I found some more quotes from singers and teachers of the Old Italian School on the mouth position in singing.
They all advocate a smile, or beginning of a smile when singing.
I can attest to the fact that in my own work as a singer, a slight smile has brightened my voice considerably and facilitated greater ease and flexibility throughout the range of my voice.
It’s also especially helpful in the lowest tones of the voice, preventing the singer (especially overly muscular male singers!) from ‘dropping into’ the chest. It gives the lowest chest tones a clarity and presence that the ‘dropped’ approach does not.
Personally, I always had to squeeze more in a dark approach to get brilliance into the lowest tones.
Gaetano Nava was a Professor of Singing at the Milan Conservatory from 1837 to 1875. From his Practical Method on Vocalisation:
The rule prescribed by the good school of singing is to keep the mouth open in such a way that the upper teeth should be immediately over the lower and that without the least discomfort, and almost smiling, it should preserve in that position a natural fitness and grace.
Luigi Lablache was one of the greatest basses of the 19th Century, and a true singer of genius. He said this in his book A Complete Method of Singing:
The mouth ought to be kept in a smiling position but without grimace, and sufficiently open to admit of the end of the forefinger being placed between the lips.
Sim Reeves, another celebrated singer, was Britain’s most successful tenor. He sang the role of Edgardo in Lucia di Lammermoor in 1846. He talked about lip positions in his book The Art of Singing, written in the year 1900:
Shooting the lips out, like the bell of a trumpet, produces a contemptuous effect in the tone, as if the singer were bellowing rather than singing. All affectations in the formation of the mouth should be avoided.
Richard Mackenzie Bacon, the English voice teacher and critic, said the following in 1824:
The mouth, which the English singer causes to take a very principal direction, has little, if any, immediate influence in the formation of the Italian tone. The mouth and lips are much more visibly at rest; they assume a gently smiling character; the aperture is lengthened rather than rounded as in English singing.