All great string players have one thing in common: impeccable bow control. Training the left hand to have perfect intonation is a matter of consistent practice and muscle memory. But the art of bowing is highly personal and takes perhaps a lifetime to master. All string instruments excel at the legato bow stroke better than other instruments in the orchestral family, which is to play a long series of notes in rapid succession. This should naturally be the first thing a string player practices to master their sound production.
In French, this is called 'Son Filé', which means to have a 'spinning sound'. It is mentioned in 2 major violin pedagogical manuals: Carl Flesch’s The Art of Violin Playing and Ivan Galamian’s Principles of Violin Playing and Teaching. The exercise is presented as long, slow bow strokes with attention to evenness of sound. It deals with how does one compensate for the difficulty of moving the bow so slowly. Since the world of the violin revolves around the beauty of tone and heartfelt vibrations, one must be careful to not overdo the slowness of the bow, at the expense of constant fluidity.
What matters is how one can identify the purpose of son filé and the benefits it teaches. It has taken on many meanings, including the subtle use of fingers to keep the bow smooth, but the most important goal is sweetness and versatility of sound. There are certain details that make sounds expressive and warm, and variations on son filé to highlight them.
(A beginner student with an optimal bow hold at the frog)
The Guideline for a Spinning Tone
Make sure that the string is engaged on every bow change - the string should be fully vibrating instantaneously. This starts the sound in its purest form. Create freely vibrating tones without pressing the bow too hard. Use the bow to imitate the human voice.
For example: play three or four inches of the bow, and within that space play variety of expressive nuances. A staccato can be followed by a gentle detaché. A fast stroke can be followed by saving the bow, while retaining the same sound. A perfectly horizontal stroke can segue to a highly pressurized vertical stabbing, mimicking the changes of character in the music.
Such is the bow’s versatility that it can change a violin's sound on a dime. The exercise of son filé connects thought and preparation with an individual player's sound.
(David Oistrakh, who was well known for his silken tone)
Practice changing strings on a long, moderately fast bow speed, letting the hair 'absorb' the string without throwing off the bow’s balance. To make the exercise more useful, add a new dynamic after the string change, while keeping it musically proportional and satisfying. A bow arm that feels good will also sound good. A useful analogy to have a good feeling when drawing a sound from the instrument is like the act of brushing your own hair and making it smooth.
The traditional son filé exercise - one very long, slow bow - requires the subtle play of the fingers to keep the bow balanced while moving over the strings. This means slightly different emphasis and usage of each individual finger as one moves up or down the bow, all to maintain evenness of sound and uniform balance of the bow. For example, the pinky will be most activated when bowing near the frog as opposed to using the index finger predominantly towards the tip of the bow.
Before putting the main focus on one's own fingers, one ought to check if there are also potential adjustments with a movement from another part of the arm other than the fingers. It could be a subtle shift in the angle of the wrist, or more energy from the lower arm. The possibilities are endless, and one is as valid as another. To find what works for each player, make a conscious decision to play louder or softer, then pay attention to what one's arm does. Since the bow arm is so personal, it will speak for itself. One may even find that all they needed was a little more bow speed to affect the sound.
(Jascha Heifetz, whose sound has a unique shimmering quality)
The open string exercise with whole notes - "son filé" - is not simply an advanced exercise on how to play an extremely slow bow speed. It reveals the subtle relationship between a delicately crafted bow, which particular muscles to work at specific points of bowing. Mastering this particular exercise is a lifelong challenge for many string players but will allow any string player to acquire refined and subtle control over their sound.
Just how much energy and gravity goes into the basic bow stroke has as many explanations as there are violinists and teachers. Getting from a functional sound to one of lyric beauty involves a critical discernment of the ear, the understanding of any music’s possibilities and a comprehensive knowledge on how to manipulate one's right arm down to the smallest muscle.