Sometimes, and for different reasons bow hair needs a good cleaning. I often hear about folks using soap and water to clean bow hair, but let me explain why it's not the best method to use.
First off, pinene which is a component of rosin isn't water soluble and second, soap is an emollient. The combination doesn't break down or dissolve the old rosin, but instead softens it and moves it around.
What ends up happening is that the rosin becomes soft enough to fill in the tiny irregularities in the horsehair, thus creating an unwanted smoother surface.
People are fooled into thinking that the rosin has been removed because the bow will no longer generate useable tone, but the rosin is still there.
The only reason the hair can no longer create useable tone is because it now has a flat, smooth surface, which means there's nothing there to grab/release the string...no tone.
After cleaning bow hair that way, people apply their new rosin and then wonder why their bow no longer accepts rosin or performs the way it used to.
They then assume the hair is worn out and so they replace it, undergoing a needless expense.
The best way to clean the bow hair is by utilizing rubbing/isopropol alcohol and a clean cotton cloth.
The alcohol will dissolve the old rosin, allowing it to be completely removed by the cotton cloth.
It's an easy, safe and uncomplicated process. One that you shouldn't be afraid to carry out.
There are some precautions we need to first discuss though.
It's a good idea to cover the wood of the bow with something like plastic kitchen wrap because if alcohol comes into contact with the finish on the wood it could damage it.
Also, the ends of the horsehair "ribbon" are attached to your bow using small wooden plugs. If the alcohol or any moisture for that matter gets in there, those plugs could swell, causing the bow to crack/split. Make sure you have the frog and tip well covered!
Lastly, before starting the cleaning procedure you must be sure to fully relax the tension on the hair. If you leave the hair with tension on it and then get it damp with the alcohol it will possibly stretch and once that happens the hair will no longer be able to be brought back to playing tension. So, make sure you've completely relaxed the tension on your bow before applying the alcohol.
Ok, here we go.
Use a clean, all cotton cloth....T-shirts work great.
Now saturate an area of the cloth about 3 X 3 inches with the alcohol. Wet, but not dripping:)
Next, grab the bow hair at one extreme end or the other with the alcohol soaked portion of cloth. You want to pinch the bow hair so that the alcohol soaked portion of the cloth is in contact with both the top (playing surface) and the bottom of the horsehair.
Now, while pinching the hair, slide the cloth in one smooth, uninterrupted motion to the other extreme end of the hair. It doesn't take a lot of pinch pressure, so be gentle.
Next, rotate the cloth so that an unused, but still saturated portion can again be placed in contact with the hair.
Always start your passes with the cloth on the same end you made the first pass from and simply repeat the pinch and slide part you did on the first pass. Rotate, pinch, repeat.
I usually only takes 5 or 6 swipes like this to remove the old rosin from the hair.
Very important! - Do not tighten the bow hair to add tension until you are completely certain that the hair has dried from the alcohol!
If you tighten the hair before it has completely dried it could stretch and if that happens it will never be able to be brought to proper playing tension again.
That's it! You now have a clean bow that is ready to accept your new rosin.