Old rosin need to be removed when using new rosin
Do you need to remove the old rosin on the bow bakes before using new rosin? If so, how do you remove the rosin on the bow?
You do not NEED to remove old rosin. However, if you want to assess the efficacy of the new rosin more accurately removing the old rosin will help.
I think I should just play with my bow until the old rosin is used up?
Just put the new stuff on it and don't bother with cleaning the hair or similar shenanigans.
I'm trying to figure out what "the bow bakes" means.
I'm of the view that if it's the same price, it's the same rosin. Consequently, my bow has about 5 different rosins on it. I'm veering from dark to amber.
You can switch to your new fiendishly expensive rosin now. It will gradually displace the rosin already on your bow, or you can clean your bow hair in the manner that Andrew has described.
If you do choose to use alcohol, be sure to use enough. Insufficient cleaning with alcohol may result in a glaze of melted rosin that I believe is a source of unwanted bow noise (hissing sound) and loss of good tone. Better cleaning, or a rehair if necessary, is the way to resolve this. From my experience, I would say that cleaning with alcohol is a stopgap on the way to a rehair.
I like what you say, Charles. Indeed my teacher has said that when she was a student, washing the bowhair with soap and water was a stop-gap on the way to a rehair.
"How do you remove the old rosin?"
The nature of the rosin on bow hair changes as the bow continues to be used. Through repeat cycles of static and and sliding friction the microscopic rosin particles can approach and even endure melting.
Rosin also sloughs off. Just look at your violin after you've played for an hour. I think eventually you reach kind of a steady state in terms of the total mass of rosin on the hair. The problem is that it also attracts dirt and other crap because of its intrinsic stickiness and it's oleophilic character.
Some bass players use stiff toothbrushes, I like mani/pedicure brushes that are a hair bigger, but small enough to fit in a case without the long handle. That and a cotton handkerchief are enough for most instances.
I used a cloth to switch from Bernardel to my Leatherwood (it's amazing, by the way, but I don't get the same result on my viola as I do for the violin, but I expected that) and I noticed a difference right away between the two! I'll use the Bernardel if my violin is in the double case, since I usually forget to switch it over!