From this comes the rule that he who departs from natural usage passes from pleasure into annoyance. Above all else the young singer should avoid exercises and studies sotto voce, because not only the trill, but every other ornament of singing, more and more, when sung sotto voce, makes it impossible to execute them any other way, and every time that he wishes to produce them in full voice, huge in large and vast places, he cannot execute these passages, or if he executes them, they cannot be other than full of imperfections, and unpleasant. While it is easy to execute any ornament in a weak and soft voice, it is very difficult to execute them with a large and strong voice.
Mancini, Giambattista. “Pensieri, riflessioni practiche sopra il canto figurato (Vienna, 1774), trans. and ed.” E. Foreman as Practical Reflections on Figured Singing (Champaign, 1968): 74.
Gathering from the above passage from Mancini, it would appear that he wanted singers to sing their music, especially of a more decorative nature, in piena voce “full voice.” This is a far cry from the methods of modern practice which aim to ‘lighten’ the coloratura passages in order to render them.
Perhaps a reassessment of our relationship to ‘soft singing’ is in order, especially from the point of view of actual voice building in the studio.
Let the Master instruct him in the Forte and Piano, but so as to use him more to the first than the second, it being easier to make one sing soft than loud. Experience shews that the Piano is not to be trusted to, since it is prejudicial though pleasing; and if any one has a Mind to lose his Voice, let him try it.
Tosi, Pier Francesco. Observations on the florid song. J. Wilcox, 1742.