Portamento, a vocal glide between notes, has slowly been coming back into fashion in string playing, after quite a long period of unfashionability starting in around the 1950s. It never truly disappeared, of course, so it is easy to exaggerate the extent of these style changes, especially when nostalgia for a lost golden age is concerned.
As usual, rather than a case of a simple on/off switch, it’s probably more accurate to think of this as a series of complex shifts in the balance of how sliding is used, how it’s valued, and (therefore) how it’s understood. It’s common for commentators to trace changes in vibrato usage precisely like this: the simple version is that vibrato was deployed rather selectively in the past, and only later – well into the twentieth century – did it become a ‘core’ component of sound production. Changes in the use of portamento seem to go in the opposite direction: it appears to have been considered a "primary" option for musicians, integrated with the way in which their minds imagined certain melodic intervals (and their contexts). Perhaps, once upon a time, some listeners may have asked why you didn’t slide between those notes, just as now they might ask why you didn’t vibrate on that note. Now it is more common to treat sliding as an optional extra, still used, still important; but in moderation, to be deployed with awareness, control and taste.
One of the main differences between early recordings and modern performances, then, is simply of how often portamento is used. Interestingly, the zeal with which pedagogues have policed this has remained broadly the same over the decades, despite the huge changes in what musicians actually did! Verbally advocating restraint seems to have generated a significant amount of cultural capital. Curbing its deployment indicated respect and seriousness towards the task of interpreting great music, rather than cheapening it by exploitative, indulgent tricks of expressivity. (The treatise by Joseph Joachim, compiled with his disciple Andreas Moser, is a good example of this, but it's a trend that runs throughout the literature on string playing.)
An even more problematic pedagogical battleground than frequency is clearly the idea of ‘tasteful’ execution, which appears all over the writings of the period. Early recorded string players tend not to release the bow pressure very much as the finger moves along the string. The best players use the bow in a vast number of ways during a slide, but in general, they keep much more contact with the string than modern players are encouraged to. Another unfamiliar aspect of the technique is that the physical impulse can sometimes be a little uneven, resulting in a sound that is surprisingly uncontrolled.
You don’t have to listen to many old recordings to realise that a ‘tasteful slide’ doesn’t really mean the same thing now, as what it meant a century or more ago. It’s impossible to see portamento in isolation, too, because the character of such a vocal gesture is closely related to flexibility of timing, and how beats are accented and de-accented. In practice, there are so many ways of executing this kind of vocal gesture that the written word has almost no chance of really capturing it; as is so often the case, the sounds of performances seem to have been hugely more important than treatises in determining how slides were conventionally rendered. Consider, for instance, the parallels and differences between these two incredibly detailed recordings of very famous pieces:
1) the famous nineteenth-century singer Adelina Patti, recorded in 1905 (https://www.youtube.com/watch?v=r1fIdT3QG4E)
and 2) the cellist W.H. Squire, recorded quite a few years later (https://www.youtube.com/watch?v=3P80ja0YhcU)
Because the idea of portamento is imbued with so many ‘emotional’ associations, that tends to be the lens through which we understand how it was used. But I think that this view can be restrictive. There are all sorts of potential layers that we miss, because a gesture like a slide is not ‘only’ the isolated expression of emotion, but is intertwined with the perception of musical structure. The historical emphasis on the sentimentality and Romanticism of the early twentieth century performers often diverts our attention away from some of the fascinating grammatical-rhetorical strategies that we hear in old recordings. But one can make a good case for seeing portamento as one of the most important tools with which musicians negotiate notated structures, not least because its expressive power has the potential to generate some sophisticated patterns of tension and release.
One of the best examples I know of this is the Capet Quartet’s recording of the Ravel String Quartet.
By modern standards, there are a lot of slides in the passage that leads into to the second statement of the theme (beginning at about 0:18). But I don’t think this is an example of Romantic excess; I think it’s an example of the structural use of portamento. While the sliding could be said to imbue the material with emotions related to sighing or crying, I don’t think this necessarily true, and it certainly isn’t the whole story. My theory is that the insistently expressive vocalisation does something else: it contributes to the sensation of motion towards a goal. With each successive slide, in tandem with a subtle acceleration towards the cadence, the sense of an imminent, inevitable arrival is increased; in other words, the portamenti are used cumulatively. When they reach the cadence there is a simple ‘grammatical’ moment of closure, and then the new, familiar phrase stated cleanly, without any slides. The sophistication of the different kinds of sounds, inflections, timings, balances (and so on) of the end of the paragraph is thus highly contrasted with the neat simplicity with which the next one starts. Indeed, the new phrase – which is an adjusted repeat of the opening – is devoid of any sliding at all, right up until the music starts to deviate from the original pattern.
Again, it’s important to note that much of this effect results from their approach to timing, and the balance of evenness and unevenness; it can’t just be put down to portamento in isolation. I think, then, that it would be a mistake to always associate density of sliding with ‘Romantic weakness’ or ill-discipline; done well, it can clearly serve an important structural, as well as emotional function. Seeing portamento like this, as a tool that can help the music make grammatical sense, is an important step in its gradual rehabilitation, and may give us a chance to develop all sorts of inventive ways of deploying it that go far ‘beyond indulgence’.
n.b. Incidentally, the opening of this piece is an example of how readings of musical structure can be disproportionately affected by small articulation or dynamic markings. I know of almost no versions of the Ravel that approach the beginning as an 8-bar phrase as the Capets do here; the > hairpin in b.4 is almost inevitably treated as an invitation to make a new phrase, with a ‘colour change’ and sometimes quite a considerable amount of time taken. This does result in the strange sensation of a significant break at the very top of the melodic arch; while it’s perfectly reasonable to do this, it’s interesting that the Capet Quartet – as with so many of the older ensembles – seem to prioritise melodic and harmonic structure over other performance markings, and commit to the entire span of the contour rather than cutting it in half. It is, however, quite hard for modern players like us to turn off this reflex!