Copyright © 2017 Oliver Steiner
When it comes to all kinds of ornamentation, including trills, vibrato and portamento, I believe the underlying general principal is a definition of an ornament as something which enhances the thing that it ornaments, rather than distracts from it. Therefor Kreisler's trill is so beautiful largely because of its rhythm: the melody note is what is heard most of the time along with extremely brief flicks of the ornamenting trilling note, Hence the trilling note truly ornaments the melody pitch rather than distracts from it. Apply this principle to lipstick and we find that a small amount of lipstick enhances the appearance of the face, but 1/4 pound of lipstick smeared over the whole lower half of the face looks grotesque because it distracts from the face rather than enhances it. Now apply this aesthetic principle to portamento and we find that the gross sounding portamento sounds gross because it lacks subtlety. It is too slow and too loud, therefor it distracts us from the pitch interval which it should be enhancing. When Milstein does a portamento it's late enough, fast enough and quiet enough to be truly ornamental. - It heightens the drama of the pitch interval. For me, the violinist who uses portamento most similarly to its use in dramatic opera (e.g.: Gigli) is Yehudi Menuhin. Listening to Menuhin's second movement of Paganini D Major Concerto most vividly reminds me of Gigli's operatic drama.