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We all know that rosin is the main cause of friction on a string-instrument bow, but it also seems to have other properties.

For example, when I have a good coat of rosin on my bow hairs, they tend to break less easily.

Also, I had been noticing that my bow hairs had been loose. I owed it up to the cooler, damper weather that we've been having here. I would try to tighten the hairs, but the screw would already be all the way taut, even though the hair touched the stick with hardly any pressure. Then I applied some rosin half way through a practice, and as soon as my bow was back on the strings, it was tighter. So I loosened it and applied more rosin, and now I have the bow's full range of tightening back.

What exactly is it about the properties of rosin that make this happen? Is it just my imagination that this happens?

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    Less friction = less wear and lower likelyhood of catching. As for the tension, I think your guess about damp is correct -- the rosin probably drew out some of the moisture. I don't play anything with a bow though, so this is just my best guess. – Matthew Read Jun 13 '16 at 5:05
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    Less likely to break --> because you're not applying a ton of pressure when properly rosined. BTW, if you can't tighted your bowhair under any meteorological conditions, there's something seriously wrong and you should get it rehaired. – Carl Witthoft Jun 13 '16 at 11:16
  • @CarlWitthoft Yeah, I suppose that there is something wonky about my bow. It's only 6 months old, but it was supposedly a really high quality bow. As soon as I got it out to rosin it, five or six hairs just popped out from the tip. I guess it was in a store house for a long time... – General Nuisance Jun 13 '16 at 15:19
  • By "high quality," do you mean it cost more than $100 or more than $10,000? Is it wood or is it carbon fiber? There's an incredible range of bows out there. – Carl Witthoft Jun 13 '16 at 15:40
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Humidity is known to have an effect on bow hair - hairs stretch longer with higher humidity, as mentioned by Carl in his comment. Interesting points here on which season (!) you should get your bow rehaired in depending on atmospheric conditions, they also recommend getting it done locally... Did you buy your bow from a different region/country? The correct hair length from the bow's last location might be different to yours!

As for the rosin causing the hairs to get tighter again - I have never observed anything like this myself (UK, usually moderate humidity, no extreme temperature swings...). I would tentatively suggest that if the rosin you are using is particularly hygroscopic (readily absorbs water) that it might be drying out the hairs, causing them to tighten. However I really wouldn't have expected any such effect to be that noticeable...

This research group did work on tensile strength of horse hairs (also cool pics of hairs with and without rosin). It would be interesting (it appeals to my music and chemistry geekiness) to see similar experiments to determine the effects of humidity and amount of rosin.

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