This may be more info than you ever cared to know... but it's a good read. I'm hosting it from my site in case the original disappears. Do me a favor and download it to your local hard drive so my bandwidth doesn't go through the roof if you need to keep referring to it.
http://www.eclecticmojo.com/uspglycerin.pdf
http://www.eclecticmojo.com/uspglycerin.pdf