesting” must be avoided, and under no circumstances is it to be “modified” as it rises in pitch.

The vowel [a] should be sounded with the utmost simplicity and directness. The exercise should be done very slowly, the vowel and the steadiness of the messa di voce requiring concentration – without tension – so that the voice can unfold. Daily repetition will increase the steadiness of the sound, the degree of softness and loudness and the length of the messa di voce the scholar can sustain.

This quote from Tosi’s Observations explains it quite well:

Tosi

Let them be taught to sustain the notes without letting the voice waver or hesitate, and if the teaching begins with a note of two measures length, the profit will be greater; otherwise the inclination which beginners have for moving the voice, and the fatigue of steadying it, will accustom them to not being able to sustain, and they will indubitably have the defect of fluttering, which is always in use by those who sing with the worst taste.

With the same lesson teach them the art of giving the voice, which consists of letting it come out softly from the least piano, so that it goes little by little to the greatest forte, and then returns by the same artifice from the forte to the piano. A beautiful messa di voce in the mouth of a professor who is not stingy with it, and does not use it except on the open vowels, will never fail to make the greatest effect. There are now very few singers who esteem it as worthy of their taste, whether because they love instability in the voice, or because they wish to remove themselves from the despised past.

The “fluttering” to which Tosi refers is the modern “wobble,” not to be confused with the natural vibrato of the steady sound. (There is nowhere in the literature a suggestion that “straight tone” was desired or attempted. This is a vocal anachronism created by tension.)

Author’s Note: The Scaletta from C-A should be comfortable for every voice range, transposing an octave down for low male voices. This first exercise is tedious, but it is necessary to lay a good foundation of simple notes, sung with a steady sound and a clean [a] vowel.

When the Scaletta is mastered, this more ambitious exercise may be undertaken, transposed to suit the comfortable range of the voice. (This purports to be from Nicola Porpora, teacher of Farinelli and Caffarelli, from “Porpora’s Elements of Singing. Adopted by Righini et all eminent Masters since his time. Extracted from the Archives at Naples. Edited by Marcia Harris, Professor of Singing and the Pianoforte, London, 1858.” Several other exercises will be used below.)

The scale is to be sung slowly on [a], with a breath on each fermata, and as indicated, a messa di voce on each note.

It is hard to know how long this practice should continue, but at least as long as it takes to get it to be easy, and the voice steady, without nasality or throatiness, which are functions of a good vowel concept. The longer the student works with a “pure” [a] vowel, the better.

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.