Please note that none of these quotations from singing masters of the past indicate a pervasively serious expression, a tall oval mouth opening, protruding lips, or a long, sad face. Most of the mentions of facial expression that I have found instructed something along the lines of the lighter emotions, and the outliers were the ones suggesting a “relaxed face”.
Art of Singing (1877)
trans. J. C. Griffith
“Q. ‘What should be the expression of the mouth to most facilitate the emission of the voice?
A. The mouth should be smiling, the lips should be drawn sufficiently tight to merely show the upper row of teeth, that the sound, striking on a hard surface, may vibrate with greater intensity, and thus give a ring and brilliancy to the voice.” (p. 6)
“Observations and precepts regarding Article VI: …strive to give always to his singing a semblance of ease and elegance. To attain this end, the mouth should retain a smiling expression; but if by exaggeration the pupil should assume an aftected air, it will be better to change that expression for one of supplication.” (p. 9)
Anna Maria Pellegrini Celoni
trans. Edward Foreman
“Since the art of singing is completely consecrated to pleasing, thus everything should come together int he singer to satisfy that end. He should present himself with a smiling face, almost laughing…” (p. 8)
L’arte del canto (1841) (English title: Instructions and Progressive Exercises in the Art of Singing)
“OBSERVATIONS ON THE MANAGEMENT OF THE VOICE, ETC.
…To avoid these defects [guttural or nasal singing], the Student ought to practise the Solfeggios with the vowel A, shaping the mouth as if smiling.” (p. 7)
trans. Justin Petersen
“Manner of Positioning the Mouth… The best disposition of the mouth is that which Tosi and Mancini advised. According to them, every singer should place the mouth as: “he usually does when he smiles naturally, that is, in the manner that the upper teeth are separated perpendicularly and slightly from those of the lower teeth.“
Hints on Singing (1894)
trans. Beata Garcia
“…the expression of the face calm” (p. 12)
Pier Francesco Tosi 1723
Translated by Gallard
“Let him rigorously correct all Grimaces and Tricks of the Head, of the Body, and particularly of the Mouth; which ought to be composed in a Manner (if the Sense of the Words permit it) rather inclined to a Smile, than too much Gravity.” (pp. 25-26)
Brian Lee’s lesson notes 2010-2014
“Feeling of uplift, inside and outside. Leave the lips alone! A singer should appear to be enjoying singing.” (all paraphrased over the course of study)