Cleaning vs. Replacing Bow Hair
Bow hair becomes dirty over time, picking up dust and oil, and this takes away from your bow's ability to articulate and grab the strings and substantially diminishes the quality of its sustained tone. If the bow has lost much hair or if it can no longer be tightened sufficiently to play, then it is time for a re-hair. But, if the hair is simply dirty, there is no need to replace it. It can easily be cleaned.
Now, I will just note here that some players rail against the idea of cleaning bow hair, insisting that it will either damage the hair, or that any hair old enough to need cleaning is old enough to need replacing. And, indeed, there are players who seek out very high-end, very fresh, live stallion hair, which does have superior performance characteristics. I don't recommend such hair for the average player because it stretches considerably over the first few months and is extremely susceptible to changes in humidity and temperature. It's also hard to find, expensive, and loses its superior performance rather quickly, as the hair ages. In any event, it takes a very discerning player to note the difference at all. But, if you are using such hair, you'll likely need to re-hair regularly, not only to maintain that premium performance, but also because the hair will often stretch to the point where the bow can no longer be tightened.
However, for the rest of us, there is no need to replace a bow's hair simply due to a year or two of age and use. The usual high-quality hair (Mongolian or Siberian), widely available from dealers and bow makers, will keep performing well for years, if kept clean. And, I can assure you that cleaning will not damage or detract from the hair's performance, no matter what your teacher may tell you. They are likely only repeating what they themselves have been told, without any real basis for the opinion. Indeed, you may be surprised how a cleaning can restore the tone and vitality of your bow.
What You'll Need
You'll need some denatured alcohol, available from your local hardware store. (Careful, it's laced with poison to deter its use at frat parties, so keep out of children's reach.) You'll also need some paper towels (or a clean cotton cloth - i.e. old t-shirt) and a comb.
First, tighten the hair to a bit under regular playing tension - just tight enough to keep the hair off the shaft of the bow.
Next, apply a generous amount of alcohol to the paper towel - fairly wet, but not dripping.
Wrap the paper towel over and under the hair and rub up and down the hair to get it clean. Try not to touch the shaft of the bow while you're doing this. Disclaimer: If you have a particularly fine pernambuco bow, please take it to a professional. With that said, you don't need to worry about damaging or dissolving the varnish on your bow, as you would on your violin, because bows are not varnished. Nothing terrible is going to occur if you happen to brush up against the wood a bit while cleaning the hair. Still, alcohol could temporarily dull the shine of the French polish, slightly, so try to keep the paper towel away from the wood. Another option is to remove the frog from the bow while cleaning, in order to keep the hair completely away from the wood. I don't personally find this necessary, but feel free to do so if it makes you more comfortable.
If the hair is very dirty, it may require several minutes of treatment, in this way, before it shows clean. Add more alcohol and change paper towels as necessary.
Once clean, allow the hair to thoroughly dry. This may take a few hours.
The hair will be clumped together - not good for rosining. So, take out your comb and gently comb it smooth. I use a comb especially designed for the purpose, but a regular old hair comb can do the trick. By placing the comb under the hair, and combing up, away from the wood, you'll avoid scratching the bow. Be gentle, and don't pull too hard. You don't want to break the hair or yank it out of the frog or tip.
Now you have clean hair, but no rosin. Ideally, at this point, you'd want to "prime" by rubbing crushed rosin across and along the hair. Raw hair is slick and doesn't take rosin well, so priming would help your rosin to adhere quickly and evenly. (With a cloth or piece of leather, repeatedly dab into the rosin and rub it across the hair, then along the hair, on both sides. You want to see an even distribution of rosin across the hair when you're done.)
If you are in the routine of replacing your rosin with each hair cleaning/re-hair (which I strongly recommend), then you can take your older (but not ancient) rosin, place it in a zip-lock bag, and crush it with a hammer. (This is the sophisticated method we use in our shop, though we always use a fresh cake.) You'll just need a container of some sort to pour it into, after crushing. You can see that I've used an old, plastic parts drawer for the purpose in the photo, above.
If you don't have crushed rosin, don't despair. You can just apply your regular rosin, as usual, without priming. But, be prepared to rosin for quite a while before the bow is ready to play. Priming helps to distribute the rosin evenly. But, even so, you'll need to use 20 - 30 strokes of your regular rosin, after priming, to get it evenly applied. You'll then have way too much rosin on the hair. So, then run the hair over a clean cotton cloth (old shirt) for maybe 10-20 strokes to remove the excess. This will also help to even it out across the hair.
Once settled, you should apply only 3-5 strokes of rosin, each time you play. Add more, as necessary, if the bow doesn't grab. Use less if you see clouds of rosin coming off your strings. (Most students use way too much rosin, which dampens the tone, not to mention making a mess and damaging the violin's varnish.) If your bow sounds "scratchy," it's a good bet that you're using too much rosin, or rosin that is too old.
Fresh, High-Quality Rosin
One last note . . . Please don't go to all the trouble of cleaning your bow hair, only to apply old and dried-out rosin to the freshly cleaned hair. Ironically, though teachers tend to rail against cleaning hair, they often fail to encourage their students to replace rosin regularly. For good bow performance, and especially for good tone quality, never use rosin that is more than a few years old. The best recommendation is to replace rosin every 6-12 months, or as often as you clean/replace your bow hair. And, stay away from the ultra-cheap rosins that sell in the under $5.00 range. A $10.00 - $20.00 investment in a decent cake of rosin can make a world of difference to your performance and tone. Or, spend a little more on a really fine rosin, and I promise you, you'll notice the difference!