http://www.braintan.com/barktan/index.htm
Andrew, the link above has probably the most information on tanning with bark, leaves or vegetable matter that is all in one place on the web.
The term “acid” refers to the place on the ph scale. If it’s anywhere below neutral or 7 on the scale then it is considered an acid such as vinegar or lemon juice. Tannins are on the acidic end of the scale. Tannins are found in all kinds of plant material such as oak, hemlock, willow, or fir bark, leaves of plants such as sumac, dock, alfalfa.
Anything above 7 on the scale is considered alkaline or base. Such as wood ash, hydrated lime and lye.
When tanning a hide that you want to keep the hair on with bark or tannins, you must be sure to make your tannin solution strong enough or acidic enough to hold the hair. If it is too weak the hide will be wet too long before the acid can set the hair and the hair will start to slip. Consequently many hair on hides done with bark are “pickled” before being put in the tannng solution. You can make the pickle with vinegar and salt. I don’t remember the propoortions off the top of my head. The vinegar lowers the ph and the salt keeps the pickled hide from swelling too much.
After tanning is complete remember that a hide that is tanned is not necessarily soft. It gets soft by oiling and working the hide after the tanning is completed. Trying to get the hide soft without oiling will be frustrating and likely will result in a torn skin.
Anyway check out Matt’s bark tanning pages and look for the bark tanning section on the Hide Out. That’s the tanning forum linked to braintan.com . There’s lots of bark tanning info there as well.