Hi @tabby,
I think bullying is a really complex issue. I read somewhere that most bullies aren't even aware that they are bullying people!! It's just that their behavior or personality is such that it is easy to take offence to things they say or do. They may be abrupt or passive/aggressive without even knowing it, becuase it has become habitual behavior for them.
To be honest with you, I believe I have been bullied most of my life, in pretty much every job I've ever had! I don't believe I ever confronted a bully, other than trying to be super-nice to them, and when that didn't work, trying to avoid them or just sort of smile blankly at them and move on.
Yes, usually the bullies lose interest in me quickly. I am not a very reactive person (at least not on the outside). Internally, I am usually a bundle of nerves, but other people have told me that they apparently don't see it. They see me as very calm.
If you feel up to it, invite the bully out for a coffee after volunteering and ask her all about herself. You will probably find that she is a very a unhappy, confused, frustrated or anxious person.
This is not to say that bullies like this don't do a lot of harm! They absolutely do!
If you are not up to talking to her directly (I probably wouldn't want to do that, myself) then you might like to talk to one of your fellow volunteers about the situation, just to get another opinion on it. Try not to denigrate the bully in any way. Don't come straight out and say "She is a bully". Just say that you feel you might have a little 'personality clash' with her. That way you are not really pointing fingers, so to speak. You are making it a mutual problem.
Ok, this is sort of telling a 'little white lie' but it could be for the best.... especially if you want to stay there and keep volunteering.