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I'm sure there are plenty of veteran tanners/taxidermists who will correct me if I'm wrong, but my understanding is that the pickling crystals are intended strictly for pickling. At least, that is what is implied in the instructions. After the hide is pickled and neutralized, the hide is treated with a separate tanning solution (Lutan or EZ-tan, for example). If you're new to tanning (and no offense intended if you're an experienced tanner), an step-by-step overview of the general procedure can be found at
www.amystaxidermy.com/tanning2.html