We have looked already at the various reasons why bow hair needs to be changed from time to time – stretching, unevenness, excessive breakage and sheer wearing out.
But what do we know about the last item on the list? How can bow hair “wear out”? What does it look like when it’s in good condition?
Here is a close up look at fresh bow hair, before it has been rosined.
With the naked eye, it looks smooth, but here you can see that the surface is a mass of interlinking scales. These scales by themselves aren’t enough to pluck at the strings as you bow – anyone who has picked up a re-haired bow from their luthier and forgotten to rosin will know the bizarre sensation of that first, silent down-bow stroke!
But without all those scaly corrugations on the surface of the hair, the rosin wouldn’t stick nearly as well. So that lovely sound of bow hair on string is very much a team effort.
So what happens when your bow hair no longer wants to hold the rosin? There are two possibilities. The first is that the rosin has caked hard on the surface of each hair, effectively smoothing it over. A quick clean by your luthier will help give the hair a new lease of life. But often, the hair surface has taken a battering – those little surface scales have worn away or disintegrated. Often the hair, overall, has become brittle in the process, so you will find you are breaking hairs more often.
For more technical information, you can check out the following pdf: ANALYSIS OF BOW-HAIR FIBRES by Alexander Mayer, Helga Pöcherstorfer, and Gregor Widholm at http://www.fineviolinbows.com/pdf/Analysis_bowhair.pdf